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Editor Work2016 Vol. 24, No. 5
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2016, 24(5): 721-731.
Abstract
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The curve landslide cumulative displacement is usually nonlinear. Hence, it is challenging to build predictive models with less error. In this paper, we propose a new methodology of embedding wavelet analysis with basic extreme learning machine(ELM) and online sequential extreme learning machine(OS-ELM) to predict the cumulative displacement. Firstly, by wavelet transformation, the cumulative function of displacement is discretized into periodic displacement and trend displacement. Secondly, basic ELM and OS-ELM are selected to predict the periodic displacement and trend displacement. Lastly, the cumulative displacement function is computed by ensembling the predicted periodic and trend displacement values. For basic ELM,a sigmoid function is selected as the kernel function and a single hidden layer with 33 nodes performs best. For OS-ELM,the prediction error reaches its minimum with 100 hidden nodes when the RBF function is selected as the kernel function. RMSE for ELM is 0.1423 and for OS-ELM is 0.1315. This methodology with high predictive accuracy performs better in comparison with other methods.
The curve landslide cumulative displacement is usually nonlinear. Hence, it is challenging to build predictive models with less error. In this paper, we propose a new methodology of embedding wavelet analysis with basic extreme learning machine(ELM) and online sequential extreme learning machine(OS-ELM) to predict the cumulative displacement. Firstly, by wavelet transformation, the cumulative function of displacement is discretized into periodic displacement and trend displacement. Secondly, basic ELM and OS-ELM are selected to predict the periodic displacement and trend displacement. Lastly, the cumulative displacement function is computed by ensembling the predicted periodic and trend displacement values. For basic ELM,a sigmoid function is selected as the kernel function and a single hidden layer with 33 nodes performs best. For OS-ELM,the prediction error reaches its minimum with 100 hidden nodes when the RBF function is selected as the kernel function. RMSE for ELM is 0.1423 and for OS-ELM is 0.1315. This methodology with high predictive accuracy performs better in comparison with other methods.
2016, 24(5): 741-751.
In recent years, many landslide dams are triggered by frequent tectonic activities and extreme climate. They are seriously threating the lives and properties in both upstream and downstream areas. Constructing spillway is the most common risk mitigation measure. The excavation volume of spillways is largely restricted by the limited available time and extremely terrible transportation conditions. It is urgently demanded to optimize spillway design and reduce the risk of dam failure to the minimum. This paper provides a breaching analysis method for landslide dams by considering the effect of different spillway design, which is applied to a case study of the Tangjiashan Landslide Dam. The instantaneous erosion rate during dam breaching can be obtained based on the interaction of hydraulic and soil erosive parameters. The influence of different spillways on the whole breaching process as well as the optimal spillway design are subsequently achieved. The case study shows that peak outflow rate could be reduced from the recorded 6500m3·s-1 to 1700m3·s-1 with the optimal design. The main reason is that a large longitudinal gradient incurs significant erosion in the spillway before dam breach, resulting in two peak outflow rates of largely reduced values. Comparing to a single trapezoidal spillway, combined spillways have smaller hydraulic radius, which reduces the less erosion and higher water level before dam breach, leading to higher peak outflow rate. Therefore, the risk mitigation effect of combined spillways is not as good as that of a single spillway with the same excavation volume.
In recent years, many landslide dams are triggered by frequent tectonic activities and extreme climate. They are seriously threating the lives and properties in both upstream and downstream areas. Constructing spillway is the most common risk mitigation measure. The excavation volume of spillways is largely restricted by the limited available time and extremely terrible transportation conditions. It is urgently demanded to optimize spillway design and reduce the risk of dam failure to the minimum. This paper provides a breaching analysis method for landslide dams by considering the effect of different spillway design, which is applied to a case study of the Tangjiashan Landslide Dam. The instantaneous erosion rate during dam breaching can be obtained based on the interaction of hydraulic and soil erosive parameters. The influence of different spillways on the whole breaching process as well as the optimal spillway design are subsequently achieved. The case study shows that peak outflow rate could be reduced from the recorded 6500m3·s-1 to 1700m3·s-1 with the optimal design. The main reason is that a large longitudinal gradient incurs significant erosion in the spillway before dam breach, resulting in two peak outflow rates of largely reduced values. Comparing to a single trapezoidal spillway, combined spillways have smaller hydraulic radius, which reduces the less erosion and higher water level before dam breach, leading to higher peak outflow rate. Therefore, the risk mitigation effect of combined spillways is not as good as that of a single spillway with the same excavation volume.
2016, 24(5): 752-759.
Engineering practice and statistics show that reservoir bank collapse usually occur in loose accumulation when slope is greater than 15 degrees. Reservoir bank collapse is small possibility when bank slope is 10 degrees or so. But it happens in Qiaoqi hydropower station reservoir area on Sichuan Baoxing River. As an factual example, the Mahuang valley 5# accumulation body is composed of gravelly clay. Particle size of less than 0.075mm is accounted for about 60%.The collapse mode of reservoir bank is slow creeping deformation after impoundment. Deformation range gradually expands backward. After more than nine years, deformation continues. This paper takes Mahuang valley 5# accumulation body as an example. It is on the basis of exploration and test work about soil structure and soil particle composition, reservoir operation mode and deformation monitoring results, and coupling analysis of finite element analysis of transient seepage and stability analysis. It studies the collapse mechanism of gently inclined slope on reservoir bank. The results show that fine-grained soil structure and weak water permeability are the main causes of accumulation of creep deformation in Qiaoqi reservoir area. The gently inclined slope stability and the periodical change of water level have an obvious correlation. With the rise and fall of water level change, the reservoir bank presents different steady states and the creep deformation rate differently.
Engineering practice and statistics show that reservoir bank collapse usually occur in loose accumulation when slope is greater than 15 degrees. Reservoir bank collapse is small possibility when bank slope is 10 degrees or so. But it happens in Qiaoqi hydropower station reservoir area on Sichuan Baoxing River. As an factual example, the Mahuang valley 5# accumulation body is composed of gravelly clay. Particle size of less than 0.075mm is accounted for about 60%.The collapse mode of reservoir bank is slow creeping deformation after impoundment. Deformation range gradually expands backward. After more than nine years, deformation continues. This paper takes Mahuang valley 5# accumulation body as an example. It is on the basis of exploration and test work about soil structure and soil particle composition, reservoir operation mode and deformation monitoring results, and coupling analysis of finite element analysis of transient seepage and stability analysis. It studies the collapse mechanism of gently inclined slope on reservoir bank. The results show that fine-grained soil structure and weak water permeability are the main causes of accumulation of creep deformation in Qiaoqi reservoir area. The gently inclined slope stability and the periodical change of water level have an obvious correlation. With the rise and fall of water level change, the reservoir bank presents different steady states and the creep deformation rate differently.
2016, 24(5): 760-767.
Based on the dislocations of earthquake faults, the geostress measurement and GPS regional displacements, this paper aims to explore the variation of geostress in the Longmenshan mountainous area before and after the 2008 Wenchuan earthquake using numerical inversion analysis method. The results of numerical simulation demonstrate that the average tectonic goestress in the Longmenshan mountainous area ranged from 12.6 MPa to 12.8 MPa before the Wenchuan earthquake. The direction of maximum principal stress ranges from NW to NEE.An clear change of geostress had been caused by the Wenchuan earthquake. It behaves mutation effect, hanging wall and footwall effect, and distance effect. The tectonic geostress of the hanging wall of the earthquake faults reached an average of 11.30MPa, whereas the geostress of the footwall at an average of 6.50 MPa. Meanwhile, the direction of maximum principal stress ranged from NWW to NEE.The geostress had clear mutation characteristics, which appeared to be the release or local increase of the geostress. Especially the geostress was released obviously at the foot wall. Different degree of geostress releases or locally rises at the hanging wall. With the increase of distance to the earthquake faults, the mutation effect of the geostress was gradually weakened.
Based on the dislocations of earthquake faults, the geostress measurement and GPS regional displacements, this paper aims to explore the variation of geostress in the Longmenshan mountainous area before and after the 2008 Wenchuan earthquake using numerical inversion analysis method. The results of numerical simulation demonstrate that the average tectonic goestress in the Longmenshan mountainous area ranged from 12.6 MPa to 12.8 MPa before the Wenchuan earthquake. The direction of maximum principal stress ranges from NW to NEE.An clear change of geostress had been caused by the Wenchuan earthquake. It behaves mutation effect, hanging wall and footwall effect, and distance effect. The tectonic geostress of the hanging wall of the earthquake faults reached an average of 11.30MPa, whereas the geostress of the footwall at an average of 6.50 MPa. Meanwhile, the direction of maximum principal stress ranged from NWW to NEE.The geostress had clear mutation characteristics, which appeared to be the release or local increase of the geostress. Especially the geostress was released obviously at the foot wall. Different degree of geostress releases or locally rises at the hanging wall. With the increase of distance to the earthquake faults, the mutation effect of the geostress was gradually weakened.
2016, 24(5): 768-774.
The mining slope in mountains area of southwestern China has some common characteristics. They include high and steep terrain,"hard stratum over soft stratum" slope structures, gently inclined strata, well-developed joints with high dip angle, and strong mining activities beneath the slope. The slope has not obvious movement basin. The slope cracks caused by underground mining are large and corresponding to the mined-out area. It has a complicated formation mechanism. This article takes the case of Jieniangping deformable rock mass in Guizhou province. It analyses the genetic mechanism of slope cracks using numerical simulation. Result shows that slope occurs cracks along joints and all the way to surface. Due to the impact of high and steep terrain and underground mining, the cracks continues to expand with repeated mining. The slope has a toppling failure trend toward surface. It proposes that slope deformation process includes three stages: mining disturbance, the tension at the crest, deformation increase through numerical simulation. Due to repeated mining in multi-coal strata, slope deformation has experienced four stages, namely initial deformation stage, constant deformation stage and accelerated deformation stage and stable deformation.
