Best Practices: Find the Right Dewatering Strategy

As technology expands options, now is a good time to consider dewatering tactics.

Best Practices: Find the Right Dewatering Strategy

Crews from a project in Michigan excavated swampland, replaced it with compacted sand and set up a wellpoint system before excavation for the project could begin.

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Dewatering is a necessary evil in construction. “Keeping a dry hole is definitely required and necessary to maintain a smooth construction schedule,” says Ken Albaugh, who leads the dewatering commercial team in North America for Xylem, a global water technology player.

Dewatering lowers groundwater levels within the vicinity of the excavation. But a one-size-fits-all approach to dewatering a construction job site simply won’t work. “Each site is truly a unique environment,” Albaugh says. 

Paul Schmall, vice president of Keller Specialty Services with Keller North America concurs. “A site-specific evaluation, including historical records if available, and a local understanding of the groundwater behavior is vital to determine the optimum solution,” he says. “If groundwater control is a critical path activity, this is not a good setting for do-it-yourselfers and certainly not for first-timers. Experience is paramount.”

Sump pumps

Sump pumps are a common dewatering solution for construction sites. Groundwater enters the site and is collected in a sump or pit and removed using powerful pumps that can handle not only water, but soil and other solids.

“The contractor has to know how much water is coming into the hole, and how fast,” says Tony Chirico, national sales manager for Gorman-Rupp. “That will help narrow down the size of the pump required for the application along with calculating the total dynamic head.”

The type of material you are pumping will influence the type of pump used as well as the pumping distance. “For example, when pumping water mixed with sand or silt you might want to choose a pump with hardened components or coatings,” Chirico says. 

According to Chirico, there are three primary types of pumps used in the construction market: self-priming pumps (wet prime), prime-assisted pumps (dry prime) and submersible pumps. Self-priming pumps require water to prime and reprime, while prime-assisted use an external priming device (compressor/venture, diaphragm and vacuum style) to prime and reprime. “Prime-assisted (dry prime) is typically a product of choice for most rental companies and contractors alike because of its ease of priming and run-dry capability,” Chirico says.

Wellpoint systems

Some sites may require 24/7 pumping if the water table is higher than the excavation subgrade. On these sites, the water level can be lowered to create a safe, dry environment for excavation using wellpoint pumps and piping. A wellpoint system features wells which are spaced around an excavation and connected to a common header, which is attached to many more wellpoints. According to Schmall, they are best suited to relatively shallow excavations (less than 15 feet) and can be installed in a wide range of soils including sands, gravels, silts, soft clays and stratified soils.

Deep wells

Deep well dewatering is typically utilized with deep excavations, where the height of static water above subgrade is greater than the typical 17-foot suction lift limitation for a wellpoint system. Tunnels and dams are good examples of deep dewatering work.

Wells can vary in size from 3 to 24 inches in diameter and range in depth from 20 feet to several hundred feet. Each deep well holds a submersible pump. By operating deep wells around the vicinity of the excavation site, the groundwater can be lowered without equipment in the hole, a major advantage for the general contractor.

Cut-off

Another dewatering method that can sometimes meet requirements is installation of a cut-off where a barrier excludes water from the excavation area.

“Cut-offs can take several forms such as barrier walls made from steel sheeting, chainsaw trencher soil mix walls, conventional slurry trenching, or diaphragm walls for deep excavations,” Schmall says. “Jet grouting and permeation grouting are also options.”

Dewatering expertise

According to Schmall, any project where there is no tolerance for a hiccup, or when multiple options are available to achieve the desired result, should involve a ground engineering specialist.

“An experienced groundwater control engineer can also weigh the options of dewatering versus groundwater cut-off such as steel sheet piling or a soil mix barrier wall.”

Xylem’s services cover a wide spectrum, and depending on client needs, it will provide anything from rental equipment to complete turnkey services. “I’m seeing growing demand for turnkey solutions,” Albaugh says. “Anybody can rent a pump. We try to be that partner that will give them a solution on how to dewater at an efficient cost.”

No matter what dewatering system is utilized, contractors need to be mindful of local discharge requirements and regulations. Water quality can be changeable too depending on site activities.

Technology is allowing contractors greater control over releases of dirty water. “We can now put sensors into the pipelines to check for pH and turbidity,” Albaugh says. “We can change the turbidity to make sure we are not releasing dirty water.”

Connected technology

Smart technology now allows contractors to monitor pumps remotely. “We can see when we have a dry or wet hole from our smartphone,” Albaugh says. Weather monitoring systems can trigger the pump to speed up or slow down to meet the conditions.

“Contractors want a reliable pump without too much maintenance, and they want to get it started without any additional resource,” Chirico says. Remote access through telematics allows them to easily check to see that pumps are properly maintained and fueled. Rental companies can see where their assets are located and how many hours they are being run.

According to Schmall other advances that the ground engineering community has seen in recent years include the effective combined use of partial cut-off and pumping as well as innovation in drilling techniques. It is now possible to install angled wells, horizontal wells, wells from inside tunnels, from basement interiors, and to drill wells into rock and difficult conditions.

With technology expanding your options, this may be the year to look at strategies to dewater your job sites with an eye toward improving efficiency, reducing pump maintenance costs and keeping projects on track.

ABOUT THE AUTHOR

AEM is the North American-based international trade group representing off-road equipment manufacturers and suppliers, with more than 950 companies and 200-plus product lines in the agriculture and construction-related sectors worldwide. AEM has an ownership stake in and manages several world-class exhibitions, including CONEXPO-CON/AGG. 



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