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OCWD Team Overcomes Wet Well Excavation Challenges

Home > 2021 > OCWD Team Overcomes Wet Well Excavation Challenges

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Since December 2019, Shimmick has made significant progress on the $200 million Orange County Water District (OCWD) Groundwater Replenishment System project, involving expansion at two Orange County water treatment plants. The project will allow the water treatment facility to process approximately 130 million gallons a day, a 30 percent increase from its current output.

Over the past six months, the team has focused efforts on the excavation of a 40 foot by 60 foot by 35 foot pump station wet well, which will store water before it is pumped from the Orange County Sanitation District (OCSD) plant to the OCWD plant. Due to its ocean-side location, the excavation site was at a water table of approximately -5, significant given the project involved digging 35 feet deep. Because the project required digging to an elevation of -25, keeping the shoring pit dry and workable was incredibly difficult.

To install the perimeter shoring of the pit, the team drove steel beams 57 feet deep, then installed steel plates and wooden lagging between the beams to keep the silt and water from entering the pit. Additionally, to minimize such silt and water leakage, an intensive dewatering system was necessary. The team installed six deep dewatering wells and two de-silting tanks around the outside of the excavation, used to desaturate the surrounding soil. However, after weeks of consistent rain in February, the team added shallow dewatering pumps inside the well to keep the shoring pit dry. 

Now that the excavation is complete and the pit floor is dry and level enough to avoid differential settlement, the team has placed filter fabric and two feet of rock over the entire pit floor. Next, the team will place a large concrete slab and six distinct phases of concrete walls before the shoring can be removed. Once the wet well has been built, the team will construct the top half of the pump station and the piping and mechanical work, which will likely take over a year.