Numerical And Experimental Analyses Of Underlying Voids Stability With Complex Conditions

With the acceleration of China's urbanization process and the rapid increase of urban population,urban land resource shortage,traffic congestion is becoming more and more serious.Full development of underground space can expand the urban space efficiently,which is urgent affairs.On account the complexity of stratum conditions,many accidents including stratum collapse and ground subsidence contribute to underground engineering often occur.In view of this,it is necessary to evaluate the effect of underlying voids on the stability of the stratum and the foundation to reduce the probability of disasters mentioned above.At present,vertical loads are mainly considered in void stability issues,and the soils are assumed to be ideal homogeneous which is unreasonable.Hence,this thesis adopted finite element limit analysis to carry out a large number of numerical simulations to investigate the void stability issues considering complex load(i.e.inclined load and eccentric load)and complex strata conditions(i.e.layered soils ? spatially variable soils and rock masses).Meanwhile,the author designed a set of void-eccentrically loaded footing laboratory experiments for deeper insight into this problem.In this thesis,the upper and lower bound limit analysis problem were firstly transformed into corresponding mathematical programming problem by mathematical variational principle.On this basis,the nonlinear programming model of finite element limit analysis(FELA)was developed.Then the feasible arc interior point algorithm was introduced to increase the length of iteration step for reducing the iteration number,and Wolfe inexact searching algorithm was adopted to replace the traditional search algorithm for higher efficiency.Furthermore,an adaptive element remeshing technique based on the advanced surface technique was introduced to further improve the efficiency and precision of this FELA code.Secondly,based on finite element limit analysis,the bearing capacity problems of voids whose shear strength varies with depth linearly and the dual voids at different depths in layered soils were investigated.A dimensionless bearing capacity factor was proposed to characterize the stability of the system and to expand the application scope of the research results.And the influence curves of different parameters on bearing capacity factors were summarized via parameter analysis.After investigation,it can be found that there exists an angle-fixed “weakest zone” of system stability in layered soil that shifts with the varying soil strength ratio,providing a reference for the site selection of new tunnels constructed near the existing tunnels.Thirdly,the Hoek-Brown failure criterion was introduced into the finite element limit analysis code,in order to accurately investigate the bearing capacity of eccentrically loaded or inclined loaded strip footings lying on voids.Through a large number of numerical simulations about the influence of rock strength parameters,the location of voids,the void inclination and the load eccentricity on the bearing capacity of the footing,design charts represented by dimensionless factor and failure envelope were proposed.In addition,based on the cloud of shear dissipation,the effects of load eccentricity and load inclination on the failure mode were discussed,respectively.Fourthly,considering the complex spatial variability of soils,the random field theory was used in the finite element limit analysis code to discretely divide the strata into heterogeneous media with random strength distribution.Monte-Carlo simulation was used to analyze the reliability of void-footing system and dual void system subjected to surcharge load.By a series of numerical simulations,the void reliability analysis method was put forward,and then the design table including different parameters were complied.Meanwhile,the shear dissipation cloud of the limit state of the system was summarized,and the relationship between the failure mode of the system and the spatial variability of soils was discussed.Finally,for deeper insight into the stability of void-eccentrically loaded footing system,a series of laboratory model tests were designed.The stability of single void,symmetrical configuration and parallel configuration under eccentric load were comprehensively analyzed.The ultimate bearing capacity of the footing under various working conditions was summarized.Then the design tables for the stability of the void-footing system is compiled.In addition,high performance Digital Image Correlation technique was adopted to capture the soil micro displacement during the test process,and the cloud of soil displacement was output to reveal the variation trend of soil disturbance path and the failure mechanism of void-footing system.Furthermore,the results of model tests were compared with the results of numerical simulation for verification purpose.