Parameter design of yielding layers for squeezing tunnels

Abstract

Tunneling in a soft rock mass under high in situ stress often results in squeezing deformation. Supports that use yielding layers are suggested to address the squeezing deformation of tunnels. However, feasible design methods for the yielding layer have not been proposed, especially for the determination of the yielding stress Ps and thickness d. In this study, a laboratory test is conducted to verify the effect of the yielding support. The test results show that the pressure of the secondary lining is reduced with the installation of the yielding layer. The comparisons between the laboratory test and uniaxial tests of confined and unconfined conditions showed that the whole stress–strain curve of the yielding layer can be obtained by its confined compression tests. The whole stress–strain curve of the yielding layer can be divided into three stages: elastic stage, yielding stage and failure stage. Compared with other yielding lining, a polyurethane (PU) yielding layer with density (70–100) is more suitable for the yielding layer as it has high deformation capacity and adjustable yielding stress. A design method of a PU yielding layer considering its density is proposed, and the key parameters of the yielding layer for a squeezing tunnel are determined.