Well point dewatering involves installing small diameter wells around an excavation area and connecting them to a pump via header pipes to drain permeable ground and allow excavation. It is commonly used for foundations, basements, tunnels and other underground construction. The well points must be properly spaced and installed, and the system regularly monitored, to safely and effectively lower the water table during excavation work within permitted timelines.

2. INTRODUCTION
• Almost all deep excavations are now associated with the removal of
subsoil water for laying foundations of structures or underground
construction. Most of the structures have deep foundations involving
a substantial dewatering job. The overall land area around the world
remaining constant, the development arrow has begun to point
vertically upwards. Horizontal expansion has given way to
vertical expansion. As a result, the importance of using dewatering
equipment has increased manifold.
• In many projects, dewatering of foundations plays the key role in
deciding the overall completion period. Till the time the dewatering
equipment is successfully installed and foundations come up,
uncertainty prevails over the commissioning date of a project.
completion schedule suffers a setback. Thus, it is very important to
assess the quantum of dewatering work involved; choose the right
dewatering equipment; and keep standby arrangements in place.

3. WELL POINT DEWATERING
A method for draining permeable deposits around an
excavation that requires small cones of depression. Tubes
about 100 mm in diameter, with wire mesh screens, are
sunk into the ground and connected by a header pipe to a
suction pump at the top. Usually a series of well-points are
connected to one header pipe. When these are used in a
staged excavation a considerable depth can be drained and
the drawdown restricted, because each well-point acts as a
hydrogeologic boundary.

4. WELL POINT DEWATERING
•

5. WELL POINT DEWATERING
 PICTURES SHOWING
WELL POINT

6. AREA OF APPLICATION
1. Hydro projects
2. Excavation of foundations for buildings
3. Basement construction
4. Laying of deep sewer lines
5. Tunnel work
6. Construction of subways
7. Water supply projects
8. Land reclamation projects
9. Canal construction
10.Thermal power project structures with not too deep foundations
Underground tank construction
11. Bridge construction

7. PRELIMANARY REQUIREMENTS
• Dimensional plan of area of excavation should be prepared.
Proposed depth of foundation should be known and lowest depth of
excavation should be ascertained.
• 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. Coefficient of
permeability may vary from 1 cm per second for very fine sands to even
3,000 cm per second for gravel and coarse sands.
• Chemical properties of groundwater may be determined only if
dewatering equipment has to remain in position for a considerably
long period.

8. MULTI-STAGE WELL POINT SYSTEM
• Well-point system is suitable for depths up to 6 m if the
pump is installed at ground level. Soils have to be coarser.
If the sand content of soil is more than 20 per cent, the
well-point system can work in it. For excavations deeper
than 6 m, multi-staged dewatering equipment can be
used. In this case, each stage has its own pump. Number of
stages can vary but more than three stages of well points
don't seem practical. More the number of stages, more is
the excavation width required. Sometimes, a single-stage
well-point system is attached with jet-eductor pump - this
system allows dewatering up to 25 to 30 m depth. This is
preferable over a multi-stage well-point system. In case of
a multi-stage well-point system, care should be taken that
the header of the lowermost stage is not more than 4 m
above the excavation bed so that vacuum in the lines is not
affected.

9. MULTI-STAGE WELL POINT SYSTEM
•

10. EDUCTOR SYSTEM
 The eductor system is generally used in areas with soils of low
permeability and to avoid using a multi-stage well-point system. In
comparison to deep wells, it proves cost-effective too. It is well suited
for deep excavations with stratified soils. The number of eductors in
one system activated by a single pump is around 100. The volume of
water pumped out by this system is normally low, in the range of 15 to
20 litre per second. Under the system, a series of eductor wells are
installed and connected to two parallel headers. One header is a high
pressure water supply line and other is a low pressure return line. Both
lines run to a central pumping station that feeds water under pressure
to eductors located at the bottom of the wells. The system uses a
venturi to draw groundwater into the screen of the well and further to
the ground surface.
 While a large diameter pipe forms the well casing, a smaller
diameter inner pipe forms the return line. Water is pumped under
high pressure between the two pipes and forced through the venturi.
The ground water is now recovered through the well and into the
return pipe.

11. SYSTEM DETAILS
• 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 ½ 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 litre per second; a 8 inch
diameter header gives 60 litre per second; a 10 inch diameter
header gives 110 litre per second; and a 12 inch diameter header
draws up to 190 litre per second.

