What is a Wellpoint Dewatering System?

What is a Wellpoint Dewatering System?

Wellpoint dewatering is one of the most effective and most widely used engineered dewatering methods for construction projects in challenging groundwater conditions. When simple sump pumping is not enough to keep an excavation dry a wellpoint system can lower the water table across a large area before and during construction.

Here is everything contractors and project managers need to know about wellpoint dewatering systems.

What is a Wellpoint System?

A wellpoint system is an engineered dewatering system that uses a series of small diameter wells installed around or within an excavation perimeter to lower the groundwater table below the excavation level before and during construction.

Each wellpoint is a small diameter perforated pipe — typically 1.5 to 2 inches in diameter — installed in the ground at regular intervals along the excavation perimeter. The wellpoints are connected to a common header pipe that runs along the surface. A high capacity vacuum pump connected to the header pipe applies suction to all the wellpoints simultaneously drawing groundwater up from the soil and discharging it away from the work area.

The result is a lowered water table in the vicinity of the excavation that allows dry working conditions without the need for continuous sump pumping inside the excavation.

How Does a Wellpoint System Work?

Installation — wellpoints are installed at regular intervals — typically 3 to 6 feet apart — around the excavation perimeter using a jetting rig that uses high pressure water to advance the wellpoint to the required depth. Installation is fast — an experienced crew can install dozens of wellpoints per day.

Connection — each wellpoint is connected to the surface header pipe with a swing connection — a short flexible hose with a ball valve that allows individual wellpoints to be adjusted or shut off.

Vacuum application — the vacuum pump connected to the header pipe draws suction on all wellpoints simultaneously. The vacuum helps overcome capillary tension in fine grained soils that would otherwise resist drainage.

Groundwater drawdown — as the system operates groundwater flows toward the wellpoints from the surrounding soil. The water table gradually lowers in the vicinity of the wellpoints creating a cone of depression that extends into the excavation area.

Continuous operation — the system runs continuously throughout the excavation and construction period maintaining a lowered water table until the structure is complete and dewatering can be discontinued.

When is a Wellpoint System the Right Choice?

Wellpoint systems are most effective in specific soil and site conditions:

Permeable soils — wellpoints work best in sandy and silty soils with good permeability that allow groundwater to flow freely toward the wellpoints. Gravelly soils also respond well. Clay soils have low permeability and do not respond well to wellpoint dewatering.

Shallow to moderate depth — a single stage wellpoint system can lower the water table approximately 15 to 18 feet below the pump. For deeper excavations multi-stage wellpoint systems or deep wells are required.

Large area dewatering — wellpoints are particularly effective for dewatering large areas where sump pumping would require numerous pumps spread across the excavation floor. A perimeter wellpoint system protects the entire enclosed area with a single coordinated system.

Slope stability requirements — lowering the water table with wellpoints improves the stability of excavation slopes by reducing pore water pressure in the soil. For deep excavations in saturated sandy soils this can be critical for slope stability.

Situations where wellpoints are not appropriate:

• Clay soils with low permeability — water cannot flow fast enough to make wellpoints effective
• Very deep excavations requiring more than 15 to 18 feet of drawdown without staging
• Rock formations where permeability is controlled by fractures rather than soil pores
• Sites where lowering the water table would cause unacceptable settlement of adjacent structures

Wellpoint System Components

Wellpoints — small diameter perforated pipes with a ball valve tip that opens during jetting and closes when suction is applied. The perforated section allows water to enter while a filter sleeve prevents fine soil particles from entering and clogging the system.

Riser pipes — connect each wellpoint to the header pipe at the surface. Typically small diameter flexible or rigid pipe.

Swing connections — flexible hose connections between the riser pipes and the header pipe with individual ball valves for adjustment and isolation of individual wellpoints.

Header pipe — the main collection pipe that runs along the excavation perimeter connecting all wellpoints to the vacuum pump. Sized for the total flow of the system.

Vacuum pump — a high capacity self priming centrifugal pump with vacuum capability connected to the header pipe. The pump simultaneously applies vacuum to draw water from the wellpoints and discharges the collected groundwater away from the site.

Discharge system — piping or hose routing collected groundwater from the pump to an approved discharge point.

Wellpoint vs Deep Well Dewatering

Wellpoints and deep wells are both engineered dewatering systems but they work differently and suit different conditions.

Wellpoints — shallow installation, close spacing, vacuum assisted. Best for sandy and silty soils at moderate depths. Lower cost per unit but more units required. Fast installation with jetting equipment.

Deep wells — large diameter drilled wells with submersible pumps. No depth limitation — can dewater very deep excavations. Works in a wider range of soil conditions. Higher cost per unit but fewer units required. Used when wellpoints cannot achieve required drawdown depth.

For most construction dewatering applications in sandy soils at depths of 15 to 30 feet wellpoints are the cost effective choice. For deeper excavations or more challenging soil conditions deep wells are the appropriate system.

Settlement Considerations

Wellpoint dewatering lowers the groundwater table which can cause settlement of adjacent structures founded on compressible soils. Before installing a wellpoint system on an urban site assess the potential for settlement impacts on nearby buildings, utilities, and infrastructure.

In sensitive urban environments the settlement risk from wellpoint dewatering may require mitigation measures or alternative dewatering approaches that do not lower the regional water table.

How Flowcor Equipment Supports Wellpoint Dewatering Projects

Flowcor Equipment sources wellpoint pumps, header pipe, and dewatering equipment for contractors and project managers across the U.S. Tell us your excavation dimensions, soil conditions, and required drawdown depth and we will recommend the right system and get you a quote within 1 business hour.

Submit a quote request at flowcorequipment.com or call us at 610-241-6770.