Services
DEWATERING & GROUNDWATER CONTROL
DFI/CSCE 13th Annual Geotechnical Seminar: Earth Slope Engineering
September 17th, 2010
Dam Safety 2010
September 19th, 2010
The 10th International Workshop on Micropiles
September 22nd, 2010
Beacon Hill Station, Seattle, WA: Deep well dewatering to depths of 190 feet to drain and stabilize granular deposits in advance of SEM tunneling.
Predrainage Dewatering
Predrainage, the lowering of the groundwater table below the proposed construction subgrade, is often a requirement to allow deep excavation or soft ground tunneling operations to be accomplished in the dry. The primary tools of predrainage are deep wells, wellpoint systems, and/or ejector systems. The selection of the optimum system is site specific and a function of the characteristics of the soils and groundwater as well as the needs of the project.
Deep Wells
A deep well is, quite simply, a dewatering device equipped with its own submersible pump. Deep wells can vary from 3 to over 24 inches in diameter, be installed from 20 feet to hundreds of feet deep, and pump from fractions of a gallon to thousands of gallons per minute. Deep wells are best suited to homogeneous aquifers that extend well below the bottom of the excavation.
Wellpoint Systems
The wellpoint system is the oldest method of practical predrainage and has been in general use for more than 80 years. The basic components are the wellpoints themselves, a shared wellpoint pump and associated header piping. Wellpoint systems have been installed requiring thousands of wellpoints and multiple pumping stations. 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. In practice, wellpoints are best suited for relatively shallow excavations in stratified soils.
Ejector Systems
An ejector system is an adaptation of the residential jet pump arrangement whereby multiple wells can be powered by a single pumping station. 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. 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.