Shear behavior of saturated silt under complex unloading paths
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摘要: 为研究基坑开挖卸荷路径下土体的力学特征，开展了饱和粉土卸荷路径三轴排水剪切试验，获取了不同应力路径的应力应变曲线，分析了卸荷比、围压和应变水平对割线模量的影响，提出了饱和粉土卸荷模量简化公式。结果表明：不同卸荷应力路径的应力应变曲线均呈应变软化型，卸荷模量随卸荷比的减小、围压的增大而增大。K0固结侧向卸荷路径（EB）的峰值剪应力高于K0固结双向卸荷路径（EC、FD）；应力路径对割线模量衰减的影响程度差异显著，K0固结双向卸荷路径（EC、FD）对割线模量衰减的影响最大。说明基坑坑底土体强度较高，受开挖的影响较小，抗变形能力较大，在基坑工程设计中，应考虑基坑不同区域对应的割线模量。Abstract: The excavation of foundation pit engineering has different unloading effects on the surrounding soil. The existing specifications mainly use the consolidated undrained strength index based on the conventional triaxial tests, which cannot describe the mechanical characteristics of the excavation soil under the unloading path. The triaxial drainage shear tests on the unloading path of saturated silt are conducted, the stress-strain curves for different stress paths are obtained, the effects of unloading ratio, confining pressure and strain level on the secant modulus are analyzed, and a simplified formula proposed for the unloading modulus of saturated silt. The results show that the stress-strain curves of different unloading stress paths all exhibit a strain softening type, and the unloading modulus increases with a decrease in the unloading ratio and an increase in the confining pressure. The peak shear stress of K0 consolidation lateral unloading path (EB) is higher than that of K0 consolidation bidirectional unloading path (EC, FD). The influences of the stress paths on the attenuation of the secant modulus vary significantly, with K0 consolidation bidirectional unloading paths (EC, FD) having the greatest impact on the attenuation of the secant modulus. The strength of the soil at the bottom of the foundation pit is relatively high, and it is less affected by excavation, with a greater capability to resist deformation. In the design of foundation pit engineering, the corresponding secant modulus in different areas of the foundation pit should be considered.
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Keywords:
- excavation of foundation pit /
- saturated silt /
- stress path /
- unloading ratio /
- secant modulus
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图 3 10-3轴向应变下卸荷模量-卸荷比-围压关系图
Figure 3. Relationship among unloading secant modulus (εa=0.1%), unloading ratio and mean effective stress
表 1 粉土基本物理性质指标
Table 1 Basic physical properties of silt
含水率w/% 天然干密度ρ/(kg·m-3) 天然重度γ/(kN·m-3) Gs 液限wL/% 塑限wP/% 塑性指数Ip 液性指数IL 33.4 1.34 17.8 2.70 35.2 25.5 9.7 0.81 
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表 2 饱和粉土三轴试验方案
Table 2 Triaxial test protocol for saturated silt
试验名称 卸荷路径 卸荷速率/(kPa·min-1) σ1/kPa(轴向) σ3/kPa(侧向) σ1(轴向) σ3(侧向) EB-100 不变 100→0 0 -0.1 EB-200 不变 200→0 0 -0.1 EB-400 不变 400→0 0 -0.1 EG-100 208→0 不变 -0.1 0 EG-200 433→0 不变 -0.1 0 EG-400 833→463 不变 -0.1 0 EC-100 208→183 100 →0 -0.025 -0.1 EC-200 416→366 200 →0 -0.025 -0.1 EC-400 833→733 400 →0 -0.025 -0.1 FD-100 208→158 100 →0 -0.05 -0.1 FD-200 416→316 200 →0 -0.05 -0.1 FD-400 833→633 400 →0 -0.05 -0.1
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