DEWATERING TECHNIQUES

What is dewatering on a construction site?

• Dewatering is the term for the control of groundwater by pumping.

• On construction sites it may be known as ‘construction dewatering’. The method is also used on mine sites – ‘mine dewatering’

• The process of dewatering can be defined as – pumping from wells or sumps to temporarily lower groundwater levels, to allow excavations to be made in dry and stable conditions below natural groundwater level

WHY DO WE NEED DEWATERING ON CONSTRUCTION SITE?

• As we all know that for working at a construction site, it should be completely dry and the surface should be such that one can easily work .

• For excavation and for making basements of building dewatering the site, is must, for easy working of the people and for the safety of the people working.

• Dewatering is also necessary during the rainy season, when excavation work is to be done during the rainy season, for smooth and fast working dewatering of the site is must.

• As discussed before, even in mines dewatering is must , else the mine will be prone to accidents.

PURPOSE FOR DEWATERING

• During construction stage:1. Provide a dry excavation and permit construction to proceed

efficiently.2. Reduce lateral loads on sheeting and bracing in excavations.3. Improve supporting characteristics of foundation materials.4. Increase stability of excavation slopes and side-hill fills.5. Cut off capillary rise and prevent piping and frost heaving in

pavements.6. Reduce air pressure in tunneling operations.

• Post construction stage:1. Reduce or eliminate uplift pressures on bottom slabs and permit

economics from the reduction of slab thicknesses fro basements, buried structures, canal linings, spillways, dry docks, etc.

2. Provide for dry basements.3. Reduce lateral pressures on retaining structures.4. Control embankment seepage in all dams.5. Control seepage and pore pressures beneath pavements, side-hill

fills, and cut slopes.

HOW IT IS DONE?

Collection of wate

r

Pumping

Filtering and removing silt

and impuri

ties

Discharge at

proper

location

Source of unwanted water on construction site

• Sources of surface waterRainfallConstruction operations (E.G. Concreting, washing of plant)Seepage through cut-off walls.• Other source is groundwater

Methods of surface water control

• Source controlIntercept run-off before it reaches the excavationPrevent unnecessary generation of water in the excavationCollect water as soon as it reaches the work area (or before!)• Water collectionFrench drains to intercept run offCollector drains and sumpsPumping systems (keep it simple!)

Dewatering techniques

• Construction dewatering methodsThe design of effective construction dewatering methods should be based on number of information sources:-Geotechnical and groundwater site investigations undertaken at the commencement of any project;-Information provided by any geotechnical study undertaken during the concept design stage-Groundwater information source from relevant authorities-A site risk assesments

Widely used dewatering techniques

• Sump pumping• Well points• Deep wells• Ejector wells

SUMP PUMPING• A sump is merely a hole in the ground from which water is being pumped

for the purpose of removing water from the adjoining area as shown in fig.

• They are used with ditches leading to them in large excavations. Up to maximum of 8m below pump installation level; for greater depths a submersible pump is required.

• Shallow slopes may be required for unsupported excavations in silts and fine sands. Gravels and coarse sands are more suitable.

• If there are existing foundations in the vicinity pumping may cause settlement of these foundations. Subsidence of adjacent ground and sloughing of the lower part of a slope (sloped pits) may occur.

• The sump should be preferably lined with a filter material which has grain size gradations in compatible with the filter rules.

• For prolonged pumping the sump should be prepared by first driving sheeting around the sump area for the full depth of the sump and installing a cage inside the sump made of wire mesh with internal strutting or a perforating pipe filling the filter material.

ADVANTAGES AND DISADVANTAGES OF SUMP PITS METHOD

Groundwater lowering method

Advantages Disadvantages

Sump Pits 1. Widely used method.2. Most economical method for installation and

maintenance.3. Can be applied for most soil and rock conditions.4. Most appropriate method in situation where

boulders or massive obstructions are met within the ground. Greatest depth to which the water table can be lowered by this method is about 8 m below the pump.

1. Ground water flows towards the excavation with high head or a steep slope and hence there is a risk of collapse of sides.

2. In open or timbered excavations there is risk of instability of the base due to upward.

WELL POINT SYSTEM OF DEWATERING• The most common practice of dewatering used in ahmedabad is the

implementation of a well system. Wells are systematically drilled around the construction area and submersible pumps placed into these wells. This practice appears to work effectively for many projects, especially those building projects that require excavations for deep basements.

• Setup and Procedure of working:• A well-point is 5.0-7.5 cm diameter metal or plastic pipe 60 cm – 120

cm long which is perforated and covered with a screen. The lower end of the pipe has a driving head with water holes for jetting as shown in fig. Well-points are connected to 5.0-7.5 cm diameter pipes known as riser pipes and are inserted into the ground by driving or jetting.

• The upper ends of the riser pipes lead to a header pipe which, in turn, connected to a pump. The ground water is drawn by the pump into the well-points through the header pipe and discharged as shown in fig.

• The well-points are usually installed with 0.75m – 3m spacing. • In gravels spacing required may be too close and impracticable. In

clays it is also not used because it is too slow. In silts and silt – clay mixtures the use of well points are aided by upper (0.60m – 0.90m long) compacted clay seals and sand-filtered boreholes (20cm – 60cm diameter). Upper clay seals help to maintain higher suction (vacuum) pressures and sand filters increase the amount of discharge.

