土体干缩裂隙的形成发育过程及机理

土体干缩开裂(龟裂)是一种常见的自然现象,龟裂的产生会破坏土体的完整性,极大地弱化土体的工程性质。基于室内试验结果,发现龟裂发育过程可分为3个典型阶段,具有很强的时序特征,且新生裂隙一般垂直已有裂隙生长。根据水土相互作用原理和基本土力学理论,建立了土体龟裂概念模型,对实验室和自然界中观测到的相关龟裂现象及其机理进行了分析,得到如下主要结论:(1)土体中存在收缩变形空间是龟裂发育的前提,主要与土质条件有关;(2)龟裂是土体发生张拉破坏的表现形式,孔隙水的表面张力及干燥过程中引起的基质吸力(毛细水作用力)会在土体中形成张拉应力场,这是导致龟裂的主要力学诱因;(3)当张拉应力场的大小超过土体的抗拉强度或土颗粒间的联接强度时,裂隙便会产生,导致局部区域积聚的应变能释放,应力场重新调整。从宏观上看,基质吸力和抗拉强度是控制龟裂发育的两个关键力学指标,但从微观上看,土体材料尤其是结构的非均质性对裂隙发育过程和裂隙网络的几何形态特征均有重要影响。通常情况下,大部分裂隙都是在饱和阶段产生,且裂隙产生时对应的临界含水率有可能高于液限;土体表面上的"杂点"易导致应力集中,裂隙往往率先在"杂点"处产生;(4)受表面和裂隙面张拉应力场的共同作用,表层土体边缘会发生向上卷曲变形,产生"煎饼效应"。此外,土体在收缩过程中还存在收缩核现象。

Behaviors and mechanisms of desiccation cracking of soils

The desiccation cracking of soils is a common natural phenomenon. The presence of cracks in soils can significantly destroy integrity of the soil mass and weaken their engineering properties. In this investigation, laboratory desiccation tests are conducted. It is found that the desiccation cracking process takes place at three typical stages and presents evident time-order characteristics. New cracks always start perpendicularly from the existing cracks. Based on the fundamental principles of water-soil interaction and soil mechanics, the mechanisms of desiccation cracking are discussed. A series of conceptual models are established to provide insights behind the laboratory and field observations. The following conclusions can be drawn: (1) The space for soil shrinkage deformation is the basis of cracking that is conditioned by soil nature. (2) The desiccation cracking is one form of tensile failures. Surface tension of pore water and drying-induced metric suction (capillary force) can lead to the development of tensile stress field in soils, which is the main mechanical cause of cracking. (3) Cracking occurs once the drying-induced tensile stress exceeds the tensile strength of soil, or the connection strength between soil particles. After that, the gathered local strain energy releases and the stress field tends to readjust. From macroscopic scale, the matric suction and tensile strength of soils are the two key mechanical parameters controlling the desiccation cracking behaviors, while from microscopic scale, the geometric and morphologic characteristics of crack pattern are strongly linked to the homogenous and microstructure features of soils. Generally, most of the cracks initiate when the soils are still fully saturated. The corresponding critical water content at onset of cracking is likely higher than the liquid limit. The flaws on soil surface can result in stress concentration and trigger the initiation of cracks. (4) The surface curling like “pancake effect” may occur during drying. The combined effects of tensile stress field in soil surface and crack face are responsible for this phenomenon. Moreover, the