Study On The Rock Characteristics Of Plasticity And Fluid-solid Coupling Under Confining Pressure And Its Application

Due to the special geological structure,the North China coalfields formed in the late Paleozoic are vulnerable to water inrush from the floor in the condition of confined water and cyclic loading and unloading induced by mining.The foundation of prevention and control the disaster is to study the rock properties under the above stress path,especially the characteristics of the material under confining pressure,including anisotropy,elasto-plastic coupling,non-isotropic evolution of the loading surface,volume yield,permeability properties,and effective stress changes.In this paper,the influences of the above factors of the fluid-solid coupling equation on the elasto-plastic stiffness matrix,the effective stress coefficient and the permeability coefficient are quantitatively determined by using the laboratory mechanics and seepage experiments and theoretical derivation.Then the elasto-plastic coupling mechanical model with double yield surface and the corresponding fluid-solid coupled model are developed,and the new model is verified and the parameter inversion is realized on the basis of in-situ test.Further studies on water inrush from coal seam floor are carried out through numerical simulation and engineering practice,the innovative achievements are obtained as follows:(1)The traditional loading surface hardening hypothesis and total flow law hypothesis are abandoned.Through the cycle loading and unloading test under triaxial compression and the corresponding data processing method,it is found that the two loading surfaces exhibit "three-stage anisotropic hardening" with the increase of plastic strain.At the same time,under the same plastic strain the ?3p loading surface is lower than the ?1p load surface,which means that ?3p generated preferentially and?3p>?1p under the same stress state.The above results explain the anisotropic behavior of the initially isotropic rock during the plastic deformation stage.Based on the in-situ data of mining stress,the principal stresses of the floor almost didn't rotate during the mining process,which verified the rationality of using ?1 and ?3 as plastic potential function.(2)The elasto-plastic coupling characteristics of the rock are analyzed and quantitatively described.The elastic modulus E increases with the confining pressure increasing(the speed slows down),and the generalized Poisson's ratio ? decreases with the confining pressure increasing(the speed slows down).Meanwhile,according to the overall hardening and overall softening of the loading surface,the evolution curve of E and ? along with the plastic strain can be divided into two sections,the root cause of the change is the change of average stress and generalized shear stress during the evolution of the loading surface.The simplified calculation curve of elastic parameters are put forward according to the experimental data.Thus,the elasto-plastic stiffness matrix is deduced and the incremental iteration expression of the elasto-plastic coupled constitutive model with double yield surface is established.Finally,the UMAT subroutine was developed by Fortran language.(3)Deduced and created theoretical expressions and test methods of hydraulic parameters.Applying the principle of minimum energy dissipation,the results that rock permeability in the plastic deformation stage is basically determined by the main control crack is concluded,and the criterion of the main control crack is proposed.Combining the plastic mechanics model proposed in this article and the macroscopic phenomenological damage mechanics theory,the relationship among the plastic strain,the crack width and the damage variable is derived.The evolution law of the permeability coefficient with the principal strain under different ?3 conditions has been obtained.Based on the basic theory of thermodynamics and the infinitesimal analysis,the relationship equation among confining pressure,water pressure,strain and effective stress coefficient is deduced.Based on this,we innovates the experimental method of effective stress coefficient under low pore water pressure conditions,and proposes the simplified calculation curve of effective stress coefficient.(4)Proposed the floor water inrush mechanism under the combining disturbance of grouting and mining.Taking the Chensilou mine as an example,using the hydrogeological method to count water inflow from grouting holes,it was found that long-term and large-scale floor grouting activities not only causes limestone water in the upper part of the Taiyuan formation to flow into the syncline structure,but also causes the limestone water in the middle section of the Taiyuan formation to involve in the groundwater circulation process,aggravating the geological risk to the synclinal basin.The response of the mining face floor in the syncline under the mining influence is studied by mechanics methods.It is found that the depth of plastic zone in the floor will be deepened again on the basis of shear failure due to the combined effect of rock elasto-plastic coupling material properties and hanging roof area.The results are verified by the in-situ tests of three North-China type coal mines.The low-pore water pressure zone will increase the water pressure gradient in the rock between the limestone and the coal seam,and further increase the risk of water inrush.As a result,the floor water inrush mechanism under the combined disturbance of grouting and mining is formed.