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Experimental study on mechanical properties and microscopic mechanism of expansive soil in a project in north Xinjiang
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摘要: 北疆某工程穿越膨胀土区域，自运行以来发生多次滑动破坏，为深入探讨其破坏机理，通过对膨胀土进行室内直剪、压缩、渗透及扫描电镜试验，从宏观角度分析其力学特性，微观上揭示其物理机制。宏观分析表明：随含水率增加，黏聚力降低，内摩擦角呈先增后减趋势，在最优含水率时达到峰值；随干密度增加，黏聚力增加，内摩擦角呈逐步增大的趋势；随含水率增加，稳定孔隙比呈下降趋势，表明土体的压缩性增强；随干密度增加，初始孔隙比减小，稳定孔隙比趋于定值。电镜扫描结果显示：随固结压力增加，土体的结构类型由絮凝结构逐渐向紊流和层流状结构演化，孔隙数量与大小均下降，颗粒聚集效应明显，膨胀土压缩性降低；膨胀土在低固结压力下渗透性较强，在较高压力（200~1 600 kPa）下较小，渗透系数量级为10−6~10−8，与孔隙比呈正相关，可用幂函数的形式表达；随固结压力增加，松散堆积结构转变为紧密结合的层流状结构，孔隙面积减少，渗透系数显著降低。Abstract: A project of north Xinjiang has encountered multiple sliding failures since running which across expansive soil area. Our primary aim is to explore the sliding failure mechanism. Therefore, the indoor direct shear test, compression test, seepage test and electron microscope scanning test of soil are carried out separately, the results of mechanical properties of soil are investigated and their affecting mechanism is discussed. The cohesion decreases with the increase of water content. The internal friction angle decreases slowly when it is less than the optimal water content, and then decreases significantly. Due to dry density increasing, the cohesion increases significantly and the internal friction angle increases more slowly. According to the analysis result, the stable void ratio decreases with increasing water content, and the compressibility of soil increases. With increasing dry density, the initial void ratio decreases, and the stable void ratio tends to be constant. With increasing consolidation pressure, the structure type of soil gradually evolves from flocculation structure to turbulence and laminar flow structure, the effect of particle aggregation is obvious, the number and size of pores decrease, and the compressibility of expansive soil decreases. All the above results are obtained based on the result of scanning electron microscopy experimental test (SEM). The permeability of expansive soil is strong under low consolidation pressure, and it is small under high pressure (200~1 600 kPa), with an order of 10−6~10−8. The permeability coefficient is positively correlated with the void ratio, which can be expressed in the form of power function. The results of scanning electron microscopy (SEM) show that the loose accumulation structure changes into a closely combined laminar flow structure with increasing consolidation pressure, and the area of pores decreases, which provides the conditions for the permeability coefficient to decrease significantly.
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Key words:
- expansive soil /
- shear strength /
- compressive deformation /
- permeability /
- micro mechanism
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图 3 不同含水率膨胀土的剪切特性
Figure 3. Shear strength of expansive soils with different moisture contents
图 4 不同干密度膨胀土的剪切特性
Figure 4. Shear strength of expansive soil with different dry densities
图 5 不同含水率下膨胀土压缩曲线
Figure 5. Compression curve of expansive soil under different moisture contents
图 6 不同干密度下膨胀土压缩曲线
Figure 6. Compression curve of expansive soil under different dry densities
图 7 不同压力下试样放大10 000 倍SEM图像
Figure 7. SEM images magnified 10 000 times under different pressures
图 9 渗透作用下放大10 000 倍试样扫描电镜SEM图像
Figure 9. SEM image of 10 000 times magnification sample under infiltration
表 1 膨胀土基本物理性质指标
Table 1. Basic physical properties
天然含
水率/%天然干密度/
(g·cm−3)液限/
%塑限 /
%塑性
指数液性
指数风干含
水率/%最大干密度/
(g·cm−3)最优含
水率/%线缩率/
%体缩率/
%自由膨
胀率%14.8 1.60 61.3 20.1 41.2 −0.13 8.0 1.67 18.9 18.4 29.5 75 
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表 2 膨胀土矿物组成
Table 2. Composition of expansive soil
蒙脱石/% 石英/% 长石/% 方解石/% 钠长石/% 杂质/% 60.3 32.7 6.0 0.5 0.4 0.1
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表 3 直剪压缩试样控制指标
Table 3. Control index of direct shear compression specimen
不同含水率 不同干密度 控制含水率/% 控制干密度/(g·cm−3) 控制含水率/% 控制干密度/(g·cm−3) 15.9 1.6 18.9 1.37 18.9 1.6 18.9 1.47 21.9 1.6 18.9 1.54 24.9 1.6 18.9 1.60 27.9 1.6 18.9 1.67
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表 4 膨胀土的不同含水率下压缩性指标
Table 4. Compressibility index of expansive soil
试样
编号含水率/
%压缩系数/
MPa−1压缩模量/
MPa压缩
指数1 16.1 0.102 15.793 0.336 2 18.8 0.109 14.802 0.328 3 22.0 0.357 4.512 0.296 4 24.2 0.375 4.292 0.396 5 27.6 0.389 4.143 0.409
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表 5 不同干密度下膨胀土的压缩性指标
Table 5. Compressibility index of expansive soil under different dry densities
试样
编号干密度/
(g·cm−3)压缩系数/
MPa−1压缩模量/
MPa压缩
指数1 1.34 0.41 3.9 0.459 2 1.48 0.15 10.66 0.424 3 1.55 0.10 16.35 0.355 4 1.61 0.11 14.29 0.275 5 1.66 0.09 17.66 0.216
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表 6 颗粒参数
Table 6. Particle parameters
压力/kPa 颗粒总数/个 总面积/μm2 平均宽度/μm 面积占比/% 平均周长/μm 0 344 43.507 0.126 40.759 1.792 100 455 56.767 0.125 53.817 1.596 200 327 72.575 0.222 67.586 0.973 400 201 71.917 0.358 68.382 1.875 800 382 76.951 0.201 73.447 0.574 1600 115 82.295 0.716 78.361 2.185
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表 7 渗透系数与孔隙比关系
Table 7. Permeability characteristic relationship
编号 固结压力/
kPa饱和后对应
孔隙比饱和渗透
系数/（cm·s−1）实测渗透
系数/（cm·s−1）1 0 0.854 — 2.78×10−3 2 100 0.671 2.25×10−6 7.09×10−4 3 200 0.611 4.23×10−7 8.95×10−6 4 400 0.548 3.35×10−8 4.47×10−6 5 800 0.473 2.48×10−8 3.66×10−7
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表 8 微观试验结果
Table 8. Micro-test results
编号 压力/kPa 总体孔隙面积/μm2 孔隙平均面积/μm2 孔隙面积占比/% 渗透系数/（cm·s−1） 1 0 11.650 1.151 10.821 2.78×10−3 2 100 10.288 0.896 9.594 7.09×10−4 3 200 4.354 0.294 4.072 8.95×10−6 4 400 9.525 0.552 8.970 4.47×10−6 5 800 6.960 1.293 5.692 3.66×10−7 6 1600 4.510 0.432 2.965 1.49×10−8
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