A 5.6m diameter Robbins hard rock TBM broke through on the first 6.1km long leg of Chicago’s Tunnel and Reservoir Plan (TARP) Phase 1 at the end of June, almost three weeks ahead of schedule.
The initial breakthrough was driven back 24m from the shaft, so the upper-half of the machine could be viewed as it progressed to the final breakthrough point. The reception chamber was partly constructed from the reception shaft, leaving the bottom half for the TBM to excavate it (see photograph). It is claimed that this is a more efficient form of construction than drill and blast, and muck-removal from the shaft.
Affholder/Jay Dee JV is excavating the 12.8km long tunnel in two drives from a central shaft. The machine will now be dismantled, with the cutterhead and grippers being transported over-ground to the launch shaft, and the remainder pulled beck through the tunnel. The second drive is expected to commence at the beginning of August.
The geology at the tunnel’s horizon – between 50m and 100m below the surface – is Racine Limestone, a fine-grained homogeneous dolomitic material with a compressive strength of between 80MPa and 100MPa. Together with the high-performance Robbins TBM, Affholder/Jay Dee JV has posted some excellent tunnelling rates. On the second drive, the JV hopes to reach weekly targets of 610m.
The US$168.7M contract is scheduled for completion in March 2006.
