Numerical simulation of vertical crack propagation in paddy fields based on nonlinear fracture mechanics

利用断口学测量的裂纹尖端张开角（CTOA）描述了裂纹形成的机理。断裂试验采用ca -膨润土与硅砂的混合物和水田土在液限处成型成矩形条状，平衡土壤吸水性在5 ~ 50kPa之间（部分50kPa的试样浸湿至5kPa），并插入0.4 x试样厚度的缺陷。从一系列图像中量化了裂纹的生长和几何形状。在稳定裂纹扩展过程中测量的CTOA随干燥而减小。当最初平衡为50kpa的试样被重新润湿至5kpa时，CTOA进一步下降。这些结果表明CTOA主要由刚度控制，尽管再润湿可能会在裂纹尖端之前改变毛细应力。在实验室尺度模型中研究了受收缩影响的垂直裂纹扩展。装在容器中的土通过压缩压力进行固结。去除压力后，在试样中心形成垂直裂纹，然后在试样两侧施加渗透性陶瓷材料的吸力。观察到的CTOA值与弯曲试验的测量值进行了比较。收缩时的CTOA值与弯曲时的CTOA值相似，表明用弯曲试验得到的CTOA值来模拟收缩开裂实际上是足够的。建立了基于土体力学与水力耦合分析的数值模型。该模型由计算土壤水分枯竭引起的土体变形的主程序和根据当前CTOA和临界CTOA确定裂纹尖端进展的模块组成。模型的试验结果表明，该模型预测的裂纹深度与现场实测的裂纹深度基本吻合，并获得了模拟参数

Mechanics of crack formation were described using Crack Tip Opening Angle (CTOA) measured with fractography soil were used. Fracture testing used deep-notch bend specimens formed by moulding a mixture of Ca-bentonite and silica sand, and a paddy soil at the liquid limit into rectangular bars, equilibrating to soil water suction ranging from 5 kPa to 50 kPa (with some 50kPa specimens wetted to 5kPa), and inserting a flaw 0.4 x specimen thickness. The growth and geometry of the cracks were quantified from a series of images. CTOA measured during stable crack growth decreased with drying. CTOA decreased even further when specimens equilibrated initially to 50 kPa were rewetted to 5 kPa. These results suggested CTOA was primarily governed by the stiffness, although rewetting probably altered the capillary stresses in advance of the crack tip.Vertical crack propagation was studied in a laboratory-scale model subject to shrinkage. Soil installed in a container was consolidated by compression pressure. After removing the pressure, a vertical flaw was formed at the center, and then the specimen was subject to water suction imposed to both sides made of permeable ceramics. Observed CTOA values were compared with those measured by the bending tests. CTOA values in shrinkage were similar to those in bending, suggesting it is practically sufficient to use CTOA obtained by the bending tests for the simulation of shrinkage-induced cracking.A numerical model was developed based on the coupled mechanical and hydraulic analysis of soil. The model is consists of a main program which calculates the deformation of soil due to depletion of soil water, and a module which determines progress of the crack tip by referring to current and critical CTOA. Test runs of the model proved that the predicted crack depths were practically corresponded to those measured in the field where parameters for the simulation were obtained