What Is Dewatering?
The process of removing water from an excavation is known as dewatering. In this process, the water is drained outside by pumping from the location.
The main purpose of dewatering is to provide a dry base for the foundation by lowering the groundwater table of a given location.
The groundwater on site should be controlled to ensure easy and safe accomplishment of the work on the construction site.
So, it is necessary to carry out dewatering before the commencement of construction activities.
Dewatering is a useful method for dealing with running sand, construction as well as repairing dams, sewers, basements, buildings, tunnels, etc.
Purpose Of Dewatering
Dewatering is required for the following purposes
- To provide a dry area and permit construction to proceed eﬃciently.
- To reduce lateral loads on sheeting and bracing in excavation.
- To reduce the pressure of air in tunneling operation.
- To control the embankment seepage in dams.
- To improve support characteristics of foundation materials.
Methods Of Dewatering
There are various methods used for controlling the groundwater during an excavation. Choosing the most suitable method of dewatering for a particular site is a critical step.
The presence of unwanted water on a construction site can lead to complications like an unsafe work area, an increase in construction cost and delay in scheduled work. So, it is important to select the right method of dewatering the undesired water on the construction site.
There are four major methods of dewatering. They are as follows :
- Well Point
- Educator wells.
- Open slump pumping.
- Deep WellPoint method
Let us study these methods in detail below
1. Well Point Method
A Well Point is basically a perforated metal or plastic pipe of 5 cm to 7.5 cm in diameter, covered with a screen. The length of the pipe varies from 60 cm to 120 cm.
The lower end of the pipe has a driving head with water holes on it. The pipes, known as riser pipes, are inserted into the ground by driving or jetting.
The upper end of the pipes is connected to a pump. The groundwater is drawn by the pump through the header pipe and is then discharged.
This water removal or dewatering method is relatively cheap, flexible, and also eﬀective under most soil and hydrologic conditions.
2. Educator Well
This method of dewatering is similar to the well point system. In this method, high-pressure water is used to draw water into the well points instead of vacuum.
High pressure is given through a Venturi tube, creating a reduction in pressure which draws the water through a pipe of large diameter.
The educator well system can operate many well points from a single pump station. Due to this, the water table can be lowered in a single stage from depths of 10 m to 45 m. In soils of lower permeability, this method proves to be an eﬀective option.
3. Open Slump Pumping
Open slump pumping is one of the easiest, cheapest, and most eﬀective methods of dewatering. In this method, drains and slumps are constructed at one or more sides or corners of the excavated pit. The groundwater gets collected into the slump through drains.
The collected water is evacuated either manually or mechanically from the slump. If the quantity of water is less, it may also get evaporated on its own after a few hours.
But if the quantity of water to be removed is large, the dewatering is done using mechanical equipment such as pumps and is discharged at a proper location.
4. Deep Well Method
This method is applicable when a deep excavation is needed and a large quantity of groundwater is to be removed.
The deep well point method can drain out water up to a depth of 24m. Educators can be considered when well points and deep wells are not suitable options for dewatering.
Precautions For Dewatering
Without proper planning and supervision, dewatering should not be done as it can cause complexities like soil erosion and other such problems at the construction sites. Given below are few tips to consider when choosing a discharge area:
1. Dewatering should be done under special supervision; if any signs of erosion or instability are found, stop dewatering instantly.
2. The water should not be pumped directly into slopes.
3. Wooded buffer should be used in dewatering, if possible.
4. Select a proper area for water discharge.
5. If possible, the water from the construction site should be discharged and directed to a forest buﬀer zone.
6. Channels to be used for dewatering must be stable and protected with grass or vegetation.
7. Dewatering should be avoided under heavy rain.
8. Water contaminated with oil, grease, and chemical products should not be discharged directly. In such cases, an oil/water separator can be used.
9. Investigations of local permits and requirements for dewatering are necessary before discharging the water to a particular location from the construction site.
10. Check and understand the water table condition of the dewatering location because the underground water may be situated near the surface, so the plan might not work.