The mining slope in mountains area of southwestern China has some common characteristics. They include high and steep terrain,"hard stratum over soft stratum" slope structures, gently inclined strata, well-developed joints with high dip angle, and strong mining activities beneath the slope. The slope has not obvious movement basin. The slope cracks caused by underground mining are large and corresponding to the mined-out area. It has a complicated formation mechanism. This article takes the case of Jieniangping deformable rock mass in Guizhou province. It analyses the genetic mechanism of slope cracks using numerical simulation. Result shows that slope occurs cracks along joints and all the way to surface. Due to the impact of high and steep terrain and underground mining, the cracks continues to expand with repeated mining. The slope has a toppling failure trend toward surface. It proposes that slope deformation process includes three stages: mining disturbance, the tension at the crest, deformation increase through numerical simulation. Due to repeated mining in multi-coal strata, slope deformation has experienced four stages, namely initial deformation stage, constant deformation stage and accelerated deformation stage and stable deformation.
2016, 24(5): 775-787.
In recent years, some large and extra large hydropower projects have been built or are under construction in China. Underground caverns of these hydropower projects usually have problems of complex geological conditions and high geostress. High geostress often leads to failure of different degrees and relaxation of extraordinary depth in surrounding rock during the construction of caverns. In this paper, the development of relaxation in surrounding rock is studied through analyzing geophysical prospecting data while excavating of main powerhouse at Jinping I hydropower station. Optimal supporting time is studied through analyzing displacement and plastic zone of the surrounding rock through numerical analysis. It is shown that the plant axis should be parallel with the maximum principal stress, which can effectively reduce relaxation depth caused by the time-dependent deformation.80% of the relaxation depth in surrounding rock is formed after the excavation interval of 2 to 3 layers. Compared with the backward support, the displacement and plastic zone are obviously smaller when immediate support is applied. So, immediate support is beneficial to the stability of the surrounding rock. These research results can be used for reference when selection of plant axis, support design and supporting time determination of large underground caverns under high ground stress.
In recent years, some large and extra large hydropower projects have been built or are under construction in China. Underground caverns of these hydropower projects usually have problems of complex geological conditions and high geostress. High geostress often leads to failure of different degrees and relaxation of extraordinary depth in surrounding rock during the construction of caverns. In this paper, the development of relaxation in surrounding rock is studied through analyzing geophysical prospecting data while excavating of main powerhouse at Jinping I hydropower station. Optimal supporting time is studied through analyzing displacement and plastic zone of the surrounding rock through numerical analysis. It is shown that the plant axis should be parallel with the maximum principal stress, which can effectively reduce relaxation depth caused by the time-dependent deformation.80% of the relaxation depth in surrounding rock is formed after the excavation interval of 2 to 3 layers. Compared with the backward support, the displacement and plastic zone are obviously smaller when immediate support is applied. So, immediate support is beneficial to the stability of the surrounding rock. These research results can be used for reference when selection of plant axis, support design and supporting time determination of large underground caverns under high ground stress.
2016, 24(5): 788-797.
Shear zone has direct influence on stability of dam foundation, which is an important engineering geological issue, and is very common in the construction of hydroelectric project. Studying the engineering geological characteristics especially development and distribution characteristics of shear zones has significant and positive influence on the process of construction. In this paper, the sedimentary environment and facies in the dam area are investigated by the result of lithology, mineral composition, mineral formation, grain size parameters, sedimentary structure and so on. Development mechanism and distribution characteristics of shear zone then are studied based on the regional geological structure and the results of palaeosedimentary environment and sedimentary microfacies of protosomatic weak intercalation. Taking the distribution characteristics of shear zones into account, stability of dam foundation further is analyzed and evaluated. The results display that development and distribution of shear zones are determined by sedimentary environment and facies. It arises at the top of hemicyclothem frequently. Due to the multiple lithofacies, uncontinuity of spatial distribution of shear zones is very common. The types of the same shear zone in different position are also various. In addition, XRD and XRF results show that shear zones are highly dispersed system which is composed of various clay minerals, which demonstrates different engineering geological characteristics and strength parameters. Furthermore, FLAC3D states that the uncontinuity of spatial distribution and behaviors of shear zones have great influence on the stability of dam foundation, concerning to the project duration and cost directly. Consequently, studying the shear zones from the perspective of sedimentary environment and facies is available to have a clear understanding about development and distribution characteristics of shear zones, having significant influence on the evaluation of engineering geological condition and stability analysis of dam foundation.
Shear zone has direct influence on stability of dam foundation, which is an important engineering geological issue, and is very common in the construction of hydroelectric project. Studying the engineering geological characteristics especially development and distribution characteristics of shear zones has significant and positive influence on the process of construction. In this paper, the sedimentary environment and facies in the dam area are investigated by the result of lithology, mineral composition, mineral formation, grain size parameters, sedimentary structure and so on. Development mechanism and distribution characteristics of shear zone then are studied based on the regional geological structure and the results of palaeosedimentary environment and sedimentary microfacies of protosomatic weak intercalation. Taking the distribution characteristics of shear zones into account, stability of dam foundation further is analyzed and evaluated. The results display that development and distribution of shear zones are determined by sedimentary environment and facies. It arises at the top of hemicyclothem frequently. Due to the multiple lithofacies, uncontinuity of spatial distribution of shear zones is very common. The types of the same shear zone in different position are also various. In addition, XRD and XRF results show that shear zones are highly dispersed system which is composed of various clay minerals, which demonstrates different engineering geological characteristics and strength parameters. Furthermore, FLAC3D states that the uncontinuity of spatial distribution and behaviors of shear zones have great influence on the stability of dam foundation, concerning to the project duration and cost directly. Consequently, studying the shear zones from the perspective of sedimentary environment and facies is available to have a clear understanding about development and distribution characteristics of shear zones, having significant influence on the evaluation of engineering geological condition and stability analysis of dam foundation.
2016, 24(5): 798-805.
The Nanning metro line 1 partly passes through the complex strata of gravel-cobble and medium expansive rock. In order to obtain the evolution process of shield attitude when tunneling in this complex strata, the influence of water-swelling expansive rock on shield attitude is analyzed by measured data. The result shows that the mixture of water-swelling mudstone and synchronous grout can adhere under the shield when encountering water, and makes it difficult to control the shield attitude. The attitude in head part of shield can be descend by enlarge shield thrust in upper region when tunneling. But the attitude in postmedian of the shield can ascend. The water-swelling mudstone can raise the head part of shield when assembling shield segments. The thrust difference between upper and lower regions should be kept in a reasonable value when tunneling. Other control measures should be taken as well.
The Nanning metro line 1 partly passes through the complex strata of gravel-cobble and medium expansive rock. In order to obtain the evolution process of shield attitude when tunneling in this complex strata, the influence of water-swelling expansive rock on shield attitude is analyzed by measured data. The result shows that the mixture of water-swelling mudstone and synchronous grout can adhere under the shield when encountering water, and makes it difficult to control the shield attitude. The attitude in head part of shield can be descend by enlarge shield thrust in upper region when tunneling. But the attitude in postmedian of the shield can ascend. The water-swelling mudstone can raise the head part of shield when assembling shield segments. The thrust difference between upper and lower regions should be kept in a reasonable value when tunneling. Other control measures should be taken as well.
2016, 24(5): 806-814.
Amoy is a city with complex ground-environment. With the development of science and technology, urban rail transit project gradually becomes a sign of the level of development. In recent years, the construction of urban rail transport projects in the country has emerged in large and medium cities. Metro Line 2 in Amoy was built on January 31st, 2016. As we all know, excavation not only influences the foundation pit itself, but also is related to the surrounding buildings and structures, as well as the safety of various municipal facilities. The purpose of this paper is to study the stability of long and deep foundation pits in operation and the rule when soil is destabilized by excavating foundation pits. Especially this engineering project is in the complex ground-environment of Amoy coast. By measuring the monitoring data of the foundation pit in Haicang road station of Amoy Metro Line 2,this article conducts the research from three aspects: horizontal displacement of the underground continuous wall, surface subsidence and groundwater level changes. The article concludes that:(1) The maximum horizontal displacement of the underground continuous wall is 24.5mm, about 0.35% of the depth of foundation pit(H),slightly lesser than the other soft soil area. (2) Surface subsidence trough shows "√" shape distribution. The maximum appears at a distances of 0.5 times of the foundation pit depth(H) from the pit side, the range of influence is about 1.76 times of the depth of foundation pit. The maximum surface subsidence of monitoring sections is 22.4mm. (3) During the construction of excavation, the groundwater level of coastal areas shows fluctuations as a change in performance, showing a rising trend in whole. The maximum cumulative variation is about 0.72cm. (4) Change of seawall horizontal displacement and sedimentation change is unstable, but always small. For the long and deep foundation pit under the environment of coastal complex formation, the research results have reference value and guiding significance.
Amoy is a city with complex ground-environment. With the development of science and technology, urban rail transit project gradually becomes a sign of the level of development. In recent years, the construction of urban rail transport projects in the country has emerged in large and medium cities. Metro Line 2 in Amoy was built on January 31st, 2016. As we all know, excavation not only influences the foundation pit itself, but also is related to the surrounding buildings and structures, as well as the safety of various municipal facilities. The purpose of this paper is to study the stability of long and deep foundation pits in operation and the rule when soil is destabilized by excavating foundation pits. Especially this engineering project is in the complex ground-environment of Amoy coast. By measuring the monitoring data of the foundation pit in Haicang road station of Amoy Metro Line 2,this article conducts the research from three aspects: horizontal displacement of the underground continuous wall, surface subsidence and groundwater level changes. The article concludes that:(1) The maximum horizontal displacement of the underground continuous wall is 24.5mm, about 0.35% of the depth of foundation pit(H),slightly lesser than the other soft soil area. (2) Surface subsidence trough shows "√" shape distribution. The maximum appears at a distances of 0.5 times of the foundation pit depth(H) from the pit side, the range of influence is about 1.76 times of the depth of foundation pit. The maximum surface subsidence of monitoring sections is 22.4mm. (3) During the construction of excavation, the groundwater level of coastal areas shows fluctuations as a change in performance, showing a rising trend in whole. The maximum cumulative variation is about 0.72cm. (4) Change of seawall horizontal displacement and sedimentation change is unstable, but always small. For the long and deep foundation pit under the environment of coastal complex formation, the research results have reference value and guiding significance.
2016, 24(5): 815-822.
