Study On Seepage-damage Constitutive Model And Numerical Simulation Of Fractured Limestone

Fractured rock mass is the main form of rock in nature.Both natural geological activities and external forces from engineering disturbances can create rock structures with different scales and properties.Additionally,due to the presence of groundwater,the mechanical properties of fractured rock mass in stress and seepage fields become more complex.To further understand the mechanical and permeability characteristics of fractured rock mass under the coupled effects of seepage and stress,this paper considers the complex fracture characteristics of engineering rock mass.Through laboratory mechanical tests,specimens of complex fractured gray limestone formed under three-axis stress conditions were prepared,and a series of seepage-stress coupling tests were conducted to reveal the mechanical and permeability characteristics of fractured gray limestone under seepage-stress coupling.Furthermore,the microscopic structure characteristics of fractured gray limestone were revealed through CT scanning and image reconstruction analysis.A constitutive model for the seepage-damage of fractured rock mass was established,considering initial damage and seepage water pressure.Finally,using numerical simulation methods and a microscopic structure reconstruction model of complex fractured rock mass,the seepage law of fractured rock mass under seepage-stress coupling was revealed.The main results of this study are summarized as follows:(1)The deformation and strength characteristics of the fractured limestone are influenced by confining pressure and water pressure,showing the characteristics of confining pressure strengthening and water pressure weakening.Confining pressure can effectively increase the elastic modulus and strength of both fractured and intact limestone while limiting lateral deformation and dilation.Under the action of water pressure,the elastic modulus and strength of fractured limestone decrease with the increase of water pressure,while the Poisson’s ratio increases with the increase of water pressure.With the increase of confining pressure,the weakening effect of water pressure on strength and deformation gradually decreases.(2)The permeability of fractured limestone under seepage-stress coupling exhibits significant confining pressure and water pressure effects.Under low-permeability water pressure conditions,with the increase of confining pressure,the permeability of fractured limestone significantly decreases,but as the seepage water pressure increases,the limiting effect of confining pressure on permeability gradually decreases.Under the same confining pressure conditions,the permeability of fractured limestone increases with the increase of water pressure.(3)The microstructural characteristics of complex fractured limestone and their correlation with permeability were revealed by CT scanning.The distribution of fractures in fractured limestone is characterized by a small number of main fractures combined with a large number of small fractures,and the average pore radius values under different stress conditions are similar.The uniformity and spatial structure of pore size affect permeability,with uniformly sized pores contributing to good permeability,and vice versa.(4)The strength of microelements containing fractured limestone is determined based on the Weibull distribution and the Mohr-Coulomb failure criterion.A constitutive relationship for the coupling of seepage and stress in fractured limestone is constructed according to the Lemaitre strain equivalence hypothesis and Biot effective stress principle.This constitutive equation can effectively describe the deformation behavior of fractured limestone under seepage-stress coupling conditions.(5)Using COMSOL Multiphysics and a high-precision geometric model obtained from CT scanning,a simulation experiment was conducted to reveal the seepage-stress coupling characteristics of fractured limestone.When the seepage field acts alone,the permeable channel that penetrates the rock mass is the key factor affecting the permeability.Under seepage-stress coupling conditions,the penetrating channel is still the main channel for fluid existence and transport;stress and strain concentration are likely to occur around the fractures of the limestone,and the influence area of the seepage field gradually expands around the fractures as the external load increases,resulting in a distribution of main fractures combined with smaller fractures in the fractured limestone.