Clearwell Going Underground

Story by Carl Molesworth | September 28, 2010

The Everett Clearwell No. 2, which Stellar J Corp. is building at Lake Chaplain near Sultan, Wash., can be considered an underground utility project; it just isn't under the ground yet.

The 22-foot-high walls of Clearwell No. 2 are poured-in-place concrete.

Lake Chaplain serves as the sole source of drinking water for the city of Everett, Wash., and parts of surrounding Snohomish County. The city operates a water filtration plant at the south end of the lake and pipes the water to Everett, which is about 20 miles west. To better serve the fast-growing Everett area, the city decided it needed to double the storage capacity for potable water at the plant. Carollo Engineers, of Seattle, was hired to design a 6.5-million-gallon concrete reservoir with associated structures and piping.

In May 2007, Stellar J won the contract from the City of Everett Public Works Department to build the project with a bid of just under $18 million. The contractor, which is based in Woodland, Wash., started work in August, said Tim McRoberts, project superintendent.

The first order of business was to set up a shop area at the remote project site, McRoberts said. The project location presents some challenges, not the least of which is the fact that it averages about 100 inches of precipitation per year, most of it coming in the form of rain. Knowing that, Stellar J trucked in its portable shop to create a dry place for the workers to perform maintenance and repairs to equipment, take breaks and change clothes.

"Some days the guys go through three sets of rain gear," McRoberts observed.

The shop is a nifty piece of design, consisting of two trailers with roof trusses set on top and the work space between them. A third trailer closes in the back side.

Because the project is located in a secured area with a gate that is locked at night, security is not much of a concern. With hills surrounding the lake, there is not much wind to hamper operations, either, but the hills also blank out cellular telephone service, which is "a little bit limiting," McRoberts said.

Building the Clearwell

Stellar J excavated 36,000 cubic yards of material to create the footprint for the Clearwell No. 2 structure, which measures 180 feet by 280 feet. These materials are stored in two stockpiles on the site and will be used for backfill later in the project. Two Link-Belt excavators have been used on the project, a 290 and a 330.

McRoberts described the land under the structure as "weak," much of it consisting of fill. To stabilize the site, subcontractor Hayward Baker of Seattle drilled 3-foot-diameter, 50-foot-deep compressed stone columns every eight feet throughout the area. In addition, areas were drilled and jet-grouted to support existing utilities.

The concrete walls of the structure were poured in place and stand 22 feet high. When PB&E visited the site in April, crews were hard at work pouring the concrete floor slab and columns placed 20 feet on center to support the roof. Later, an elevated concrete slab will form the roof.

Cadman Inc. is supplying about 7,000 cubic yards of concrete for the total project from its batch plant in nearby Monroe, Wash.

"They have done a really good job of servicing us," McRoberts said. Ralph's Concrete Pumping Inc. of Seattle is providing concrete pumps for the job.

As it stands now, Clearwell No. 2 is an imposing structure at the entrance to the filtration plant, but that will change. Once the big box is completed, it will be filled with water to check it for hydrostatic issues, McRoberts said.

Then the reservoir will be backfilled and bermed up, with grass planted over the entire structure.


Still to come is construction of the Clearwell Flow Distribution Structure, which will distribute water from the filtration plant to the existing Clearwell No. 1 and new Clearwell No. 2. Potholing in the location of the CFDS has revealed an array of existing utilities there, none of which can be disturbed without disrupting the operation of the plant. So Stellar J has chosen an unusual method of shoring to complete the 34-foot-deep excavation.

After casings have been drilled around the perimeter of the CFDS, Soil Freeze Inc. of Seattle will come in to freeze the ground and create a 40-foot shoring wall. Then the excavators will carefully dig out within the 75-feet-by-50-feet footprint, taking special care to avoid damaging any existing utilities crossing the area.

The existing line connecting the filtration plant to Clearwell No. 1 passes through the location of the CFDS, so it will be bypassed to Clearwell No. 2 by means of a hot tap to keep potable water flowing to the city of Everett throughout the construction of the CFDS.

The CFDS will be a large concrete box, connecting to the two clearwells via 84-inch-diameter, concrete-lined and -cased pipe. In all, Northwest Pipe of Portland is supplying about 500 linear feet of the 84-inch pipe for the job.

Key participants in the project include Tim Starbuck and Alex McCoy, project managers for Stellar J Corp.; Jack Baker and Bill Fisher, construction managers for the city of Everett; and Charlie Dougherty P.E. and Adam Schuyler P.E. of Carollo Engineers.

McRoberts said the estimated completion date for the project is April 2009.