During the phased construction of large water-sealed oil storage, the construction of a newly built underground water-sealed oil storage has a certain degree of impact on the water curtain system and the seepage field of the nearby under-production similar underground structure. And then may cause safety risks to the already built storage such as oil and gas leakage. However, for the time being, domestic and foreign research on this issue seems little. This paper is based on a large underground water-sealed strategy oil storage's extension phase-three project. The change of seepage field which is influenced by the new-construction of extension phase-three poses a safety impact on under-production extension phase-two project. This paper back-calculates the permeability tensor based on the field measured data. Then it simulates the stable seepage field of the under-production underground water-sealed oil storage using the software COMSOL Multiphysics. It views the seepage field as the initial seepage field. And it then simulates the evolution law of existing seepage field changes during new cavern excavation. It views the underground water level, water pressure and cavern area line rock seepage as the evaluation index. The analysis is about the new-construction cavern's impact on the water seal reliability of the under-production cavern. The research results show that when the separation distance of these two phase cavern is more than 200m, even if without setting up the central vertical separation of water curtain system, new-construction cavern has no impact on the water seal reliability of the under-production cavern. The minimum safe storage cavern separation distance is 200m. The research results provide a reasonable theoretical support for the feasibility and impact of the construction of similar oil storage in the vicinity of the underground water-sealed oil storage.
During the phased construction of large water-sealed oil storage, the construction of a newly built underground water-sealed oil storage has a certain degree of impact on the water curtain system and the seepage field of the nearby under-production similar underground structure. And then may cause safety risks to the already built storage such as oil and gas leakage. However, for the time being, domestic and foreign research on this issue seems little. This paper is based on a large underground water-sealed strategy oil storage's extension phase-three project. The change of seepage field which is influenced by the new-construction of extension phase-three poses a safety impact on under-production extension phase-two project. This paper back-calculates the permeability tensor based on the field measured data. Then it simulates the stable seepage field of the under-production underground water-sealed oil storage using the software COMSOL Multiphysics. It views the seepage field as the initial seepage field. And it then simulates the evolution law of existing seepage field changes during new cavern excavation. It views the underground water level, water pressure and cavern area line rock seepage as the evaluation index. The analysis is about the new-construction cavern's impact on the water seal reliability of the under-production cavern. The research results show that when the separation distance of these two phase cavern is more than 200m, even if without setting up the central vertical separation of water curtain system, new-construction cavern has no impact on the water seal reliability of the under-production cavern. The minimum safe storage cavern separation distance is 200m. The research results provide a reasonable theoretical support for the feasibility and impact of the construction of similar oil storage in the vicinity of the underground water-sealed oil storage.
2016, 24(5): 823-832.
The load on the arch dam mostly applies on the abutment rock mass of both sides of dam. Dam abutment is key part to maintain stability of arch dam and its stability is one of the important engineering geology factors in the process of arch dam construction. Two geological discontinuities with long persistence are found during the excavation of slope at left abutment of Wudongde hydropower station. According to the engineering geological conditions and data of feasibility study, in combination with special exploration and exposure of excavation surface, the distribution of discontinuities is obtained. The quantitative research about the physical composition and character of infillings of discontinuities is achieved through field investigation in detail and geophysical exploration test. Based on the results, the adverse impact of discontinuities to the stability of abutment is researched through engineering geological evaluation and numerical analysis. The results show that these discontinuities are two tectonic faults formed in the earlier stage and superimpose the modification of dissolution later. It is a typical phenomenon in limestone zone. Most of the infillings exist with calcium cementation. Partial infillings are poorer because of dissolution. Thus, the distribution of soft material of infillings is discontinuous. Hard rocks contact each other directly. The discontinuities have no impact on the abutment stability, but may cause the stress concentration in abutment rock mass partly. The developed approach is scientific and comprehensive. It can be applied to study the engineering geological characteristics of discontinuities and evaluate their impact to the stability of abutment rock mass in the process of dam construction. It also provides theoretical and practical significance to the survey and design of arch dam project.
The load on the arch dam mostly applies on the abutment rock mass of both sides of dam. Dam abutment is key part to maintain stability of arch dam and its stability is one of the important engineering geology factors in the process of arch dam construction. Two geological discontinuities with long persistence are found during the excavation of slope at left abutment of Wudongde hydropower station. According to the engineering geological conditions and data of feasibility study, in combination with special exploration and exposure of excavation surface, the distribution of discontinuities is obtained. The quantitative research about the physical composition and character of infillings of discontinuities is achieved through field investigation in detail and geophysical exploration test. Based on the results, the adverse impact of discontinuities to the stability of abutment is researched through engineering geological evaluation and numerical analysis. The results show that these discontinuities are two tectonic faults formed in the earlier stage and superimpose the modification of dissolution later. It is a typical phenomenon in limestone zone. Most of the infillings exist with calcium cementation. Partial infillings are poorer because of dissolution. Thus, the distribution of soft material of infillings is discontinuous. Hard rocks contact each other directly. The discontinuities have no impact on the abutment stability, but may cause the stress concentration in abutment rock mass partly. The developed approach is scientific and comprehensive. It can be applied to study the engineering geological characteristics of discontinuities and evaluate their impact to the stability of abutment rock mass in the process of dam construction. It also provides theoretical and practical significance to the survey and design of arch dam project.
2016, 24(5): 833-846.
Due to the blend of multi-source oil and gas as well as multi-period tectonics in Tarim Basin, the origin and relative contribution of these marine oil and gas has not been clear. According to oil-source correlation, source rocks of O1+2 are the main origin of these oil & gas. However, the fact that the distribution of source rocks with high TOC(>0.5%) is quite partial, can not reflect the large-scale found reserves. Furthermore, the found gas reserves in Tarim Basin have exceeded the result of the 3rd resource evaluation. These contradictions show the limitation of oil-source correlation in the superimposed basin. For this study, forward modelling method is used to simulate the processes of hydrocarbon generation and expulsion of potential marine source rocks. Effective quantity of hydrocarbon accumulation namely quantity of resources can be obtained by deducting the critical saturation of residual hydrocarbon, loss of diffusion phases, loss of hydrocarbon residued in the migration and damaged by tectonic movements from the total amount of hydrocarbon expulsion, which can be used to assess the relative contributions of oil & gas of each set of source rock. Studies show that the amount of resources can be provided by the Cambrian source rocks is higher than that by Ordovician source rocks. The ratio is about 63%to 37%. The amount of resources that can be provided by source rocks with low abundance is lower than those by source rocks with high abundance. The ratio is about 32%to 68%.The reason that the scale of reserves in Ordovician is larger than the predicted may be the contribution of part oil & gas coming from the Cambrian or source rocks with low TOC(≤0.5%).
Due to the blend of multi-source oil and gas as well as multi-period tectonics in Tarim Basin, the origin and relative contribution of these marine oil and gas has not been clear. According to oil-source correlation, source rocks of O1+2 are the main origin of these oil & gas. However, the fact that the distribution of source rocks with high TOC(>0.5%) is quite partial, can not reflect the large-scale found reserves. Furthermore, the found gas reserves in Tarim Basin have exceeded the result of the 3rd resource evaluation. These contradictions show the limitation of oil-source correlation in the superimposed basin. For this study, forward modelling method is used to simulate the processes of hydrocarbon generation and expulsion of potential marine source rocks. Effective quantity of hydrocarbon accumulation namely quantity of resources can be obtained by deducting the critical saturation of residual hydrocarbon, loss of diffusion phases, loss of hydrocarbon residued in the migration and damaged by tectonic movements from the total amount of hydrocarbon expulsion, which can be used to assess the relative contributions of oil & gas of each set of source rock. Studies show that the amount of resources can be provided by the Cambrian source rocks is higher than that by Ordovician source rocks. The ratio is about 63%to 37%. The amount of resources that can be provided by source rocks with low abundance is lower than those by source rocks with high abundance. The ratio is about 32%to 68%.The reason that the scale of reserves in Ordovician is larger than the predicted may be the contribution of part oil & gas coming from the Cambrian or source rocks with low TOC(≤0.5%).
2016, 24(5): 847-855.
Abundant water resources stores in the Qinghai-Tibet Plateau and its vicinity, which has huge hydropower development space. The unique topography, geological conditions, earthquake and climate of the Qinghai-Tibet Plateau decide that the hydropower project construction in this area will face a series of engineering geological challenges. So we have to innovate the exploration research methods, study the giant engineering geological problems in advance, fully understand the complex geological conditions, take a positive, prudent, practical work attitude, adopt appropriate engineering scale, scientific planning, appropriate exploit manner, proper dam type and pivot arrangement and effective risk control and other measures. We can effectively avoid the geological risk and hazard. It is possible to safely develop hydropower resources in the region with extremely complex geological conditions.
Abundant water resources stores in the Qinghai-Tibet Plateau and its vicinity, which has huge hydropower development space. The unique topography, geological conditions, earthquake and climate of the Qinghai-Tibet Plateau decide that the hydropower project construction in this area will face a series of engineering geological challenges. So we have to innovate the exploration research methods, study the giant engineering geological problems in advance, fully understand the complex geological conditions, take a positive, prudent, practical work attitude, adopt appropriate engineering scale, scientific planning, appropriate exploit manner, proper dam type and pivot arrangement and effective risk control and other measures. We can effectively avoid the geological risk and hazard. It is possible to safely develop hydropower resources in the region with extremely complex geological conditions.
2016, 24(5): 856-862.
Desiccation cracking is one of the typical engineering geological characteristics of expansive soil and has a major impact on engineering properties of expansive soils. It can lead to many engineering geological problems directly or indirectly. After indoor wetting-drying circle tests on remolded expansive soil, obtained crack digital images are disposed by a series of pre-processing operations with the image processing technique. A series of parameters are propose to describe the morphological structure of the desiccation crack pattern quantitatively. It includes the crack ratio of surface, crack number, total crack length, average crack width and absolute shrinkage ratio. And the impact of wetting-drying circles on characteristics of expansive soil crack development are analyzed. The results show that wetting-drying cycles and moisture content are some significant factors affecting the development of expansive soil desiccation cracks. With decreasing moisture content, there is an increasing trend of the development of remolded expansive soil cracks overall. The surface crack ratio, number of cracks and total length increase with decreasing moisture content and absolute shrinkage ratio grows greater, but there is no obvious regularity between average crack width and water content. Desiccation cracks develop further with increasing wetting-drying circles and the numbers of cracks, total crack length and surface crack ratio increase, and absolute shrinkage ratio increases in the mass. Nevertheless, there is a reduced phenomenon on average crack width, which is related to the development of micro-cracks later.
