Pre-supporting mechanism and supporting scheme design for advanced small pipes in the silty clay layer

Abstract

This study presents a design method for the advanced support of subway tunnels for different water content conditions in the silty clay layer. The influence of particle size distribution and water content on the mechanical properties of silty clay is studied through laboratory tests. In addition to the actual construction of the Harbin subway with a silty clay layer, a numerical calculation model is established to study the surface settlement, crown settlement, and supporting structure internal force during the tunnel excavation and support process under different working conditions. Furthermore, the mechanism of action of the advanced small pipe support is clearly defined, and a stability analysis model of the top and bench excavation in a silty clay layer is proposed to evaluate the stability of the tunnel face. The necessity and rationality of the pre-support is quantitatively analysed using the stability coefficient. The laboratory tests illustrate that within a certain range of clay content, the shear strength of silty clay decreases with an increase in water content. The silty clay was further divided into subgrades. The pre-support transfers the loose soil load to the initial support, which improves the early utilization of the lining structure. The advanced small pipe grouting increases the range of the bearing arch, shortens the interval distance of the supporting arch feet, and reduces the vertical load acting on the tunnel face. For the condition of subgrade V in silty clay strata, the stability of the tunnel face can be maintained without the advanced small pipe support. For the condition of subgrade VI1, the advanced small pipe support must be applied to ensure the stability of the tunnel face, but the grouting reinforcement measures are not required. For the condition of subgrade VI2, the advanced small pipe support and grouting reinforcement must be used simultaneously to ensure the stability of the tunnel face.