drainage water ranges from 0.01 to 0.05 per cent. Due to this
low sand wash factor the drainage wells are not silted.

Fig. 2 illustrates the most progressive methods used in the
construction of gravel filters for high-debit wells. Preference is
given to the suction method of big-diametre drainage well cons-
truction with inverse washing.

Experience accumulated in the operation of artesian wells
shows that by fitting big-diametre wells with Tlilters incorpora-
ting a sufficiently thick gravel packing it is possible to improve
the performance of the pumping and power equipment and the
water intake section of the well, to raise the water intake capa-
city, to increase the period of non-maintenance operation and
reduce pressure losses in the filter zone.