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

extent of about 500 mm to 600 mm above the main sewer to low level mansewer with a minimum amount of disturbance.14.3.5.1 The construction of drop manhole avoids unnecessary steep gradient of branchsewer and it thus reduces the quantity of earthwork.14.3.5.2 When a sewer connects with another sewer, where the difference in levelbetween water lines (peak flow levels) of main line and the invert level ofbranch line is more than 600 mm or a drop of more than 600 mm is required tobe given in the same sewer line and it is uneconomical or impractical toarrange the connection with in 600 mm a drop connection shall be provided forwhich manholes maybe built incorporating a vertical or nearly vertical drop pipefrom the higher sewer to the lower one.14.3.5.2.1 This pipes maybe either outside the shaft and enclosed in concrete orsupported on brackets inside the shaft, which should be suitably enlarged. Ifthe drop pipe is outside the shaft, a continuation of the sewer should be builtthrough the shaft wall to form a rodding and inspection eye, which should beprovided with a half blank flange.14.3.5.2.2 If the drop pipe is inside the shaft, it should be in cast iron and it would beadvantageous to provide adequate means for rodding and water cushion of150 mm depth should also be provided. The diameter of the backdrop shouldbe at-least as large as that of the incoming pipe14.3.5.3 The drop pipe should terminate at its lower end with a plain or duck-foot bendturned so as to discharge its flow at 45 degree or less to the direction of theflow in the main sewer and the pipe, unless of cast iron, should be surroundedwith 150 mm of concrete.14.3.5.4 In the case of sewer over 450 mm in diameter the drop in level maybeaccomplished by one of the following methods: -14.3.5.4.1 A. cascade: -This is a steep ramp composed of steps over which the flow is broken up andretarded. A pipe connecting the two levels is often concreted under the steps toallow small flow to pass without trickling over the steps. The cascade stepsmaybe made of heavy-duty bricks of class- I quality (IS: 2180-1985) cementconcrete with granolithic finish or dressed granite.14.3.5.4.2 B Ramp: -A ramp maybe formed by increasing the grade of the last length of the uppersewer to about 45 degrees or by constructing a steeply graded channel orculvert leading from the high level to the low level sewer. In order to break upthe flow down the ramp and minimize the turbulence in the main sewer the floorof culvert ramp should be obstructed by raced transverse ribs of either bricks orconcrete at 1.50m intervals and a stilling pool provided at the bottom of theramp.Urban Administration & Development Department Page 161