Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

5.3.2 The shape of the openings should be such as to prevent clogging and bridging,which can be diminished by V-shaped openings with the larger end towards theinside of the well. Long, narrow, horizontal or vertical slotted pipes arepreferred for large diameters. The openings should be placed as close togetheras the strength of the screen will permit.5.4 The total area of the openings in a screen should be such as to maintain anentrance velocity less than necessary to carry the finest particle of sand that isto be excluded by the screen.5.4.1 In general, it should be less than about 4 to 6 cm per second with gravelshrouding. It is generally desirable that the length of the screen is made slightlyless than the thickness of the aquifer penetrated and placed centrally in respectof the aquifer.5.4.2 The length, diameter and total area are inter-dependent dimensions that mustbe adjusted to give the desired entrance velocity. Some margin of safety inscreen size is desirable to allow for incrustation and clogging and to prolongthe life of the screen.5.5 Where the water-bearing sand stratum contains little or no gravel, it is veryadvantageous to insert a layer of fine gravel between the strainer and the sandstrata, thus permitting the use of larger orifices in the strainer and greatlydecreasing ground friction.5.5.1 The gravel wall so provided may very in the thickness to suit the size and depthof the boring. It may very from 10 cm to 25 cm, but it is usually 10 cm. The sizeof the gravel to be provided would be decided by the particle size distribution inthe layer penetrated and the slot size in the well screens proposed to beadopted.5.5.2 Since screen sizes can now be custom-tailored to fit any grading of desiredgravel, there is a shift from the former multiple (Concentrically placed) gravelpacks to single ones.5.6 Well effectiveness and performance may be adversely affected if the gravelpack ratio, that is, the mean size of gravel divided by mean size of formationmaterial, exceeds 5. Beyond this limit, wells may require longer time fordevelopment or, if the ratio is excessive, they may turn out to be sand pumpersultimately resulting in failure. The gravel size should be related to the size ofthe formation material in the finest section of the aquifer material against whichscreen is provided.5.7 In gravel packed wells the screen size should be related to the gravel in aboutthe same manner as it is related to the aquifer material in a non-gravel pack(for natural pack) well, i.e. it should correspond to the size that separates 90percent of coarser fractions of gravel.Urban Administration & Development Department Page 212