Temporary Dewatering Systems

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a PUMPS & SYSTEMS repriint www.pump-zone.com MAY 2011 1

Mention dewatering to anyone outside of the under-

ground construction trade, and typically, with ablank stare, theyll ask what is D-watering? Mention

dewatering to anyone in the underground construction trade,and theyll likely state that it is one of the most importantphases in the project, as a well designed and installed dewater-ing system will allow them to complete their project in a safeand timely manner.

Groundwater exists at various depths below grade (groundlevel), and in simple terms, dewatering is the lowering of theexisting water table so that work can be done below this depth.Te lowering of the groundwater table can be achieved by dif-ferent types of dewatering systems and the type of installation

is directly dependent upon the soil type and the soils perme-ability (ability to pass water through pore spaces).

Soils can range from: Highly permeable, such as

coarse sands and gravel that will allow for thepumping of high volumes of groundwater

Low permeability, such as silts with lower groundwaterpumping ability

Impermeable soils, such as clays

Te type of system or systems installed will vary dependingon the excavation depth, size and type or types of soil presentat the jobsite. When it comes to dewatering, one size does notfit all.

Generally, when a project is going to be designed and

built, if there is any excavation below grade, a geotechnical firmwill gather all the necessary geotechnical information, which

Temporary Dewatering

SolutionsJerry Soto, Griffin Pump and Equipment, Inc.

Construction projects require different pumps and systemsfor safe and timely dewatering.

This project (CRC Rice University) consisted of a multistory building next to the Rice University campus inside Houston Medical Center. The

pump company installed a 1,400-linear-foot eductor system to lower the groundwater below the deepest cut (40 feet deep from the street

level). These eductors were 65 to 80 feet deep and drilled outside the excavation area along the property line. The eductor system with two

pump stations has lowered the groundwater more than 5 feet below the deepest excavation.

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Mining & Dewatering

will include exploratory bor-ings to get, among other items,a soils profile and classification,at which time, the ground waterlevel is also measured and noted.Te firm will also gather infor-mation to determine the feasi-bility of building the project.

Te dewatering contractorwill then take this informationto design a system that will beappropriate for the soil type andfor project being built.

Te soil type, grain sizeand associated permeability willdictate the type of system to beinstalled and the well or well-point centers (distance between

wells/wellpoints).Low permeable soils will

require close spacing, whilehighly permeable soils allowfor greater spacing. A reputabledewatering contractor will be intune with the work to be performed so that does not get in the

way of the construction work being done.

Types of SystemsTe dewatering contractor will decide which of the systems dis-cussed in this section will be most suitable for the project.

Wellpoint DewateringA wellpoint dewatering system is one of the most versatile sys-tems because it can be used in a range of soil types. It will gener-ally be installed at a jobsite in which the excavation is relativelyshallow (no more that say 15 feet to 20 feet) as it is dependenton the suction lift capacity of the pump. Excavations deeperthan the suction lift capacity of the pump and system can beachieved with a second (or multiple) tiered system below thefirst system.

A wellpoint system consists of wellpoints (typically a pieceof 1-1/2-inch or 2-inch slotted or screened perforated pipe that

allows water to pass through) that are installed into the groundby jetting, self jetting or drilling into place. Tey are generallyspaced between 3 to 15 feet apart, depending on the soils per-meability below grade, to achieve a curtain that will interceptand remove groundwater.

Te wellpoints will have a pipe connected to them up tothe surface where they will be connected to swingjoint witha valve for tuning the wellpoint. uning a wellpoint is regu-lating the flow from each wellpoint to reduce or eliminate theintrusion of air as the groundwater level is lowered closer to the

wellpoint. Te swingjoint is then connected a common collec-tor or header pipe via a swingjoint with a valve for tuning

the wellpoint.

Tis header pipe is then connected to a specialized well-point pump. Te pump itself is a specialized piece of equip-ment as it needs not only to handle the groundwater, but alsohandle larger volumes of air that is present in wellpoint systems.Tis air handling capability is generally achieved by the use ofa vacuum unit and large air/separator chamber. As the air andgroundwater enter the separation chamber, the air is removedby the vacuum unit, and the water is pumped out by the pump.

Rotary lobe, diaphragm pumps and piston pumps that donot require a vacuum unit can also be used as wellpoint pumps.However, the head and flow capacities are usually significantlyless than conventional wellpoint pumps. Pumps equipped with acompressor/venturi or diaphragm priming devices generally willnot be effective wellpoint pumps because they have insufficientair handling capacities or dont have a large enough air/waterseparator. If the air/water separator is too small, it will not effec-tively remove the air causing a detrimental effect on the overallpumping system. oo small of an air handling capacity will notallow the system to pump continually because it will lose prime.

Some variations of wellpoint systems are vacuum can well-point systems, draft wells and vacuum wick wellpoint systems.

Sock DewateringA sock dewatering system is generally used in shallow excava-tions with low ground water flow applications and in area with-out underground utilities or obstructions. Sock dewatering sys-tems consist of a perforated tube covered by a filter fabric that isinstalled in a trench. Te depth is limited to the depth that thetrencher or other machinery can reach, which is generally 20feet with some going a bit farther. Te trench is backfilled overthe fabric cover tube, and a pump is connected to the tube and

Installation of the dewatering system just completed to bypass the Galena River in LaPorte County,

Indiana, just south of the Michigan border. The 6-inch, variable-use, non-clog pump was used to sump

out the jet water. Being environmentally friendly, some stray fish were trapped inside the cofferdam

and were hand removed. The construction worker shown is looking for fish.


