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Millions of tourists enjoy Yosemite National Park with its spectacular waterfalls and Lake Tahoe in California annually, but few consider where that water goes. The answer for much of it is to the San Joaquin River Delta approximately 75 miles east of San Francisco Bay. Here multiple rivers from the Sierra Nevada Mountains empty into a massive wetlands area distinguished by a river delta.

It is here along the San Joaquin River that the city of Stockton, Calif., is building the Delta Water Supply Project, which will include a water treatment plant and an intake pump station that is approximately 11 miles from the plant. Managing construction of the intake pump station and the yard piping at the station and the plant to transport all that water is the job of Preston Pipelines.

CDM is the design/build contractor for the treatment plant and the pipeline, but with more than 30 years as an underground engineering contractor, Preston Pipelines now is sticking its corporate head above ground and serving as a general contractor on portions of the project. Additionally, the part of the job on which Preston’s Project Manager/Estimator Chad Hutchinson bid in July 2009 included approximately 50,000 cubic yards of fill for a setback levy behind an existing levy on an island in the river where the pump station would be built.

“When I bid this job, we had never undertaken a $16.5 million job this complicated where we were acting as general contractor,” Hutchinson explains. “This is the largest pump station project for Preston. The islands themselves have been built up over years and are mostly organic soils – we call it peat. So what you have is the ground itself isn’t very solid at all. It’s very unstable and doesn’t have any structural capacity.”

Consequently, the 50,000 cubic yards of soil had to be spread out over six months. “We built it in lifts, and after each lift, we brought in anywhere from 5,000 to 7,000 cubic yards of soil, placed it and let it settle out, because we couldn’t bring in the whole entire mass at one time – that might have caused a levy failure or a breach of the levy,” Hutchinson explains.

Piles in the Peat

Because of the soft soil, approximately 100 24-inch-diameter piles were driven 45 feet deep into the bottom of an excavation measuring roughly 75 feet square and 30 feet deep, of which 25 feet was below the water. “From a soil capacity standpoint, the soil report said that whole entire peat layer was negligible, almost like you’re driving through water,” Hutchinson says. “We did the excavation all in the wet, and the base slab of the concrete wet well itself is about 400 yards of concrete sitting on 100 piles. So after we did the excavation in the wet, we drove the 100 piles down beneath the base slab also in the wet, and before we dewatered the intake we placed a 3-foot slip tremie slab.”

The depth of the excavation resulted in some innovative engineering. “Because we were so deep, the engineer was worried about the hydrostatic head differential between the top and bottom of the excavation,” Hutchinson notes. “So he wanted the tremie to be reinforced with rebar. To do that, you have to send in divers around the piles, and the water is murky. It couldn’t be constructed. So we added a wire mesh to the actual tremie concrete. We basically got the tensile strength that the engineer was looking for. I don’t know if anybody has ever done that before.”

Snaking Around

The Delta Water Supply Project has several environmental considerations. One is massive screens approximately 15 feet tall by 5-1/2 feet wide over the water intakes on the pump station that will have a fine mesh with openings about 8 millimeters wide. These are to keep smelt larvae from being drawn into the intakes. At several times throughout the day, a fish screen cleaner operates automatically like a giant toothbrush to clean the front faces of the screens.

Additionally, no ground-disturbing activities could be conducted from Oct. 1, 2011, to April 30, 2012, when the project is due for completion, because it would disturb the giant garter snake, a threatened species. Construc­tion started in May 2011.

Preston Pipelines used approximately six major subcontractors on its portion of the project. “The sheet pile and pile-driving sub did a really good job,” Hutchinson maintains. “They drove all the piles inside of an inch from where they’re shown on the plans. HDR Inc. is the design engineer and Carollo Engineers is the construction manager. They have both been good to work with.”

The pump station itself houses pumps and electrical equipment with a generator along with a small temporary office and an air-handling unit. It is constructed primarily of concrete masonry units on a concrete foundation.

Hutchinson is enthused about working on a project like the Delta Water Supply Project for Preston Pipelines, whose work is centered in Northern California. “We have an ambitious group,” he says. “Some people maybe watch football or work on their cars or go fishing for fun. For the people I work with, this is fun to them – this is what they talk about. The culture of Preston is just that we want to do this stuff because we enjoy it. If you want to be successful at it, you’ve got to enjoy what you’re doing.”