Manhole: necessity and construction

A manhole is an access chamber, built on a sewer line, that allows a person to enter the sewer for inspection, cleaning, testing and repair.

Necessity of manholes

1. For inspection and cleaning of the sewer (removing silt and blockage).
2. For joining two or more sewers (junctions) and for changes in direction, gradient or size of sewers.
3. For ventilation of the sewer through the cover.
4. For testing (smoke, mirror and lamp) of the sewer.
5. To allow sewers to be laid in straight lengths between manholes, which makes laying, inspection and cleaning easy.
6. For flushing and for house connections.

Manholes are provided at every change of alignment, gradient and diameter, at junctions, and at intervals of 30-90 m on straight lengths (smaller sewers need closer spacing, e.g. 30 m for 300 mm).

Construction (components)

        Cast iron cover & frame
         ________________
        |_______________|   <- Cover slab / ring
         | |         | |
         | |  Neck   | |    <- Shaft (min 0.9 m dia.)
  Steps  |-|         | |       with CI/GI steps at 0.3 m
         | |         | |
        /               \    <- Chamber widens (working
       /     Chamber     \      chamber about 1.2-1.5 m)
      |  Inlet-->____     |
      |        ( channel )--> outlet
      |__Benching____Base_|
          concrete base

1. Base (foundation): cement concrete 1:3:6, 15-30 cm thick, laid on a firm bed to carry the manhole.
2. Walls (chamber): brickwork in cement mortar 1:3 (or stone masonry, or RCC), 20 cm thick for shallow and 30-45 cm for deep manholes, plastered inside and out with 1:2 mortar to make it watertight.
3. Channels and benching: semicircular channels of half-round pipes (or cement concrete) in the base carry the flow smoothly from the inlet to the outlet. The sides (benching) are sloped at about 1 in 6 towards the channel, in 1:2:4 concrete, so that the workers can stand and solids do not stay.
4. Shaft (access shaft): above the chamber, the manhole narrows to a shaft of 0.9 m diameter or 0.9 x 0.6 m rectangle, to take a cover (corbelling at the top).
5. Steps: CI or galvanised iron steps (foot rests), fixed 30 cm apart in the wall, for going down.
6. Cover and frame: heavy cast iron (or RCC for light traffic) cover, with frame set in the top slab, circular of 50-60 cm diameter, kept flush with the road.
7. A drop pipe is added where an incoming sewer is much higher than the outgoing one (drop manhole).

Depth classification: shallow (up to 0.9 m), normal (0.9-1.5 m) and deep (>1.5 m) manholes.