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

14.5.3.4 Where the diameter of the sewer is increased, the crown of the entering &leaving pipes shall be fixed at the same level and necessary slopes given in theinvert of the manhole chamber. In exceptional cases and where unavoidablethe crown of the entering sewer maybe fixed at lower level but in each casesthe peak flow level of the two sewer shall be kept the same.14.5.3.5 A slab of plain cement concrete at least 150mm thick should be provided at thebase to support the walls of the manhole and to prevent the entry of foul water.14.5.3.5.1 The thickness of the base shall be suitably increased up to 300mm, formanholes on large dia sewers, with adequate reinforcement provided towithstand excessive uplift pressures. In the case of larger manholes, the flow inthe sewer should be carried in U-Shaped smooth channel constructedintegrally with the concrete base of the manhole.14.5.3.5.2 The side of the channel shall be equal to the dia. of the largest sewer pipe. Theadjacent floor should have a slope of 1 in 10 draining to the channel. Wheremore than one sewer enters the manhole the flow through channel should becurved smoothly and shall have sufficient capacity to carry the maximum flow.14.5.3.6 It is desirable to place the first pipe joint outside the manhole as close aspracticable. The pipe shall be built inside the wall of the manhole flush with theinternal periphery protected with an arch of masonry or cement concrete toprevent it from being crushed.14.5.3.7 The sidewalls of the manhole are usually constructed of cement brickwork250mm thick and corbelled suitably to accommodate the frame of the manholecover.14.5.3.8 The inside and outside of the brickwork should be plastered with cement mortar1:3 (1 cement: 3 coarse sand). Inside finished smooth with a coat of neatcement.14.5.3.9 Where subsoil water condition exist, a richer mix may be used and it shallfurther be water proofed with adequate quantity addition of approved waterproofing compound as per manufacturer's specifications.14.6 Hazards & Safety of manhole work:-The element of the sewer maintenance is ignored very often, which requirescareful attention and protective measures is the manhole work.14.6.1 The staff should be trained for the comparatively easy act of removing amanhole cover, not only to avoid smashed toes and fingers, but also to preventmore serious back injuries.14.6.2 In addition, the approaching drivers on the road should be warned from adistance about the manhole work in progress by installation of suitable signalsof light.Urban Administration & Development Department Page 164