dewatering in different soil conditions, methods, explanation of dewatering methods, : open sumps & ditches, vaccumm method deppwell point method electro osmosis metheod

1. BUILT
IT
UP
New Horizon Institute Of Technology & Management (Thane)
BRANCH: B.E. CIVIL ENGINEERING. SEMESTER: 3RD
PRESENTED BY:
Ms. Tejaswini Narayane
Mst. Vaibhav Phatangare
Mst. Dipesh Dhadave
Mst. Dishank Raut
PROJECT GUIDE:
Prof. Dhanashri Joshi

2. TOPIC
DEWATERING IN DIFFERENT
SOIL CONDITIONS:
ELECTRO – OSMOSIS METHOD

3. CONTENTS
• WHAT IS DEWATERING?
• IMPORTANCE OF DEWATERING.
• CONSEQUENCE OF DEWATERING.
• BASIC PROCESS OF DEWATERING.
• DEWATERING METHODS.
• ELECTRO – OSMOSIS METHOD OF DEWATERING.
• WATER MANAGEMENT & DISPOSAL OF
DEWATERED WATER.
• SUMMARY OF METHODS OF DEWATWERING
WITH SUITABILITY.
• EXAMPLES.

4. WHAT IS WATER TABLE?
• Water Table – The Upper Surface of Ground Water.
• Ground Water – Sub-surface water, but not all sub-
surface water is ground water.
• Phreatic zone – is where ground water occurs.
NOTE: Strictly speaking, only water found in the
saturated zone is ground water.

5. WHAT IS DEWATERING?
• ‘Dewatering’ – Artificial means of removing
groundwater or surface water.
• Dewatering Process – done by pumping or evaporation
methods.
• On construction sites it may be known as
‘CONSTRUCTION DEWATERING’.
• Dewatering means “the separation of water from the
soil,” or perhaps “taking the water out of the
particular construction problem completely.”

6. WHY DO WE NEED
DEWATERING ON
CONSTRUCTION SITE?
• Completely Dry Surface.
• Easy Workability of construction Workers.
• During Excavation – Safety of Workers & to Achieve
Stability of Foundation.
• During Monsoons – for Smooth & Fast Working
Progress.
• Other Essential Reasons – Washing of plant, etc.

7. IMPORTANCE OF
DEWATERING
• Subsurface excavations may be
jeopardized due to water table
level.
• Permits excavation and
construction.
• 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.

8. 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 for
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.

9. CONSEQUENCES OF
IMPROPER DEWATERING
• Ground Subsidence
1. Vertical deformation of rock
formation without loading and also
the lowering of the land surface
elevation from changes that take
place underground
• Flooding
1. Excess water flowing into
discharge areas.
2. 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
1. Increases load on foundation soil
below original groundwater table
2. As most soils consolidate upon
application of additional load,
structures located within the radius
of influence of dewatering system
may collapse
• Groundwater Depletion
1. Withdrawal of water at greater
rates than replenishment
2. Long–term water level decline
caused by sustained groundwater
pumping

10. SOURCES OF
UNWANTED WATER ON
CONSTRUCTION SITE
• Sources Of Surface Water:
1. Rainfall
2. Construction operations (E.G. Concreting,
washing of plant)
3. Seepage through cut-off walls.
• Other Source Is Ground Water

11. BASIC PROCESS OF
DEWATERING
COLLECTION
OF WATER
TO BE
DEWATERED.
PUMPING:
ANY
DEWATER-
ING METHOD
FILTERATION
OF
IMPURITIES,
SILTS &
OTHER
SCRAPS.
DISCHARGE
&
DISPOLSAL
AT PROPER
LOCATION &
CARE.

12. PERMEABILITY OF SOIL
SOIL PERMEABILITY
COEFFICENT (K)
CM/SEC
RELATIVE
PERMEABILITY
COARSE GRAVEL EXCEEDS 10^-1 HIGH
CLEAN SAND 10^-1 TO 10^-3 MEDIUM
DIRTY SAND 10^-3 TO 10^-5 LOW
SILT 10^-5 TO 10^-7 VERY LOW
CLAY LESS THAN 10^-7 IMPERVIOUS

13. METHODS OF
DEWATERING
Factors such as Type of Soil & Nature of Construction
Site will all influence, which Dewatering Method will
be Best Suited for the Project.
• OPEN SUMPS AND DITCHES
• WELL POINT SYSTEMS
• DEEP WELL DRAINAGE
• VACUUM DEWATERING SYSTEMS
• ELECTRO OSMOSIS

14. OPEN SUMPS & DITCHES
• Simplest & Commonly Used Method Of
Dewatering
• Shallow Pits called “SUMPS” are Dug
Along the Periphery of the Area &
Connected by Drains of Semi-Circular In
Shape & 20cm diameter.
• The Water from the Slopes Flow Under
Gravity & is Collected in Sumps from
which It is Pumped Out.

