Basic Dewatering Principles For Construction

Basic Dewatering Principles For Construction

Planning/Design (Before You Pump)Planning/Design (Before You Pump)

•Obtain All Permits

•Separate Stormwater Runoff From Pumping Water

•Maintain All Erosion, Sediment, Turbidity BMPs

•Select Pumping System Carefully

•Design A Complete System

•Involve Environmental Agencies

•Have A Back-up Plan For Treating Water

Pumping TechnologiesPumping Technologies

•Sump Pump (Rim Ditch)

•Sock System (Horizontal Well)

•Well Point System

•Deep Well System(Vertical Sock)

Insert design picture hereSump Pit / Rim Ditch

Well-point

Pump

Sock / Horizontal Well

Well Point Dewatering System –

Well Point Dewatering System –

A well point dewatering system is the most versatile form of lowering water levels in excavations, creating a dry and stable working environment

It consists of following – •Arrow of several well points, installed along periphery of foundation pit.•Riser pipes.•Swing joints.•Header pipelines.•Pumps & discharge pipes.

•A well point system consists of a closely spaced series of small-diameter shallow wells. •The well points are connected to a common headermain and are pumped with a high-efficiency vacuum dewatering pump.•The well points are usually installed using a jetting tube and pump. •The two combined are a rapid and cost effective method of installing disposable well points; the use of filters reduces undermining and produces clean discharge water, this system can be designed to suit most excavations.

PROCEDURE

•Excavations for basements and foundations of Buildings•Foundations of Bridges & Underground tanks•Laying of deep sewer lines•Excavations and lining works for canals and channels etc•Tunnel work•Sub-ways (Tube Railway etc.)•Reclamation of land•Water supply

Uses/Area of application –

•Effective, economical for construction sites where deep well dewatering is not suitable.•Well point dewatering is used to reduce the hydrostatic pressure off the back of the sheet pile wall, in conjunction with lowering the water table to below the maximum excavation depth required for the construction site.•Excavation in dry is less expensive than in wet soil.•Lowering the ground water table permits earthwork excavations all year around.•Each well point can be excluded from the general operation of the well point system, inspected & repaired.

•Advantages -

•The initial installation cost is relatively high.•Replacement cost in case of failure is high.•If well is not properly gravel-treated; there is a danger of sand being pumped into the well point.•The construction site is jammed with pumps, pipes, & associated equipments.

Disadvantages –

The deep well system is a versatile pre-drainage dewatering system which can pump high and low volumes of groundwater

Deep Well Dewatering system –

•Deep well systems consist of one or more individual wells, each of which has its own submersible pump at the bottom of the well. •Such wells are used for depths greater than about 7.5 m.•These wells operated successfully at depths greater than 100m.•There is a cased borehole having diameter about 300mm.•Inner well casing is inserted after completion of borehole.•A perforated screen is provided where dewatering is required.•Graded gravel is placed between well casing & outer borehole.•Outer casing is withdrawn in stages.

PROCEDURE

•This method is best suited to homogeneous aquifers that extend well below the bottom of the excavation.•The Deep well system is employed to lower groundwater levels to provide stable working conditions in excavations. •Deep well dewatering systems are used when large volumes of groundwater must be pumped, highly permeable soils result in rapid recharging of the aquifer, and/or the depth of excavation excludes other dewatering methods.

Uses/Area of application -

Ability to penetrate strata impervious to the jetting method of wellpoint systems.

Installation of up to 100 feet deep or more in a single stage.

Capable of pumping tens to thousands of gallons per minute per well.

Deep Wells can be effective when placed outside of the jobsite work area.

Advantages –