Abstract: This study focuses on investigating the steady seepage characteristics of fracture network with free surface. A visual two-dimensional fracture network seepage test device was designed to conduct a series of fracture network seepage tests by adjusting the upstream and downstream heads of the fracture network model, and recording the height of free surface head and seepage flow. Additionally, a two-dimensional fracture network numerical model with equal proportions was established using the Darcy interface of the finite element software COMSOL Multiphysics. The test conditions were reproduced in the numerical simulation and compared with the test results. The findings demonstrate that the test results are basically consistent with the simulation results, which verifies the effectiveness of the test results and the accuracy of the numerical simulation. The results show that the main factors affecting the change of free surface are the height of upstream and downstream heads and the height difference between them. A larger seepage hydraulic gradient leads to a faster drop in the position of the free surface. Under the same hydraulic condition of seepage hydraulic gradient, higher the upstream and downstream water heads result in higher the free surface water heads and steeper falling curve of free surface. Notably, if the upstream and downstream water head heights are not at similar levels, their influence on the position of the free surface differs greatly. The basic laws of free surface change align with Dupuit formula. The hydraulic gradient of steady seepage is linearly correlated with the average flow rate only when the flow rate is small, while with increasing hydraulic gradient, the relationship becomes noticeably nonlinear.

Key words: steady seepage, fracture network, free surface, permeable layer, seepage hydraulic gradient, water head