Dewatering Methods for Foundation
Construction of Footings of buildings, powerhouses, multistory buildings and many other structures requires excavation below the water table into water-bearing soils. Such excavations require lowering the water table below the slopes and bottom of the excavation to prevent raveling or sloughing of the slope and to ensure dry, firm working conditions for construction operations so for this purpose Dewatering is carried out.
Purpose of Dewatering:
Construction sites are dewatered for the following purposes:
To provide suitable working surface at the bottom of the excavation.
To stabilize the banks of the excavation thus avoiding the hazards of slides and sloughing.
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.
Methods of Dewatering:
Following methods are used for Dewatering:
Well point systems with suction pumps
Shallow wells with pumps
Deep wells with pumps
1) Sumps & Sump Pumping:
A sump is merely a hole in the ground from which water is being pumped for the purpose of removing water from the adjoining area. They are used up to 8m with ditches leading to them in large excavations.
Pumping from Sump
For prolonged pumping the sump should be prepared by first driving sheeting around the sump area 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.
2) Well Point Systems:
A well point is 5.0-7.5 cm diameter metal or plastic pipe 60 cm – 120 cm long which is perforated and covered with a screen. The lower end of the pipe has a driving head with water holes for jetting. Well points are connected to 5.0-7.5 cm diameter pipes known as riser pipes and are inserted into the ground by driving or jetting. The upper ends of the riser pipes lead to a header pipe which, in turn, connected to a pump. The ground water is drawn by the pump into the well points through the header pipe and then discharged.
Well Point Dewatering System
This type of dewatering system is effective in soils constituted primarily of sand fraction.
The well points can lower a water level to a maximum of 5.5 m below the centerline of the header pipe. In silty fine sands this limit is 3-4 m. Multiple stage system of well points are used for lowering water level to a greater depth. A single well point handles between 4 and 0.6 m3/hr depending
3) Shallow Wells:
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. Its limit is 8 m because these are pre bored. These wells are used in very permeable soils when well pointing would be expensive and often at inconveniently close centers. These can extract large quantities of water.
4) Deep Wells:
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 then the well inner casing. These systems are used in gravels to silty fine sands and in water bearing rocks.
Typical Deep Well
5) Eductor System: (Jet Eductor System)
It is similar to well point system. Instead of employing a vacuum to draw water to the well points, it uses high pressure water, each about 30-40 mm in diameter. A high pressure is supplied through a venturi tube creating a reduction in pressure which draws water through the large diameter pipe. The high pressure main feeds off the return water. Its advantage 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. This method becomes economically competitive at depth in soils of low permeability.
6) Drainage Galleries:
Drainage galleries are used for the removal of large quantities of water for dam abutments, cut-offs, etc. Large quantities of water can be drained into gallery (small diameter tunnel) and disposed of by conventional large – scale pumps
7) Electro Osmosis:
It is used in low permeability soils (silts, silty clays, etc) when no other method is suitable. In this method direct current electricity is applied from anodes (steel rods) to cathodes (well-points, i.e. small diameter filter wells)
8) Other Methods:
Ground freezing with ammonium brine or liquid nitrogen. It is used for all types of saturated soils.
Slurry trench cut-off walls with bentonite or native clay. It is used for all types of soils.
Impervious soil barrier. Used for all soils. Relatively shallow applications (5-6m max.)
Sheet piling. It is used for all soils except soils with large boulders.
Compressed air. It is used for all types of saturated soils and rock. Its applications is in tunnels, shafts and caissons.