David Lees and Associates was appointed by The Babtie Group to act as Designer's Representative for the construction of the NATM back shunt tunnel for the Changhi Tunnel contract T-01.

The construction was carried out in Old Alluvium.  The heading was a very coarse grained clayey sand which was often damp by dried out quickly upon excavation. There was a hard calcareous band just below axis level, beneath which the material was generally a hard silty clay but included lenses of soft organic clay which often were associated with minor ingress of water.

The cycle of work was essentially excavate a single 1m advance in the heading and install the first layer of mesh and lattice arch, then apply 100mm of shotcrete. Another 1m of advance was taken in the heading after a few hours to allow the shotcrete to attain some initial strength. Following installation of mesh and the lattice arch in the second metre of advance the second layer of mesh was installed in the previous section. The initial 100mm of shotcrete was then applied to the new excavation and the second layer of shotcrete to the previous section. The excavation and support of two 1m advances in the heading took about 15 hours. In general the heading was only advanced a maximum of 5m ahead of the bench and invert closure.   Survey control was provided by measuring with EDM to five points on the lattice arch, both in the heading and in the bench, and then adjusting the arch within 30mm of the required position. To assist with excavation profiling a peg was surveyed in the berm prior to each cut and a string was used to check the arc length.