Stability And Deformation Effect Of Defects In Complicated Strata

Complicated geology condition will be encountered in the construction of urban subway, among which voids are a common defection of ground. The existence of voids in ground will exert great influence on strata deformation and engineering safety. Therefore, the paper conducted a systematic research on voids in ground, including the classification, mechanism of instability, influential factors, deformation effect and prediction. The main work and conclusions are listed below.(1) The paper clarified the definition of complicated strata and summarized the typical shape of voids in ground. Strata in urban area is the composite structure of continuous soil mass and defected geological structure, and it performed in an integral way. Strata defection is a broad concept of ground voids, and can be classified into three forms: rotundity, horizontal ellipsoid and vertical ellipsoid.(2) Based on experimental model test, three failure modes of voids are concluded. For circular voids, they fails progressively, with slippage of the two sides first and followed by the broken down of the top part. The failure modes is similar to beam and arch for elliptic voids with dip angle over and below45°respectively. Hence, from the perspective of failure mode, voids in ground can fall into three classification:hole, beam and arch.(3) The paper analyzed the instability mechanism of the three kinds of voids. Under the geostatic stress and the tool of complex variables functions, the circular and vertical elliptic voids perform better than the horizontal elliptic voids. Usually the failure of horizontal elliptic voids dues to tensile stress. The failure of voids is the result of tensile and shear strength. Utilizing grey relational analysis, the paper obtained the conclusion that the scope of tensile-plastic zone is determined by the size of voids while the scope of shear-plastic zone is determined by external loads.(4) By theoretical deduction, the paper analyzed the instability mechanism of voids disturbed by tunneling. There are four deformation pattern of circular voids:radial shrinkage, ovalization, vertical settlement and horizontal lateral displacement. The plastic zone near voids is affected by its deformation pattern greatly. Instability will occurred when the non-uniform settlement of the two sides of "beam voids" exceeds the critical value. For voids whose failure mode is similar to arch, instability will occurred when the vertical settlement or the horizontal lateral displacement exceed the critical value.(5) The deformation characteristics of strata with voids in it is analyzed from the perspective of strata stability. The influence of voids on strata deformation are classified, and three sort of indicator and calculation method of strata deformation are also put forward. The paper initiates three aspects of strata deformation disturbed by tunneling, which are the elastic deformation of material, structural deformation of voids and solidification. The calculation method considering both time-space effect is provided for the prediction of complicated strata condition. The method was applied in the construction of Huangzhuang station of Beijing subway and met the requirement of engineering.