REPORT ON

Dewatering Techniques

Submitted By:
Priyanka Bist (11BCL083)
Yashvi Patel (11BCL085)
Piyush Gondalia (11BCL095)

Submitted to:
Trudeep Dave, Lecturer Civil Engineering Department, PDPU

B. Tech. Program in Civil Engineering

Pandit Deendayal Petroleum University
Gandhinagar 382007

1.0 DEWATERING
Dewatering is a process in which groundwater contained within the sites soil is
extracted , ensuring a Stable foundation.
1. To provide suitable working surface of the bottom of the excavation.
2. To stabilize the banks of the excavation thus avoiding the hazards of slides and sloughing.
3. To prevent disturbance of the soil at the bottom of excavation caused by boils or piping. Such
disturbances may reduce the bearing power of the soil.
Lowering the water table can also be utilized to increase the effective weight of the soil and
consolidate the soil layers. Reducing lateral loads on sheeting and bracing is another way of use.
A number of methods are available for controlling the inflow of water into an excavation; the
choice of method will depend on the nature and permeability of the ground, the extent of the area
to be dewatered, the depth of the water table below ground level and the amount by which it has
to be lowered, the proposed methods of excavation and ground support, the proximity of existing
structures, the proximity of water courses etc.

1.1 Methods of groundwater control:

1. Surface water control
2. Gravity drainage
3. Sump pumping
4. Well point systems with suction pumps
5. Shallow (bored) wells with pumps
6. Deep (bored) wells with pumps
7. Eductor system
8. Drainage galleries.
9. Electro-osmosis.

1.2 Exclusion methods

1. Ground freezing
2. Slurry trench cut-off walls with bentonite or native clay and Diaphragm concrete walls
3. Impervious soil barrier.
4. Sheet piling
5. Secant (interlocked) piling or tangent piling with grouting
6. Compressed air
7. Grouted cut-offs

1.1.1 SURFACE WATER CONTROL

Sources of surface water

1. Rainfall and storm water

2. Snow melt
3. Seepage from pit walls

Detrimental effects of poorly-managed surface water

1. Risk of localised flooding

2. Softening of soil or rock exposed in pit walls

Source control:Intercept run-off before it enters the pit

Use surface water drainage ditches and bunds around the perimeter of the pit to prevent surface
water entering the pit from the surrounding land

Water collection:Collector drains, ditches and sumps used to divert water away from working areas
Sumps to temporarily store storm water while it is pumped away

1.1.2 SUMP PUMPING

Sump Pumps are used in applications where excess water must be pumped away from a particular
area.
3

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.

For sump pumping gravel and coarse sands are more suitable.

They generally sit in a basin or sump that collects this excess water

This classification includes bilge and ballast pumps, centrifugal pumps, cantilever pumps, sewage
pump pumps, submersible sump pumps and utility pumps, among others.

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 in the
space outside the cage and at the bottom of the cage and withdrawing the sheeting

Fig.1 sump pumping

(Source:ground water engineering Pvt.Ltd)

1.1.3 SHALLOW WELLS

Comprise surface pumps which draw water through suction pipes installed in bored wells drilled by
the most appropriate well drilling and or bored piling equipment.

The limiting is about 8 m.

Because wells are pre bored, this method is used when hard or variable soil conditions.

Used in very permeable soils when well pointing would be expensive and often at inconveniently
close centres.

Can be used to extract large quantities of water from a single hole. On congested sites use of
smaller number dewatering points is preferred

Hence shallow wells may be preferred to well points. Since the initial cost of installation is more
compared to well points it is preferred in cases where dewatering lasts several months or more.

Another field of application is the silty soils where correct filtering is important.

Fig.2 shallow wells

(Source:engineersdaily.com)

1.1.4 Deep well system

When water has to be extracted from depths greater than 8 m and it is not feasible to lower the
type of pump and suction piping used in shallow wells to gain a few extra meters of depth the deep
wells are such and submersible pumps installed within them.

