结构性对膨胀土收缩特性影响的试验研究

Experimental study of the influence of structure on the shrinkage characteristics of expansive soil

膨胀土的收缩性明显,容易引发边坡与地基开裂,但有关结构性对收缩特性影响的认识甚少。采用收缩自动试验装置,在恒湿恒温条件下对原状膨胀土和重塑膨胀土开展了收缩对比试验和扫描电镜(scanning electron microscope,简称SEM)测试分析,结果表明:与原状土相比,重塑土在土中水流动阶段的蒸发速率较小,蒸汽扩散阶段收缩稳定速率较慢,最终体积收缩应变量更大;重塑土体积收缩-含水率关系曲线的线性段较长,斜率较大,直线段与稳定段之间的过渡不明显,而原状土则反之;重塑土和原状土的收缩特征曲线(soil shrinkage characteristic curves,简称SSC)在较高含水率段基本重合,随着含水率下降,重塑土的SSC下降更快,对应的含水率范围更宽,最后进入残余-零收缩阶段时,孔隙比明显较小;Chertkov收缩模型适用于原状膨胀土,但不适用于重塑膨胀土。SEM测试结果表明,原状膨胀土较重塑膨胀土具有更强的原生结构性,初始密度与湿度相同情况下,两者颗粒排列、接触方式、胶结状态、孔隙大小与分布特征等微观结构上差异明显,导致蒸发过程中重塑土的水分迁移速率较小、基质吸力较大,是重塑土收缩更剧烈的内在原因。研究结果可为膨胀土边坡的坡面工程防护设计提供参考依据。

Shrinkage is inherent to expansive soil, usually resulting in slope and foundation cracking, but there is little understanding about the influence of structure on shrinkage characteristics. Using an automatic shrinkage test device, comparison tests on shrinkage and scanning electron microscope(SEM) tests for undisturbed and remolded expansive soil were carried out under the same humidity and constant temperature. The results show that compared with undisturbed soil, remolded soil has smaller evaporation rate in the water flow stage, slower shrinkage stability rate in the vapor evaporation stage and larger volume shrinkage strain in the end. For the remolded soil, the linear section of the curve of volume shrinkage versus water content is longer, the slope is larger, and the transition between the linear section and the stable section not is obvious, while the undisturbed soil is on the contrary. The soil shrinkage characteristic curves(SSC) of remolded and undisturbed soil basically coincide in the section at higher water content;with the decrease of water content, the SSC of remolded soil decreases faster and the corresponding water content range is wider, and when entering the residual-zero shrinkage stage, the void ratio is obviously smaller. Chertkov shrinkage model is suitable for undisturbed expansive soil, but not for remolded one. SEM test results show that the undisturbed expansive soil has stronger primary structure than remolded expansive soil. Under the same initial density and humidity, there exists obvious differences in microstructure, such as particle arrangement, contact mode, cementation state, pore size and distribution characteristics between the undisturbed and remolded expansive soil, resulting in smaller water migration rate and larger matrix suction of remolded soil during evaporation, which is the internal reason why remolded soil shrinks more violently than undisturbed soil. The research results can provide a reference basis for the design of slope engineering protection for expansive soil slopes.