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EVALUATION OF SHEAR STRENGTH OF UNSATURATED LOESS USING CONVENTIONAL DIRECT SHEAR TEST
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摘要: 为了研究一种简化的方法评价非饱和黄土抗剪强度，本文利用常规直剪仪对非饱和重塑黄土进行固结快剪试验，Whatman's No.42型滤纸测试固结前和剪切后土样吸力，研究同一正应力下初始吸力和剪后吸力的变化趋势及机理，并用非饱和直剪测试结果对常规直剪和滤纸法测得的吸力与强度进行了效验。研究表明，土-水特征曲线的进气值和残余值对固结前和剪切后干密度和吸力的变化起控制作用，获得初始吸力与剪后吸力的变化关系。基于Vanapalli模型，提出了利用土样初始土-水特征曲线、有效黏聚力和有效内摩擦角预测非饱和黄土抗剪强度的公式，研究结果简化了非饱和土强度测试方法，节约了时间，为非饱和土抗剪强度理论在工程实践中推广应用提供新思路。Abstract: In order to investigate the simplified method for evaluating the shear strength of unsaturated loess, we conduct a series of conventional consolidated quick shear test and the suction test before consolidation as well as after shearing using filter paper method. The data is validated by unsaturated direct shear test. Then we discuss the variation of initial suction and shear suction under normal stress. The research shows that air entry value and residual value of the soil-water characteristic curve control the change of dry density and water content within consolidating. Based on the Vanapalli's model, a simple method to predict the shear strength of loess is gained. The proposed model using the initial soil-water characteristic curve, effective cohesion and effective internal friction angle of soil simplifies the procedure and time of shear strength measurement for unsaturated soil. The model provides a novel technique for the application of unsaturated shear strength theory in engineering practice.
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Key words:
- Direct shear test /
- Shear strength /
- Filter paper method /
- Suction /
- Soil-water characteristic curve
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图 3 剪后吸力与初始吸力关系对比图
Figure 3. Comparison of the relationship between shearing suction and initial suction
图 4 剪后体积含水率与初始体积含水率关系
Figure 4. Relationship between volumetric moisture content after shearing and initial volumetric moisture content
图 5 剪后干密度与初始体积含水率关系
Figure 5. Relationship between dry density after shearing and initial volumetric moisture content
图 6 抗剪强度与体积含水率关系
Figure 6. Relationship between shear strength and volumetric moisture content
图 7 非饱和直剪强度与常规直剪强度与剪后吸力的关系
Figure 7. Relationship between unsaturated direct shear strength and conventional direct shear strength and suction after shearing
图 8 抗剪强度与初始吸力的拟合关系
Figure 8. The fitting relationship between shear strength and suction before shearing
图 9 抗剪强度与初始吸力的拟合关系
Figure 9. The fitting relationship between shear strength and initial suction
表 1 试验土样的基本物理指标
Table 1. Basic physical indicators of test soil samples
干密度ρd/g·cm-3 含水率w/% 天然密度ρ/g·cm-3 比重Gs 孔隙比e 液限WL/% 塑限WP/% 塑性指数IP/% 1.32 8.3 1.43 2.70 1.06 27.5 18.3 9.2 
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表 2 不同含水率下黄土的抗剪强度与吸力值
Table 2. Shear strength and suction value of loess under different water content
ρd/g·cm-3 ρd′/g·cm-3 W/% (ua-uw)/kPa 第1组 第2组 τ/kPa (ua-uw)′/kPa w′/% τ/kPa (ua-uw)′/kPa w′/% 1.570 1.577 2.0 38791.0 132.5 17747.6 2.2 129.0 10102.0 2.4 1.570 1.584 4.0 5183.9 112.8 1826.7 4.4 101.4 1469.0 5.0 1.570 1.591 6.0 1187.2 102.1 369.8 5.5 80.8 — — 1.570 1.594 8.0 414.5 55.0 60.8 7.8 65.5 79.0 8.6 1.570 1.595 9.0 248.5 67.7 62.6 8.5 65.0 55.1 9.2 1.570 1.599 12.0 90.2 62.6 40.0 12.0 60.6 — — 1.570 1.600 15.0 43.0 54.0 31.6 13.9 62.6 31.8 13.6 1.570 1.601 18.0 20.9 58.1 23.6 17.3 52.0 21.6 17.4 1.570 1.605 21.0 11.1 45.4 12.8 20.2 50.9 14.2 19.4 1.570 1.615 23.0 6.3 34.5 8.6 21.9 45.0 13.4 20.9 1.570 1.635 25.0 2.7 38.7 5.4 22.2 38.0 4.8 22.9 1.570 1.637 27.2 1.0 36.2 1.7 25.2 36.1 — — ρd为初始干密度；ρd′为剪后干密度；w为初始质量含水率；w′为剪后质量含水率；τ表示抗剪强度；ua-uw为初始吸力；(ua-uw)′为剪后吸力
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表 3 非饱和直剪试验结果
Table 3. Unsaturated direct shear test results
剪后质量含水率w′/% 吸力(ua-uw)′/kPa 抗剪强度τ/kPa 28.8 10 18.9 13.9 40 71.9 8.3 80 75.3 7.8 200 79.3
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表 4 SWCC参数拟合值
Table 4. Fitting parameters of SWCC
曲线类型/拟合参数 a/kPa m n θr/% R2 初始土-水特征曲线 12.1 0.71 2.15 14.0 0.98 剪后土-水特征曲线 10.4 0.81 1.15 14.0 0.99
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