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If a relatively thin impervious formation of a stiff clay layer is encountered at a shallow depth underlain by an extensive thick confined alluvial aquifer, then it is an excellent location for a cavity well. A hole is drilled using the hand boring set, and casing pipe is lowered to rest firmly on the stiff clay layer, Fig. 5.54. A hole of small cross-sectional area is drilled into the sand formation and is developed into a big hollow cavity by pumping at a high rate or by operating a plunger giving a larg.
In the initial stage of pumping, fine sand comes along with water resulting in the formation of a cavity. During development, the size of the cavity increases till the velocity of ground water flow at its perimeter becomes small enough to retain the aquifer material in place. With further pumping ultimately equilibrium condition is reached when clean water is discharged.
The depth of the cavity at the centre varies from 15-30 cm with 6-8 m radius of the cavity. The flow of water into the cavity is spherical and the yield is low. Cavity wells have shorter life and failure is caused usually due to the collapse of the clay roof. Since the depth is usually small, deep-well pumps are not necessary and thus the capital costs of construction, development and installation of pumpset of a cavity well are low.
Steady Flow into Cavity Well:
Assuming the cavity to be a segment of a sphere of radius ‘rw‘ resting on the top of a confined, homogeneous and isotropic aquifer of infinite areal extent and extensive thickness, the well yield for steady state flow condition in given by-
Unsteady Flow into Cavity Well:
As the pumping progresses, assuming no recharge into the confined aquifer, water is drawn from storage of the aquifer, from a wider and wider area. The cone of depression expands and the drawdown increases with time for the constant pumping rate, Q, given by-
Where s = drawdown in the observation well at a distance V from the cavity well; Q = constant pumping rate and Ss = specific storage coefficient (i.e., for unit aquifer thickness).
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Example:
The following data are obtained from a cavity tube well:
Determine the radius and width of cavity.
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Solution:
