Investigation of the pressure distributions around quasi-rectangular shield tunnels in soft soils with a shallow overburden: A field study

摘要

The magnitudes and distributions of the pressures around a tunnel lining are the most significant variables in shield tunnel design, as they determine the safety of the lining structure and influence the investment cost of the tunnel project. The grouting process has a negligible effect on the pressures exerted on the shield tunnel segmental linings when the boring machine is drilling. However, improper grouting control might result in large bending moments, local damage of segments and leakage at joints, leading to a reduction of the linings' safety, durability and serviceability. On the other hand, more and more special-section shield tunnels are developed to solve the problems associated with infrastructure construction under complex environmental conditions. How-ever, previous studies on the pressure distribution around tunnel linings all focused on circular tunnels. Few research about the loads during construction on special-section shield tunnels was conducted. In this study, three in-situ monitoring tests for the total pressure distributions of shallow buried quasi-rectangular tunnels in soft soils were carried out, which is deemed to be the first field study for special-section shield tunnels. Different grouting situations were adopted and the temporal and spatial distributions of the lining pressures during and after the construction were obtained. When the lining segments are pushed out of the shield tail, the adverse pressure distributions caused by grout ejections were observed which are typical non-uniform and unsymmet-rical. The effect of different grouting situations attenuates quickly in the longitudinal direction within a range of 9 rings. Additionally, the influence of the following back-up equipment and transportation carriages on the pressures outside the linings were first observed and studied by means of these field tests. The outside pressures fluctuated in different stages with the coincident pace of construction activities and performed as a counterforce and to resist the inside loads, which was different from the active loads exerted on the lining when the segmental rings were pushed out. From the long-term perspective, the pressure distributions in different tests tend to stabilize at a similar state. The influence of the grouting process or other construction loads on the lining pressures needed about 50 days to be relieved. A comparison between the measured results and the theoretical long-term pressures was conducted and the applicability of the proposed load distributions in the design model for quasi-rectangular tunnels was demonstrated. Additionally, a pressure mode combining cosinusoidal pressures and theoretical long-term pressures is proposed to predict the grouting pressures when lining segments are pushed out of the shield.