Research On Tunnel Face Stability In Saturated Layered Soil

In urban underground tunneling, the complicated hydro-geological conditons and execuation conditions somehow influence shield tunneling and make it difficult to control the soil deformation. The tunnel face instability which causes the catastrophical soil deformation and failure attracts broad attations. On the background of urban underground EPB shield tunneling, the tunnel face stability in saturated layered soil is investigated by using upper bound analysis and numerical simulation. The support pressure for the face stability without seepage application and that for the face stability under groundwater seepage application have been obtained respectively. The content of the research is divied into4parts as the following,(1) On the basis of Mohr-Coulomb constitution and associated flow rule, the3D kinemetically admissible mechanism for the failure range in the layered soil is proposed. Using the upper bound theorem, the formula of the support pressure for the face stability in the layer soil is obtained.(2) The upper bound solution presents the same support pressure as the Leca upper bound solution in the study of the face stability in the homogeneous soil. In the layerd soil, support pressure resists both of the crossed layer and cover layers. The influence of crossed layer and cover layers on the face stability is investigated. As the shear strength in cover soil is bigger than that in the crossed soil, the face stability is influenced mainly by the shear strenghth variation of the crossed soil, but as the shear strength in the cover soil is smaller than that in crossed soil, the face stability is influenced by both of the cover soil and crossed soil.(3) For the saturated layered soil, the seepage applied on the kinematically admissible mechanism is introduced into upper bound analysis. Consequently, the support presseure for the face stability under the groundwater seepage application is obtained.(4) The code FLAC3D is used to study the groundwater seepage. In the layered silty or sany soil, the seepage becomes steady-state easily in each excavation period and ratio of the seepage forces on the tunnel and hydrostatic pressure at the middle of the tunnel keeps stable. Meanwhile, the seepage in the crossed layer is horiztonal toward the tunnel face while the seepage in cover layer is vertically downward. The distribution of the seepage gradients in the layers is obtained by using the semi-aquifier model. The seepage forces on the tunnel face is bigger than the seepage forces in the failed soil, and the seepage forces becomes the key factor of the face stability as the groundwater level increases.