Experimental parameters.

Mountainous areas in southwest China are rainy in summer. The rainfall infiltration process involves complex soil thermal-hydraulic-mechanical (THM) coupling problems. The researches on soil THM coupling are mostly focused on numerical simulations, whereas the corresponding model tests are relatively few, and the existing model test studies often ignore the effect of temperature gradients in the soil. However, temperature gradients in the soil can cause water migration and affect the THM behavior of soil, so it cannot be ignored. This paper describes an experimental device that can test the changes of temperature, moisture and displacement in unsaturated soil columns with temperature gradients under rainfall infiltration conditions. By using the apparatus, the model tests of homogeneous soil column (H), homogeneous soil column with infiltration (HI), and preferential flow soil column with infiltration (P) under different temperature gradients are respectively conducted, and the results of moisture and heat migration and deformation properties in soils under different conditions are presented and discussed. A rainfall of low intensity and long duration is applied in the experiments, and the temperature of infiltration rainwater is consistent with that of the soil upper boundary. The results show that: (1) The infiltration of rainfall will increase the temperature of the soil column. The appearance of preferential flow results in faster heat transfer within the soil column, but causes the steady-state temperature to be lower than that of the homogeneous soil (HI); (2) Under infiltration conditions, the preferential flow soil column has an earlier outflow time but a later time for water field to reach steady state, while its water distribution is different from that of the homogeneous soils, with accumulation occurring near the end of preferential flow channel; (3) Under the action of temperature gradient, water migration occurs in homogeneous soil column (H), accompanied by soil settlement, while the infiltrated columns (HI and P) exhibit an increase in both water content and top displacement. In addition, the larger the temperature gradient, the more obvious the thermally induced hydraulic-mechanical response. The research results in this paper can provide experimental evidence for the theoretical study and numerical simulation of the soil THM coupling problems.

中国西南山区夏季多雨。降雨入渗过程涉及复杂的土热-水-力（THM）耦合问题。当前针对土THM耦合的研究多集中于数值模拟，相应的模型试验相对匮乏，且现有模型试验研究往往忽略了土体中的温度梯度效应。然而，土体中的温度梯度可引发水分运移并影响土体的THM性状，因此不容忽视。本文介绍了一种可在降雨入渗条件下，测试带温度梯度的非饱和土柱内温度、含水率与位移变化的试验装置。利用该装置，分别开展了不同温度梯度下均质土柱（H）、入渗均质土柱（HI）及入渗优先流土柱（P）的模型试验，并呈现与讨论了不同工况下土体的水热运移与变形特性。试验采用低强度长历时降雨，且入渗雨水的温度与土柱上边界温度保持一致。试验结果表明：（1）降雨入渗会提升土柱温度；优先流的出现会加快土柱内部的热传递，但会使稳态温度低于均质入渗土柱（HI）；（2）在入渗条件下，优先流土柱的出水时间更早，但水分场达到稳态的时间更晚，且其水分分布与均质土柱存在差异，优先流通道末端附近会出现水分积聚；（3）在温度梯度作用下，均质土柱（H）会发生水分运移并伴随土体沉降，而入渗土柱（HI与P）的含水率与顶部位移均有所增加。此外，温度梯度越大，热诱发的水-力响应越显著。本文研究结果可为土体THM耦合问题的理论研究与数值模拟提供试验依据。