Investigation On The Induced Surface Settlements,Tunnel Interaction Mechanisms And Control Countermeasures During The Construction Of Two Parallel Shield Tunnels

Tunnelling systems are often constructed in pairs to facilitate travel in opposite directions.These arrangements are well known as twin-tunnelling.Among them,the side by side geometry arrangement is most often considered.Due to time,cost or logistical constraints,it is usual for the two tunnels to be created sequentially.This means a construction delay between the tunnels,and that the first tunnel should be regarded as an existing structure during the construction of the second.Consequently,it is of much concern to ensure the safety of the pre-existed tunnel when boring a new one.As the tunnels are not constructed simultaneously,the ground movements above closely spaced parallel tunnels can be significantly different from those observed above single tunnels,and this must have a significant impact on the settlement of structures and services in the vicinity.Therefore,it is also of great importance to predict the ground movements subjected to such a twin-tunnel type operation.This article mainly serves to solve the two key problems,which can be divided into three parts:In the first part,ground movements induced by a single tunnel is first investigated using physical model tests,which were well designed to consider the operating principles of EPBSs in practice.The tests can be divided into two categories: collapse tests(CTs)and advance tests(ATs),which investigated catastrophic failure and normal tunnelling situations,respectively.The tests were repeated tests to provide confidence in the results with the characteristic of ground movements fully determined.To explain the ground response observed in the tests,coupled Eulerian-Lagrangian(CEL)analyses were performed since these have superior performance in solving problems with severe deformations compared with a conventional Lagrangian scheme.A “tracer plate” method was developed to obtain the displacement field from the Eulerian domain and this showed good agreement with test data.The ground movements,including both settlements and horizontal movements,induced by EPBS tunnelling are presented and the disturbance mechanism of a single EPBS tunnelling are clarified.The second part reports a case history of closely spaced twin EPBS tunnels in typical soft ground and presents field-observed deformations of a preceding tunnel reinforced by various measures during the succeeding tunnelling.In view of the conclusions from single tunnelling,the face pressure of the EPBS was strictly kept below the horizontal total ground stress.However,the preceding tunnel still received a squeeze load from the succeeding tunnel when the cutting face was approaching.In order to address the reason for this counterintuitive observation,a sophisticated 3D FEM model was developed to simulate and analyse the complete twin EPBS tunnelling process.The simulation is verified to be reasonable through a comparison with field data,and the deformation characteristics of the preceding tunnel are systematically summarised.Reinforcement measures,such as inner support and grouting reinforcement,are employed in this project also to increase the safety of the preceding tunnel.Because of a lack of documentation on the effects of these reinforcement measures,the FEM model elaborately considers the reinforcement measures.As a fundamental purpose of the research,the article conducts assessments of the performance of the reinforcement scheme and evaluates the effectiveness of each reinforcement measure adopted in the project to provide references to similar projects.The third part performs numerical analyses to investigate the conclusive data from a group of reasonable centrifuge tests performed by Sam Divall at City,University of london,which aimed to provide a full insight into the behaviour of ground movements in a twin tunnelling process.Following the principle from simple to complex,both the Modified Cam Clay model and the Three-Surface Kinematic Hardening model were adopted in the numerical analyses.The Modified Cam Clay model was shown insufficient to reproduce the test data because of an elastic behaviour assumed for soil states inside the yield surface,which may significantly under-predicted the stress changes in response to the tunnel construction.By contrast,the Three Surface Kinematic Hardening model can reinvoke a high stiffness if a stress reversal was detected in a multi-staged simulation,which guaranteed a considerable stress alteration.More importantly,the Three Surface Kinematic Hardening model can naturally take the effect of recent stress history into account.This enabled the influence due to the first tunnel on the soil stiffness around the second to be properly dealt with and further the primary cause for the increased settlements observed in the tests to be reasonably addressed.