In this study, a hydro-mechanical coupled algorithm is proposed to solve unsaturated soil problems in FLAC3D. The seepage model is extended using the in-built FISH language. The SWCC for unsaturated loess was best-fitted using MATLAB. The obtained parameters were written as a subroutine and invoked in each step in the seepage model to simulate seepage in unsaturated soil. Water infiltration at one-dimensional condition was modeled and compared using both the finite difference method and the finite element method, suggesting that the extension of the seepage model is appropriate. A non-linear elasto-plastic model was then defined to consider wetting induced variations in soil modulus and shear strength. Based on above improvement, a hydro-mechanical coupled analysis was performed to simulate sinkhole formation in unsaturated loess. The findings are meaningful in the investigation of the unsaturated soil theory and in the application of FLAC3D to simulate unsaturated soil problems.


Water has been recognized as a major cause of geological disasters in numerous cases, in forms of intense rainfall, irrigation, snowmelt, and water-level changes etc. Under natural conditions, soil exposing on slope, pit and tunnel is commonly at partially saturated state, which shows different properties with saturated soil in aspects of permeability, modulus and strength etc. (Fredlund & Rahardjo 1993, Mao 2003). Due to common existence of thick soil layers in field, it cannot obtain reasonable numerical results without consideration of the hydraulic and mechanical behaviors of unsaturated soil, especially for infiltration problems.

FLAC3D is a general finite difference program for modeling soil and rock behavior produced by Itasca (Itasca 2002). FLAC3D provides a flexible tool, i.e. the in-built FISH language, to solve complicated geotechnical problems. It has been applied to a broad range of projects and scientific research. However, the current standard FLAC3D cannot be used reasonably to analyze unsaturated soil problems due to absence of some constitutive equations. To overcome above limitation, a hydro-mechanical coupled algorithm is proposed for unsaturated soil in this study. The algorithm is verified firstly, and then applied to a field case of sinkhole formation in unsaturated loess layer.

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