Abstract：The permeability and its changing characters in physicochemical environment of porous media like rock and soil are very important for all kinds of civil engineering projects. But until to now, most theories about this problem were based on a phenomenological method, which can not essentially display the innate character and variation properties of fluid seepage in porous media induced by the changing of environment.Based on the hypothesis that the pore number follow exponential distribution with pore radius, rectified the gamma without change its mathematical features, and fitted it for mathematic distribution function exactly, proposed a capillary bundle model for depicting the microscopic peculiarity of porous media's seepage. With this model, the effect of water temperature, the upper limit of Darcy Law, flow rate beyond Darcy Law's upper limit, and the infection of porosity on permeability etc issues were discussed theoretically. An instance was cited by the calculating of a permeability coefficient and the critical hydraulic gradient of China red sandstone, and the results were discussed.

Keyword：porous media;capillary bundle model;seepage;upper limit of Darcy Law;critical hydraulic gradient;