12. WELL POINT PUPMS-1
• 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.
For this, self priming centrifugal pumps are attached
with vacuum pumps. A diesel set is kept as a standby
in case of power failure. A float- actuated air water
separator tank is provided with the pump. A vacuum
pump throws out the air separated by the separator
from the water being removed. The location of the
pump in the header line is governed by the fact that
the pump is able to develop the required vacuum in
the pipeline.

13. WELL POINT PUPMS-2
• If the header pipeline is too long, i.e. more than 150
m, the pump may have to be located at its centre. If
the length is smaller, the pump may be located at its
end. In any case, development of maximum vacuum
can't be sacrificed. An effort should always be made to
set the pump intake at the level of the header line. In
any case, it shouldn't be kept more than 5 m above the
level of the bottom of the excavation. The pump is
equipped with a water discharge pipe that leads to a
drain or basin at a sufficient distance from the
excavation area to avoid its effect on the dewatering
exercise.

14. WELL POINT PUPMS-3
• The spacing of well points depends upon the type of soil
being dewatered. The more the permeability of soil, the
less the spacing of well points as more discharge is
required to be removed. Thus, spacing is less in sandy soils
and more in silt soils. In sandy soils with gravel, spacing
may be as less as only 1 ft. Generally, the spacing varies
between 1 to 4 ft. The length of the header pipe and rate of
discharge also matter in deciding the location and spacing
of well points. Well-point systems may be in metal (steel
or aluminium) or PVC. Earlier, metallic pipes, brass
strainers and MS header pipes were used. Now, more and
more manufacturers are switching over to PVC because of
its flexibility, corrosion-proof quality and cost-
effectiveness. Well-point system in PVC also has the
advantage of being lightweight and easier to handle.
Another advantage is low friction losses.

15. INSTALLATION
• Well points are equipped with self jetting tips and require
water under pressure for self-installation. About 900 litre
of water are specified in the IS code for each well-point.
Water pressure has to be up to 14.5 kg per sq cm; if the
pervious layer is underlain by a clay layer, jetting does not
prove effective and augering of clay layer is first done. IS
9759 advises an important safeguard of 'sanding in' of well
points to avoid clogging of the system by fine materials
from the ground. Under this process, the water pressure
for jetting is reduced just to keep the hole around the
point open, and coarse sand is filled around the annular
space to act as a supplementary filter. Now, the jetting
water supply can be safely closed.

16. MERITS AND DEMERITS
• The well-point system has the advantage of getting
installed along any flexible line or shape. When a large
area is to be dewatered, well points can be installed
around the area. For long excavations for pipelines or
cables, well points can be installed along a line and prove
quite effective.
• In such a case, a continuous exercise of installing new well
points in the area ahead and taking out well points from
the area completed is on. Just one header pipe laid along
the length of excavation connects the well points.
However, the well-point system is not suitable for very
deep areas. Excavation width becomes very large.
Sometimes, air leakage into suction pipes is common and
this adversely affects dewatering efficiency.

17. PLAYING SAFE
 Before beginning the dewatering operation, the well-point system
should be checked to ensure it is leak-proof. All leakages, however
minor, should be plugged by use of adhesives or paints available for
the purpose. Always provide each well-point with a stop cock so that
each individual well-point can be connected to or disconnected from
the network. See that all vertical risers run full and air leakage is not
allowed.
 Keep all the joints in the system airtight. Try to restrict the number of
well points connected to a pumping unit to 50, maximum 60. Provide
a solid base to the pump. See that the area of installation doesn't
become slushy. Keep an eye on the vacuum gauge fitted to the pump.
If any fluctuations are noted, these are an indication of trouble from
well points. Discharge from well points may reduce in this case. This
problem should be corrected by adjusting the plug cocks and
checking the vacuum pump outlet.
 Always provide each well-point with a stop cock so that it can be
connected or disconnected from the network.

18. QUICK BITES
 Deep excavations are associated with removal of subsoil
water for laying foundations of structures.
 To complete the projects on time, it is very important to
assess the quantum of dewatering work involved.
 Two types of dewatering equipment are used -
well-point equipment and deep bore well equipment.
Well-point equipment consists of a number of wells
installed around the area of excavation.
 Well points are equipped with self jetting tips and require
water under pressure for self-installation.
The well-point system has the advantage of getting
installed along any flexible line or shape but is not suitable
for very deep areas.