Combined Sewage Storage Tunnel (CSST)

City of Ottawa
Dragados Tomlinson JV
$232 million CAD
February 2016 — ongoing
Services performed
Dr. Sauer & Partners provided geotechnical engineering, waterproofing & water control as well as instrumentation & monitoring support.
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.
The alignment of the tunnels mainly crossing limestone formations with laminated black shales of varying thickness and locally glacial till.
Ottawa, Canada


  • Shielded TBM working in open mode
  • Application of Jet Grouting
  • Boreholes through ports of TBM to examine soil ahead along critical sections
CSST Map (

CSST Map (

Excess wet weather flow is sent to CSST (

Excess wet weather flow is sent to CSST (