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

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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