If you're working anywhere near water, and especially near the coast, dewatering a site prior to construction tends to be an integral part of the project. In the Hampton Roads area of Virginia, where the water table can be less than three feet below the surface, wellpointing is often used to control the ground water at construction sites. Bryant Baker, branch manager of Thompson Pump in Chesapeake, Virginia, and his team have become experts at wellpointing and assisting contractors with this critical part of many projects.
Dewatering by wellpointing involves installing a series of small wells (wellpoints) into the ground. These wellpoints are constructed of either PVC or galvanized steel pipe about 1.5 inches in diameter. The wellpoints are driven about 3 feet to 6 feet apart to the depth at which water must be eliminated or the water table must be lowered. Each wellpoint is connected to a common PVC header pipe up to 6 inches in diameter. The header pipe is then connected to a pump with a large amount of vacuum power. The vacuum power produced by the pump sucks the ground water up through thin slits cut into the walls of the wellpoints. The water then travels to the header pipe, through the pump and is discharged through an outfall line away from the site.
"One of the things I've learned is that you can't underestimate the need for dewatering," says Baker. "There have been contractors who have lost (multimillion) dollar jobs because they failed to put dewatering into their bid."
Of course, it is not just a matter of setting up a wellpoint system; it can get complicated. Baker points out that "working with a geotechnical report is very important when doing any dewatering. If you don't look at the report, you can get on site and lose your shirt on the project because of unexpected conditions."
For example, clay and rock on a site can cause problems. Sandy soil is, of course, ideal for dewatering, but you can dig down a few feet and suddenly encounter clay. Why is this important?
"Water can't percolate through clay," Baker explains. "It's like trying to suck water through a plastic bag; it just isn't going to work."
When Miller & Long Concrete Construction Company and project superintendent Nick Gaston experienced clay on their project, the Hampton University Proton Therapy Institute (HUPTI) in Hampton, Virginia, Thompson Pump helped them design a wellpoint system with an additional sump pump at the bottom of the excavation to keep the water out of the hole.
On this project, Thompson Pump set up a 1,000-foot perimeter with 196 wellpoints. The water table was approximately 6 feet down; at a depth of approximately 23 feet, Miller & Long encountered clay and there was a problem with water in the hole because of it.
"A lot of contractors think that just installing a dewatering system will solve their problems," says Baker, "but if there is clay you will need both a wellpoint system and a small sump pump in the bottom of the hole.
"We tell contractors to slope the grade at the bottom of the hole and place gravel at one end. The water will naturally go to this end of the hole, and it can be pumped out of one location. This eliminates water throughout the hole."
On this project Baker points out that they actually buried a portion of the dewatering system, which is uncommon on a project.
"We had to bury the header pipe after it was hooked up to the wellpoints. There were so many cranes and equipment that needed access, and because of the soil conditions they did not want to put in any ramps. We buried the header pipe underground, put in 45-degree elbows and made a ledge where that one side of the wellpoint system actually went down into the ground."
When referring to the geotechnical report to plan for dewatering on a project, it is also very important to consider when construction will actually start on site. Soil borings and testing for the geotechnical report are often done months before construction is actually scheduled to begin, and ground conditions can change dramatically during that time.
For example, if the geotechnical report is generated in the summer, which is a dry season, actual construction may not begin until spring, which is a wet season. "If you do your borings in the summer, you might not hit the water table until 21 feet, and you may be only planning to go 18 feet," explains Baker. "But if you're not starting construction until the rainy season, the water table is going to rise. You need to plan for an additional 5 to 6 feet on the water table. This means you need to wellpoint the site — even though the geotechnical report doesn't indicate that you need to do so. Use the geotechnical report as a guide, but understand that the seasons will greatly affect the water table."
Pizzagalli Construction Company is using several wellpoint systems at the Atlantic Treatment Plant in Virginia Beach, Virginia, which is in the midst of an expansion and upgrade. (See "Atlantic Treatment Plant Expansion," Construction, October 23, 2006, at www.constructionequipment.com.) At this site, Shane Snow, Pizzagalli project superintendent, explains they hit the water table only 18 inches underground. A conglomeration of many small wellpoint systems, most averaging 300 feet to 400 feet in length, is in place and continuously operating to remove the ground water.
Pizzagalli is also using sheeting on this job to hold back the sand while a continuously operating pump keeps the water out. Although it is not the case on this project, Baker points out that a wellpoint system can also be used with sheeting to eliminate water infiltration and reduce pressure on the sheeting, which can cave in due to the weight of the water.
"If you have sheeting around the perimeter of your excavation, you still may be better off with the addition of a wellpoint system," says Baker. "The wellpoint system is installed around the exterior a few inches away from the sheeting. This takes the water pressure off of your sheeting. If too much water pressure builds up behind the sheeting, it can cave in."
As the experts at Thompson Pump point out, ground conditions play a major role in every project, and knowing how to overcome soil condition problems can mean the difference between a successful and an unsuccessful project.