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EVALUATION OF STRENGTH CHARACTERISTICS OF COMPACTED LOESS SUBJECTED TO FREEZE-THAW CYCLING BY WAVE VELOCITY
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摘要: 如何评价冻融循环条件下压实黄土的力学特性是我国黄土高原挖填方工程设计中亟需解决的重要科学问题，为此，本文对冻融循环次数、含砂量、含盐量和含水率等几个关键影响因素进行研究，通过开展无侧限压缩试验，获得了压实黄土的单轴抗压强度(UCS)。同时还开展了压实黄土的波速测试，分析了不同冻融循环条件下压实黄土的剪切波波速(VS)和压缩波波速(VP)的变化规律。研究结果表明：(1)随着冻融循环次数的增加，黄土试样的剪切波波速和压缩波波速减小，且含砂量较低的试样的波速降低更加明显；(2)含盐量高的黄土试样在经历第1次冻融循环后波速大幅衰减，但继续冻融循环时，其强度降低幅度不大；(3)随着冻融循环次数和含水率的增加，土样破坏的形式呈现出由脆性破坏向塑性破坏过渡的趋势；(4)根据试验结果，研究建立了黄土波速与单轴抗压强度的经验关系式，研究成果为评价冻融循环作用下现场压实黄土的强度劣化提供了新思路。Abstract: Characterizing the mechanical properties of loess while considering the influence of freeze-thaw cycling is of great importance in geotechnical designs at Loess Plateau in China. Several key influencing factors are studied in a collective manner. They include the number of freeze-thaw cycles, the sand content, the salt content and the water content. In this paper,we carry out the unconfined compression tests,and obtain the uniaxial compressive strength of loess. At the same time,we also measure the shear wave velocity and the compression wave velocity of loess specimens subjected to different freeze-thaw cycles. The following are found: (1)the shear wave velocity and compression wave velocity decrease with an increase of freeze-thaw cycles. The reduction is more pronounced in loess samples with lower sand content. (2)The velocity of loess with high salt content decreases after the first freeze-thaw cycle,and it becomes less significant with further increase of the freeze-thaw cycles. (3)With the increase of freeze-thaw cycles and water content,the failure forms of soil samples change from brittle failure to plastic failure. (4)Based on the test results,we propose an empirical correlation between the wave velocity and the uniaxial compressive strength of loess. The research provides a new insight into evaluation of strength deterioration of loess under freeze-thaw cycle.
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图 3 带有弯曲元装置的三轴试验仪器及工作原理图
Figure 3. Triaxial test apparatus equipped with a pair of bender/extender element
图 4 弯曲元装置中S波、P波测试原理图
Figure 4. Schematic diagrams of S-wave and P-wave test in bending element device
图 5 5 kHz激振条件下的接收波信号
a. 压缩波；b. 剪切波
Figure 5. Wave signal at excitation frequency of 5 kHz
图 6 砂黄土的应力-应变关系(含水率为9.5%)
a. 不同的冻融循环次数；b. 不同的加砂量；c.不同离子浓度
Figure 6. Stress-strain relationship of sandy loess(water content: 9.5%)
图 7 冻融循环下砂黄土的单轴抗压强度
a. 含水率≈9.5%；b. 含水率≈13.5%
Figure 7. Uniaxial compressive strength of sandy loess under freeze-thaw cycle
图 8 冻融循环下含盐黄土的单轴抗压强度
a. 含水率≈9.5%；b. 含水率≈13.5%
Figure 8. Uniaxial compressive strength of saline loess under freeze-thaw cycle
图 10 UCS与P波、S波波速之间的关系
a. 加砂黄土; b. 含盐黄土
Figure 10. Relationship between UCS and P-wave and S-wave velocities
表 1 文献中含盐分黄土的离子种类和离子浓度
Table 1. Ion types and concentrations of saline loess in literature
文献中含盐分的黄土 离子种类和离子浓度/mg·g-1 Cl- SO42- K+ Na+ Ca2+ Mg2+ 渭北黄土(张蓉蓉等，2018) 0.47 0.71 0.01 0.23 0.10 0.05 黑方台黄土(Zhang et al.，2018) 2.63 2.45 0.02 2.52 0.20 0.09 
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表 2 试验材料的物理性质
Table 2. Physical properties of tested materials
土样 不均匀系数 平均粒径/um 塑限/% 塑性指数 比重 黄土 — 13 17.7 9.9 2.65 石英砂 2.0 182 — — 2.64
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表 3 重塑黄土波速-强度拟合曲线参数表
Table 3. Parameter table of wave velocity-strength fitting curve of remolded loess
土样类型 含砂重塑黄土 含盐重塑黄土 经验系数 α β α β P波 1.94 0.01 6.77 0.007 S波 2.66 0.02 5.96 0.015
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