ADVANTAGE DISADVANTAGE OF WELL SYSTEM

Groundwater lowering method

Advantages disadvantages

Well systems

1. Effective in sandy soils2. Provide drawdown up to 5-6 meters in

sand and 4 meters in silty soil.3. Relatively cheap and flexible4. Installation is very rapid.5. Water is filtered and carries little or no soil

particles.6. There is less danger of subsidence of the

surrounding ground than with open-sump pumping

1. Not effective beyond 4-6 meters of drawdown

2. Might require stages of well installation.

DEEP WELL• Deep well systems consist of one or more individual wells, each of

which has its own submersible pump at the bottom of the well shaft. Such systems are particularly suitable. Where large volumes of water in highly permeable sand and gravel areas permitting rapid recharging of ground water from surrounding areas exist. The range of permeability under which the deep well system is applicable.

SPACING OF DEEP WELL POINT SYSTEM• Normally, individual wells are spaced at an approximate distance of 15 m (50

feet) apart. However, depending upon soil conditions and the dewatering plan the spacing may need to be just a few meters apart.

DEWATERING CAPACITY OF DEEP WELL POINT SYSTEM• Individual well capacities are from 2100 to 3000 gallons per minute and with

total systems the capacities can be as high as 60 000 gallons per minute. • Deep well pumps can lift water 30 m (100 feet) or more in a single stage and

the variation of the typical deep well system is a pressure within an aquifer.• Deep well points require no pump as the water is forced to the surface by its

own pressure. To boost the water flow a vacuum pump is frequently used.

ADVANTAGE DISADVANTAGES OF DEEP WELL SYSTEM

Groundwater lowering method

advantages Disadvantages

Deep wells 1. No limits on drawdown

2. Fewer wells required than well system

1. Expensive to install

DITCHES AND TRENCHES

• Ditches and french drains (commonly known as trenches in construction projects in ahmedabad) are also used on qatar’s construction sites.

• The ditches are formed in a pre-planned manner, allowing groundwater flow to surface in the deeper level trenches. Perforated pipes are then placed in the trenches, and groundwater extracted through these pipelines to be filtered. These trenches were particularly common in infrastructure network projects.

• Graded aggregates and geotextile layer to be used when laying out the pipelines.

ADVANTAGE AND DISADVANTAGE

Groundwater lowering method

Advantages Disadvantages

Ditches/French drains 1. Relatively inexpensive2. Controls shallow

groundwater and over bleed.

3. Effective in controlling groundwater after casting of foundations.

1. Possible obstruction of traffic

2. Groundwater will only be controlled at shallow depth

3. Unlikely to be effective in fine grained soil.

EDUCTOR SYSTEM• This system also known as the ‘jet eductor system’ or ‘ejector system’ or ‘eductor

wellpoint system’ is similar to the wellpoint system.• Instead of employing a vacuum to draw water to the well-points, the eductor

system uses high pressure water and riser units, each about 30-40mm in diameter. • A high pressure supply main feeds water through a venturi tube immediately above

the perforated well screen, creating a reduction in pressure which draws water through the large diameter rise pipe. The high pressure main feeds off the return water.

• The advantage of the eductor system is that in operating many well points from a single pump station, the water table can be lowered in one stage from depths of 10-45 m.

• This method becomes economically competitive at depth in soils of low permeability.

WORKING PROCEDURE OF EDUCTOR SYSTEM

• Supply pumps at ground level feed high-pressure water to each eductor well head via a supply main.

• The supply flow passes down the well and through a nozzle and venturi in the eductor. The flow of water through the nozzle generates a vacuum in the well and draws in groundwater.

• The supply flow and extracted ground water mix, return to the surface and feed back to the pumping station via a return main. The return flow is used to prime the supply pumps and the excess water extracted is discharged by overflow from the priming tank.

• A single pumping station can be used to operate up to about 75 eductor wells installed in an appropriate array around the works.

ADVANTAGES OF EDUCTOR SYSTEM

1. They are flexible in level and layout.2. Stable in operation.3. Able to run dry without damage.4. Not limited by depth. Also effective to greater depths.5. Best in low-yielding wells.6. Energy intensive.7. Venturi in base of well creates vacuum.

THINGS TO BE TAKEN CARE DURING DEWATERING!!!

• Dewatering activities must be done properly to avoid eroding the soil on the construction site.

•  It is also important to choose the best location for discharge, even when you might be far away from water bodies or catch basins.

• Water should not be pumped directly into slopes.• Pay special attention and discontinue dewatering if the area

shows signs of instability or erosion.

• Channels used on dewatering must be stable and better if they have been protected with grass or vegetation.

• Avoid dewatering under heavy rains because the infiltration rate is at a minimum and water will move slower or just the dewatering process will not function.

• Never discharge water that has been contaminated with oil, grease, chemical products directly. In such instances an oil/water separator may be necessary.

• Additional permits and requirements might be needed from state, local or federal agencies.

REFRENCES

• HTTP://WWW.GRIFFINDEWATERING.COM/DEWATERING/EDUCTOR_SYSTEM.HTML#.VBIGKVMSXQU