Numerical Investigations And Centrifugal Model Tests On Face Stability Of Shield Tunnel In Dry And Saturated Sandy Soils

In order to solve the problem of the heavy traffic in the cities, more and more cities begin to construct the urban rail transits. Nowadays, most interval tunnels of the urban rail transits are constructed by the shield machines, due to the complicated constructional conditions such as the severely variable engineering geology and hydrologic geology, the densely distribution of the buildings and underground pipelines, the problem of face stability of the shield tunnel has become increasingly important. Based on the engineering requirement, two typical tunnel face stability problems under complicated constructional conditions were mainly investigated in this thesis, one is for tunneling in the dry sandy stratum, the other is for tunneling in the saturated sandy soils (e.g. the sandy silt stratum) with the steady state seepage.The major works and results of this dissertation are as follows:(1) Relatively in-depth DEM studies on the problem of tunnel face stability in the dry sandy ground was performed. According to the DEM investigations, it is found that during the tunnel face failure, with the increase of the face displacement, the support pressure firstly decreases to the limit support pressure and then increases gradually to a stable value (i.e., the residual support pressure). It is also obtained that when the cover-to-depth ratio C/D is no less than1.0, the limit support pressure is almost not influenced by the C/D value. For one typical buried tunnel (i.e., C/D=2), the failure patterns of the limit state and the residual stated were investigated. In the limit state, it is found that significant soil arching occurs in the upper part of the failure zone (which is above the tunnel crown), meanwhile, the soil becomes loosened in the failure zone.(2) An onboard centrifuge apparatus for investing the problem of tunnel face stability in the single phase soils (e.g. the dry sand) was developed. Based on this apparatus, a series of centrifugal model tests on tunnel face stability in dense sand were performed. The relationships between the support pressure and face displacement revealed by the DEM analysis are confirmed, it is obtained that the limit support pressure increases with the increase of relative depth CID and then remains almost the same. By using the PIV technology, it is found that in the limit state, a "wedge-prism" failure pattern occurs in front of the tunnel face, and it is also found that the relative depth has effects on the height of the failure "prism" in the limit state.(3) A world-first onboard centrifuge apparatus for investing the problem of tunnel face stability under the steady state seepage was developed. Based on this apparatus, a series of world-first centrifugal model tests on tunnel face stability in the saturated sandy silts stratum under the steady state seepage were conducted. During the tunnel face failure, with the increase of face displacement, it is found that the support pressure firstly decreases to the limit effective support pressure and then increases linearly and gradually. The limit effective support pressure increases linearly with the increase of the water head difference in the shield’s chamber and the far ground. In the limit state, a "wedge-prism" failure pattern occurs in front of the tunnel face, while the height of the failure "prism" is not affected by the relative depth of the tunnel (i.e., C/D). It is also found that the pore pressure increases with the horizontal distance to the runnel face, and when the horizontal distance to the tunnel face is no less than0.75D, the pore pressure seems to keep invariant.(4) Based on the investigations via DEM simulations and centrifugal model tests, some modifications on the classical "wedge-prism" limit equilibrium model (Anagnostou&Kovari1996) from the perspectives of the failure pattern and soil arching were conducted to build the improved "wedge-prism" model, which is more suitable for calculating the limit support pressure (or limit effective support pressure) on the conditions of the dry cohesionless soils (e.g., the dry sand), dry cohesive-frictional soils, and two phases cohesive-frictional soils (e.g., the saturated sandy silts) in steady state seepage. The accuracy of the improved wedge-prism model were verified by the results of the centrifugal model tests, and it was successfully applied in the interval tunnels of the line No.1of the Hangzhou metro.This research will obviously improve the engineers’cognitions on the problems of the tunnel face stability under complicated constructional conditions, and also will help to guarantee the face stability of the shield tunnels in the practical engineering.