土体干缩裂隙发育方向及演化特征的层间摩擦效应研究

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

自然界中的土体通常成层分布,在干旱条件下,表层土体的干缩开裂过程极易受层间接触条件的制约。为了探究土层间摩擦效应对土体干缩裂隙发育方向及其演化特征的影响,开展了一系列室内干燥试验。试验共配置了3组初始饱和的泥浆样,在30℃的室温条件下干燥失水,通过在试样底部铺设不同粗糙度的砂纸来模拟不同土层间的摩擦效应。试验过程中对试样表面及侧面进行定时拍照,从不同角度记录了裂隙发育全过程,通过分析,获得了一些新发现:(1)干缩裂隙不仅能从土体表面向下发育,而且还可能率先从土体底部向上发育,这不同于以往的习惯性认识;(2)初始裂隙的发育位置受土质条件及基底摩擦条件的共同制约,对于非均质性比较严重或者表面存在明显"杂点"的土体而言,裂隙往往首先从表面"杂点"处产生并向下发育,而对均质性较好的土体而言,在基底摩擦效应的影响下裂隙可以率先从土体底部生成并逐渐向上发育,且土体底部的裂隙发育程度甚至会高于表面的裂隙发育程度;(3)底部生成的裂隙以斜向上发育居多,这可能与裂隙发育过程中受到的剪切应力作用有关;(4)干燥过程中,土体呈向心收缩,存在明显的收缩核现象;(5)基底摩擦效应能改变土体干缩开裂过程中内部应力场的演化,从而对土体的横向和纵向收缩应变及剖面含水率的空间分布产生影响,如基底摩擦越大,土体横向收缩应变越小,而纵向收缩应变越大。

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 the bottom of soil appears at the bottom of the soils.(5) The substrate friction effect can change the internal stress field during the desiccation cracking process of soils, and therefore affects the transverse and longitudinal strains and profile water content distribution of the soils. For example, the larger the substrate frictional degree, the smaller the transverse strain of soils, but the greater the longitudinal strain of soils.