Multiple Stage Wellpoint Dewatering
Case Study, Staging Stability
When Thompson Pump Company was contracted to dewater three acres of land in South Carolina in late 2006, it expected to complete the task in three stages. But as the project neared completion, Thompson’s team ran into an unexpected obstacle below the surface and had to add another stage to its pumping process. Fortunately, the company was able to adapt to the site conditions and still successfully prepare the site within a five-month timeline.
Planning to Pump
At the project site in Fairfax, South Carolina, a team with Thompson’s Charleston, South Carolina, branch set out to clear and dewater land for a pressboard plant. The plant’s foundation was to be a 40-foot-wide, 150-foot-long structure containing 300,000 pounds of steel. But before this extensive structure could be constructed, an area 52 feet below natural ground needed to be cleared and dewatered. This would involve installing wellpoint systems in three stages.
“Wellpoints are shallow wells that go around the perimeter of the site. Each one is six feet apart, and they attach to a common header line,” explains Sales Representative Don Polzin, who headed the team that worked closely with site contractor Phillips & Jordan Construction of Myrtle Beach, South Carolina. “Then you put a big pump on the end, and it pulls from all pumps simultaneously and lowers the water table in that area. Realistically, you can only get about 18-foot out of a stage, so that’s why we had to do three stages.”
Installing the first stage took three days. The crew used two 8-inch wellpoint systems with two Thompson 12-inch rotary pumps and had one backup pump on hand in the event of a primary unit failure. Meanwhile, Phillips & Jordan’s crew used a Cat D9 dozer to push scraper pans across the site to excavate the dirt. After a week of pumping and excavating, the area was 18 feet deeper and ready for stage two. This stage involved installing two 450-foot-long, 8-inch systems around the perimeter with a Thompson 12-inch rotary pump on each system and another backup pump. After a week with the stage two pumps functioning, another 18 feet of earth was excavated, totaling 36 feet in depth.
Confronting the Conditions
Phillips & Jordan had initially planned for Thompson to complete a three-stage project at a target depth of 47 feet. Stage three started successfully with two 350-foot-long, 6-inch systems, again utilizing two Thompson 12-inch rotary pumps. Despite the fact that the pumps were placed at a depth of 36 feet and discharging through 500 feet of Thompson galvanized pipe, they quickly reached the original target depth of 47 feet. But, at 44 feet, an unexpected challenge was met: A very soft, impermeable layer of clay 5 feet thick was shelving water past the wellpoints and making the banks of the final cut unstable.
Thompson used a multi-pronged approach to find a successful cure for this situation. “First, we put shallow points in to catch water running across the clay layer,” Polzin explains. “Then we used sand wicks to puncture the clay layer and allow water to drain through.” Finally, the Thompson team installed a rocked French drain around the grade perimeter of the final slope. “We dug a ditch along the edge of the wall, filled it with rock along the edge and then put a pump at the lowest part. Whatever water makes it past the clay layer then falls into the rock.”
A Thompson 4-inch vacuum-assisted pump and one backup pump kept the French drain sumped out at its lowest point. Because the clay layer made the center of the excavation floor unstable and unsuitable for building, a fourth stage wellpoint system was installed to solidify the excavation floor. Using one Thompson 12-inch rotary pump to pump against 50 feet of static discharge head and 540 feet of discharge piping, stage four had the area dewatered and ready for building within a week.
After the area was cleared to 52 feet below natural ground elevation, a 2-foot layer of stone was poured across the entire excavation floor and a 1-foot mud mat was laid. Six weeks later, the stage four system was buried in concrete and the pump was removed. Twelve weeks after stage four was removed, the stage one pumps were pulled and removed.
In total, Thompson Pump’s team used 13 pumps and pumped an average of 1.3 million gallons of water a day. At the project’s completion, 195 million gallons had been pumped through 3,150 feet of wellpoint system and 3,600 feet of discharge pipe. The successful dewatering efforts made it possible for Phillips & Jordan to continue the project, knowing they could rest on a stable foundation.
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