Study On Three-dimensional Identification And Reconstruction Of Shallow Buried Karst Cave And Bearing Capacity Of Upper Plate

In the past two centuries,with rapid urban expansion,the dimensions of engineering construction and speed have been accelerated.Accordingly,the occupation of land increased.In southwest China,limestone is the major constituent of the bedrock,and buried karst caves distributed widely.Owing to lack of understanding the detailed information of limestone strata with karst caves,accidents caused by them happen frequently,and threaten the safty of human life and property.Therefore,based on integration of model test,theoretical analysis,numerical simulation,a study on three-dimensional identification and reconstruction of shallow buried karst cave and bearing capacity of upper rock plate of karst cave in limestone strata was performed.The specific research content and results could be summarized as follows:?1?A method for identification and reconstruction of three-dimensional geologic structure was proposed,combing geological investigation,GPR survey and data fitting.To testify the validity and application of the method,a model test was performed.The results indicate that GPR signal patterns were related to the material properties and shapes of internal objects.Highly disturbed reflections and overlapping diffraction hyperbolas in the radargrams are the reflection characteristics of similar limestone material.Inter-formational multiples appear from a cavity's roof to its floor.The reflected signal strength drops with the fourth power of the travel time due to the absorption and dispersion of radiation energy by the media.The degree of absorption is governed by the electrical conductivity of the materials.A strong convex reflector is the primary cavity characteristic.Circular cross-section of spherical objects results in hyperbolic reflections from the top and bottom of the cavity.Square cross-section of cubic object results in two parallel quasi-hyperbolic reflectors and flat radar reflections from the boundary.Straight lines indicate the presence of a hard structural plane.Weak layers result in fluctuating reflectors from the top and bottom of the weak layer,which are related to overlapping sections of different materials.?2?To analysis the three-dimensional stress distribution of surrounding rock around karst cave in limestone strata,limestone strata containing shallow buried cylinder and spherical karst cave was selected as research objects respectively.Firstly,considering gravity,internal filling and the external load,the equivalent calculation model was simplified into spatial axisymmetric models.Currently,in terms of elastic theory,Love displacement function was established.Love function was established,including polynomial and logarithmic functions.Applying the boundary conditions and continuity conditions for this problem,the elastic general solutions of stress components were obtained through theoretical analysis.Afterwards,the change of stress components affected by various influencing factors was discussed.For imestone strata containing cylinder karst cave,depth?z?and the distance from center?r?are the two influence factors.The distribution of radius stress and tangential stress are similar to paraboloid,and the value tends to constant finally.However,the trend of value change is opposite between them.In addition,vertical and shear stress were only affected by depth?z?and the distance from center?r?respectively.For imestone strata containing spherical cylinder karst cave,radius?R?and angle???are the two influence factors.The value of stress components is continious,and the three-dimensional distribution of stress components presents symmetry.To quantitatively evaluate the validation of the analytical solution proposed,the comparison between theoretical analysis results and numerical simulation results was finished in the same condition.The results indicate the solved general solution could reflect the distribution of stress accurately.?3?For limestone strata containing shallow buried cylinder and spherical karst cave,combing the analytic solution of stress components with Mohr-Coulomb strength theory,the mathematical expressions of bearing capacity of limestone roof were obtained.The mathematical expressions indicate the property of rockmass??,?₁,c?,the size of karst cave?R,r,H?and the thickness of overlying strata?h?are the main influencing factors of bearing capacity of limestone roof.?4?Taking a project in Nan'an district in Chongqing as an example,a method for identification and reconstruction of three-dimensional geological structure and the mathematical expressions of bearing capacity of limestone roof proposed in this paper were applied comprehensively.Firstly,combing ground penetrating radar survey with the field investigation data,spatial distribution characteristics was determined and numerical simulation model was estabilished.Then,on the basis of the established model,under the same working condition,the numerical simulation of loading is carried out,and the value of the load and the corresponding displacement at the key point is obtained,and the P-S curve is monitored.The ultimate bearing capacity of limestone roof containing cylinderial and spherical karst cave is determined by combining P-S curve and the change of plastic zone analysis.The comparison among the calculation result by the mathematical expression,P-S curve and the change of plastic zone indicate the bearing capacity of limestone roof indicate the error are 3.5%and 5%,so the proposed mathematical expressions of bearing capacity of limestone roof are reasonable.