Unsaturated shear strength characteristics of coarse-fine mixed soils in a wide range of degree of saturation: experimental phenomena
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摘要: 粗细混合土是自然界和工程中普遍存在的土体类型，其强度对边坡、基坑、挡土墙等岩土工程稳定性至关重要。然而粗细混合土的强度在宽饱和度范围内的演化规律不明，尤其是缺少低饱和度时的试验数据和演化规律。为了阐明粗细混合土强度随饱和度变化的一般规律，采用室内直剪试验研究了7种不同细粒含量的粗细混合土的强度与变形特性。试验结果表明：①在宽饱和度范围内，由于毛细作用和吸附作用对抗剪强度贡献不同，非饱和粗细混合土的抗剪强度随饱和度的变化普遍存在“山峰效应”，即在非饱和残余区内出现了强度极大值，其强度随饱和度或吸力的演化规律可以划分为缓慢增长区、快速上升区和峰后下降区3个阶段。②当饱和度低于残余饱和度时，非饱和土的剪切破坏还出现了典型的脆性破坏特征。③粗细混合土的强度变形特征与细粒含量密切相关，随着细粒含量的增加，土骨架由粗颗粒控制逐渐转变为细粒控制，非饱和强度最大值所对应的细粒含量为50%~80%。Abstract: The coarse-fine mixed soils are widely distributed in natural and geotechnical engineering practices. Their strength is vital to the stability of geotechnical engineering (e.g., slopes, foundation pits and retaining walls). However, the unsaturated shear strength characteristics of the coarse-fine mixed soils are not evident in a wide range of degree of saturation (Sr), especially the lack of experimental data in the low range of Sr. In order to clarify the general rules that the unsaturated shear behaviors of the coarse-fine mixed soils in the full range of Sr, a series of direct shear tests are carried out on seven unsaturated coarse-fine mixed soils to study the unsaturated shear strength and deformation characteristics. The test results show that: (1) Based on the contribution of capillary water and adsorbed water to the unsaturated shear strength, the unsaturated shear strength exhibits a "peak behavior", where the shear strength reaches its peak at the residual zone, the shear strength characteristics with Sr or suction can be divided into three stages: slow growth zone, rapid rise zone and post-peak decline zone. (2) Brittle failure occurs when Sr is lower than the residual saturation. (3) The fines content (FC) significantly affects the strength and deformation characteristics of the coarse-fine mixed soils. As FC increases, the soil microstructure changes from a fines-controlled to a coarse-controlled structure. The FC corresponding to the maximum unsaturated strength is between 50%~80%.
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图 4 粗细混合土的强度试验结果(FC=36%, σv=100 kPa)
Figure 4. Results from direct shear tests on coarse-fine mixed soils (FC=36%, σv=100 kPa)
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图 5 低饱和度非饱和土的脆性破坏特征(FC=100%)
Figure 5. Brittle failure of unsaturated soils with low degree of saturation (FC=100%)
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图 7 不同FC粗细混合土的脆性指数随饱和度变化规律
Figure 7. Variation of law of brittleness index with degree of saturation
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图 9 不同FC混合土体的剪胀角比较
注：图 9和图 11中的饱和度取平行土样初始饱和度的平均值。
Figure 9. Dilation angles of coarse-fine mixed soils
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图 10 非饱和强度随饱和度和吸力的变化规律
Figure 10. Variation of unsaturated strength with degree of saturation and suction
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图 11 不同FC下粗细混合土的强度对比
Figure 11. Comparison of shear strengths of coarse-fine mixed soils
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图 12 不同类型土的强度随饱和度和吸力的变化规律
Figure 12. Variation of shear strength of different types of soils with saturation and suction
表 1 粗细混合土的制样干密度和含水率
Table 1 Dry densities and water contents of test soils
FC/% 16 24 36 50 65 80 100 ρd/(g·cm-3) 1.89 1.94 2.04 2.00 1.94 1.81 1.68 wopt/% 6.7 7.5 8.8 10.3 11.8 13.4 15.5 
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表 2 土-水特征曲线的主要参数
Table 2 Main parameters of soil-water characteristic curves
FC/% ψa/kPa ψr/kPa Sre/% 16 2.5 9.0 15.8 24 6.2 98.6 14.4 36 8.5 450.0 14.3 50 16.0 700.0 15.5 65 16.6 750.0 15.6 80 20.0 1000.0 14.6 100 25.0 1300.0 14.6 注：ψa为进气值，ψr为残余值，Sre为残余饱和度。
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