A three-acre site has been put in place for the constructionof an Oriented Strand Board (OSB) Plant in Fairfax, South Carolina, USA. As part of the development of the site, Wabi America, the general contractor, worked with dewatering expert Thompson Pump on an extensive, five-month dewatering project. Don Polzin, Thompson Pump sales representative from the company's Charleston, branch, worked closely with general contractor Wabi America to prepare the site for constructionof the plant. It needed to be a concrete cast in place structure spanning 40 ft wide, 150 ft long, 6 ft thick and containing 300,000 lbs of steel. Before this extensive foundation could be built, an area 52 ft below natural ground elevation had to be cleared. Thompson Pump worked in four stages, utilizing 13 pumps and pumping an average of 1.3MGD.
The first stage of the project took three days to install and utilized two, 550 ft by 8 in wellpoint systems with one 12 in rotary pump on each system and one backup pump. Within a week, the area was 18 ft deeper. As this first system was installed, Don Polzin was instructed to discharge the water to a ditch over a quarter mile away from the press pit. Instead of running individual piping for each of the stages into the ditch, he created a collection pit 400 ft away from there using a Thompson 8 in jet pump to discharge the water 1200 ft through a 6 in Thompson Galvanized Pipe line to the ditch.
Stage two involved installing two 450 ft by 8 in systems with one Thompson 12 in rotary pump on each system and a backup pump. After a week during which the stage two pumps ran throughout, the area was another 18 ft deeper, totaling 36 ft of clearance in 10 days.
Stage three began successfully with two 350 ft by 6 in systems utilizing another 12 in rotary pump on each system and one backup. Despite being benched down to a depth of 36 ft, subject to 36 ft feet of static head, and discharging through 500 ft of Thompson galvanized pipe, these pumps were quickly reaching the original target depth of 47 ft.
An unexpected challenge
At 44 ft deep the contractor was faced with an unexpected challenge. A soft, impermeable layer of clay, 5 ft thick, was shelving water past the wellpoints and providing severe instability for the banks of the final cut. It took time to install more shallow points, jet in some sand wicks, and find a successful solution. Thompson installed a rocked French drain around the grade perimeter of the final slope and placed a 4 in vacuum-assisted pump and backup pump to keep the French drain sumped out at its lowest point. The clay layer made the centre of the excavation floor too soft and unsuitable for building. To remedy this, a fourth stage was installed that solidified the excavation floor. Utilizing one Thompson 12 in rotary pump with 50 ft of static head and 540 ft of discharge piping, stage four had the area ready for building within a week.
After the area was cleared to 52 ft below natural ground elevation, a 2 ft layer of stone was poured across the entire excavation floor and a 1 ft mud mat was laid. Six weeks later, the stage four system was buried in concrete and the pump was removed. After nearly five months, 195 million gallons of water was pumped through a 3,150 ft wellpoint system and 3,600 ft of Thompson galvanized pipe to complete the foundation of the OSB Plant.
