膨胀土收缩开裂过程及其温度效应

Shrinkage and desiccation cracking process of expansive soil and its temperature-dependent behaviour

温度作为主要的外部环境因素之一,对膨胀土水-力学性质有重要影响。为了了解膨胀土在不同温度条件下的干缩开裂特性,开展了一系列干燥试验。试验中共配制了9组初始饱和的泥浆试样,干燥环境温度分别为22℃,60℃和105℃。在干燥过程中,实时监测试样的含水率变化及表面裂隙的演化过程,利用数字图像处理技术,对裂隙图像进行了定量分析,获得了表面裂隙率Rsc;采用填充法,测量了裂隙的体积。结果表明:在干燥初期,试样中水分蒸发速率为常数,且随温度的增加而增加,试样达到残余含水率所需时间随温度的增加而减小;裂隙出现时对应的临界含水率wIC受温度的影响非常明显,当温度从22℃增加到105℃时,wIC从38.0%增加到90.9%;在裂隙发展前期,Rsc增速较缓,随后进入快速发展期,较小的含水率变化可导致Rsc迅速增加;当含水率接近终期临界含水率wFC或缩限时,裂隙发展趋于稳定,且wFC基本不受温度的影响;在高温环境下,膨胀土的裂隙发育程度较低温环境高,存在明显的温度效应。

During drying, expansive soil shrinks and induces desiccation cracks on surface due to water evaporation. The presence of cracks significantly weakens soil structure and stability, and results in various engineering problems. Better understanding the processes of shrinkage and desiccation cracking is essential in analyzing drought effects on buildings constructed on expansive soil. As temperature is one of the most important external factors that control the hydro-mechanical behaviour, desiccation tests are therefore conducted on initially saturated expansive soil slurry to investigate the temperature effects on shrinkage and cracking characteristics. Nine specimens are prepared and dried under 22~C, 60~C and I05~C respectively. Water loss and surface crack evolution are monitored during the whole drying period. The image processing technique is employed to quantitatively characterize the crack pattern, and surface crack ratio Rsc is determined to describe surface cracking extent. In addition, the filling method is used to measure the crack volume by using pure sands. It is found that the initial evaporation rate of soil water is constant and increases with increasing temperature; the drying period that needs to reach the residual water content decreases with increasing temperature; and the initial critical water content Vlc at the onset of cracking is significantly influenced by temperature： as temperature increases from 22 to 105 ℃, wic increases from 38.0 to 90.9%. After crack initiation, it is found that the ratio Rsc increases slowly at the beginning stage, and then increases quickly even when water content decreases slightly. However, as the water content reaches the final critical water content WFC or shrinkage limit, Rsc gradually reaches stabilization, and the effect of temperature on WFC is insignificant. The experimentalresults also show that the desiccation cracking behaviour of expansive soil significantly depends on environmental temperature; in general, the higher the temperature, the more the development of cracks on specimen surface.