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Combined Sewage Storage Tunnel (CSST)

Dr. Sauer & Partners provided geotechnical engineering, waterproofing & water control as well as instrumentation & monitoring support.
Estimated cost: Commencement:
February 01, 2016
Completion date:
110 Laurier Avenue West
Ottawa, ON K1P 1J1
Technical data:
Geology description:

The alignment of the tunnels mainly crossing limestone formations with laminated black shales of varying thickness and locally glacial till.

Geology types:
Bid cost: Final cost:
$232 million CAD
Service areas:
Construction methods:
Tunneling under:

The Combined Sewage Storage Tunnel (CSST) project is part of the Ottawa River Action Plan (ORAP) and will greatly reduce the frequency of sewage overflows during storms from entering the Ottawa River. The project consists of the design and construction of two TBM tunnels running east-west and north-south along 6.2km in total, also access shafts incorporating drop structures. The storage volume will be 43,000m3. Dr. Sauer & Partners supported the contractor with the preparation of the feasibility study on the TBM utilisation and also assisted with the clarifications on ground water drawdown risks and pre-excavation grouting criteria justification.

CSST map (
Excess wet weather flow is sent to CSST (