Pressures on the lining of a large shield tunnel with a small overburden: A case study

The pressure on a tunnel lining is an important issue in shield tunnel design, because it is related to the safety and durability of the lining. There is an increasing need for short (about 1 km) but wide traffic tunnels in parts of China near rivers. In order to satisfy the gradient requirement of the road over such a short distance, the depth of the overburden is inevitably very shallow. The non-uniform distribution of the pressure on the lining of this kind of tunnel may result in damage to the tunnel segments. However, little published research is available on this topic. In this study, in-situ monitoring was carried out to measure the pressure on the lining of a large cross-river shield tunnel (diameter D 11.36 m) with a shallow overburden (0.7 D) in silty-sandy ground. A pad type earth pressure gauges and pore-water pressure gauges were used to measure the pressures on the tunnel lining, including the tail brush pressure, grouting pressure, earth pressure and pore-water pressure. The time history and the distribution of pressures on the lining during tunnel construction and post-construction were obtained. A comparison between the measured values and theoretical values was carried out. This showed that the tail brush pressure had a considerable effect on the segments while the shield machine was passing through. Non-uniform tail brush pressures caused cracking and water leakage in the concrete segments. The maximum pressure induced by the backfill grouting was twice that of the theoretical earth pressure. However, the non-uniformity of the backfill grouting induced pressure was not as great as that induced by the tail brushes. In the silty sandy ground, the earth pressures were generally stable 24 h after segment assembly. The final stable values were close to the total overburden pressure, particularly at the tunnel crown. The pore-water pressure generally was equal to the hydrostatic pressure and accounts for a large proportion of the total earth pressure. (C) 2017 Elsevier Ltd. All rights reserved.