Stability Analysis Method On The Voids Subjected To Vertical Loads

The presence of underground voids due to two reasons: the one is hum an activities such as tunneling,mining and subway excavation;another one is dissolution of soluble material such as karst caves and soil caves.As the foundation was constructed above the voids,it is very important to evaluate the stability of the voids.At present,the investigation of stability for foundations above voids is still at the stage of regular exploration,and there are no definite formulas for calculation and related design tables.Therefore,it is necessary to make a further study for the subject.Based on the different types of the foundation loading,this paper investigates the stability of the void subjected to surcharge loading,the strip footing loading and the pile foundation loading,respectively.The main study contents are shown as follows:(1)Based on complex variable function method,the analysis model of the voids with arbitrary shapes subjected to surcharge loading was built.The stresses can be obtained by using conformal mapping method,and the stability of the voids was examined based on the material failure criterion.To further clarify the analysis process,the stability of arbitrary rectangular karst caves subjected to subgrade loading was examined.Meanwhile,the method proposed in this paper is verified by the numerical method and case study.(2)According to the theory of the traditional limit analysis method,combining the finite element method,the analysis problem is transformed into a mathematical programming problem,and the finite element limit analysis program is developed.The method does not need to construct the static and velocity fields artificially,and does not need displacement load iteration to obtain the ultimate bearing capacity.The upper bound and lower bound can be obtained directly,which establish a basement for investigate the complex multiple void stability problems.In addition,in order to reduce the relative errors between the upper bound and the lower bound,an adaptive mesh generation technique is developed.(3)The stability of two circular tunnels in soils subjected to surcharge loading has been investigated by using finite element limit analysis,and the tunnel stability number has been calculated.The influence of the different depth of the voids on the stability number has been investigated emphatically.At last,the calculation results are summarized as design tables for engineering use.(4)The stability of two circular and square tunnels in rock masses subjected to surcharge loading has been investigated by using finite element limit ana lysis.To describe the nonlinear characteristics for the rock mass,the generalized Hoek-Brown failure criterion was adopted,which has been directly incorporated into the finite element limit analysis program.A dimensionless stability number was defined to evaluate the effect different parameters on the voids stability,and the failure modes are presented.At last,the calculation results are summarized as design tables for engineering use.(5)Finite element limit analysis is used to study the undrained stability of strip footing above voids in two-layered clays and in rock masses.The effects of the material parameters,the void location,the void size and the void distribution on the bearing capacity of strip footing are investigated,and the influence of parameters on failure modes is also discussed.At last,The equations and design tables are provided to calculate the bearing capacity of strip footing.(6)Based on the axisymmetric model and the research of ultimate failure mechanism at the tip of rock-socketed pile,the critical failure conditions of the cave were determined in accordance with the position relationship between the cave and the ultimate failure line of pile tip.Secondly,the failure models of rock socketed pile tip in Karst areas have been analyzed by combining the force-transferring mechanism of the rock at the tip of pile in critical condition.Then,the punching and collapse failure conditions of the cave roof have been given in polar coordinate system.Moreover,the calculation formula of ultimate bearing capacity at the tip of rock-socketed pile in Karst areas have been induced.(7)In order to provide a stability assessment method of cavity under the pile tip load at different locations.Firstly,the equation of formation stress i n semi-finite space under the pile tip load was introduced by Mindlin solution.Secondly,the formation stress in the semi-finite space with a void under gravity by the complex variable function.Then,the formation strss of cavity under the pile tip load was superimposed on the formation stress under gravity,thus obtaining the equation of formation stress.On this basis,the major principal stress and minor stress were induced,and the cavity stability was evaluated based on Griffith failure criterion.(8)By using the finite element limit analysis method,the stability of the bridge double-piles foundation above voids has been studied.To describe the nonlinear characteristics for the rock mass,the generalized Hoek-Brown failure criterion is adopted,which has been directly incorporated into the finite element limit analysis program.The effects of rock parameters,the void size and the void location on the bearing capacity of bridge double-piles foundation,and the influence of parameters on failure modes is also discussed.