Numerical prediction of long-term settlements over tunnels in clay

Abstract

The long-term settlements over tunnels can be rather significant, particularly when tunnels are embedded in soft and compressible soils. The major factors, which have very significant influence on the development of long-term (post-construction) settlements, can be mainly attributed to the magnitude and distribution of excess pore pressure around the tunnel due to shield tunneling, the drainage condition of the tunnel linings during the evolution of long-term settlement, the vibration damage of the vehicle to the surrounding soils, and the stress-strain behavior of the soils where the tunnels are embedded. It is widely accepted that the effects of time on strength and deformation characteristics of soils, especially of saturated cohesive soft soils, play an important role in a variety of geotechnical engineering activities, including tunneling where either rapid or long-term response is concerned. Consequently, the long-term settlement can be predicted based on the stress-strain-time relationship of soft clay using numerical method. An elastic-viscoplastic constitutive model was undertaken with the framework of Perzyna and modified Cam-clay model based on extensive experimental data of undisturbed samples with k0 pre-consolidation conditions. The constitutive model coupled with Biot's consolidation theory was correspondingly implemented into the FEM code to carry out the numerical simulation. The adopted elastic-viscoplastic constitutive model with few parameters was an advantage in studying the time-dependent stress-strain behavior of soft clay. The evolution of long-term settlement with time was simulated with the elastic-viscoplastic constitutive model. A multiple-step numerical procedure was designed to simulate the different behavior and mechanical characteristics of the tunnel and the surrounding soils in the long term. The influences of drainage condition of tunnel linings on the evolution of long-term surface settlement as well as ground loss are simulated with the multiple-step numerical procedure. Consequently, the development of surface settlements, ground loss, and transverse surface settlement troughs with time can be detected, and conclusions can be drawn from the numerical simulations. Finally, a case study was also performed and a satisfaction agreement between the predicted long-term settlement and the observed one was obtained. (A) Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.