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In some instances, there may be a significant level difference between the incoming pipe and the invert level of the manhole. To accommodate this, and to avoid having to excavate to full depth for the incoming pipe, a Drop shaft connection may be used.

Download this drawing in cad format

Backdrop in Autocad format

A backdrop to a manhole is a method of connecting two substantially different drain line invert levels in a manhole. This can be done either internally or externally by using the following 110mm or equivalent 160mm fittings, as follows.
Installation of Backdrops

  1. For an internal backdrop, use a Socket Plug (4D.296) or a Screwed Access Cover (4D.292), plus an Equal Access Junction (4D.593), vertical pipe to suit, a Short Radius Bend (4D.161) and a Channel Branch Bend (4D.782/3) or a Sealed Access Fitting to suit (see Figures 37 and 38).
  2. Fix internal vertical pipe securely to the manhole wall with OsmaSoil Brackets (4S.083).
  3. For an external backdrop, use a Screwed Access Cover (4D.290), plus a 45° Equal Junction (4D.213), a Short Radius Bend (4D.163), vertical pipe to suit and either a Long Radius Bend (4D.581), or a Short Radius Bend (4D.161) plus a short length of pipe connected to either a Channel Branch Bend (4D.782/3) or a Sealed Access Fitting (see Figure 38). 
  4. Alternatively, ramped backdrop can be used, for drops of less than one meter, by means of two Long Radius 45˚ Bends (4D.583) and a raking length of pipe.
external backdrop for manhole
Figure 38. Open Channel Manhole with stepped invert and external backdrop

internal backdrop for manhole
Figure 37. Sealed Access Manhole with internal backdrop