Dewatering

Construction dewatering, the temporary lowering of the groundwater, has been practiced by Keller for more than 90 years and the company is recognized as the industry's preeminent dewatering contractor and an innovator in the field.

Common uses

Groundwater control

Pre-drainage for excavations

Dam rehabilitation

Expansion of existing structures

Stabilizing wet fly ash and bottom ash ponds

Process

Under some circumstances, artificial recharge can be used to minimize the potential side effects of excessive groundwater lowering during construction.

Groundwater control can also be provided by the installation of subsurface barrier walls or by ground modification methods. In addition to construction applications, groundwater control methods can be used for environmental containment and structural seepage control or stabilization of waste products such as coal combustion residuals.

There are three primary dewatering systems.

Deep wells

Since the installation of its first deep well system in the mid-1950s, Keller has successfully engineered numerous dewatering projects to allow excavation in the dry, including the largest single dewatering effort ever accomplished in the United States for the construction of Lock & Dam 26 across the Mississippi River. This depth of experience and engineering expertise is unmatched by any other dewatering contractor.

A deep well is a dewatering device equipped with its submersible pump. Deep wells can vary from 3 to over 24 inches in diameter, can be installed from 20 feet to hundreds of feet deep, and pump from a fraction to thousands of gallons per minute. Deep wells are best suited where permeable soil extends well below the bottom of the excavation. Deep wells are equipped with individual submersible pumps. Unlike well points, deep wells are not limited by suction and can be installed from 20 feet to hundreds of feet deep. Since each deep well involves a higher unit cost than other pre-drainage systems, deep wells are best suited to homogeneous aquifers that extend well below the bottom of the excavation.

Deep well dewatering in conjunction with well points is also proving to be successful in stabilizing wet fly ash and bottom ash ponds at existing or former coal-fired generating stations for fast, safe, and effective clean closure in compliance with EPA regulations.

Deep wells can be constructed by several different methods including jetting with pressurized water, direct rotary drilling, bucket drilling, and duplex drilling. Moretrench determines the best installation method based on site geology and other project specifics.

Well points

The wellpoint system is the oldest method of predrainage. Over the last 90 years, Keller has completed numerous wellpoint dewatering projects and is recognized as North America's leading wellpoint contractor.

The basic wellpoint system consists of the well points themselves, header piping, and a centrally located pumping station serving the whole system. Wellpoint systems are very cost-effective where close spacing is required. However, as a vacuum-type system, their effectiveness is limited by depth and available suction lift to 15-20 ft.

Well points are typically installed around the perimeter of the excavation by self-jetting methods, with a hole puncher used to penetrate coarse gravels and cobbles, and boulders. In difficult ground, penetration can be facilitated by various drilling methods.

Ejectors

An ejector system is somewhat of a hybrid system between well points and deep wells. Ejectors are typically used where the groundwater must be lowered more than 15 feet and the soil is of low hydraulic conductivity so that vacuum application is of benefit to improve soil drainage.

Unlike deep wells which are equipped with individual submersible pumps, an ejector system (also called an eductor system) allows multiple wells to be powered by a single pumping station. Single pipe and two-pipe ejector systems are available. The single-pipe system is typically used when the well casing is less than three inches in diameter, while the two-pipe system requires a minimum 4-inch well screen and casing. The two-pipe ejector is the simplest to install and can be easily removed for maintenance.

However, ejectors are inherently inefficient and their application requires accurate knowledge of site conditions and practical restraints.

Since they are not limited in suction lift and have a lower unit cost than deep wells, ejectors are well suited for deep excavations in stratified soils where close spacing is required. Single-pipe ejectors are installed using methods similar to well points while two-pipe ejectors are typically installed by mud rotary drilling techniques.