Reinforced concrete slurry walls provided initial ground support and groundwater cutoff and allowed excavation of the ground within the shaft perimeter without the use of any internal bracing.

The Project

The City of Portland West and East Combined Sewer Overflow (CSO) Tunnel projects consist of over 50,000 feet of tunnel in soft ground conditions requiring the construction of 13 deep shafts for tunnel access. Additionally, the Pump Station Shaft was the deepest and largest of the 13 shafts requiring a permanent ground support system with a deep jet grout ground water cut-off to 335 feet deep. The Pump Station Shaft has a diameter of 137 feet and an internal excavated depth of 163 feet.

The Solution

Typically, the reinforced concrete slurry walls provide initial ground support and groundwater cutoff and allow excavation of the ground within the shaft perimeter, resulting in a ring made of multiple reinforced concrete panels cast in-situ in slurry-filled trenches excavated from the ground surface to the required target depths. This self-supporting ring is achieved without the use of any internal bracing. Additionally, on the East side shafts, Bencor installed ground improvement plugs where the TBM’s enter and exit the shafts. All break-in and break-out ground improvements were constructed after the shaft slurry wall was completed from the surface using vertical overlapping panels. All of the shafts on the East side and two on the West side were constructed using hydromill technology. The remaining shafts were constructed using conventional slurry wall equipment.

  • Wall Type: (West) - 6 circular shafts, various ranging from 36 inches reinforced slurry wall to 48 inches reinforced slurry wall; (East) - 7 circular shafts, 42 inches reinforced slurry wall; tunnel break-in break-out plugs of un-reinforced lean concrete slurry wall for ground improvement.
  • Wall Area: (West) - 250,000 square feet; (East) - 189,670 square feet.
  • Max Depth: 205 feet with slurry wall and 335 feet with jet grouting below the slurry wall shaft.

Project Facts


The City of Portland

Main Contractor(s)

(West)-Impregilo-Healy, JV
(East) - Kiewit-Bilfinger Berger, JV

Keller Business Unit(s)