Desiccation cracking is one of the typical engineering geological characteristics of expansive soil and has a major impact on engineering properties of expansive soils. It can lead to many engineering geological problems directly or indirectly. After indoor wetting-drying circle tests on remolded expansive soil, obtained crack digital images are disposed by a series of pre-processing operations with the image processing technique. A series of parameters are propose to describe the morphological structure of the desiccation crack pattern quantitatively. It includes the crack ratio of surface, crack number, total crack length, average crack width and absolute shrinkage ratio. And the impact of wetting-drying circles on characteristics of expansive soil crack development are analyzed. The results show that wetting-drying cycles and moisture content are some significant factors affecting the development of expansive soil desiccation cracks. With decreasing moisture content, there is an increasing trend of the development of remolded expansive soil cracks overall. The surface crack ratio, number of cracks and total length increase with decreasing moisture content and absolute shrinkage ratio grows greater, but there is no obvious regularity between average crack width and water content. Desiccation cracks develop further with increasing wetting-drying circles and the numbers of cracks, total crack length and surface crack ratio increase, and absolute shrinkage ratio increases in the mass. Nevertheless, there is a reduced phenomenon on average crack width, which is related to the development of micro-cracks later.
2016, 24(5): 863-870.
With the rapid development of marine exploitation activities, the sediment strength in the bed slope of Northern South China Sea has attracted more and more attention. The vane shear tests and penetration tests are conducted on ship with the sediment samples collected by gravity corer and box corer in the Yingqiong Slope, Shenhu Slope, Dongsha Slope and Taiwan Slope. Sampling results show that the sediments in Dongsha Slope are sandy and the carbonate nodules with the diameter of 22cm are found. Besides, living bivalve is found on the seabed and the organic remains are found 110cm below the seabed in the Shenhu area. The sediment strength results show that the penetration resistance is less than 0.1N and the vane test strength is no more than 10kPa within 20cm below the seabed. Generally, the sediment strength increases with the depth below the seabed. Furthermore, sediment strength stratification is found within 200cm below the seabed, especially in the Taiwan Slope. The sediment strength is low above the interface and increases sharply under the interface. The sediment strength interface is the potential sliding surface.
With the rapid development of marine exploitation activities, the sediment strength in the bed slope of Northern South China Sea has attracted more and more attention. The vane shear tests and penetration tests are conducted on ship with the sediment samples collected by gravity corer and box corer in the Yingqiong Slope, Shenhu Slope, Dongsha Slope and Taiwan Slope. Sampling results show that the sediments in Dongsha Slope are sandy and the carbonate nodules with the diameter of 22cm are found. Besides, living bivalve is found on the seabed and the organic remains are found 110cm below the seabed in the Shenhu area. The sediment strength results show that the penetration resistance is less than 0.1N and the vane test strength is no more than 10kPa within 20cm below the seabed. Generally, the sediment strength increases with the depth below the seabed. Furthermore, sediment strength stratification is found within 200cm below the seabed, especially in the Taiwan Slope. The sediment strength is low above the interface and increases sharply under the interface. The sediment strength interface is the potential sliding surface.
2016, 24(5): 871-880.
Uniaxial compression experiments on shale specimens of Wufeng-Longmaxi region are carried out to study the mechanical properties within bedding planes under different depths. The deformation feature, compressive strength and five fundamental failure modes are summarized. The result shows that the mechanical parameters of shale within bedding planes under different depths are turned out to be significant difference. Shale blocks within bedding planes present obvious brittle failure characteristics, which mainly forms of splitting failure accompanied by partial or localized shear failure. On the whole, as the sampling angle β enlarges, the elastic modulus and Poisson's ratio increase at first and then decrease. Comparative analyses are carried out for the mechanical parameters varieties of the transverse and longitudinal diversity. The diversity factor of compressive strength is quite larger than that of elastic modulus and Poisson's ratio. The effect of shale bedding planes is one of the main factor for the diversity of strength and failure mode. Bedding plane can determine shale compressive strength and failure mode to some extent. The research provides references for the wellbore stability analysis and hydraulic fracturing design in the shale gas exploitation.
Uniaxial compression experiments on shale specimens of Wufeng-Longmaxi region are carried out to study the mechanical properties within bedding planes under different depths. The deformation feature, compressive strength and five fundamental failure modes are summarized. The result shows that the mechanical parameters of shale within bedding planes under different depths are turned out to be significant difference. Shale blocks within bedding planes present obvious brittle failure characteristics, which mainly forms of splitting failure accompanied by partial or localized shear failure. On the whole, as the sampling angle β enlarges, the elastic modulus and Poisson's ratio increase at first and then decrease. Comparative analyses are carried out for the mechanical parameters varieties of the transverse and longitudinal diversity. The diversity factor of compressive strength is quite larger than that of elastic modulus and Poisson's ratio. The effect of shale bedding planes is one of the main factor for the diversity of strength and failure mode. Bedding plane can determine shale compressive strength and failure mode to some extent. The research provides references for the wellbore stability analysis and hydraulic fracturing design in the shale gas exploitation.
2016, 24(5): 881-890.
This paper presents the increasing-amplitude cycle loading experiment of plagiogranite through the MTS815 rock mechanics testing machine. It examines the effects of number of cycle, confining pressure and moisture content on elastic modulus, and analyzes the variation of work stress,elastic strain energy and dissipated energy with increasing number of cycle. The process of rock compression deformation and failure is divided into four stages according to the energy change law. It researches the relationship between tangent modulus and dissipated energy. The experimental results show that:(1) The tangent modulus at the same stress level is increased and then decreased at the cycle times increases. Its decreasing amplitude increases as cycles and stress level increase. (2) The weakened amplitude of tangent modulus decreases with increase of confining pressure. (3) The weakened amplitude of tangent modulus of saturated samples is bigger than that of natural samples, which shows that water is aggravated damage on saturated samples. (4) The increase of dissipated energy is the cause of the tangent modulus weakening. The change rules of dissipated energy and tangent modulus are correlative.
This paper presents the increasing-amplitude cycle loading experiment of plagiogranite through the MTS815 rock mechanics testing machine. It examines the effects of number of cycle, confining pressure and moisture content on elastic modulus, and analyzes the variation of work stress,elastic strain energy and dissipated energy with increasing number of cycle. The process of rock compression deformation and failure is divided into four stages according to the energy change law. It researches the relationship between tangent modulus and dissipated energy. The experimental results show that:(1) The tangent modulus at the same stress level is increased and then decreased at the cycle times increases. Its decreasing amplitude increases as cycles and stress level increase. (2) The weakened amplitude of tangent modulus decreases with increase of confining pressure. (3) The weakened amplitude of tangent modulus of saturated samples is bigger than that of natural samples, which shows that water is aggravated damage on saturated samples. (4) The increase of dissipated energy is the cause of the tangent modulus weakening. The change rules of dissipated energy and tangent modulus are correlative.
2016, 24(5): 891-898.
The discontinuities are the key factors that differentiate the rock mass and intact rock. The mechanical behaviours of rock mass are strongly affected by length, occurrence and consistency of the discontinuities. Characteristics-deformation, strength, failure modes of rock samples with various discontinuities under uniaxial compression have been studied based on the numerical modelling FLAC3D.Rock samples with different groups of penetrated discontinuities and different unpenetrated discontinuities have been set up. On the basis of these models, a number of numerical uniaxial compression tests have been carried out, in which the shear and tensile strength criterion(Mohr-Coulomb criterion) has been used. During the uniaxial compression simulation tests, a servo-control program is used to restrict the maximum unbalanced force through adjusting the applied velocity. The results of simulation tests are shown as follow: rock samples with fewer than 4 groups of penetrated discontinuities are anisotropic while those with 4 groups of penetrated discontinuities are nearly isotropic. The deformation and strength parameters get lower as the number of penetrated discontinuities get higher in rock sample with same size. Stress drop is observed after peak stress in the stress-strain curve of rock sample with unpenetrated continuity. Uniaxial compressive strength derived from simulation is different from that calculated by analytic method based on shear strength and damage theory, which indicates that shear strength criterion may be not suitable for rock mass with unpenetrated discontinuities. The deformation and strength parameters get lower as the continuity trace length bigger. The failure modes of rock samples with discontinuities can be divided into three types: discontinuity-controlled, discontinuity-half controlled and discontinuity-uncontrolled. As the number of groups increases the failure are more likely to be discontinuity-controlled.
The discontinuities are the key factors that differentiate the rock mass and intact rock. The mechanical behaviours of rock mass are strongly affected by length, occurrence and consistency of the discontinuities. Characteristics-deformation, strength, failure modes of rock samples with various discontinuities under uniaxial compression have been studied based on the numerical modelling FLAC3D.Rock samples with different groups of penetrated discontinuities and different unpenetrated discontinuities have been set up. On the basis of these models, a number of numerical uniaxial compression tests have been carried out, in which the shear and tensile strength criterion(Mohr-Coulomb criterion) has been used. During the uniaxial compression simulation tests, a servo-control program is used to restrict the maximum unbalanced force through adjusting the applied velocity. The results of simulation tests are shown as follow: rock samples with fewer than 4 groups of penetrated discontinuities are anisotropic while those with 4 groups of penetrated discontinuities are nearly isotropic. The deformation and strength parameters get lower as the number of penetrated discontinuities get higher in rock sample with same size. Stress drop is observed after peak stress in the stress-strain curve of rock sample with unpenetrated continuity. Uniaxial compressive strength derived from simulation is different from that calculated by analytic method based on shear strength and damage theory, which indicates that shear strength criterion may be not suitable for rock mass with unpenetrated discontinuities. The deformation and strength parameters get lower as the continuity trace length bigger. The failure modes of rock samples with discontinuities can be divided into three types: discontinuity-controlled, discontinuity-half controlled and discontinuity-uncontrolled. As the number of groups increases the failure are more likely to be discontinuity-controlled.
