On Stability Of Deep Excavation Caused By Surcharge On Surface Ground

Deep excavation is still a key problem in geotechnical engineering.Up to now, the design of deep excavation depends on the method ofsemi-experience and semi-theory. It is no strange that casualty and lossrelated with failures of deep excavation presents on highlight ofnewspaper. Generally, the factors induced the failure are loading,mechanical properties of soil, water. Loading, especially surcharge onedges of the excavation, is a trigger to the failure, which is inevitable inimplementation. So this thesis will focus on study of the influence uponthe deep excavation caused by surcharge on the edges of the excavation.Chapter 1 list some accidents of deep excavation related withsurcharge in China in recent years. The facts make us recognized that it isworthy to work on this area. Then, kinds of excavating methods with andwithout bracing system are summarized, and their applied conditions arecompared. Next, the history and the present state of the research on deepexcavation is summarized, and some shortcomings are pointed out, whichbased on literatures and data at home and abroad. At the final of thischapter, the research goal, procedure and implementation is presented.Chapter 2 will mainly concern with the general theory of deepexcavation, which consists of stress analysis and failure criterion. Thereexist gravity stress and additional stress in the soil mass, the calculationof gravity stress is easier than that of additional stress by different typesof the loading. The Boussinesq and Mindlin solutions are used here forcalculating additional stress in soil, and other formulae could be obtainedthrough integration of the two solutions. Then we choose Mohr-Coulombfailure criterion to judge whether the soil is on failure state aftercalculating the stress of the soil by all loading concerned. Though thismethod is based on the elastic theory, it is effective for design. If plasticproperty of soil is considered, numerical analysis method is needed. Sowe introduced the process of FEM briefly.In order to further understand the performance of deep excavationslope, micro-and meso-scopic properties of the silt are discussed, andelastoplastic constitutive model is studied, and its parameters arecalibrated. In this thesis, elastic modulus tensor and elastoplastic modulustensor are gotten by virtue of triaxial compression test and consolidationtest . Besed on the data, young's modulus E , cohesion c and internalfriction angle ? of the silt are yielded. Poisson ratio ν is selected fromthe 'engineering geology manual' based soil type. Finally, we yield allparameters of associated Mohr-Coulomb elastic-plastic constitutivemodel.Except formed time, the deep excavation slope is the same as thenatural slope. So stability analysis methods of natural slope are valid forthe deep excavation slope. So the characters of the deep excavation slope,failure mechanism and phenomena are reviewed. Moreover, classicalmethods analyzing deep excavation slope are summarized. In order tofurther understand the influence due to surcharge on the deep excavationslope, we adopt ABAQUS software to study plastic zone and plastic strain,shear stress of deep excavation slope with the associated idealelastic-plastic Mohr-Coulomb constitutive model. The factors ofsurcharge, we considered, consist of the magnitude, acting position anddistribution state. The results demonstrate that the slope stability dependsmost on the magnitude and acting position, less on distribution state ofsurcharge.Based on stability analysis, if the slope is on unstable state, bracingsystem has to be considered. Chapter Ⅴ analyze an engineering exampleof deep excavation which concerns with many technology codes and soilconditions of in situ. With software ABAQUS, we analyzed thedeformation of the bracing systems, heave of bottom, axial force of thestrut, and the evolution of the plastic zone by surcharge. The resultsdemonstrate that the magnitude, acting position and distribution statehave same order of contribution to the stability of the deep excavation.Finally, the results in this thesis are presented and the furtherresearch is brought forward.