Drop manhole: need and construction

A drop manhole is a manhole in which the incoming sewer enters at a higher level than the outgoing sewer, and the sewage is led down to the lower sewer through a vertical pipe (drop pipe) built on the outside or inside of the manhole.

Necessity (need) of drop manhole

1. Steep ground: when a branch sewer joins a main sewer at a much lower level (difference more than about 0.6 m), a direct fall would splash the sewage, cause erosion of the manhole base and release foul gas.
2. Avoid high velocity: sewage falling freely produces a high velocity and scouring of the lower sewer.
3. Level difference at junction: when a shallow sewer must join a deep main sewer, the drop pipe avoids deep excavation of the entire branch and reduces cost.
4. Gradient control: it lets the sewer follow a flatter gradient than the ground slope.
5. Protection of workers: the sewage does not fall on a person who enters the manhole.

Purpose

To bring the sewage safely from a higher sewer to a lower sewer without splashing and damage, and to avoid deep trenching.

Construction

   Ground level
   ________________          Cover
  |  |          |  |   <- Manhole shaft
  |  |   Inlet  |  |
  |  |  sewer-->|  |
  |  |__ ____   |  |   Vertical drop pipe
  |        |  |    |   (outside the shaft, in
  |        |  |    |    concrete encasement)
  |        v  |    |
  |  Outlet ---> main sewer
  |____ Channel ___|

• It is built like an ordinary brick/RCC manhole (brickwork in 1:3 cement mortar, circular or rectangular chamber with steps and a cast iron cover).
• A vertical drop pipe of the same diameter as the branch sewer (CI or stoneware) is fixed on the outside wall of the manhole and connected to the branch sewer through a tee/bend at the top. A cleaning eye with a cover or plug is provided at the top for rodding.
• The bottom of the drop pipe discharges into the manhole base through a bend, in a half-round channel (benching) leading to the main sewer.
• The drop pipe is encased in concrete 1:2:4 to protect it from load and impact.
• The base is built of concrete with a cushion to take the impact of falling sewage.