2. What is Dewatering?
Dewatering is used to describe the artificial means of
removing groundwater or surface water for
favourable condition of any construction. Normally
dewatering process is done by pumping or
evaporation. It is usually done prior to excavation for
footings or to lower water table that might be causing
problems during excavations.
3. Importance of Dewatering
Ground Excavation
•Plays a vital role in ground excavation
•Subsurface excavations may be jeopardized due to water table level
•Permits excavation and construction within a relatively dry environment
Stable Foundation
•Controls the hydrostatic pressure and seepage thereby increasing the stability of excavation slopes so as to
make them suitable for supporting structures
•Can also be utilized to increase the effective weight of the soil and consolidate the soil layers
•Reduces lateral loads on sheeting and bracing
Irrigation
•Excess water extracted from construction sites may be used for irrigation
Needs (moneysaving and environmentally friendly option)
4. Three Dewatering Methods
1. Active Dewatering:
Controlling groundwater by pumping, to locally lower groundwater levels in the
vicinity of the excavation.
i. Sump Pumping
Installation of sumps within the excavation, from with water entering the excavation can
be pumped.
HOWEVER, seepage into excavation can create risk of instability and other
construction problems. To ensure stability of excavation side slope and base it may
be necessary to lower groundwater levels in advance of excavation.
This is known as ‘pre-drainage’.
Geology, groundwater conditions, and type of excavation all
influence the selection of dewatering technology.
5. Three Dewatering Methods
Pre-drainage methods include:
ii. Deep wells
An array of bored wells pumped by submersible pumps. Pumping from each well lowers the
groundwater level and creates a cone of depression or drawdown around itself.
6. Three Dewatering Methods
iii. Well Point System
A typical well point system consists of a series of small diameter tubes (known as well
points) with slots near the bottom that are inserted into the ground and connected via a
header pipe. The pump creates a vacuum in the header pipe, drawing water up out of
the ground.
7. Three Dewatering Methods
iv. Horizontal Wells
Uses a horizontal flexible perforated pipe, pumped by a well point pump, to lower
groundwater levels. The method can be very effective for dewatering long pipelines.
excavations.
8. Three Dewatering Methods
2. Interception Dewatering:
Intercept the inflow of surface water or groundwater by preventing it from
getting to the excavation site.
i. Spillways around cofferdams
9. Three Dewatering Methods
ii. Grout Curtain
The purpose of the grout curtains are used under dams where the foundation would
otherwise pass too much seepage, reduce the seepage erosion potential and reduce
leakage through the dam foundation. Grout curtain is restrict seepage to such an amount
that it does not cause too much loss of storage, and does not dislodge the foundation
downstream or erode the base of the dam.
12. Three Dewatering Methods
3. Isolation (Exclusion) Dewatering:
Groundwater can be excluded from the working area by a very low permeability physical cut-
off wall or barrier installed around the perimeter of the excavation. Ideally the barrier
penetrates down to a very low permeability stratum that forms a basal seal for the excavation.
i. Steel Sheet-piling
Is an earth retention an excavation support technique that retains soil, using steel sheet
sections with interlocking edges.
13. Three Dewatering Methods
ii. Coffer dams
Is a temporary enclosure built within, or in pairs across, a body of water and constructed to
allow the enclosed area to be pumped out.
14. Three Dewatering Methods
iii. Freezing
Change the water in the soil into a solid wall of ice which is completely impermeable.
15. Three Dewatering Methods
iv. Grouting
Consists of injecting a fluid material at a certain pressure into soil or rock in order to decrease
the permeability and/or strengthen the formation.
16. Advantages and Disadvantages
Method Advantages Disadvantages
Sump Pumping Simple and cheap Sump takes up space within excavation
Potential water pollution due to silt-laden discharge water
Well Point System Flexible and effective method of dewatering in sands or
sands & gravel
Drawdown limited to 5 to 6 m due to suction lift limits
Deep Well System Effective in a wide range of ground conditions, sand,
gravels, fissured rocks
Drawdown limited by well depth and soil stratification
Interceptor Drain Good coverage
Good option to intercept groundwater perched above
relatively impermeable soil
Widely practiced technique
Pipes often undersized
Must be able to slope pipe in trench
Ground Freezing Versatile
Penetration of a freeze does not vary greatly with
permeability, therefore more effective as cut-off than
grout
Takes time to develop and initial costs are high
Highly energy intensive process
Requires copious amounts of monitoring
Steel Sheet Piling Light weight and easier to lift and handle
Reusable and recyclable
Pile length is easily adaptable and can be welded or
bolted to make it work
Can be used to retain either soil or water
Difficult to install in soil that is rocky or has large boulders
Driving the sheets may cause neighbourhood disturbance
Grouting Depths greater than 200 ft can be achieved
Little waste material is generated
Costly
Relatively high hydraulic conductivity values are obtained
17. Dewatering Method for Soil Type
Method Suitability of Soil
Sump Pumping Gravel or well graded sandy gravel, partially cemented material, porous rock formation
Well Point System Sandy soil, sand & gravel
Deep Well System Effective in a wide range of ground conditions, sand, gravels, fissured rocks
Interceptor Drains Relatively impermeable soil layer
Ground Freezing Applicable to entire range of soils, provided soil is near saturation or completely saturated
Also applicable to difficult ground conditions
Steel Sheet Piling Soft soils
Grouting Can be used in almost any ground condition
18. Basal Heave
Basal heave arises from the weight of soil outside the excavation zone exceeding the bearing
capacity of soil below the excavation bottom, causing the soil to move and the excavation bottom to
heave so much that the whole excavation collapses.
Basal heave failure analyses are only applicable to saturated clayey soils.
20. Consequences of Uncontrolled
Dewatering
Ground Subsidence
•Vertical deformation of rock formation without loading and also the lowering of the land – surface elevation from changes that take
place underground
Flooding
•Excess water flowing into discharge areas can cause minor flooding
•Discharge of turbid water into storm drains or bodies of water can cause clogging of existing drainage facilities, which causes flooding
during storm events
Structural Collapse
•Increases load on foundation soil below original groundwater table
•As most soils consolidate upon application of additional load, structures located within the radius of influence of dewatering system
my collapse
Groundwater Depletion
•Withdrawal of water at greater rates than replenishment
•Long–term water level decline caused by sustained groundwater pumping
