Wellpoint Dewatering Plays Key Role in Treatment Plant Project

If your jobsite has ever been near water — whether it’s a lake, river or ocean — then you know the importance of implementing the proper dewatering system to prevent your project from being a washout.

Dewatering, which sounds basic and easy enough, is a critical part of completing any project that encounters water. By developing a proper dewatering system, you will save your project time, money and headaches.

Such was the case for a treatment plant construction project that is under way in North Carolina. State Utility Contractors realized before it began construction of a new sewer treatment plant in Beaufort, N.C., that dewatering was essential to getting the work done and took the necessary steps to implement a dewatering program.

The town of Beaufort, N.C., is a small town situated near the midpoint of North Carolina’s coastline, with a population of approximately 4,100. Beaufort officials are in the process of constructing a new wastewater treatment plant, directly adjacent to the existing one. With a price tag of approximately $16 million, the new plant will offer Beaufort residents a treatment plant with increased sewer capacity and reliability. Completion date is projected for December 2009.

With deep structures a part of the construction plans — namely two 65-ft diameter clarifiers and a 91-ft wide by 200-ft long oxidation ditch — dewatering would be essential. The jobsite posed a challenge for primary contractor State Utility Contractors, as it was surrounded by water; in this case, the Atlantic Ocean was on one side, with a sound and marshes along the remaining three sides.

Thompson Pump & Mfg. was asked to do a site evaluation to recommend a dewatering program to keep the water table below the underground structures. After digging a series of test holes about 20 ft deep to evaluate the water table and the ground conditions, Thompson Pump’s  Mid-Atlantic region manager Tim Creech and North Carolina representative Alan Worley knew what the crews would be facing in terms of water and soil and determined that a wellpoint dewatering system was required to keep the water under control.

“Even in a drought situation, the area had a high water table due to tidal influences,” explains Creech, who was onsite for the hole testing. “The existing water table was at 5 ft below sea level when we dug the test holes. The challenge for us was to lower the water table to 11 ft below sea level. You can only imagine how much of a challenge that might be when the ocean itself is less than a half mile away from the site. Tidal influences, which is the effect of the ocean rising and falling as much as 5 ft twice a day, was also a factor.”
The test holes also showed Creech that the ground conditions were a mixed bag of soils but were mainly clay and sand. Clay is hard for water to permeate, while with sand, the water comes through it pretty quickly.

“We determined that the site was a mid-level situation in which it will have a good amount of water coming back into the hole after being pumped out,” Creech says. “We determined that using a wellpoint dewatering system would be the best solution to control the water table issues.”

Thompson Pump designed a 780-ft wellpoint dewatering system with 117 wellpoints, which would encompass the clarifiers and also have some effect in the area where the oxidation ditch was constructed.
Dewatering using wellpoints involves installing a series of small wells (wellpoints) into the ground to a pre-determined depth, as determined by the pump specialist. A wellpoint, constructed of either PVC or galvanized steel, is driven into the ground with a high-pressure jetter at a pre-determined distance between each one to the depth at which the water table must be lowered.

In this case, the wellpoints were installed 6 ft apart around the entire perimeter of the two clarifiers with the exception of a ramp area for vehicles entering the excavation. The wellpoints were then connected to the suction header pipe, which is a manifold to collect the water from all the wellpoints. The header pipe was connected to a Thompson Pump 12-in. rotary electric wellpoint pump, which handles a large amount of air and water simultaneously to effect a lowering of the ground water table. The 12-in. pump is capable of pumping 3,000 gpm.

The pump ran continuously until the water table was lowered well beneath the subgrade elevation of -8.50 ft. The discharge water was channeled through a filter bag to prevent silts from escaping the worksite. Once all this was accomplished, State Utility Contractors could begin construction of the underground structures.
“Once we started excavating, if we didn’t have the water down below our subgrade to where we needed it to be, we would have been in sand and water,” said State Utility Contractors field superintendent Jimmy Coore. “But having the dewatering system pump the water down below our subgrade before digging allowed us to work in dry soil.”

Coore further notes that having the dewatering system in place before starting was critical. “It is very important to have the proper dewatering system,” he says. “If we didn’t have it, there would be time delays, additional costs and the conditions would be unsafe to work in.”

Why Wellpointing

Wellpointing 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 the sides caving in creating a much larger hole than needed does not create a problem for you,” 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 on for about eight months and is expected to continue into early 2009, when construction of the clarifiers and oxidation ditch is 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.  

Sharon M. Bueno is managing editor of Trenchless Technology.
// ** Advertisement ** //

See Discussion, Leave A Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.