Laboratory characterization and discrete element modeling of desiccation cracking behavior of soils under different boundary conditions
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摘要: 为了探究土体干缩开裂过程的边界效应问题,采用不同底面粗糙度的容器开展了多组干燥试验,发现干缩裂隙存在从顶面向下和从底面向上两种典型的发育形式。并且,裂隙发育程度与土样/容器界面接触条件密切相关,从而验证了裂隙发育过程的边界效应。通过理论分析,阐明了上边界的蒸发条件及下边界的接触条件对裂隙发育形式的控制作用。为了能更深入地理解土体干缩开裂边界效应的内在机制,在试验的基础上建立离散元模型,创新性地引入了沿深度的失水速率梯度参数,模拟土样上边界的蒸发条件变化。通过设置底面摩擦系数,模拟土样下边界的接触条件变化。将模拟结果与试验结果进行了对比分析,发现二者具有较好的吻合度。总体上,土体干缩裂隙的发育过程是顶面蒸发失水与底面摩擦两种边界条件共同作用的结果。当底面摩擦系数相对较小时,裂隙发育由蒸发失水主导,大部分裂隙由顶面向下发育。随着底面摩擦系数的增加,底面接触条件对裂隙发育过程的主导作用逐渐增强,由底面向上发育的裂隙数量所占比重也相应增加。Abstract: In order to explore the boundary effect of the desiccation cracking process, multiple sets of drying tests are carried out using the containers with different bottom roughnesses. Two different forming patterns can be observed in the laboratory tests, initiating from the top/bottom, and there is propagation closely related to the sample/container interface contact conditions. This verifies the boundary effect of the crack propagation. In order to understand the internal mechanism of the desiccation boundary effect of the soils more deeply, a discrete element model is established based on the drying tests. A water loss rate gradient parameter along the depth is introduced innovatively to simulate the change of the evaporation condition of the upper boundary of the soil samples. By setting the friction coefficient of the bottom surface, the contact condition of the lower boundary of the sample is simulated. The simulated results are compared with the experimental ones and found to have good agreement. In general, the initiation and propagation of desiccation cracks are the result of the combination of water loss due to surface evaporation and bottom friction. When the coefficient of friction of the bottom surface is relatively small, the development of the fracture is dominated by water loss, and most of the fractures develop from the top surface. With the increase of the friction coefficient of the bottom surface, the effect of the contact condition of the bottom surface on the development of the crack gradually increase, and the proportion of the number of cracks developed from the bottom surface increases accordingly.
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Keywords:
- desiccation cracking /
- boundary conditions /
- DEM /
- bottom friction /
- evaporation
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图 1 裂隙从顶面向下发育过程（E1组侧面局部放大）
Figure 1. Downward propagation of crack from top (side view of E1, enlarged)
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图 2 裂隙从底面向上发育过程（E1组侧面局部放大）
Figure 2. Upward propagation of crack from bottom (side view of E1, enlarged)
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图 7 3组摩擦系数对应不同失水速率梯度条件下的裂隙发育情况
Figure 7. Variation of crack development with water loss gradient under three different friction coefficients
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图 8 （a）模拟中裂隙从顶面向下发育情况；（b）模拟中裂隙 从底面向上发育情况（线段越粗表示颗粒间连接受力越.大,应力集中越明显）
Figure 8. Simulated results of crack initiating from (a) top and crack initiating from bottom (b) simulation results (greater line weight indicates stronger force in contact)
表 1 室内试验参数
Table 1 Parameters of laboratory tests
试样编号 砂纸规格/目 砂颗粒粒径/mm 摩擦系数 初始厚度/cm E1 80 0.180 大 1 E2 120 0.120 中 1 E3 240 0.063 小 1 
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表 2 室内试验裂隙条数统计
Table 2 Number of cracks in laboratory tests
试样编号 摩擦系数 总裂隙条数 顶面发育裂隙条数 底面发育裂隙条数 E1 大 9 5 4 E2 中 5 3 2 E3 小 3 2 1
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表 3 数值模拟裂隙条数统计（λ=0.05）
Table 3 Number of cracks in numerical tests (λ=0.05)
编号 摩擦系数 总裂隙条数 顶面发育裂隙条数 底面发育裂隙条数 ① 0.3 8 5 3 ② 0.2 5 4 1 ③ 0.1 3 3 0
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