Study On Some Problems Of Soil Mechanics In Deep Excavation Engineering

Nowadays,a new era of exploring the underground space has come.As both the knowledge of engineering problems and the theory research develops,a lot of new excavation problems have turned up.However,the further consideration for the original questions is still being carried on.This article has done deep research on the following three aspects on excavation engineering:the earth pressure of retaining wall,the numerical simulation for deformation and temperature fluctuations influence on the axial force of structs.The first chapter has drawn a conclusion on the recent research situation of the foundation pits.And the second chapter has put forward an analytical expression of the passive earth pressure for cohesionless soil,which takes soil arching effect into account.Then in the third chapter,based on the Mindlin's elastic solution,the initial rigidity of this model has been solved.After that,the ultimate stress of this very model has been calculated using the Log spiral's theory with considering the three-dimension effect modification.In the fourth chapter,based on the Duncan-Chang model theory and the Kordner's hyperbolic curve assumption's being banished,the expression of improved elastic non-linear tangent modulus has been established.And then,based on ABAQUS user subroutine,a corresponding numerical program has been written.This program has been employed to research the variation of the lateral displacement in the excavation in Wen Zhou.The fifth chapter,based on the Winkler's elastic foundation beam model,established the simplified calculation method for calculating temperature stress in multi-layer for deep excavation considering temperature variation.Finally,the sixth chapter has concluded all the research in the article.And some promising researching directions have been put forward,through the theoretical analysis and the calculation examples.(1) The lateral earth pressure coefficient considering soil arching effect is related to the rotation angle of principal stress.And this new lateral earth pressure is closer to the actually measured earth pressure.As the friction angle of the wall-soil interface decreases,the action points of the active earth pressure resultant and the passive one infinitely approach to the H/3 of the wall from the upper and lower part of the wall respectively. (2) As the friction angle increases form 0 toφ,the distribution of the passive earth pressure turns from linear to non-linear,and the earth pressure on the upper part of the wall becomes smaller whereas the pressure on the lower part turns larger.Especially,when the friction angle equals 0,there is no soil arching effect,and the passive earth pressure coincides with the Coulomb theory.(3) Through the theory of subzones of excavation,the hyperbolic earth pressure model and the Duncan-Chang's constitutive model has been connected.In a real engineering example,the calculation based on this combined model can illustrate the relationship between the earth pressure and the lateral displacement in the active zone and passive zone.The subgrade resistance coefficient varies with the initial resistance coefficient,the stress failure ratio R_f,the failure stress p_f,the displacement of the bracing system and the depth z,of which the initial subgrade resistance coefficient is the most significant factor.(4) Based on the Mindlin's elastic solution,the lateral displacements of some rectangular region underground have been calculated when it is bearing lateral loads.Through this solution, the initial rigidity of the hyperbolic non-linear earth pressure model was defined.And the Log Spiral's theory was used to calculate the ultimate stress,which has taken the three-dimension effect modification into account.In addition,by an anchor experiment,the relationship between earth pressures and displacements is well reflected.(5) Because of no Kordner's hyperbolic assumption,the expression of improved elastic non-linear tangent modulus is more reasonable than that of the Duncan-Chang.Through the numerically simulated triaxial test,the results can be controlled within a certain range using this expression of improved modulus.What's more,by a simulated foundation pit's excavation,it shows this expression of improved elastic non-linear tangent modulus can be applied to practice, and the solution of the geotechnical engineering can be embodied to some extent.(6) Two simplified calculation methods for calculating temperature stress of bracs when temperature fluctuations.The models consider the process of brace-pile-soil interaction.There are many influential factors,such as structural forms of the support,the integral rigidity of the bracing system,the deformational modulus of the soil,and so on.Through the analysis of a pit with two-layer struts,the errors accounts for 10.4%and 7%of the total axial force respectively. In general,with the fluctuation of temperature,the axial force at the bottom changes the most whereas the upper one changes less.