A new adaptive compressible element for tunnel lining support in squeezing rock masses

The so-called "Non-Deformable Support System" and "Yielding Support System" have been used in tunnel ground support designs in squeezing rock conditions. According to the convergence confinement method, it serves as a pivotal role to use yielding elements in the primary support for expressway tunnels in squeezing rocks. The elements of yielding support are able to convert the radial displacement into circumferential tangential closure without affecting the support capacity of the lining system, which prevents from damage in the primary lining composed of mesh, steel arch and shotcrete. This paper introduces a new adaptive compressible element that significantly improve the deformability of support lining in squeezing rock. The new compressible element features by its straightforward mechanistic principle, compact design form and convenient construction and installation. The load-displacement curves of the compressible element were obtained by laboratory compression tests. Field tests were carried out in Qinghe tunnel of Zhaolu Expressway in Yunnan Province, China. The settlements of the tunnel in three sections, the contact pressure between the surrounding rock and the ground support and the stress in steel arches were monitored during the excavation of the tunnel. Based on the monitoring data, it is concluded that when two symmetrical and longitudinal compressible elements are installed in the lining, the support pressure required at the equilibrium is greatly reduced, and the support pressure at the rock support contact is more evenly distributed. The construction measures of the compressible elements were elaborated through the field application. Finally, the advantages and suggestions for the improvement of the compressible element were summarized.