Lessons learned from the West Trunk Sewer Pipe-Mississauga, Ontario

Abstract

The West Trunk Sewer Pipe (WTSP) has a 9,777 m, 300 mm thick Cast-in-Place (CIP) plain concrete liner. Mining of the WTSP using a Tunnel Boring Machine started in 2013 and was completed in 2018. A minimum 90-day delay time between the tunnel mining and casting of the CIP liner was allowed to relieve some of the shale relaxation. Due to numerous reported leaks, the Region of Peel had significant concerns about the performance of the CIP liner under the effects of the rock Time Dependent Deformation, other loading aspects of the rock media, and the fluctuation of the underground water table. Visual inspection has confirmed that the CIP liner exhibits structural signs of distress along the tunnel’s springlines, and at the crown in some areas. Most of the cold joints and cracks were found to have active leaks. Sealing these leaks was very difficult due to the inherent structural deficiency in the CIP liner. The Region of Peel decided to reinforce/strengthen only the affected areas where structural cracks appeared. The proposed strengthening of the tunnel using Carbon Fibre Reinforced Polymers was a challenge due to the difficulties in maintaining the required environmental conditions recommended by the manufacturer. New cracks appeared between areas of CFRP due to changes in tunnel stiffness and the accumulation of negative hydrostatic pressure. Lessons learned throughout this project are presented in this paper, providing the audience with important keynotes on the design and protection of concrete in sewer tunnels constructed within squeezing shale bedrock.