ABSTRACT
At each end of an immersed tunnel, it is necessary to construct some in situ works to form the tunnel approaches. These tunnel approaches generally comprise a length of cut and cover tunnel construction containing the tunnel portal and a length of open ramp structure that retains the ground on either side of the alignment as the road or railway lowers toward the tunnel portal. Some of the most complex engineering issues that arise on immersed tunnel projects occur in the tunnel approaches. This is due to the varying geotechnical conditions that are often encountered and the desire to create dewatered excavations in which to build the approach structures. Because of the proximity of the waterway, the approach structures are always built below, or partly below, the groundwater table. The method of excavation therefore requires the groundwater flow to be cut off by some means of temporary works, so that water cannot enter the excavation. In addition, the tunnel approach areas are often used as a temporary construction yard in which to build the immersed tunnel elements, and the need to optimize the arrangement to suit both temporary and permanent requirements may generate some unusual design conditions to consider.