A cased borehole can be sunk using well drilling or bored piling rigs to a depth lower than the
required dewatered level.

The diameter will be 150 200 mm larger than the well inner casing, which in turn is sized to accept
the submersible pump.

The inner well casing has a perforated screen over the depth requiring dewatering and terminates
below in 1 m of imperforated pipe which may serve as a sump for any material which passes the
filter.

After the slotted PVC or metal well screen (casing) has been installed it is surrounded by backfill
over the imperforated pipe length and with graded filter material over the perforated length as the
outer casing progressively withdrawn.

As with the shallow wells the initial pumping may involve twice the volumes when equilibrium is
achieved.

Deep well systems are of use in gravels to silty fine sands and in water bearing rocks.

They are priority or use with deep excavations and where artesian water is present below an
impermeable stratum.

If this type of installation is to be designed economically the ground permeability must be assessed
from full scale pumping tests. Because of their depth and the usually longer pumping period these
installations are more likely to cause settlement of nearby structures, and the use of recharge
methods may have to be considered.

Fig.3 Deep well Details

source : Google images

1.1.5 Well Point System

A well point system consists of a closely spaced series of small-diameter shallow wells.

The well points are connected to a common header main and are pumped with a high-efficiency
vacuum dewatering pump.
6

For drawdowns in excess of 6 m further stages of well points are required, installed at successively
lower levels as excavation proceeds.

Rapid and cost-effective well point installation may be achieved in sandy soils by jetting using highpressure water; drilling installation may be necessary in coarse or cohesive soils.

Fig.4 Diagram of well system in well

point method

Source: Google images

Process involved:

Well points are about 1-m-deep slotted pipes carrying brass mesh screens over them. These act as
filters or strainers and thus throw out only clear water. The diameter of well points is only 2 to 3
inch.

Each well-point has a self-jetting nozzle at the bottom to help it drive into the ground to the desired
depth. Sometimes, it takes minutes to sink the well points to the desired depth.

Vertical riser pipes connected to the well points are of 2 to 2 inch diameter. These are connected
to the horizontal header with flexible swing joints.

The header pipe has plug cocks to receive the flexible connections. These connections are equipped
with non-return valves.

The horizontal pipe connected to the vertical pipes is of 6 inch to 1 ft. diameter. In certain cases, it
may be of larger diameter. One well-point system has 50 to 60 well points.

All the points and pipe system are connected to the pump. A 6 inch diameter header pipe provides
a flow of 30 liter per second; a 8 inch diameter header gives 60 liter per second; a 10 inch diameter
header gives 110 liter per second; and a 12 inch diameter header draws up to 190 liter per second.
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The dewatering pumps are centrifugal pumps driven by electricity or diesel.

The pumps are able to produce a high vacuum and have good air handling capacity.

Fig.5 well point system

source: Google image

Soil

Typical spacing(m)

Time(days)

Silt sand

1.5-2

7-21(could be longer)

Clean Fine to coarse

1-1.5

3-10

0.5-1

1-2

sand and sandy

gravel
Fine to coarse gravel

Preliminary Requirements

Dimensional plan of area of excavation should be prepared.

Geo-technical investigation data should be collected and position of subsoil water should be
known.
If river or stream is running in the vicinity of the site to be dewatered, its distance, discharge,
direction and high flood level (HFL) should be known.
Characteristics and type of soil to be dewatered should be ascertained. Thickness of various strata
should be known.
Permeability of porous strata should be determined.
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 Chemical properties of groundwater may be determined only if dewatering equipment has to

remain in position for a considerably long period.

Areas of Application
1) Hydro projects
2) Laying of deep sewer lines
3) Tunnel work
4) Construction of subways
5) Water supply projects
6) Canal construction
7) Underground tank construction
8) Bridge construction

1.1.6 Eductor system

This system also known as the jet eductor system or ejector system or eductor well point system is
similar to the well point system.

