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

provided with cover of medium duty casting and when built within the width ofthe road under vehicular traffic, these shall be provided with cover of heavyduty casting.14.3.4 Types of manholes- Following is the general classification of manholes-14.3.4.1 Straight-through manholes-The simplest type of manhole is that built on astraight run of sewer with no side junctions. Where there is a change in thesize of sewer, the soffit or crown level of the two sewers should be the same,except where special conditions require otherwise.14.3.4.2 Junction Manholes- A manhole should be built at every junction of two or moresewers, and the curved portions of the inverts of tributary sewers should beformed within the manhole.14.3.4.2.1 To achieve this with the best economy of space, the chamber may be built of ashape other than rectangular. The soffit of the smaller sewer at a junctionshould be not lower than that of the larger sewer, in order to avoid thesurcharging of the former when the letter is running full, and the hydraulicdesign usually assumes such a condition.14.3.4.2.2 The gradient of the smaller sewer may be increased from the previous manholesufficiently to reduce the difference of invert level at the point of junction to aconvenient amount.14.3.4.3 Side Entrance Manholes: In large sewer or where it is difficult to obtain directvertical access to the sewer from ground level, owing to existing services, gas,water etc. the access shaft should be constructed in the nearest convenientposition off the line of sewer and connected to the manhole chamber by alateral passage.14.3.4.4 In the tunneled sewer the shaft and the lateral access heading may be used asa working shaft, the tunnel being broken out from the end of the heading, oralternatively the shaft and heading may be used as a working shaft, the tunnelbeing broken out from the end of the heading, or alternatively the shaft andheading maybe constructed after the main tunnel is completed, provisionhaving been made for breaking in from the access heading to build thechamber.14.3.4.5 The floor of the side-entrance passage, which should fall at about 1 in 30towards the sewer, should enter the chamber not lower than the soffit level ofthe sewer. In large sewer where the floor of the side entrance passage isabove the soffit either steps or a ladder (which should be protected either by aremovable handrail or by safety chains) should be provided to reach thebenching.14.3.5. Drop Manholes-(See Drawing No.-16)The term drop manhole is used to indicate the manhole on sewer line which isconstructed to provide a connection between the high level branch sewer toUrban Administration & Development Department Page 160