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

off. This operation is an essential part of the boring and should be carefullyperformed. Important differences in quality and yields are discovered in thisway.4.3 When quality stratification exists, which may be ascertained from geophysicallogs or drill-stem rests, blank casings should be provided against zonescontaining undesirable quality of water and the annular space between thecasing and hole wall should be sealed with cement grout or packers. This willensure that the fresh water aquifers are not contaminated by leakage.4.4 Large casing is generally made of welded or riveted steel pipe. For smallersizes of pipes which are to be driven, the standard wrought iron pipe isordinarily used, but for heavy driving extra strong pipe is necessary.4.4.1 The life of good heavy pipes is ordinarily long but they are liable to rapidcorrosion due to the presence of excess amount of carbonic acid. The use ofrust resisting alloys would be economical is such special cases. Non-reinforcedplastic, usually PVC, casing up-to 100 mm dia and reinforced plastic casingand fiber glass for longer dia up to 400 mm are coming into vogue.5. Well Strainer and Gravel Pack5.1 In providing the strainer arrangement are constructed whereby water isadmitted and sand or gravel excluded, it is desirable to make the openings ofthe strainer as large as practicable in order to reduce friction, while at the sametime preventing entrance of any considerable amount of sand.5.2 The openings in well strainers are constructed in such a fashion as to keepunwanted sand out of the well while admitting water with the least possiblefriction.5.2.1 In fine uniform strata, the opening must be small enough to prevent theentrance of the constituent grains. Where the aquifer consists of particles thatvery widely in size, however, the capacity of the well is improved by usingstrainer opening through which the finer particles are pulled in to the well, whilethe coarser ones are left behind with increased void space.5.2.2 A graded filter is thereby created around, with the aid of back-flushingoperations or by high rates of pumping.5.3 The selection of the well screen is important on it depends the capacity and thelife of the well. The size of the openings may be selected, after a study of themechanical analysis of the aquifer, to permit the passage of all fine particlesrepresenting a certain percentage, by weight, of the water-bearing material.5.3.1 It is common practice to use openings that will pass about 70 percent or moreof the sand grains in the natural aquifer whose uniformity coefficient shouldrange between 2 to 2.5. For soil with a uniformity coefficient less than 1.5,gravel shroud should be used.Urban Administration & Development Department Page 211