2016, 24(5): 899-906.
The main reason of the occurrence of collapsibility of loess under pressure soaked by water is the loss of the cementation within its specific pore system, which leading to the deformation caused by the relative sliding among practices. The main cement in loess is the clay minerals attaching to the surface of the skeletal debris particles. There are a variety of clay minerals in loess. Due to their different hydraulic properties, the impact on the collapsible loess are also different. So the composition and its proportion of clay mineral, to some extent, can reflect the collapsibility of loess. Based on the spectrum analysis of main mineral elements at different depths in Yan'an, element mass conservation equations have been directly obtained. Then it is converted into an optimization problem. The main clay mineral contents of the loess are calculated by the means of simplex evolutionary method of standard linear programming. Moreover, a feasibility study of the method is carried out by comparing the calculating results with chemistry analysis result. The results show that the method meets request of precision and is proved to be an effective way of measuring clay mineral contents of loess. The acquisition of data is convenient and the quantitative analysis at fixed point under SEM platform can be realized.
The main reason of the occurrence of collapsibility of loess under pressure soaked by water is the loss of the cementation within its specific pore system, which leading to the deformation caused by the relative sliding among practices. The main cement in loess is the clay minerals attaching to the surface of the skeletal debris particles. There are a variety of clay minerals in loess. Due to their different hydraulic properties, the impact on the collapsible loess are also different. So the composition and its proportion of clay mineral, to some extent, can reflect the collapsibility of loess. Based on the spectrum analysis of main mineral elements at different depths in Yan'an, element mass conservation equations have been directly obtained. Then it is converted into an optimization problem. The main clay mineral contents of the loess are calculated by the means of simplex evolutionary method of standard linear programming. Moreover, a feasibility study of the method is carried out by comparing the calculating results with chemistry analysis result. The results show that the method meets request of precision and is proved to be an effective way of measuring clay mineral contents of loess. The acquisition of data is convenient and the quantitative analysis at fixed point under SEM platform can be realized.
2016, 24(5): 907-914.
Quartz-epidotization alteration is an important geological problem in Dagangshan hydropower station. Taking into account the advantage of geological survey and laboratory experiment, this paper analyses its geological distribution, mineralogical and structural characteristics and physical and mechanical properties. The study reveals that ranite in SN/E∠17°~26° low-angle shearing belts often develops quartz-epidotization alteration formed by the dynamic metamorphism, which always grows scratches and order step. Alteration produces sericite, chlorite, epidote and calcite weak mineral content of about 18%. Chemical analysis shows little change in its composition. Physical and mechanical tests show that the physical and water properties and strength characteristics of altered rock have a certain degree of weakening, natural and saturated compressive strength loss rate of 8.3% and 5.9%. Quartz and epidote produced by alteration have an impact on the shear strength of the structure. In-situ direct shear tests show shear intensity decreases. Research has some innovative, can be a reference on engineering impact and process of quartz-epidotization alteration.
Quartz-epidotization alteration is an important geological problem in Dagangshan hydropower station. Taking into account the advantage of geological survey and laboratory experiment, this paper analyses its geological distribution, mineralogical and structural characteristics and physical and mechanical properties. The study reveals that ranite in SN/E∠17°~26° low-angle shearing belts often develops quartz-epidotization alteration formed by the dynamic metamorphism, which always grows scratches and order step. Alteration produces sericite, chlorite, epidote and calcite weak mineral content of about 18%. Chemical analysis shows little change in its composition. Physical and mechanical tests show that the physical and water properties and strength characteristics of altered rock have a certain degree of weakening, natural and saturated compressive strength loss rate of 8.3% and 5.9%. Quartz and epidote produced by alteration have an impact on the shear strength of the structure. In-situ direct shear tests show shear intensity decreases. Research has some innovative, can be a reference on engineering impact and process of quartz-epidotization alteration.
2016, 24(5): 915-923.
With the exploration and development of various kinds of resources in area of desert-loess plateau, poor understanding about the dynamic characteristics of sandy loess of aeolian sand to loess transifional origin has become the constraint more than ever in this region. This issue is particular for development of earthquakes and artificial power engineering construction. In this paper, under isotropic consolidation condition, consolidated undrained dynamic triaxial tests under different confining pressures are tested for the aeolian sand-loess transitional sandy loess. Kinetic parameters are obtained. Based on them, the dynamic stress-strain relationship of aeolian sand-loess transitional sandy loess was drawn. The change characteristic of dynamic shear modulus and damping ratio with dynamic strain of their relation curve and fitting model is explored. Equivalent dynamic visco-elastic constitutive model is further established for applicable to aeolian sand-loess transitional sandy loess. Results show that the relation curve between dynamic shear modulus and dynamic strain fits well for aeolian sand-loess transitional type of sandy loess, according to the Harding hyperbolic model. Besides, the power function model is good for fitting the relationship between damping ratio and dynamic strain. The results achieved in this paper provide basic data for the future research of dynamic characteristics of aeolian sand-loess transitional sandy loess.
With the exploration and development of various kinds of resources in area of desert-loess plateau, poor understanding about the dynamic characteristics of sandy loess of aeolian sand to loess transifional origin has become the constraint more than ever in this region. This issue is particular for development of earthquakes and artificial power engineering construction. In this paper, under isotropic consolidation condition, consolidated undrained dynamic triaxial tests under different confining pressures are tested for the aeolian sand-loess transitional sandy loess. Kinetic parameters are obtained. Based on them, the dynamic stress-strain relationship of aeolian sand-loess transitional sandy loess was drawn. The change characteristic of dynamic shear modulus and damping ratio with dynamic strain of their relation curve and fitting model is explored. Equivalent dynamic visco-elastic constitutive model is further established for applicable to aeolian sand-loess transitional sandy loess. Results show that the relation curve between dynamic shear modulus and dynamic strain fits well for aeolian sand-loess transitional type of sandy loess, according to the Harding hyperbolic model. Besides, the power function model is good for fitting the relationship between damping ratio and dynamic strain. The results achieved in this paper provide basic data for the future research of dynamic characteristics of aeolian sand-loess transitional sandy loess.
2016, 24(5): 924-930.
With the vigorous development of underground space, the ground settlement caused by dewatering has an impact on the stability of surrounding buildings and underground piping systems, which has attracted attentions of domestic and foreign scholars. This article uses typical silt of Wuhan to carry out dehydration process of SWCC test with the pressure plate apparatus. The following results are obtained: SWCC of unsaturated soil; mass moisture content, the axial displacement variation with the change of matric suction and their error correction; water and axial displacement variation with time under different matric suction. According to the measured curve, the water content of the unsaturated soil is reduced with the increasing matric suction before it reaches the residual moisture content; the soil shrinks because of water loss. The axial displacement increases with the increasing matric suction.,at a residual moisture content, axial displacement increase stops changing. During pressurization, axial deformation appears, accompanied by a certain degree of radial deformation. In addition, a detailed analysis is carried out about the instrument itself and the error that may occur during the test, which provides a reference to the pressure plate test.
With the vigorous development of underground space, the ground settlement caused by dewatering has an impact on the stability of surrounding buildings and underground piping systems, which has attracted attentions of domestic and foreign scholars. This article uses typical silt of Wuhan to carry out dehydration process of SWCC test with the pressure plate apparatus. The following results are obtained: SWCC of unsaturated soil; mass moisture content, the axial displacement variation with the change of matric suction and their error correction; water and axial displacement variation with time under different matric suction. According to the measured curve, the water content of the unsaturated soil is reduced with the increasing matric suction before it reaches the residual moisture content; the soil shrinks because of water loss. The axial displacement increases with the increasing matric suction.,at a residual moisture content, axial displacement increase stops changing. During pressurization, axial deformation appears, accompanied by a certain degree of radial deformation. In addition, a detailed analysis is carried out about the instrument itself and the error that may occur during the test, which provides a reference to the pressure plate test.
2016, 24(5): 931-940.
Soil mechanical properties change during the consolidation progress, which is related to the changes for pores of saturated clays. Therefore, a thorough investigation of pore evolution characteristics during the process of saturated clay consolidation has important significance for understanding and applying saturated clay drainage in construction. The pore evolution characteristics of the Daya Bay soil are investigated. In order to decrease the void ratio efficiently, saturated fine-grained soil under the pressure gradient is tested. Firstly, cylindrical soil samples are dried by a cryogenic chamber. Secondly, Synchrotron Radiation CT is used to scan the dried samples to obtain high-precision slices. Then three-dimensional models are restructured with Avizo software using grayscale threshold method. Furthermore, porous structures are quantitatively estimated with several of morphological algorithms. Studies show that pores, under the weight settlement, more than 90% are located in 4~10μm and only a few are larger than 40μm. Soil samples, under the pressure of 100kPa and with large pore content will decrease rapidly while its small pores will increase rapidly, demonstrating that pores are more sensitive to pressure under the initial state. Medium and large pores can be split into small micropores or easily eliminated after being compressed. The results show that aperture with increasing pressure changes gently, small pores and micro pores have more advantages. It can be conclude that small and micro pores have stronger anti-pressure ability. After the pressure gradient, the original flocculation structure gradually changes to the flat stacked structure, particles repel parallels, porosity abundance decreases gradually and pores become elongated in a level direction. In this study, a three-dimensional model with real pore structure characteristics is established using the combination of CT scanning and Avizo(an advanced 3D visualization software).It analyzes the consolidation mechanism of the saturated fine-grained soil from the microscopic perspective.