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groundwater is pumped out. A variation ofa sock system is French drains.

Eductor DewateringAn eductor dewatering system is installedinto soils with lower permeability and onexcavations that are deeper than a single-tiered wellpoint system can effectively drawdown the groundwater (generally excava-tion depths greater than 20 feet). Whilethe typical installation for a single eductormay have originally been designed to befor a single homeowners water well, it hasbeen incorporated into an effective dewater-ing method for deep excavations with lowerpermeable soils.

An eductor dewatering system consistsof wellscreens (typically a 2-inch to 4-inch

diameter piece of slotted or screened perfo-rated pipe that allows water to pass through)that are installed into the ground by either

jetting, self jetting or drilling into place.Tey are generally spaced 3 feet to 15 feetapart depending on the soils permeabil-ity below grade to achieve a curtain thatintercepts and removes groundwater. Te

wellscreen will have a solid pipe connectedto it, up to the surface to create a well. Intothis well, the eductor will be installed to therequired depth. Te eductor itself consists

of a special casted body into which a nozzleand venture combination is installed.

Te eductor will have an inlet for highpressure water, and an outlet for the returnof the water mixed with the groundwater. Atthe surface, there will be two pipelines. One will be the driv-ing (pressure) pipeline which will have an outlet at each educ-tor well site. Tis pipeline will be connected to a high pressurepump that will provide the water needed by the eductor at highpressure.

Te second pipeline is the return pipeline, which will havean inlet from each eductor well site and will be connected to a

recirculating tank. Te recirculating tank will be connected tothe high pressure pump, to recycle the water to the pump, andalso from which the extracted groundwater will be removedor pumped off. In the eductor system, water from the recircu-lating tank is pumped at high pressure into the eductor. Te

water is directed through the nozzle into the venturi diffuser.Tis combination creates an area of low pressure, into whichgroundwater flows, and is pushed (pumped) to the surface andthen to the recycling tank.

Deep WellA deep well dewatering system is generally installed in soils

with higher permeability and a potential to pump significantlyhigher groundwater volumes per well than could be pumped bya wellpoint or eductor. A deep well dewatering system consistsof wellscreens (4-inch diameter and larger depending on thevolume to be pumped) that are installed into the ground byeither jetting, self jetting or drilling into place. Te wellscreenmay extend to the surface or will be attached to a solid piece ofpipe that extends to the surface to create a well.

Due to the more permeable soils, well spacing is gener-ally farther apart than would be in other types of dewateringsystems and could vary from a few feet to over a hundred feet,depending on the subsurface soils permeability. An electricsubmersible pump is installed into each well with a dischargepipe attached to the surface. Electrical cable to provide powerfor the pump will need to be brought to each well location.Discharge from each well will be connected to a discharge pipeand directed to the discharge point.

Ground FreezingA ground freezing dewatering system is used in low permeable

This construction project was located in San Diego County for the city of El Cajon. The

dewatering system at this excavation consisted of a drilled well system installed around

the perimeter of the excavation and pumped by a series of electric submersible pumps.


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soils that will not drain using conventional methods and will generally terminateinto an impermeable material such as bedrock to form a bottom seal. ypically aground freezing system consists of an array of freeze pipes that are installed into theground around the perimeter of the excavation, usually in a circular pattern. A supercooled brine solution is pump through to freeze the pipes, which freezes the waterbearing soils around the pipes to create a frozen wall. Extreme care must be taken tomake sure that the freeze is complete because any groundwater seepage though the

wall or from below the excavation depth will have a catastrophic effect. Because ofits complexity and cost, it is not widely used as a conventional dewatering system.

Slurry WallsSlurry walls are not a dewatering type of system but rather a method of creating a

bath tub to prevent groundwater infiltration into a project. A trench is dug in animpervious layer (typically to bedrock) and filled with a slurry generally consistingof bentonite and water to create a water tight wall. Te slurry wall will be builtaround the project after which any water trapped within the Bathtub can be sumppumped out. A variation of a slurry wall is pressure grouting.

ConclusionA construction project may use one or a combination of the dewatering systemsdescribed in this article, as some projects sub-surface soils can vary greatly andrequire the use of multiple systems. It would not be uncommon to see a deepwellsystem used in conjunction with a wellpoint system as each would be aimed at adifferent soil type.

Te types of dewatering systems may seem straight forward. However, dewa-tering should be completed by a contractor specialized in construction dewateringas only experience will ensure that a project is completed on time and in a safemanner.

P&S

Lake Fork Pump Station (east of Dallas, Texas). Deep wells were installed at an

over 65-foot excavation on Lake Fork.

Jerry Soto has more than 25 years experience in pumps and their applications.He is involved in installations, shop testing of electric submersible pumps, design,estimating, project management of groundwater control systems and bypass pump-ing operations, as well as handling the logistics and coordination of Griffins fleet of

pumps for large projects nationwide. He can be reached [email protected].

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