Basal Stability Of Deep Excavations In Anisotropy Soft Clay Using Upper Bound Method

The stability and displacement are two classical problems in geotechnicalengineering. For deep excavations in soft clay, the design of the lateral earth supportsystem is often controlled by stability requirements. In current practice, basicallythere are three methods used to perform stability calculations for braced excavations:(1)limit equilibrium methods; (2)nonlinear finite-element methods; (3)upper andlower bound limit analysis.The method proposed in this paper is used to analyze the base stability of deepexcavations in anisotropic soft clay. It is derived from the upper bound theory andstrength reduction theory assuming the Prandtl's soil slide failure mode suitable for acritical basal stability which can consider the anisotropy and nonlinear feature ofclays. The comparison between the result of the current method and the numericalresults presented by Hashash and Whittle(1996) for an excavation in normallyconsolidated Boston blue clay, demonstrates the method's ability to simulate thestrength anisotropy and nonhomogeneity of soil on the basal stability of deepexcavations. Furthermore, this method also shows good accuracy in back-analyzingthe safety factor for two base failure cases in Shanghai and Taipei respectively.Generally the conclusion of the paper can be summarized as follows.The main achievements of this thesis are as follows:1. Anisotropy strength formula which can take into account the strength inrelation to the vertical effective stress are induced from the anisotropy boundingmodel (Wei and Huang, 2006).2. The necessities of considering the anisotropy feature of clays for thecalculation of basal stability of deep excavations in soft clays.3. Without taking into consideration of strength anisotropy of clays the safetyfactor against base heave from the traditional mehods is higher than the real one.4. When the anisotropy ratio of soft clay equals to one, the result from themethod based on the Terzaghi failure mode is the rigid upper one.5. The parameters used in the current method are easily obtained by simple labtests (eg. triaxial compression test and extension test or plain compression andextension test). Additionally the method can consider the relationship between soilstrength and vertical effective stress. Finally, further studies on the research are discussed briefly.