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[sspawar]

Dear Sefians,
There many type of circular manhole chambers are in practice. But what should be the slanting angle of brickwork closing at mouth from larger diameter in lower side, when we are using M7.5 one brick wall with mortar 1:4 and plastering inside 1:3 12mm thick.
what should be the minimum slanting angle of circular frustum for brickwork of circular manhole chamber? how much top collar vertically can be erected safely above tip of circular frustum?

Regards

[vikram.jeet]

As per Book ' NABHI'S PRACTICAL HANDBOOK FOR BUILDING CONSTRUCTION '  by Er MKGupta ( Retd CPWD)  an angle of 1 (H) : 5(V) is indicated at page 641 giving details of manhole.

Further from design point of view , It is better to have small dia circular bars  reinforcement in masonry mortar at start of junction of taper & vertical masonry,   at least in two courses mortar. Reinf can be two nos circular bars along masonry in two courses making total 4 nos . Bars must be closed with lap Length.  This can take hoop force generated at junction  even if passive resistance of soil insufficient.

Other way is to provide higher section of masonry at junction  say 345x345 or 460x460 as ring beam  to bear any net Tensile stresses in horizontal direction as result of hoop tension at junction.