Pipe network model for unconfined seepage analysis in fractured rock masses

• A UPNM method is developed to study unconfined seepage in fractured porous medium.
• Only boundary intersecting meshes are treated thereby reducing computational cost.
• An Average Acceleration Method is proposed to accelerate the convergence.
• Effects of fracture distributions on unconfined seepage in rock slope are studied.
• Effectiveness of water curtains for oil storage in rock tunnels is investigated.

An efficient unconfined seepage analysis method is developed for analysing fluid flow in fractured rock mass. The proposed method discretises both porous rock matrix (a low permeability medium) and discrete fracture networks (a high permeability medium) as permeable pipe networks. A finite volume based approach is used to determine the equivalent flow parameters in the undirected pipes for the porous rock matrix such that seepage analysis in a fractured rock mass is carried out in a consistent mathematical framework which gives excellent computational accuracy and efficiency. Using an auxiliary boundary method, pipe networks in the vicinity of the phreatic surface can be adapted in unconfined seepage problems. Validated by finding the phreatic surface in dam structures, it has been found that the fracture orientation, hydraulic aperture, fracture set number and fracture distribution can affect the phreatic surface significantly in a fractured rock mass. The proposed method is applied to investigate the effectiveness of the water curtain system in a water-sealed underground crude oil storage cavern. Grouting effect can also be conveniently investigated by the unified pipe network system. Results show that the proposed method is robust in seepage analysis for both fractured and porous rock masses.