In this article we will discuss about the formation of cavity in wells with the help of a suitable diagram.
When the water is not withdrawn from the well, the water level inside the well is equal to the static water table outside the well. But when the water is being drawn from the well at some constant rate, the water level inside the well starts going down and at a level it will become stable.
The difference of head between the water level inside the well and outside the well will cause flow of water inside the well to meet the requirement of the pumping. The difference in head causing flow is known as depression head.
Now as the water from the soil travels towards the well, it will cause gradual head loss and the water surface will drop towards the well. As the came discharge has to pass through the reducing soil areas when it reaches the well, the velocity of the water gradually increases as it reaches the well. As per Darcy’s law the velocity can gradually increase only if the hydraulic gradient line gradually increase.
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Therefore, the water surface will have less slope in the beginning, but it will have sharp slope as it will reach the well, to fulfil the above condition. Fig. 4.10 also shows the hydraulic gradient line. The surface of the water table around the well takes the shape of curved cone, and it is called cone of depression. The distance from the centre of the well to the base circumference of the cone is known as radius of influence of the well.
The velocity of the water entering the well all around directly depends on the rate of pumping or withdrawn of water. When the rate of withdrawal is very high, it will increase the depression head, thereby causing increase in the velocity of percolating water.
If this velocity is more than the critical velocity for the soil, soil particles will make hollows in the well bottom. Now when more and more sand particles will flow inside the well, the effective area will be increased due to formation of the percolating water below critical velocity.
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The formations of the hollows below the wells are dangerous and may cause sinking of staining of the well and damage it. To reduce the possibility of formation of hollows or cavity below the wells, the maximum rate of water withdrawal from the wells is limited.