15. • The Well Point System is Perforated Pipe of
5 – 8 cm Diameter & 1m Long Pipe covered
by Cylindrical Wire Gauge Screen Known as
Strainer.
• Pipes are jetted in the Ground 1 – 2 meters a
part.
• Well point → Riser Pipe → Swinger Arm →
Header.
• It is Suitable for Lowering the Water Table by
5 – 6 meters in Soil.
WELL POINT SYSTEM

16. DEEP WELL
• Adopted when the depth of excavation
precedes more than 16 meters or where
Artesian Water is Present.
• 15 – 16 cm diameter of bore hole is done, & a
Casing with large screen is Provided. A row
of Well Points are Frequently Installed at the
Toe of the Side Slope of Deep Excavation.
• A Submersible Pump is Installed at the
Bottom of the Well, of casing of minimum dia
of 150mm.
• The Discharge Pipes of the Submersible
Pumps of a Number of Adjacent Wells are
Connected to Common Delivery Main. The
water is raised from the well by the Multi
Staged Pump.

17. VACUUM DEWATERING SYSTEMS
• When draining is required for silt / clay which
has size > 0.05mm. That time Vacuum Pump
System is used for Dewatering.
• The well points are driven & 25cm diameter is
provided around the well point.
• Installed in the ground by forcing a jet of
water under sufficient pressure.
• The sand of medium – coarse size is forced
into this hole as rapidly as possible, this
sand then forms a filter medium.

18. • In the upper most 600 – 900 mm, an impervious
material such as clay is tamped to form the seal
of the upper portion.
• Pumping is then carried out by means of
equipment capable of maintain a vacuum in Well
Points & the Surrounding Filter.
• In this way, the pressure around the Well Points
is reduced to a small fraction of the Atmospheric
Pressure. The ground is acted upon the
Atmospheric Pressure. Thus, the soil becomes
consolidated under a pressure which is nearly
equal to atmospheric pressure.

19. ELECTRO – OSMOSIS SYSTEM OF
DEWATERING
• This system is used for fine grained
cohesive soils (such as clay), which
can be drained or stabilized using
electric current. This method was
developed by L. Casagrande (1952).
• If Direct Current is Passed between 2
Electrodes driven into a natural soil
mass, the soil water will travel from
the Positive Electrode (ANODE) to the
Negative Electrode (CATHODE).

20. • The Cathode is made in the form of
Well Point / metal tube for pumping
out the seeping water.
• A Steel Rod, a Pipe or a Steel Piling
of excavation can serve as a
Cathode.
• The Arrangement of Electrodes is
done in such a way that the natural
direction of flow of water is
reversed away from the excavation
thereby, increasing the strength of
the soil & stability of the slope.

21. • The potentials generally used in the
process are from 40 – 180 volts, with
electrodes spacing 4 – 5 meters.
• This dewatering system is very
expensive in nature as it calls for
impeccable experienced team to do
the job.
• The flow of ions in Electro –
Osmosis Processes of Dewatering is
known as Ionic Flow or Phreatic
Flow.

22. ASSUMPTIONS OF
ELECTRO – OSMOSIS
METHOD OF DEWATERING
The model of electro-osmotic dewatering is based on the following
assumptions:
• The sludge is homogeneous and fully saturated
• There is no movement of particles by electrophoretic migration
• The water in the sludge moves electro osmotically in the one-
dimensional , direction between electrodes.
• The electroosmotic permeability(k), is constant with time; No
electrochemical reactions occur during electro-osmotic
dewatering; Darcy's law and Ohm's law are applicable during
electroosmotic dewatering.

23. WATER MANAGEMENT &
DISPOSAL OF DEWATERED
WATER
• Water should never be pumped directly into slopes.
• Discontinue dewatering if the area shows signs of instability
or erosion.
• Channels used for dewatering need to be steady and
protected by grass and vegetation.
• Never dewater during heavy rains. The water and infiltration
rate will be slower during the dewatering process or it will not
function entirely.
• Never discharge water that contains oil, grease, or chemical
products.
• Make sure to acquire any additional permits from state, local,
or federal agencies.

24. METHOD SUITABILITY
SUMP PUMPING FOR SHALLOW EXCAVATION IN
COARSE GRAINED SOIL.
WELL POINT SYSTEMS SUITABLE FOR LOWERING THE
WATER TABLE BY 5-6 METERS
IN SOILS.
DEEP WELL POINT SYSTEM FOR COARSE GRAINED SOILS
& DEPTH OF EXCAVATIONS OF
MORE THAN 16 METERS.
VACUUM PUMP DRAINING SILTY SANDS WITH
FINE SANDS
ELECTRO – OSMOSIS
DEWATERING
SUITABLE FOR FINED
GRAINED COHESIVE SOILS
SUCH AS CLAYS
SUMMARY OF
DEWATERING METHOD