A numerical procedure for unsaturated seepage analysis in rock mass containing fracture networks and drainage holes

It is computationally difficult to model unsaturated flow through a highly fractured rock system that contains porous rock matrix, densely distributed fracture networks and narrowly spaced drainage holes. We present an efficient numerical procedure that uses a three-dimensional, finite-element model to solve Richards' equation with embedded zero-thickness elements for fractures and line elements for drainage holes. A mixed boundary condition is applied at potential seepage surfaces and a water retention relation with extremely narrow unsaturated zone is specified to precisely locate the phreatic surface. The procedure compares favorably with results from other numerical solutions and experimental data. A case study is developed to evaluate seepage through a densely fractured rock slope. The simulated results demonstrate that fractures disorder the hydraulic potential distribution with fracture patterns acting as significant influential factors and drainage holes dramatically lower the phreatic surface due to the effects of drainage and depressurization. The proposed numerical procedure shows to be capable of investigating the unsaturated flow in highly fractured rock mass.