Wastewater / Water
Millrace Pump Station
*A 120 MGD regional wastewater pumping station with a 58-foot-deep wet well and 60-inch force main, built in an urban floodplain next to an operating interceptor.
- Client
- Treasure Valley Water Reclamation District
- Engineer
- Sawtooth Engineering Associates
- Location
- Boise, ID
- Value
- $25M–$100M
- Delivery
- Design-Bid-Build
- Status
- Completed
- Completed
- 2018
Spec sheet
By the numbers
- Firm capacity
- 120 MGD peak
- Pumps
- Six 700 HP vertical turbine units with VFDs
- Wet well depth
- 58 ft, 64 ft diameter caisson
- Force main
- 5,200 LF of 60-in. steel
- Standby power
- Three 2.0 MW generators
- Contract value
- $39.7 million
Story
Challenge, solution and outcome
Challenge
The wet well had to be sunk through 40 feet of saturated gravel and cobbles with a groundwater table two feet below the surface, only 70 feet from an active 84-inch interceptor that could not be shut down.
The station was designed to be fully operational before the spring peak flows of 2018, a date that left no room for slippage.
Solution
We used a caisson sinking method with a bentonite lubricant and deep-well dewatering, setting 36 monitoring points on the interceptor and neighboring buildings. Settlement stayed under 0.4 inches, well below the 1-inch alarm threshold.
While the structure was being built, pumps, switchgear and generators were released for early fabrication and staged off site, so that we could set equipment within three weeks of the building being weathertight.
Outcome
The station passed its 72-hour full-load acceptance test on April 3, 2018, 41 days ahead of schedule, and handled the spring peak flow that year without a bypass event. The force main passed hydrostatic testing at 150 psi with no leaks.
The project closed with 0 lost-time incidents over 219,000 hours.
Safety
Safety highlights
- 0 lost-time incidents over 219,000 work hours
- Caisson sinking protocol with rescue plan and gas monitoring
- Interceptor protection plan with real-time settlement alerts
- Weekly hot-work permit audits
From the owner
“Sinking a 58-foot caisson 70 feet from an interceptor we could not shut down made everyone nervous. The settlement data never moved, and the station was running before the spring peak.”
Gallery
Project gallery
