The Lowdown on Groundwater Drawdown

Wellpoint dewateringIn any underground construction project, accounting for groundwater is a must. If groundwater is present, dewatering is commonly used to lower the groundwater table and ensure a dry and safe working environment.

Wellpoint dewatering is the process of temporarily removing groundwater from soils in a localized area to accomplish the construction of a pipeline, foundation, or any other below-ground structure — or to perform soil remediation. Dewatering by wellpointing, is considered one of the oldest methods of practical pre-drainage.

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Common Wellpoint Dewatering Applications
• Remediation of land
• Reclamation of pollutants
• Excavations for canals or channels
• Laying of water, sewer or utility lines
• Excavations for structures, basements and foundations of buildings
• Foundations of bridges, dams, power houses & underground tanks

A Wellpoint System
A series of shallow wells known as wellpoints are installed at a pre-determined depth and spaced along a trench or around an excavation site. Dewatering experts apply several installation techniques such as: installing or jetting with high pressure water; installing the wellpoints by hand; using a casing or punch; or utilizing drilling methods such as augers or drill rigs.

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The vacuum produced by the pump draws groundwater through notches in the wellpoints, and then travels from the wellpoints through the swing joints into the header pipe to the pump. The groundwater is then discharged away from the site or to other processes to remove unwanted properties such as contaminants.

Dewatering Pumps Are Important
As water is collected, a large amount of air is collected as well. Wellpoint pumps such as a rotary, piston, vacuum and vacuum-assisted are designed specifically with high air-handling capability to handle groundwater, air and soluble gases. In wellpoint dewatering, control of air is important, as excessive air causes cavitation that considerably reduces pump efficiency.

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From filtering to remediation, pumps handle various harsh elements in an effort to keep potentially harmful liquids controlled. A site can be dewatered quicker by simply employing a larger volume pump. Furthermore, more than one pump can be added to a system.

Invention of the Rotary Dewatering Pump
In 1970, George A. Thompson of Thompson Pump & Mfg. saw an opportunity to adapt the rotary pump to handle wellpoint applications more efficiently. High adaptation allowed the rotary pumps to pump thoroughly even with a large number of air leaks in the suction system, subsequently revolutionizing the wellpoint dewatering industry. Rotary wellpoint pumps operate by rotating a pair of rotors inside the pump housing, creating a vacuum. Prior to Thompson’s redesign, rotary pumps were used primarily for moving heavy liquids in controlled conditions with clean fluids free of air and only in ideal suction conditions.

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Design Considerations
System design depends on many factors including: soil conditions, methods of construction, header elevation, water volume, pump and discharge location. Wellpoint dewatering is best suited for relatively shallow excavations in stratified soils and permeable sandy soils. The wellpoint system is utilized to dewater soils drained by gravity flows.

Single-stage wellpoint systems have an effective suction lift of 15 ft at sea level, and under particular circumstances, lifts can be increased up to 22 ft. Thus, groundwater drawdowns are limited to an average of 18 ft; however several wellpoint system stages can be utilized as the depths of dewatering are increased. Wellpoints can usually handle a range of flows from a fraction of a gallon per minute to 100 gallons per minute.

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Professional dewatering and pump companies are available for complete groundwater control services including site investigation, design, calculations, and comprehensive subsoil knowledge, along with recommending and installing appropriate dewatering systems.

Wellpointing Advantages
• Cost effective
• Temporary or permanent installation
• Readily available materials
• A wide range of applications
• Can dewater and stabilize fine to silty soil conditions
• Various lengths available for monitoring specific zones
• Versatility of regulating well depth and vacuum amounts

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Wellpointing Limitations
• Suction lift limitations
• Only applicable in certain soil conditions
• May require use of a drill rig or other machinery for installation
• Cannot be used in soil with boulders or cobbles
• If remediating, wellpoints may carry down contaminants if not properly installed

Wellpoint Dewatering Applied
Thompson Pump was contracted to provide site dewatering of a coal yard for the construction of a tunnel for a coal-powered power station in the southern United States. To create a dry foundation for construction in a localized area with permeable soil conditions, a wellpoint dewatering system was recommended and employed. A Thompson Pump 8RW, an 8-in. rotary wellpoint dewatering pump, was utilized along with over 250 ft of header pipe to complete the system. The 8RW pumped up to 1,550 gallons per minute with vacuum up to 29 Hg and air handling up to 275 cfm. The highly efficient and low fuel consuming pump, coupled with an auxiliary fuel tank, allowed for continuous operation to complete construction of the tunnel.

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An additional Thompson Pump 4DD, a 4-in. double diaphragm pump, was kept on hand for potential heavy rainfalls.

This article was contributed by Thompson Pump. 

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