Fig.6 Typical Eductor systems

(Source:slideshare.net)

Process involved:
1. In a typical eductor well system a series of wells are installed with spacing related to the soil
condition.
2. The wells are equipped with a feed pipe, a venturi ejector and a return pipe.

3. At the head of the well, the feed pipe is connected to a high pressure feed line and the return pipe
is connected to a low pressure evacuation line.
4. The two lines are connected to a special pumping plant which supply the feed line with high
pressure water and collect and evacuate the water from the evacuation line.
5. The high pressure water going through the venturi will draw the ground water through the well
screen and push it up to the surface through the return pipe.
6. This system can lower the water table to approximately 30 meters in conditions where the
permeability of soil is low.

Fig.7 Typical Eductor systems

Source:slideshare.net

Difference between Well Point System and Eductor 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 30-40 mm in diameter.

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.

Eductor system is cost effective when compared to deep wells where close spacing is necessary
because of stratification and/or low permeable soils.

Eductor system not limited in vacuum limitation as in well point systems.

It is also effective in soil stabilization by applying a high vacuum to fine grained soils.

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1.1.7 Ground Freezing technique

A process of making water-bearing strata temporarily impermeable and to increase their compressive and
shear strength by transforming joint water into ice.
Freezing is normally used to provide structural underpinning; temporary supports for an excavation or to
prevent ground water flow into an excavated area.

PURPOSE:

Below the ground water table

Ground water hinders the excavation process, as water is seeping through the pores into the
excavated area .

In any soil or rock formation regardless of structure, grain size or permeability.

Best suited for soft ground rather than rock conditions..

Earth support

Temporary under pinning

Stabilization of earth for tunnel excavation

Arrest land slides

Stabilize abundant mine shafts

PROCESS:
Freezing may be:

Indirect, by circulation of a secondary coolant through tubes driven into the ground

Direct, by circulation of the primary refrigerant fluid through the ground tubes

Direct, by injection of a coolant into the ground, such as liquid nitrogen.

In these systems the primary refrigerant is circulated through the system of tubes in the ground, extracting
directly the latent heat, therefore having a higher efficiency than the indirect process.
Direct freezing time is similar to that for the indirect process. The choice will depend on plant availability,
estimates of cost and perhaps personal preference.
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Primary refrigeration plant is used to abstract heat from a secondary coolant circulating through pipes
driven into the ground. The primary refrigerant most commonly used will typically be some alternative to
Freon
. The secondary coolant, circulated through the network of tubes in the ground is usually a solution of
Calcium Chloride. With a concentration of 30% such as brine has a freezing point well below that of the
primary coolant..
With this method a large portable refrigeration plant is not necessary, and the temp is much lower and
therefore quicker in application The nitrogen under moderate pressure is brought to site in insulated
containers as a liquid which boils at 196C at normal pressure and thereby effects the required cooling. It
can be stored on site.
There is a particular advantage for emergency use, i.e quick freezing without elaborate fixed plant and
equipment. This may be double advantageous on sites remote from power supplies. In such conditions the
nitrogen can be discharged directly through tubes driven into the ground, and allowed to escape to
atmosphere. Precautions for adequate ventilation must be observed.

The speed of ground freezing with N2 is much quicker than with other methods, days rather than
weeks, but liquid nitrogen is costly.

The method is particularly appropriate for a short period of freezing up to about 3 weeks. It may be
used in conjunction with the other processes with the same array of freezing tubes and network of
insulated distribution pipes, in which liquid nitrogen is first used to establish the freeze quickly and
is followed by ordinary refrigeration to maintain the condition while work is executed.

1.1.8 Electro-osmosis

When an external electro motive force is applied across a solid liquid interface the movable diffuse
double layer is displaced tangentially with respect to the fixed layer. This is electro osmosis.

