SVCW Emergency Outfall Repair

LOCATION:

Redwood City, CA

DESCRIPTION :

This was a design-build emergency repair at Silicon Valley Clean Water (SVCW), a wastewater treatment plant in Redwood City, when their 63-inch effluent outfall burst a high-volume leak.

The facility was built in 1980 atop young bay mud, a highly liquid clay soil that yields significant movement. The main facility is on deep pile foundations (essentially, on solid ground). The effluent outfall, however, connects to the main building and extends a mile offshore into San Francisco Bay, and is subject to settling in the young bay mud. A 16-inch differential settlement is what caused this leak.

PEC responded to the leak immediately by installing a temporary grout plug from inside of the pipe.  Afterwards, they quickly moved forward with the design-build of a permanent solution. The final design was a permanent underground chamber that contains a non-settling pipe support system and keeps the pipe free of soil loading on top.

To start, the construction area needed to be excavated 20-feet below grade to fully expose a portion of the 63-inch HDPE outfall pipeline. This was accomplished by first driving sheet pile 60-feet deep then forming and pouring reinforced concrete pile caps at grade for upper excavation bracing and as permanent traffic-rated loading support. A wide flange waler system was then installed for lower bracing and additional excavation ensued down to 20-feet below grade.

For primary vertical support of the pipe, 80-foot long coated H-Piles were driven to the stiff sand lens beneath the young bay mud. Next a steel support system was installed above the H-Piles which utilized adjustable slings to cradle the HDPE pipe while also providing the ability to make height adjustments in the future.

At the end, the entire excavated construction area was covered with a traffic-rated concrete slab, personnel access points, and settlement monitors. The entire underground structure was built with an allowance for settling as well as cathodic and drainage protection.

The project successfully restored the effluent pipe to the appropriate elevation and reseated the critical flange connection adjacent to the facility.  It also provided SVCW with the ability to easily monitor the long-term condition of the effluent pipe and infrastructure at this sensitive location.