Peel Hwy 50 Trunk Sanitary Sewer - Tunnelling Complications with Rolling Rocks and Damaged Pipes
The Region of Peel’s Highway 50 Trunk Sanitary Sewer project involves the installation of a new 1.4 km (1 mile) long sanitary sewer from Old Castlemore Road to Coleraine Drive in the City of Brampton, Ontario, to provide additional system capacity and address sewage collection system constraints associated with future development. The sewer was designed as three separate tunnel drives approximately 500m (0.3 miles) in length using conventional Tunnel Boring Machine (TBM) or Microtunnelling methods. Upon contract award, the contractor presented a value engineering change proposal that involved one-pass tunnelling with a 1880mm (74”) outer diameter Earth Pressure Balance TBM using 1500mm (60”) internal diameter jacked microtunnel pipe.
During construction of the first tunnel drive, there were two events where the contractor claimed to encounter “rolling rocks” which appeared to damage a steel tailing can and two microtunnel pipes. After these issues were encountered, the contractor installed internal bracing, probed from within the tunnel to locate potential boulders, translated the obstruction position to the surface using survey equipment, and then installed emergency rescue shafts. The contractor also replaced the damaged microtunnel pipes from the pipe string before successfully completing the tunnel drive.
The “rolling rocks” and damaged microtunnel pipe conflict resolution involved examining the Contractor’s customized tunnelling methods and equipment, reviewing real-time TBM output data, acquiring evidence of the obstruction material, and evaluating the merit for additional materials testing. This case study will provide an overview of the project and design considerations, including the “rolling rocks” and damaged pipe complications, investigation approach, outcomes and lessons learned.