Process Involved:

As the surface of fine grained soil particles causes negative charge, the positive ions in solution are
attracted towards the soil particles and concentrate near the surfaces.
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 Upon application of the electro motive force between two electrodes in a soil medium the positive
ions adjacent to the soil particles and the water molecules attached to the ions are attracted to the
cathode and are repelled by the anode. The free water in the interior of the void spaces is carried
along to the cathode by viscous flow.
By making the cathode a well, water can be collected in the well and then pumped out.
For low permeable soils, the normal pumping methods of dewatering may not be adequate. In such cases,
the electro-osmosis procedure may be helpful. In any event, it is much cheaper and faster than freezing.

Fig.8 Control of ground water

by electro-osmosis

Source: slideshare.net

If an anode is penetrated in the soil close to the excavation edge and a cathode is located far
enough from the excavation, the groundwater flows from the excavation side away from it.
Hence, if a perforated tube is located near the position of the cathode, it collects water, which
seeps away from the excavation site towards that tube. Water can be pumped from the pipe to the
ground level.

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Methods

Advantages

Disadvantages

Application

1. Sump
pumping

Simple,
effective &
economic

Disposal of the
water from
sump pumping
can also create
problems,
because
pumped water
may have a high
sediment load,
which can cause
environmental
problems at the
disposal point.

gravel and coarse sands

2. Shallow
wells

preferred in
cases where
dewatering
lasts several
months or
more

initial cost of
installation is
more compared
to well points

silt soils

3. Deep well
system

Installation up
to 100 feet or
more in a
single stage

It causes
settlement of
nearby
structures, and
the use of
recharge
methods may
have to be
considered.

homogeneous aquifers

water bearing rocks

Capable of
pumping 101000 gallons
per minute per
well

Effective when
placed outside
of the jobsite
work area

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4. Ground
freezing
method

5.Electro-osmosis

Temporary
underpinning
of adjacent
structure and
support during
permanent
underpinning

costly method

Shaft sinking
through waterbearing
ground
Shaft
construction
totally within
non-cohesive
saturated
ground

Tunnelling
through a full
face of
granular soil

Tunnelling
through mixed
ground

soil
stabilisation

much cheaper
and faster than
freezing.

Limited to lower
permeable soils

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Soft ground rather than rock

conditions

Temporary underpinning of
adjacent structure and support
during permanent underpinning

Shaft sinking through waterbearing ground

Shaft construction totally within

non-cohesive saturated ground

Tunnelling through a full face of

granular soil and mixed ground

soil stabilisation

For low permeable soils

6.Well point system

7.Eductor System

The pumps are

able to
produce a high
vacuum and
have good air
handling
capacity

Effective in soil
stabilization by
applying a high
vacuum to fine
grained soil
operating
many well
points from a
single pump
station, the
water table
can be
lowered in one
stage from
depths of 1045 m.

higher
installation cost

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shallow foundations and trench

works

Hydro projects

Laying of deep sewer lines

Tunnel work

Construction of subways

Water supply projects

Canal construction

Underground tank construction

Bridge construction

low permeability.

Suited for deep excavations with

stratified soils

1.1.9 Design:

Most important input parameters for selecting and designing a dewatering system:
-the height of the groundwater above the base of the excavation
-the permeability of the ground surrounding the excavation

Height of Free Discharge Surface hs =

Influence Range :

; Ollos proposed a value of C = 0.5.

WELL 1
WELL 2

C = 3000 for wells or 1500 to 2000 for single line well points
H ,hw In meters and k in m/s

x2
X1

Forchheimer Equation for Multi wells

X

WELL 3

Circular arrangement of wells:

(eq.1)

Obtain an estimate of the total quantity of water to be pumped from Eq.1. The values of H, y and R are determined
by the type of aquifer, the required draw down and soil type. If a is the radius of the equivalent circular area and X
and Y are the dimensions of the excavation,

The number of wells is obtained by dividing the total yield with that of yield of a single well.

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