Water held in soil is termed groundwater. It may be in saturated or unsaturated form.
The water table is the line showing the change of water level in the soil. Below the water table, the soil is saturated with water.
Water held in the soil acts as a lubricant. It increases the tendency of soil to slip or slide. This property is dangerous and causes difficulties when excavations to be done during construction works.
Construction of foundation below the subsoil water level creates problems due to waterlogging. Therefore, it is necessary to dewater the area of excavation and keep it dry.
Several operations have to be carried out within the excavation. For example, laying bed concrete, laying of RCC raft slab, construction of masonry works are a few of them. Work can be carried out efficiently when the excavation area is kept dry.
Reducing Water Level below the Excavation Bottom
For keeping the area of excavation in dry condition, the water table should be maintained at least 0.5 m below the bottom of the excavation.
The following two methods are frequently used for lowering the water table during construction.
Dewatering for Shallow Foundations
The simplest and easiest method is to excavate drains along the edges of the excavation, collect water to sumps and remove using pumps. This method is suitable for fairly dense soil and shallow excavations. This is an economical method and is possible of being performed with unskilled labour and simple equipment.
Dewatering for Large Excavations and Deep Foundations
Wellpoint system
Wellpoint systems are used when dewatering to be done in large excavations such as for raft foundations. In this method, the flow of groundwater into the excavation is cut off using pipe wells driven deeper than the excavation around the perimeter.
A wellpoint comprises a 120 cm long and 4 cm diameter perforated pipe, with a valve which is used to regulate the flow and a screen preventing entry of mud, etc.
These wellpoints are installed along the outside edges of the excavation at the required depth, normally deeper than the bottom level of excavation. The spacing of the wellpoint is to be decided on the type of soil, normally around 1 meter.
The wellpoint tube is connected to a pipe known as riser pipe having 5 to 7.5 cm. These are sunk into the ground by jetting. Then, these riser pipes are connected to a header pipe which is connected to a high capacity suction pump.
The groundwater is drawn out by the operation of the pump and is discharged away from the excavation area.
When installing well points using the process of jetting, water is forced down through the wellpoint at the rate of 20 to 25 liters per second. The high-pressure water jet displaces the surrounding soil, enabling the well point to be sunk to the required depth.
After the wellpoint has been driven into the required depth, the water jet is allowed to run for some time. It ensures washing all sand or silt out of the hole, till the return water from the hole is clean. Then, the water jet is stopped and the annular space formed around the wellpoint due to the jetting action of water is filled with coarse sand and gravel. It forms a filter zone around the wellpoint. Entering of fine particles from the surrounding soil into the wellpoint is prevented due to the filter zone. And also it prevents clogging of the wellpoint screen. Normally, the filter sand should be filled up to the water table. The depth of the hole above the water table up to the ground level can be filled with tamped clay. This acts as a clay seal reducing air getting into the wellpoint through the sand filter.
After installing and connecting all the wellpoint, the suction pump is operated.
Due to the suction, the ball valve in the wellpoint gets closed; the groundwater is drawn in through the wellpoint screen and is sucked up through the riser pipes, flows through the header pipe, and is discharged away from the excavation area.
This method can be successfully adopted for a modest depth of excavation, normally up to 18 m.
However, the suction pump is normally not used to lift water above 6 m depth. Therefore, in deep excavations, a multi-stage system of wellpoint can be used.