Interfacial friction dependence of propagation direction and evolution characteristics of soil desiccation cracks

As the soils in nature are distributed in layers, the desiccation cracking process of the top soil is easily constrained by the contact conditions between the soil layers under drought conditions. A series of laboratory desiccation tests are therefore conducted to investigate the influences of interfacial frictional effect between soil layers on the developing direction and evolution characteristics of soil desiccation cracking. In the tests, three slurry samples with the initial saturation are prepared and dried under a constant room temperature of 30℃. Different interfacial friction conditions are designed at the bottom of the samples to simulate the frictional effect between soil layers in nature. During drying, photos of the surface and side of the samples at regular intervals are taken to record the development process of soil cracks from different angles. Some new discoveries show: (1) The soil desiccation cracks can develop from the surface downwards and may be firstly generated from the bottom of the soil and gradually develop upwards, which is different from the previous habitual understanding; (2) During the drying process, the initial development position and development degree of cracks are constrained by both the soil properties and the substrate friction conditions. For the soils with severe inhomogeneous or obvious flaws on the surface, the cracks are generated and develop from the surface flaws of soils, while for the relatively homogeneous soils, the cracks can be firstly generated from the bottom of the soil and gradually develop upwards under the influences of the substrate friction. Moreover, after the drying is completed, the development degree of the bottom cracks of the soils is even higher than that of the surface cracks. (3) The bottom cracks mostly propagate in inclined direction, and they are probably related to the developed shear stress. (4) During drying, the soil shrinks concentrically, and noticeable shrinkage nucleus at