Soil mechanical properties change during the consolidation progress, which is related to the changes for pores of saturated clays. Therefore, a thorough investigation of pore evolution characteristics during the process of saturated clay consolidation has important significance for understanding and applying saturated clay drainage in construction. The pore evolution characteristics of the Daya Bay soil are investigated. In order to decrease the void ratio efficiently, saturated fine-grained soil under the pressure gradient is tested. Firstly, cylindrical soil samples are dried by a cryogenic chamber. Secondly, Synchrotron Radiation CT is used to scan the dried samples to obtain high-precision slices. Then three-dimensional models are restructured with Avizo software using grayscale threshold method. Furthermore, porous structures are quantitatively estimated with several of morphological algorithms. Studies show that pores, under the weight settlement, more than 90% are located in 4~10μm and only a few are larger than 40μm. Soil samples, under the pressure of 100kPa and with large pore content will decrease rapidly while its small pores will increase rapidly, demonstrating that pores are more sensitive to pressure under the initial state. Medium and large pores can be split into small micropores or easily eliminated after being compressed. The results show that aperture with increasing pressure changes gently, small pores and micro pores have more advantages. It can be conclude that small and micro pores have stronger anti-pressure ability. After the pressure gradient, the original flocculation structure gradually changes to the flat stacked structure, particles repel parallels, porosity abundance decreases gradually and pores become elongated in a level direction. In this study, a three-dimensional model with real pore structure characteristics is established using the combination of CT scanning and Avizo(an advanced 3D visualization software).It analyzes the consolidation mechanism of the saturated fine-grained soil from the microscopic perspective.
2016, 24(5): 941-950.
Millions of years ago, seemingly boundless sand storms covered northwestern China, which formed loess deposits several hundred meters deep. Some loess was transported thousands of kilometers away through the river system to the coastal area of the Bohai Sea, which generated the Modern Yellow River Delta. The engineering properties of loess changed from Yellow river to the sea. The physical mechanical properties, gradation composition, microstructure and dynamics properties of loess are obtained based on the achievements along the Yellow River in the past 30 years and researches on estuary sediment in the latest 10 years. Analysis of these data shows that the unit weight and water content increased in the process of transported into sea, while the plastic index decreased. The major contents of loess are silt, with little sand and clay. The clay content increased while sand content decreased. Consequently, the void was filled better, and the compressibility of loess reduced. The shear resistance shows big differences at different areas. The earthquake resistance and liquefaction resistance were increased. This paper is of great significance to better understanding the mechanism and prevention of the loess geological disasters.
Millions of years ago, seemingly boundless sand storms covered northwestern China, which formed loess deposits several hundred meters deep. Some loess was transported thousands of kilometers away through the river system to the coastal area of the Bohai Sea, which generated the Modern Yellow River Delta. The engineering properties of loess changed from Yellow river to the sea. The physical mechanical properties, gradation composition, microstructure and dynamics properties of loess are obtained based on the achievements along the Yellow River in the past 30 years and researches on estuary sediment in the latest 10 years. Analysis of these data shows that the unit weight and water content increased in the process of transported into sea, while the plastic index decreased. The major contents of loess are silt, with little sand and clay. The clay content increased while sand content decreased. Consequently, the void was filled better, and the compressibility of loess reduced. The shear resistance shows big differences at different areas. The earthquake resistance and liquefaction resistance were increased. This paper is of great significance to better understanding the mechanism and prevention of the loess geological disasters.
2016, 24(5): 959-966.
By means of large-scale shaking table test, this paper analyzes the dynamic response of stratified hard rock and soft rock slopes with anti-dipping strata or dipping strata subject to strong earthquakes. The results show that: the seismic signal of the slope shoulder contains the characteristic of the slope structure and material. The processed signal using Fast Fourier Transform Algorithm contains some feature of the slope itself. This method is practical in the shaking table test. The FFT spectrum of discrete frequency signal shows that the derivative frequency of the anti-dip slope is higher than that of the dip slope under the vertical seismic force. And as horizontal seismic force is applied, the hard rock dip slope and the soft rock anti-dip slope have higher magnitude and their predominant frequency is more concentrated than those the hard rock anti-dip and soft rock dip slope. The seismic response of the hard rock dip slope is more significant than that of the soft rock dip slope. As a natural earthquake record is applied to the model vertically, the FFT spectrum of the dip slope has the same pattern as the shaking table, and the anti-dip slope has more magnification in certain frequency. While a horizontal natural record is applied, the four models show more consistency, which means at this condition, the influence of structure and the lithology are non-significant.
By means of large-scale shaking table test, this paper analyzes the dynamic response of stratified hard rock and soft rock slopes with anti-dipping strata or dipping strata subject to strong earthquakes. The results show that: the seismic signal of the slope shoulder contains the characteristic of the slope structure and material. The processed signal using Fast Fourier Transform Algorithm contains some feature of the slope itself. This method is practical in the shaking table test. The FFT spectrum of discrete frequency signal shows that the derivative frequency of the anti-dip slope is higher than that of the dip slope under the vertical seismic force. And as horizontal seismic force is applied, the hard rock dip slope and the soft rock anti-dip slope have higher magnitude and their predominant frequency is more concentrated than those the hard rock anti-dip and soft rock dip slope. The seismic response of the hard rock dip slope is more significant than that of the soft rock dip slope. As a natural earthquake record is applied to the model vertically, the FFT spectrum of the dip slope has the same pattern as the shaking table, and the anti-dip slope has more magnification in certain frequency. While a horizontal natural record is applied, the four models show more consistency, which means at this condition, the influence of structure and the lithology are non-significant.
2016, 24(5): 967-975.
With research on "surrounding rock-roadway" system, the unloading rockburst can be recreated by the operation of horizontal stress unloading. The results indicate that the unloading rockburst has experienced evolution process of from "nonchalant stage to small particles ejection stage to flake off stage and to rockburst".The tunnel model produces a number of the process from rockburst to stress adjust then to stress adjust failure and to another rockburst. At the end it forms "V"shape rockburst hole. In the condition of unloading, it exists some relationship between horizontal stress and fracture model. The unloading operation of tunnel model of distribution stress can be obtained from theoretical derivation. Vertical crack stress in different horizontal stresses(8.9MPa, 13.3MPa, 17.8MPa) is taken by modified Griffith failure criteria. The calculation model of "V"shape is established. At the stress point(13.3MPa) when the horizontal stress is under 13.3MPa, the relationship between horizontal stress and vertical crack stress has a positive correlation. When the horizontal stress is above 13.3MPa, the relationship between the horizontal stress and the vertical crack stress is a negative correlation. When the horizontal stress is at 13.3MPa, the strain energy concentrated area is the largest, and distance between this area and side wall is the minimum.
With research on "surrounding rock-roadway" system, the unloading rockburst can be recreated by the operation of horizontal stress unloading. The results indicate that the unloading rockburst has experienced evolution process of from "nonchalant stage to small particles ejection stage to flake off stage and to rockburst".The tunnel model produces a number of the process from rockburst to stress adjust then to stress adjust failure and to another rockburst. At the end it forms "V"shape rockburst hole. In the condition of unloading, it exists some relationship between horizontal stress and fracture model. The unloading operation of tunnel model of distribution stress can be obtained from theoretical derivation. Vertical crack stress in different horizontal stresses(8.9MPa, 13.3MPa, 17.8MPa) is taken by modified Griffith failure criteria. The calculation model of "V"shape is established. At the stress point(13.3MPa) when the horizontal stress is under 13.3MPa, the relationship between horizontal stress and vertical crack stress has a positive correlation. When the horizontal stress is above 13.3MPa, the relationship between the horizontal stress and the vertical crack stress is a negative correlation. When the horizontal stress is at 13.3MPa, the strain energy concentrated area is the largest, and distance between this area and side wall is the minimum.
2016, 24(5): 976-980.
This paper uses a newly developed rotating flume experiment apparatus. The friction slope and flowing rate of the debris flow are measured to evaluate the effect of flow rate on the friction slope. In the rotating flume experiment, a stationary recirculating debris flow is produced by controlling the rotating rate v.The friction slope J of the debris flow is experimentally evaluated by measuring the tangential slope of the flume. In the experiments, the properties of debris flow are observed, including the shape of the head, the jump of large particles, the low friction slope, the segregation of solid phase from liquid phase and the flow rate effect. The experimental results show that the magnitude of J increases with the increasing flow rate v under a certain conditions on v(e.g.1.88m·s-1≥v>0.47m·s-1 in this research).The segregation of solid phase from liquid phase occurs when v is too small(e.g.v<0.47m·s-1 in this research).The displacement rate of gravels is much lower than the flow rate of the liquid phase. The experimental results provide an alternative method of producing debris flow and an alternative measurement method of friction slope. They also provide an experimental support for the further researches on the flow rate effect of debris flow.
This paper uses a newly developed rotating flume experiment apparatus. The friction slope and flowing rate of the debris flow are measured to evaluate the effect of flow rate on the friction slope. In the rotating flume experiment, a stationary recirculating debris flow is produced by controlling the rotating rate v.The friction slope J of the debris flow is experimentally evaluated by measuring the tangential slope of the flume. In the experiments, the properties of debris flow are observed, including the shape of the head, the jump of large particles, the low friction slope, the segregation of solid phase from liquid phase and the flow rate effect. The experimental results show that the magnitude of J increases with the increasing flow rate v under a certain conditions on v(e.g.1.88m·s-1≥v>0.47m·s-1 in this research).The segregation of solid phase from liquid phase occurs when v is too small(e.g.v<0.47m·s-1 in this research).The displacement rate of gravels is much lower than the flow rate of the liquid phase. The experimental results provide an alternative method of producing debris flow and an alternative measurement method of friction slope. They also provide an experimental support for the further researches on the flow rate effect of debris flow.
2016, 24(5): 981-991.
In recent years, coal mining gradually has transformed from eastern region to western region of China and the quicksand disaster had received increasing attention. This paper presents a series of experiments on the influencing factors of quicksand and the stress inside fissures caused by quicksand. Two different man-made quartz sand and other three natural sand samples are used in two quicksand test equipment set ups designed by authors. The experiment with two man-made quartz sand shows that the speed of quicksand mainly depends on the width of outlet. The total amount of quicksand mainly depends on the sand resource. Controlled by the fissure dip angle, the opening pattern of the quicksand passage can be divided in three types: the local space expanding, the local space moving and quicksand directly. The experiment with three natural sand samples show that stress decreases along the entrance to the outlet of fissure model regardless of dry sand or sand and water mixture flow. According to the stress variation, the transportation of water and sand mixture can be classified into two type: the collision transporting type, such as sand sample 1,which observed stress fluctuation; the continuous transporting type, such as sand sample 2 and sample 3,the stress changes continuously. All the dry quicksand of three natural sand samples can cause stress fluctuations to the fissure. The stress, the amplitude and period of fluctuation are larger when quicksand formed by larger particle. The stress fluctuation frequency of dry quicksand are about 3.2~3.6Hz by interpolation analysis with Matlab. For sand and water mixture flow, the stress fluctuations are only monitored in sand sample 1. When the frequency reduced to 2.7~3.5Hz and controlled by the particle size, the stress raises with little range. The outcomes can contribute to understanding the movement mechanism of quicksand and the stress dynamic inside fissure. It also has an important guiding to avoiding and preventing quicksand disaster.
