Research On Shield Tunnelling-induced Ground Surface Heave And Subsidence And Behavior Of Underwater Shield-driven Tunnels Subject To Tidal Bores

There is an ever-increasing potential for construction of underwater shield-driven tunnels nowadays in China. Research on environmental impacts induced by excavation of underwater tunnels and their structural soundness has emerged under this circumstance. This study was initiated synchronously with the construction of underwater shield-driven tunnels in Hangzhou, for the purpose of deepening the insight into their environmental disturbances and long-term structural properties. A comprehensive series of in situ tests during construction and operation of the Hangzhou Qingchun Road Underwater Tunnel combined with analytical computations were conducted to study tunnelling-induced ground surface vertical deformations and responses of tunnel structures subject to the Qiantang tidal bores.The contents of this study are broadly classified into two parts:(1) tunnelling-induced ground surface heave and subsidence, and (2) mechanical characteristics of the shield-driven tunnels subject to the Qiantang tidal bores.1. Tunnelling-induced ground surface heave and subsidenceTunnelling-induced ground surface vertical deformations were measured from extensive field instrumentations during construction of the Qingchun Road Tunnel. Besides, the shield excavation parameters were recorded in real time. Characteristics and development of ground surface deformations induced by shield tunnelling in Hangzhou soft ground were summarized by means of field observations.In addition, the applicability of available subsidence calculation methods was assessed.Based on field observations, several fundamental theories in soil mechanics and fluid mechanics were applied for calculation of heave and subsidence at the ground surface induced by shield tunnelling. The main innovations were as follows:(1) A numerical integration method based on the Mindlin’s problem was proposed for calculation of ground surface vertical deformations due to shield tunnelling along an inclined alignment.(2) The transverse ground surface heave caused by shield tail synchronous grouting accords with the Gaussian distribution through field observations. A generalized Peck formula taking both ground loss settlement and grouting heave into account was put forward.(3) A formula for calculation of tunnelling-induced ground surface settlements under different surrounding soil intrusion models was deduced from the source-and-sink method in fluid mechanics. Subsequently, an empirical modification was conducted on the basis of field observations. The modified formula provides an easy and reliable means for estimation of ground loss settlement caused by shield tunnelling.(4) An easy and practical method to distinguish the time boundary between ground settlements arising from ground loss and consolidation was presented.(5) An analytical method for calculation of excess pore water pressures and consolidation settlements arising from disturbance caused by shield tunnelling was put forward.(6) A computation theory for tunnelling-induced ground surface settlement taking influences of shield advance rate and abnormal machine halt into account was presented.(7) The efficiency of tail void synchronous grouting was evaluated with respect to the time and spacial effects. A method for calculation of ground surface vertical deformations with account of grouting efficiency and distribution was proposed.(8) A simple method was formulated for estimation of ground surface vertical deformations based on over-excavation control at the cutting-face.2. Mechanical characteristics of shield-driven tunnels subject to Qiantang tidal boresA structural health monitoring system (SHMS) was designed and set up during construction of the Qingchun Road Tunnel to measure river stage of the Qiantang River, earth pressures acting on the tunnel linings, strains of the reinforcing steel bars, convergence of the tunnel linings along the springline and longitudinal settlements of the tunnel. Based on field observations from the SHMS, innovative research as follows was carried out.(1) Correlations between changes in earth pressures acting on the tunnel linings and strains of the reinforcing steel bars and fluctuations of river stages were assessed.(2) According to differences in permeability of the overburden strata, two kinds of design models were established for underwater shield tunnel linings, one of which was verified by field observations.(3) The uniform rigidity ring method was applied for calculation of changes in strains of reinforcing steel bars induced by fluctuations of river stages. Moreover, an empirical modification was made to the computed bending moment based on field observations. In this way, the modified computed member forces can be applied for stress and strain analysis of the tunnel lining.(4) The limit allowable river stage of the Qiantang river was predicted for structural safety of the tunnel linings with respect to the two above-established design models.