In Infra works utility services like storm water line/ sewage line / MEP services lines are running. And for maintenance purpose of such lines, at some intervals the chambers / manholes are constructed.
Specially for sewage and storm water lines, with increasing the slope and length of line, the excavation depth of chamber/ manholes are also increases i,e height of brick masonry is also increased.
Generally the Chambers depth varies from 0.6 to 0.9 Mtr. and can be cleaned/ maintained from top and for that rectangular or square shape are suitable. But Manholes are circular which provided where depth varies from 1.5 to 4.0 Mtr. and man has to get down in manholes for maintenance purpose.
Mostly, Manholes are preferred in sewage lines, its circular shape helps to avoiding the corners obstacles with proper benching, coving and punning (in rectangular shape, sewage may deposit at corners even proper benching and coving is done) and getting the required working space with ease construction for more height and required stability.
So for more depth/height, only circular with tapered shape is suitable option to meet required opening at top to fix the frame with cover in circular shape instead of rectangular shape.
Minimum bending stress is developed at circular manhole
compare to square and rectangle.
No torsional stress is developed in circular but it will be there at corner of rectangle /square/or any straight edged shape