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

earing aquifer. Such tube wells shall be constructed by direct circulation rotarydrilling method or reverse circulation rotary drilling method using suitable rotarydrilling machine.17.21 The diameter of boreholes for construction of 100 mm, 150 mm & 200 mmfinished nominal diameter gravel packed tube wells shall be 300 mm, 350 mmand 400 mm respectively exclusive of pipe wall thickness. The thickness of thegravel shroud around the screen shall generally be not less than 10 cm. Suchtube wells shall be designated as 300 (100) mm dia, 350 (150) mm dia,400(200) mm dia gravel packed tube wells.17.22 The gravel tube packed tube wells shall be constructed only after obtaining thetechnical clearance of drawing & design of gravel packed tube well from thecompetent authority.17.23 The rates for drilling work are inclusive of the collection of samples of soil/rockcuttings not less than 100 grams from different strata to be collected at every 3meter depth for initial 15 meters depth and at each 10 meters depth after 15meter and at the change of strata also and depositing the same with theEngineer in charge, duly marked and packed in polythene bags.17.24 Screen length :-The length of strainer is usually determined from the thickness of the aquiferencountered. It is not necessary to screen the whole depth of the aquifer andonly that depth should be screened, which will be enough theoretically forabout 200% of the desired discharge. Wherever a full aquifer has been drilledthrough, the screened portion of the aquifer may be limited as follows:-17.24.1 In fine sand gravel aquifers, only 40% of the aquifer need be screened.17.24.2 In coarse to medium sand grade aquifer, a maximum of 60% of the aquifer bescreened.17.24.3 Thickness of aquifer % of screen lengthLess than 7.5 707.5 to 15m 75Above 15m 8017.24.4 1.5 m long bail plug of M.S. blind pipe with close bottom and lifting hook of 20mm dia bars or bail plug of approved type shall be provided at the bottom ofscreen pipe.17.24.5 Standard slot size for 150 mm dia 4.85 mm thick slotted pipes of slot size 76.2mmx3.2 mm.Design pattern (staggered and vertical)(i) No. of slots per set 3(ii) No. of sets per row 11(iii) No. of rows per m 13Urban Administration & Development Department Page 205