Study On Dissolution Characteristics And Mechanical Properties Of Rock Mass Of The Epikarst Zone

The dissolution cave and fracture of rock mass in the epikarst zone has the characteristics of the large-number,small-size,and close relationship with the discontinuity,which is of great significance to the study of its mechanical properties.The mechanical properties obtained from indoor rock tests are difficult to reflect the rock mass at engineering scale,but the conditions of the field test are extremely harsh.With the rapid development of computer simulation technology,the method of establishing the equivalent rock mass model for numerical simulation tests has gradually become one of the most effective means of research rock mass mechanics.However,how to establish an equivalent dissolution rock mass model is an important difficulty in the numerical method.Firstly,based on the results of the previous research and field investigation,the dissolution uniform-coefficient(u)was proposed to quantitatively describe the characteristics of dissolution rock mass in addition to the dissolution-rate(k).Secondly,with the aid of particle discrete element code(PFC^2D),a dissolution algorithm based on“Cellular Automata”was proposed to establish the equivalent solution rock mass model with different dissolution-rate(k)and uniform-coefficient(u).Simultaneously,a series of compression simulation tests are carried out on the models,and then the mechanical properties and quality evaluation method of the dissolution rock mass are analyzed.Finally,taking the Pusa collapse in Guizhou Province as an example,the deformation and failure mechanism of the karst slope was discussed based on the elastic-plastic theory of Hoek-Brown criterion by using the finite-difference method software(FLAC^3D).The conclusions are as follows:(1)In the epikarst zone,the dissolution-level of the vertical discontinuity is stronger than the horizontal one,and the dissolution at the intersection of the structural planes is the strongest;the dissolution-rate(k)decreases exponentially with the increase of depth(h),and the dissolution uniform-coefficient(u)is negatively related to the depth;the parameter a in the dissolution-rate function k(h)can be determined by the dissolution rate of the bedrock surface,while parameter b can be estimated by the average dissolution-rate of a depth;when the uniform-coefficient function u(h)is assumed to be linear,it can be determined by the limit dissolution depth.(2)The"Cellular Automata"algorithm regards the dissolution process of the rock mass as the state transformation process between cell particles,and the dissolution process of jointed rock mass realized by controlling the probability of state transformation.By using the algorithm,the equivalent rock mass model with the different levels of dissolution development and uniformity can be generated.(3)Under the loading condition,the stress-strain curves of the dissolution rock mass show multi-peak fluctuation,and the deformation and failure process of it can be divided into 6 stages:dissolution cave gap compaction,elastic deformation,stable fracture development,unstable fracture development,slow softening after peak and fast softening after the peak.Among them,the friction under the dilatancy effect of the failure surface is the key to maintain the strength of rock mass in the post-peak slow softening stage,however,in the post-peak softening stage,the bulge of the failure surface is quickly sheared to produce again the peak of acoustic emission event,and the rock mass is completely destroyed due to the energy release.(4)The main performance of the dissolution is to reduce the mechanical parameters of the jointed rock mass.The uniaxial compressive strength decreases exponentially with the increase of dissolution-rate,and its attenuation rate has a polynomial negative correlation with the dissolution uniform-coefficient;the deformation-modulus decreases linearly with the increase of dissolution-rate,and its attenuation rate has a linear negative correlation with the uniform-coefficient;the Geological strength index(GSI)decreases linearly with the increase of dissolution-rate,and its attenuation rate has a linear negative correlation with the uniform-coefficient.In addition,with the increase of confining pressure(0.5-5MPa),the peak strength of the rock mass increases first and then decreases.(5)The numerical model of epikarst zone with arbitrary topography can be constructed by the“vertical-strip division method”.The sliding surface tends to migrate to the corrosion zone with lower strength,resulting in a significant reduction(1.77 to1.52)of the safety factor of the landslide surface when considering the existence of epikarst zone.After the goaf of coal seam M16?M14?M10,the shear stress concentration appears on the surface from the end of the goaf up to the edge of the collapse,which is close to the actual tension fracture;at the same time,the shear stress concentration also appears on the karst zone from the end of the goaf of M16 and M14up to the slope surface,and the shear stress at the exit of the collapse shear increases continuously,which causes the collapse body to slide towards the free face.Therefore,the coupling effect of underground mining and karst is the key factor of Pusha collapse.