The process of dewatering is defined as pumping from wells or sumps to temporarily lower groundwater level, permitting excavation under natural groundwater levels to occur in dry and stable situations.
The term dewatering is used to control groundwater through pumping at construction sites, it is known as construction dining and also used at mine sites known as mine catering.
Here we will learn about dewatering, types of dewatering & much more.
Introduction to dewatering:
The process of dewatering for the foundation trenches ground of the high water table, water-logging area.
The excavations is carried out by dewatering the sub-soil water and water must be pumped into an area filled with water, for which the foundation is to be excavated.
In a waterlogged area, at the time of excavation timber and strutting are very difficult because the soil is in contact with water.
Therefore the remedy to overcome this problem is to let the area come into contact with water, offering a better solution for the foundation excavation.
Purpose for dewatering:
- This provides a dry digging and allows the construction to proceed efficiently.
- This reduces lateral load on sheeting and bracing in the excavation.
- Also, improves the supporting characteristics of foundation materials.
- This excavation increases the stability of slopes and side-hill fills.
- It cut off capillary rise and prevent piping and frosting in the pavement.
- It reduces air pressure in the operation of tunnels.
Methods of dewatering:
Ditches and Sumps:
This is a simple method, which is adopted on a large scale if the soil is shallow, coarse-grained.
The pits, called the sumps are dug and water is collected from the side current under gravity.
This water is then pumped out.
Deep Well System:
This method is adopted when the depth of excavation exceeds 16 m below the water table.
In this method, boring of 15 cm to 70 cm diameter is done with a casing, which has a long screen and a submersible pump to lift the water up.
A separate pump is provided to each well.
Vacuum dewatering system:
A well point system is followed in a well for fine grained soils.
For better dewatering results, a suction head is applied in the case of fine, non-adhesive clay.
Dewatering is done both ways and a vacuum is maintained in the well using the airtight seal for all points.
Well point system:
This method is based on gravity flow, it consists of a perforated pipe which is about 0.5 to 1 meter long and 5 to 8 cm in diameter.
This perforated pipe is covered by a cylindrical wire gauge screen.
The suction pump has the ability to bring water to the surface from a maximum depth of about 6 meters.
If there is more than 6 meters of dewatering excavation below the water table, a multistage well system is preferably adopted.
Electro osmosis dewatering:
It is the most suitable method for cohesive soils or soils in which the soil is drained or stabilized by electric current.
The electro-osmosis method is best suited for fine-grained cohesive soils.
In this case, the soil water can be traveled from the positive electrode i.e. the anode to the negative electrode i.e. the cathode, only when the current is passed between two electrodes to the saturated soil mass.
The cathode is made with a metal hose or well for the purpose of pumping out the seeping water.
A pipe or sheet piling of an excavation or steel rod acts as an anode in this method.
In order to increase the shear strength and slope stability of the soil, the natural flow direction of the water is overcome by digging the suitable arrangement of electrodes.
Importance of Dewatering:
It plays an important role in the excavation of the ground, subsurface excavation can be endangered due to the level of the water table.
Allows excavation and construction in a relatively dry environment.
It controls the hydrostatic pressure and seepage thus increasing the stability of excavated slopes to make them suitable for supporting structures.
It can be used to increase the effective weight of soil and strengthen soil layers and reduces lateral load on sheeting and bracing.
The excess water extracted from construction sites can be used for irrigation.