Centrifuge modelling of shallow and large sectional tunnel under full pipe-jacked ring

This paper reports on a series of centrifuge model tests to investigate the ground and structural response of tunnel linings, pipe ring roof and pile foundation induced by a shallow and large section tunnel supported by a roof formed of a ring of jacked pipes pre-installed before the excavation. The prototype is the Gongbei tunnel which has a cross sectional area of 336.9 m(2) and is 5 m below the busy Gongbei port and 0.5 m away from the pile foundations of a port building. The pipe ring, tunnel lining, temporary struts, a single pile are simulated in the model with similarity laws under an 80g acceleration. The tunnelling process is modelled by the staged inflight drainage of pre-injected liquid bags inserted in between model pipe ring and lining. The tested soil mass has dimensions of 900 x 700 x 250 mm(3) (width x depth x length) and comprises a homogeneous remoulded soft clay extracted from the Gongbei site and consolidated by the in-flight operation. Four test scenarios were conducted to investigate the effects of a nearby pile, drained liquid volume (representing volume loss) and the pre-installed pipe ring on the ground settlement and structural response of the tunnel linings and pile foundation. Test results indicate that the settlement trough is asymmetrically distributed along the tunnel axis due to the asymmetrical excavation sequences. The ground settlement above the tunnel axis varies almost linearly with the distance to centre of the excavation step. The pile near the tunnel tends to reduce ground settlement. The increase of drained liquid volume will produce significant increases of maximum ground settlement and pile settlement. Similar findings were also found for the case of failure of the frozen pipe ring, but the effect of a nearby pile is not as much as those effects of volume loss and pipe ring.