TN-30 : Design and performance of large diameter wells in hard rock areas

Abstract

Unsteady flow to a large-diameter well in a confined aquifer has been analysed by discrete kernel approach for the case where the pumping rate is a quadratic function of drawdown. The equation assumed to hold good between pumping rate and drawdown is of the form

Q (n) =[ 1 - (1 P

SW - C)-----

(n) S2(n) Q C--; ] Qv

in which S

SF SFF

F and C are the pump characteristaics, QI is the

initial pumping rate and Sw(n) is the drawdown at the largediameter well at time n. Results have been presented for

variation of Q (n)

, Sw(n), and recovery of well storage with

P time.

Variation of specific capacity of large-diameter well with time for different well storage has been studied. The relationships between transmissivity and specific capacity at various time after the onset of continuous pumping have been presented for different values of well storage and specific yield which can be used for estimating transmissivity.

Analysis of the design criteria for a large-diameter well has been carried out. The procedure for finding the optimum depth and diameter of the large-diameter well for which the cost of excavation is minimum has been presented. It has been found that the large-diameter wells are useful in the aquifers of low transmissivity.