In recent years, coal mining gradually has transformed from eastern region to western region of China and the quicksand disaster had received increasing attention. This paper presents a series of experiments on the influencing factors of quicksand and the stress inside fissures caused by quicksand. Two different man-made quartz sand and other three natural sand samples are used in two quicksand test equipment set ups designed by authors. The experiment with two man-made quartz sand shows that the speed of quicksand mainly depends on the width of outlet. The total amount of quicksand mainly depends on the sand resource. Controlled by the fissure dip angle, the opening pattern of the quicksand passage can be divided in three types: the local space expanding, the local space moving and quicksand directly. The experiment with three natural sand samples show that stress decreases along the entrance to the outlet of fissure model regardless of dry sand or sand and water mixture flow. According to the stress variation, the transportation of water and sand mixture can be classified into two type: the collision transporting type, such as sand sample 1,which observed stress fluctuation; the continuous transporting type, such as sand sample 2 and sample 3,the stress changes continuously. All the dry quicksand of three natural sand samples can cause stress fluctuations to the fissure. The stress, the amplitude and period of fluctuation are larger when quicksand formed by larger particle. The stress fluctuation frequency of dry quicksand are about 3.2~3.6Hz by interpolation analysis with Matlab. For sand and water mixture flow, the stress fluctuations are only monitored in sand sample 1. When the frequency reduced to 2.7~3.5Hz and controlled by the particle size, the stress raises with little range. The outcomes can contribute to understanding the movement mechanism of quicksand and the stress dynamic inside fissure. It also has an important guiding to avoiding and preventing quicksand disaster.
2016, 24(5): 992-1007.
In shale gas reservoirs, natural rock mass structural planes usually have significant influences on hydraulic fracturing treatment. This study uses the RFPA2D-Flow program. It considers the coupling seepage-stress-failure process of crack initiation, propagation, and damage. It simulates hydraulic fracturing process of rock mass in heterogeneous rock mass with different scales of natural structural planes. Though numerical simulation and discussion, the main conclusions can be drawn as follows:(1) When a hydraulic fracture encounters a natural unclosed crack, the principal stress trajectories would be deflected and a tensile stress zone would be created between the hydraulic fracture and the natural crack due to the increased pore pressure and propagation of the hydraulic fracture. Thus, the emergence and coalescence of tensile failure of the micro-units eventually causes the connection between the hydraulic fracture and the natural crack.(2) Bedding planes have pronounced effects on hydraulic fracturing. When the strikes of the maximum principal stress and the bedding planes intersect at a small angle, the bedding planes control the propagation of the hydraulic fractures. Otherwise, both the maximum principal stress and the bedding planes dominate the expansion of the fracture network. In addition, with the increase in structural planes and differential stresses, the scope and complexity of the stimulated fracture network increase simultaneously.(3) Hydraulic fracturing treatment is a short dynamic damage process. Although the existing of a fault structure can greatly affect the propagation model of the hydraulic fracturing and enhance the growth height of the hydraulic fracture, the damage to fault sealing capacity and activity caused by hydraulic fracturing is still limited compared with the depth of the reservoir.
In shale gas reservoirs, natural rock mass structural planes usually have significant influences on hydraulic fracturing treatment. This study uses the RFPA2D-Flow program. It considers the coupling seepage-stress-failure process of crack initiation, propagation, and damage. It simulates hydraulic fracturing process of rock mass in heterogeneous rock mass with different scales of natural structural planes. Though numerical simulation and discussion, the main conclusions can be drawn as follows:(1) When a hydraulic fracture encounters a natural unclosed crack, the principal stress trajectories would be deflected and a tensile stress zone would be created between the hydraulic fracture and the natural crack due to the increased pore pressure and propagation of the hydraulic fracture. Thus, the emergence and coalescence of tensile failure of the micro-units eventually causes the connection between the hydraulic fracture and the natural crack.(2) Bedding planes have pronounced effects on hydraulic fracturing. When the strikes of the maximum principal stress and the bedding planes intersect at a small angle, the bedding planes control the propagation of the hydraulic fractures. Otherwise, both the maximum principal stress and the bedding planes dominate the expansion of the fracture network. In addition, with the increase in structural planes and differential stresses, the scope and complexity of the stimulated fracture network increase simultaneously.(3) Hydraulic fracturing treatment is a short dynamic damage process. Although the existing of a fault structure can greatly affect the propagation model of the hydraulic fracturing and enhance the growth height of the hydraulic fracture, the damage to fault sealing capacity and activity caused by hydraulic fracturing is still limited compared with the depth of the reservoir.
2016, 24(5): 1008-1015.
The mechanical properties of BFRP bar are studied by tensile test, shearing test, corrosion resistance test and bond strength test with cement-based materials. The result shows the following results. The tensile strength of BFRP is greater than 890MPa. The retention rate of acid and alkali resistance is greater than 92%.The shear strength is slightly smaller than that of ordinary steel bar. The bond strength of cement-based materials is greater than 4.5MPa. Referencing to the "Technical specification for grout anchors"(CECS 22:2005),the BFRP anchor bolts supporting soil slope are designed with BFRP mechanical performance parameters. While the BFRP bar is designed as anchor bolt, the design value of tensile strength of BFRP is 750MPa, and the bond strength of BFRP with mortar is 2.0MPa. Through in-situ comparing test of BFRP anchor bolts and steel anchor bolts supporting soil slope, the effect of BFRP anchor reinforcement on soil slope is analyzed. The results show that it is similar in bolt stress and slope deformation with BFRP bolt and steel bolt. The applicability of BFRP bar as geotechnical supporting anchor bolt is verified.
The mechanical properties of BFRP bar are studied by tensile test, shearing test, corrosion resistance test and bond strength test with cement-based materials. The result shows the following results. The tensile strength of BFRP is greater than 890MPa. The retention rate of acid and alkali resistance is greater than 92%.The shear strength is slightly smaller than that of ordinary steel bar. The bond strength of cement-based materials is greater than 4.5MPa. Referencing to the "Technical specification for grout anchors"(CECS 22:2005),the BFRP anchor bolts supporting soil slope are designed with BFRP mechanical performance parameters. While the BFRP bar is designed as anchor bolt, the design value of tensile strength of BFRP is 750MPa, and the bond strength of BFRP with mortar is 2.0MPa. Through in-situ comparing test of BFRP anchor bolts and steel anchor bolts supporting soil slope, the effect of BFRP anchor reinforcement on soil slope is analyzed. The results show that it is similar in bolt stress and slope deformation with BFRP bolt and steel bolt. The applicability of BFRP bar as geotechnical supporting anchor bolt is verified.
2016, 24(5): 1016-1021.
Based on the measured data of the foundation pit, the work mechanism and environmental effect of innovative pre-stressed and prefabricated support(IPS) for retaining structures are studied in three aspects. They include pre-stress control, deformation control and stiffness control. Results show that: Compared with the traditional support, the passive soil pressure is fully motivated by the pre-stress that applied to the retaining structure using IPS before excavation. And the measured data shows that the overall deep displacement of the retaining structure is less than 10mm. At the same time, the IPS is applied to limit the deformation of foundation pit by applying pre-stress to improve the apparent stiffness. Measured surface subsidence is not more than 18mm which show it reduces the impact of excavation on the environment effectively. In addition, aimed at the stress concentration defect of traditional internal support, the IPS overcomes it through the support system full span reaction. The IPS indicates wide application in deep excavation engineering.
Based on the measured data of the foundation pit, the work mechanism and environmental effect of innovative pre-stressed and prefabricated support(IPS) for retaining structures are studied in three aspects. They include pre-stress control, deformation control and stiffness control. Results show that: Compared with the traditional support, the passive soil pressure is fully motivated by the pre-stress that applied to the retaining structure using IPS before excavation. And the measured data shows that the overall deep displacement of the retaining structure is less than 10mm. At the same time, the IPS is applied to limit the deformation of foundation pit by applying pre-stress to improve the apparent stiffness. Measured surface subsidence is not more than 18mm which show it reduces the impact of excavation on the environment effectively. In addition, aimed at the stress concentration defect of traditional internal support, the IPS overcomes it through the support system full span reaction. The IPS indicates wide application in deep excavation engineering.
2016, 24(5): 1022-1027.
Permanent displacement and plastic zone in the stable strata of a soil slope occur generally in the slope reinforced by stabilizing piles under the condition of intensive seismic action. However, the two important factors are usually omitted in the traditional aseismic design and analysis for slopes with cantilever-type stabilizing piles. Based on the principle of the rigid Newmark sliding model and limit analysis, the computational method is proposed that engineered landslide thrust on a stabilizing pile can be determined, by taking into consideration design safety factor and permanent displacement of the slope with piles. Meanwhile, according to the condition that some area in the stable strata of the slope is in the plastic state, the section of a stabilizing pile in the stable strata can be further divided into the segment in plastic zone of the strata and the segment in elastic zone of the strata. Therefore, the former can be regarded as cantilever beam model with uniformly lateral limit ground reaction; the latter can be analyzed by use of elastic foundation beam model. Their continuous conditions including internal forces and deformation of a pile and lateral ground reaction should be met at the interface between the two segments. The proposed method is demonstrated by a soil slope engineering example, in which calculation procedure and corresponding results are given in detail by the three section analysis method for the aseismic design of a cantilever-type stabilizing pile. The results show further that the proposed method is reasonable and cost effective.
Permanent displacement and plastic zone in the stable strata of a soil slope occur generally in the slope reinforced by stabilizing piles under the condition of intensive seismic action. However, the two important factors are usually omitted in the traditional aseismic design and analysis for slopes with cantilever-type stabilizing piles. Based on the principle of the rigid Newmark sliding model and limit analysis, the computational method is proposed that engineered landslide thrust on a stabilizing pile can be determined, by taking into consideration design safety factor and permanent displacement of the slope with piles. Meanwhile, according to the condition that some area in the stable strata of the slope is in the plastic state, the section of a stabilizing pile in the stable strata can be further divided into the segment in plastic zone of the strata and the segment in elastic zone of the strata. Therefore, the former can be regarded as cantilever beam model with uniformly lateral limit ground reaction; the latter can be analyzed by use of elastic foundation beam model. Their continuous conditions including internal forces and deformation of a pile and lateral ground reaction should be met at the interface between the two segments. The proposed method is demonstrated by a soil slope engineering example, in which calculation procedure and corresponding results are given in detail by the three section analysis method for the aseismic design of a cantilever-type stabilizing pile. The results show further that the proposed method is reasonable and cost effective.
2016, 24(5): 1028-1036.
Anchor system of bamboo bolt fully grouted by PS solution-based slurry is successfully applied in the conservation of rammed earth sites. However, the research on its anchoring mechanism has not been conducted so far. By means of the laboratory physical model experiment, anchor capacity, failure mode, strain distribution and load transfer along bolt-grout interface are tested. Experiments reveal that both PS-F and PS-(C+F)anchor system fail at the bolt-grout interface with ultimate pullout force of 10~15kN and 8~16kN respectively. Both of them have strong ductility. During load increment, single-peak and double-peak distributions emerge along bolt-grout interface. Meanwhile, load gradually transfers toward anchor end along with emergence of compressive strain and local peak value near anchor end. All these performance depicts the complicated mechanism. In conclusion, the researched anchor systems are suitable to the conservation of rammed earth sites in terms of anchor capacity. However, durability and structural integrity guarantee of bamboo bolt should be further studied.
Anchor system of bamboo bolt fully grouted by PS solution-based slurry is successfully applied in the conservation of rammed earth sites. However, the research on its anchoring mechanism has not been conducted so far. By means of the laboratory physical model experiment, anchor capacity, failure mode, strain distribution and load transfer along bolt-grout interface are tested. Experiments reveal that both PS-F and PS-(C+F)anchor system fail at the bolt-grout interface with ultimate pullout force of 10~15kN and 8~16kN respectively. Both of them have strong ductility. During load increment, single-peak and double-peak distributions emerge along bolt-grout interface. Meanwhile, load gradually transfers toward anchor end along with emergence of compressive strain and local peak value near anchor end. All these performance depicts the complicated mechanism. In conclusion, the researched anchor systems are suitable to the conservation of rammed earth sites in terms of anchor capacity. However, durability and structural integrity guarantee of bamboo bolt should be further studied.
2016, 24(5): 1037-1040.
The 5th China Youth's Symposium on Engineering Geology(CYSEG) was successfully held in Xianlin campus of Nanjing University from 6th to 7th of May 2016. This is the first time for the 3rd Youth Committee of Engineering Geology in China to organize such a symposium with specific theme. There are 355 participants. The theme of the symposium is "Soil engineering geological characteristics and structure",which is the specialty of Nanjing University in Engineering Geology. A total of 20 keynote speeches were presented. These speeches are rich in content with depth and breadth. This symposium is a grand event for young scholars in engineering geology and related fields to initiate academic discussions and in-depth exchange. It plays a positive role for promoting academic exchanges between young scholars, strengthening academic ties of geological engineering with geotechnical engineering and other related disciplines, training communication and organization skills of young scholars and nurturing young talents.
The 5th China Youth's Symposium on Engineering Geology(CYSEG) was successfully held in Xianlin campus of Nanjing University from 6th to 7th of May 2016. This is the first time for the 3rd Youth Committee of Engineering Geology in China to organize such a symposium with specific theme. There are 355 participants. The theme of the symposium is "Soil engineering geological characteristics and structure",which is the specialty of Nanjing University in Engineering Geology. A total of 20 keynote speeches were presented. These speeches are rich in content with depth and breadth. This symposium is a grand event for young scholars in engineering geology and related fields to initiate academic discussions and in-depth exchange. It plays a positive role for promoting academic exchanges between young scholars, strengthening academic ties of geological engineering with geotechnical engineering and other related disciplines, training communication and organization skills of young scholars and nurturing young talents.
2016, 24(5): 732-740.
Super-huge landslides in collapsed debris deposits with nearly horizontal sliding surface widely develop in Wanzhou District and Yunyang District, Chongqing Municipality in the Three Gorges Reservoir Region. Based on the two typical landslides, Taibaiyan Ancient Landslide in Wanzhou District and Laoyaopu Landslide in Yunyang District, this paper analyzes their structures and failure characteristics, and establishes a mechanical model of failure for super-huge deposit landslides with nearly horizontal sliding surface under the condition of heavy rainfall. This paper explains the failure process for this kind of landslide. Therefore, the excess pore water pressure produced in sliding band(or sliding surface) is the cause of this kind of landslide failure. Taking into account of the internal relation of pore water pressure and soil stress state, this paper deduces expressions of pore water pressure and hydraulic pressure of each soil slip in sliding surface, and obtains landslide stability coefficient expression considering the pore water pressure. The case of Laoyaopu Landslide in Yunyang District indicates the follows. Laoyaopu Landslide can be divided into two landslides by the location of zk5. The higher pore water pressure is located in the center of two landslides. The value of the pore water pressure calculated by this paper is bigger than the measure value in borehole. Its calculation error is 5.8%~10%. The cause can be the dissipated pore water pressure. The stability coefficient of Laoyaopu Landslide was 0.862 under the condition of heavy rainfall(i.e. the condition of excess pore water pressure). This landslide was unstable and has failed, which verified the induced action of the pore water pressure to landslide failure. This paper suggests that drain hole should be hit in the center of two landslides divided by zk5 in landslide project of management, which can eliminate or decrease the pore water pressure in the sliding surface. Besides Laoyaopu Landslide is governed with interception and drain engineering around landslide and in landslide surface, filling cracks.
Super-huge landslides in collapsed debris deposits with nearly horizontal sliding surface widely develop in Wanzhou District and Yunyang District, Chongqing Municipality in the Three Gorges Reservoir Region. Based on the two typical landslides, Taibaiyan Ancient Landslide in Wanzhou District and Laoyaopu Landslide in Yunyang District, this paper analyzes their structures and failure characteristics, and establishes a mechanical model of failure for super-huge deposit landslides with nearly horizontal sliding surface under the condition of heavy rainfall. This paper explains the failure process for this kind of landslide. Therefore, the excess pore water pressure produced in sliding band(or sliding surface) is the cause of this kind of landslide failure. Taking into account of the internal relation of pore water pressure and soil stress state, this paper deduces expressions of pore water pressure and hydraulic pressure of each soil slip in sliding surface, and obtains landslide stability coefficient expression considering the pore water pressure. The case of Laoyaopu Landslide in Yunyang District indicates the follows. Laoyaopu Landslide can be divided into two landslides by the location of zk5. The higher pore water pressure is located in the center of two landslides. The value of the pore water pressure calculated by this paper is bigger than the measure value in borehole. Its calculation error is 5.8%~10%. The cause can be the dissipated pore water pressure. The stability coefficient of Laoyaopu Landslide was 0.862 under the condition of heavy rainfall(i.e. the condition of excess pore water pressure). This landslide was unstable and has failed, which verified the induced action of the pore water pressure to landslide failure. This paper suggests that drain hole should be hit in the center of two landslides divided by zk5 in landslide project of management, which can eliminate or decrease the pore water pressure in the sliding surface. Besides Laoyaopu Landslide is governed with interception and drain engineering around landslide and in landslide surface, filling cracks.
2016, 24(5): 951-958.
Frost heaving hazards of saline soil in seasonal region affect engineering quality. The change of water, density and salt influences the frost heaving characteristics of seasonal soil. By taking the saline soil from dry land in Nong'an County, Jilin province, frost heaving tests are conducted to examine the frost heaving regularity of saline soil with different water contents, compaction degrees and salt contents. Under different water contents, compaction degrees and salt contents, the changing rule of the critical water content of frost heaving of saline soil is analyzed. The research indicates that higher compaction degree and higher water content are in favor of frost heaving. In lower water content and lower compaction degree, shrinkage occurs. Shrinkage disappears and then expansion appears with the increase of water content and compaction degree. Frost heaving quantity and water content have a nearly linear relationship. Therefore, a critical water content of frost heaving exists. The critical water content decreases in a linear rule with the increase of compaction degree. But as the salt content increases, the critical water content rises generally. The influence of plastic limit and compaction degree on the critical water content of frost heaving can be fitted in a linear equation. The coefficient of the equation rises overall with the increase of salt content. On the basis of the formula summed up previously, a new parameter, compaction degree has been added, which provides reference and theoretical basis for the research of the frost heaving characteristics of seasonal saline soil in future.
Frost heaving hazards of saline soil in seasonal region affect engineering quality. The change of water, density and salt influences the frost heaving characteristics of seasonal soil. By taking the saline soil from dry land in Nong'an County, Jilin province, frost heaving tests are conducted to examine the frost heaving regularity of saline soil with different water contents, compaction degrees and salt contents. Under different water contents, compaction degrees and salt contents, the changing rule of the critical water content of frost heaving of saline soil is analyzed. The research indicates that higher compaction degree and higher water content are in favor of frost heaving. In lower water content and lower compaction degree, shrinkage occurs. Shrinkage disappears and then expansion appears with the increase of water content and compaction degree. Frost heaving quantity and water content have a nearly linear relationship. Therefore, a critical water content of frost heaving exists. The critical water content decreases in a linear rule with the increase of compaction degree. But as the salt content increases, the critical water content rises generally. The influence of plastic limit and compaction degree on the critical water content of frost heaving can be fitted in a linear equation. The coefficient of the equation rises overall with the increase of salt content. On the basis of the formula summed up previously, a new parameter, compaction degree has been added, which provides reference and theoretical basis for the research of the frost heaving characteristics of seasonal saline soil in future.
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