Toronto Ashbridges Bay Treatment Plant Outfall – Special Contract Provisions for Large Tunnelling Projects

Author(s) K. Waher, S. Ziegler, G. Kramer, A. Hares, J. Teper
North American Society for Trenchless Technology (NASTT) NASTT’s 2017 No-Dig Show Washington, D.C. April 9-12, 2017


The Toronto Ashbridges Bay Treatment Plant Outfall (ABTPO) project includes detailed design and construction management services for a new tunnelled outfall that will convey treated effluent from the Ashbridges Bay Treatment Plant (ABTP) into Lake Ontario. Once the new outfall is constructed, and additional ABTP improvements are complete, flow from the plant will be directed to the new outfall. The new outfall will convey up to 3,923 MLD (1036 MGD). Outfall components include an onshore shaft, approximately 85 m deep with an outer diameter of 16 m (52 ft); a 3,500 m (2.1 mile) long tunnel, with a 7 m (23 ft) internal diameter tunnelled through rock beneath the lakebed and risers constructed in line with the tunnel along the diffuser length extending from the tunnel horizon to the lakebed.

Underground construction involves management of risks not otherwise encountered in above-ground construction. A number of specialized, contemporary contract provisions have been developed or adapted by the tunnelling industry to better manage and allocate risk in major underground construction projects, including: Geotechnical Baseline Report; Differing Site Conditions; Change Clause; Escrow Bid Documents; Dispute Resolution Board; Schedule Management and Evaluation; Value Engineering Change Proposals, and Partnering. These tunnelling-specific construction contract provisions are intended to improve risk management, attract additional bidders, promote competitive bid prices, and encourage Contractor innovation. The provisions also provide the Owner with the ability to focus on resolution of issues, rather than resolution of disputes. This paper provides an overview of these tunnelling-specific special contract clauses including the anticipated project-specific applicability and benefits as related to the planned contract implementation for the ABTPO project.