METHOD OF DEWATERING AT CONSTRUCTION SITE


WHAT IS DEWATERING 

For construction of building, dams, other structures it is required excavation for foundation most of the times foundation  depth may be below the water table at that time seepage of water disturbed the work therefore seepage of water is removed from the excavation that process is dewatering.
Dewatering process of each site varies depends upon depth of excavation geological conditions and characteristics of soil.

METHODS   OF DEWATERING

THE FOLLOWING METHODS OF DEWATERING SYSTEMS ARE APPLIED AT CONSTRUCTION SITE.

1.pumping 
2, providing sumps and side chairs 
3.well point systems
a. single stage system 
b. multible stage system 
c, vacuum system
4, chemical grouting
5. freeing process
6.electro osmosis process

PUMPING

For these methods pumps are installed in excavated foundation trenches at suitable place which point is water stagnation. The stagnation water is removed from foundation trenches through the pumping system.

THE PUMPING SYSTEM MUST HAVE THE FOLLOWING FEATURES

 The pump should be small and portable because easily moved where we want to dewatering

The pump is must have the capable of removing of water mixed with sand clay etc.

The pump should be made of quality materials and strong
The efficiency of the pump require good
Providing sumps and side drains

In this method drains are formed along the longitudinal direction of the foundation trench. For every 40m to 60m distance sumps are constructed. small gradient is provided in drain for easy drain of water to the sumps. Sump is collected water from drain, then this water is pumped through centrifugal pump. The sump is formed in semi circular shape with diameter 0.2m and above.

WELL POINT SYSTEM

Well point dewatering method series of wells are formed into required depth in excavated area from where the water has to be released  out 
In this wells are formed in series line in 2m spacing.

WELL POINT 

Well point is a pipe it is one metre long and 0.4m to 0.5m in diameter. In bottom end of the well point pipe a valve is fitted. the valve is opened when water is pumped into pipe and it is closed when suction is applied pipe and it is closed when suction is applied into well point pipe. Inflow pipe is provided into well point pipe. Inflow pipe is provided holes and these holes are covered with mesh screen.

RISER

Riser is the vertical pipe driven into the ground. riser is connecting well point pipe to the flexible swipe pipe. Responsible of the riser is discharging the water away from the excavated depth. Top end of the riser a flexible swing pipe is connected. The purpose of swinger pipe is just for understands what being pumped. Dia of riser is 40 mm to 50 mm.

HEADER

Header is the horizontal pipe. all the swinger pipes are connected with this header pipe. Header pipe is connected to water pump.

PROCESS OF WELL POINT SYSTEM.

The area to be dewatered is surrounded by number of well points. The spacing of dewatering system is depends upon nature of soil condition and ground water table. minimum spacing is 1m.

Water is forced through the well point at the rate of 20 to 25 litters per second. Well point is automatically inserted into the ground with the help of water pressure and without process of driving.

When the well point is reached the required depth water forces is stopped . for this high pressure of water make annular spacing around the well point.

Then around the spacing of well point system. Filter media of sand and gravel is filled for the purpose of preventing the entry of soil into the system. then forcing of water down is stopped and suction is applied through the well point with the help of water pumping system. Water is removed from the ground and thus the original water table is lowered.

The work of excavation is started. when the work of excavation is completed this suction process is stopped . then water is pumped force  into the pipe this make around the well point loosen the ground and the system is recovered.

SINGLE STAGE SYSTEM

The water lifting depth is about 5m or so . The depth of excavation is not to exceed 5m in this system the whole setup is not sifted until the excavation process is done.

MULTIPLE STAGE SYSTEM.

When the depth of excavation is exceed 5 metres below the water table second stage well point is provided. For first stage well point is provided up to 5m depth . In the second stage well point is provided next 5m depth from first stage excavation surface.

CHEMICAL GROUTING

This method is similar to well point system first one chemical is forced through well point system and it will be stopped when the desired depth is reached. Then another chemical is forced through pipe and the pipe is withdrawn. Two chemical are react and make the soil solidify.

TYPES OF CHEMICAL GROUTING

INORGANIC CHEMICAL – include sodium silicate lingo chemical and lingo sulphate etc.

ORGANIC CHEMICALS- include epoxy resin polyester and other resins.

FREEZING PROCESS

In this method. the surrounding area of excavation is frozen and thus a solid wall of frozen earth formed. The process of freezing process is cold water is applied into earth the cold water freezing the ground water and thus solid soil wall is formed with the help of freezing water.

PROCEDURE

A refrigeration plant is installed near to the site.

A 10 cm to 15 cm dia pipe is inserted into ground desired depth . the spacing between the pipe is 1m to 1.5m . the pipes are closed at bottom.

Another pipe of 2.5 cm to 5 cm pipe is inserted into larger dia pipe. The bottom of the pipe is not closed.

All the small pipes are connected to the single pipe and closed circuit is formed. Larger pipes are connected similarly. 

 The cold liquid temperature about -23 0 c to 300  c is passed through larger pipe and come out through smaller pipe.

This cause the ground to freeze around the pipe. A solid wall of frozen earth of considerable thickness is formed.

ELECTRO OSMOSIS PROCESS

In this process two electrode one positive and other one is negative are driven in the saturated soil. Then electricity is applied tow electrodes.

The water contained in soil is repeated by positive electrode and is attracted negative electrode the water can be collected in it and then it can be pumped out.




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