Why Wellpointing

Wellpolnting was necessary for the Beaufort project due to the space limitations of the jobsite. "A dewatering pump can be used when you have plenty of room and are able to dig a hole without the sides caving in," Creech explains. "Normally you would just drop a suction hose into the water and connect to the pump and start pumping. Wellpoints are used when you have limited room to work, ground conditions are unstable, you don't want to remove [sand] from the site or you need to dig a dry hole for any reason. In this case, two 65-ft diameter clarifiers had to be constructed in a space not much wider than the clarifier itself and in an excavation as much as 15 ft deep."

Creech says the biggest challenge Thompson Pump faced in designing the wellpoint dewatering system was that the project was located in what is called the coastal zone, which is near the ocean. Having the site so close to the ocean makes it susceptible to tidal influences. "The ocean's high and low tides influences the ground twice a day, every day," Creech says. "You may have as much as 3 or 4 ft of water rise twice a day. So even though you pump this water down, the tide comes in and the water is going to come back in your excavation. It's pretty difficult to do. You may be pumping 200 gpm for part of the day and then another part of the day it may be 600 to 1,000 gpm."

Also of note is the use of an electrical pumping system instead of the standard diesel engine system; the decision to go with the electrical one was a matter of cost. "Going with the electrical pumping system, while costly upfront to bring 480-volt power to our worksite, has saved us a great deal of money with the price of diesel fuel these days," explains Mike McLamb, vice president of State Utility Contractors.

The wellpoint dewatering at Beaufort has been going for about eight months and is expected to continue into early 2009, when the clarifiers and oxidation ditch are to be completed. But more dewatering will likely be needed in 2009 when three other structures, including filter tanks, will be constructed below grade.

"Everything has been working according to plan," Coore says.