Improvement And Application Of Stochastic Medium Prediction Model For Ground Settlement

As a method of discontinuous medium mechanics,stochastic medium theory is widely used in geotechnical engineering.For ground settlement induced by excavation,the probability integral formula of underground excavation derived from the stochastic medium theory is a complex and non-integrable double integral.A series of improvements and simplification procedures have been carried out to solve the problem.The existing research is basically applied to adjacent parallel twin circular tunnels.In order to expand the range of engineering application,stochastic medium theory simplified procedure for three circular tunnels was proposed.According to the proposed method,the ground surface settlement,the horizontal displacement,the differential surface settlements,the horizontal strains and the curvatures of surface settlement profile can be derivated in case of the three circular tunnel excavation.At last,the theoretical results are verified with the measured data of the Shanghai Metro Line 9 project.In the process of surface settlement prediction in tunnel excavation,the convergence mode of the displacement at tunnel wall has a great influence on the accuracy of the calculation results.Most of the calculation approaches assume that the displacement at tunnel wall is uniform.However,real displacement is non-uniform.Based on the stochastic medium theory,the integral limit of probability integral formula for single tunnel and double holes tunnel is obtained.Considering the boundary condition of non-uniform displacement at tunnel wall and its asymmetric convergence mode of ground loss together,the expression of the gap parameter G and tangent value of the main influence angle ? is derived out by using the parameter of ground loss ratio Vl and the width of surface settlement trough i defined by Peck.Firstly,the calculation of uniform convergence and elliptic part is considered,and then the whole subsidence part is considered.Simplified surface settlement formulas of single and double holes are obtained.An approach to calculate the ground surface subsidence induced by tunnel excavation is proposed according to non-uniform displacement at tunnel wall.In the prediction of tunnel surface settlement,the tunnel can be simplified to plane deformation condition.However,in the prediction of large-scale excavation surface settlement,the time effect of excavation should be considered together with the sptail characteristic.And the calculation is very complicated.The calculation is time-consuming and laborious by using the general method.Coupling with stochastic theory,the authors have proposed an approach to calculate the surface subsidence profile and ground movement distribution by using arbitrary shape 3D-Polygon and Knothe time model in Geographic Information System(GIS).The improved methodology results in a more applicability of the proposed model correspondingly.Through comparative analysis of research work and engineering examples,the results show that:1.In circular tunnel,the simplified program of stochastic medium theory can be extended to solve a three-hole circular tunnel proglem.Whether the three-hole circular tunnel is located at the same height or not,the theoretical result is consistent with the measured data.2.In case of an elliptical tunnel,the asymmetric convergence model,which leads to a more accurate calculation result,is more suitable compared to others.The method proposed in this paper can improve the practicability of stochastic medium theory and the accuracy of prediction results.3.The stochastic medium theory is applied to large-scale excavation,and the prediction model of large-scale excavation surface settlement can simulate the time effect of large-scale long-term excavation,analyze and calculate the surface settlement,deformation and spatial distribution under complex excavation conditions.Finally,the validity of the method is verified by case study.