Theoretically speaking, the whole manhole structures can be constructed as a whole instead of splitting into two stages of construction. With detailed calculation of longitudinal fall and cross fall of road pavement, it is possible to place and construct the manhole cover and frame in accurate levels and falls so that the whole manhole can be constructed in a single stage.
However, in actual practice it may not always be possible to accurately predict the inclination and levels of future manhole cover. As such, it is not uncommon that the construction of manholes is split into two stages:
(i) 1st Stage: The lower part of manholes is constructed up to the access shaft (about the level of sub-base). Then a rigid cover is placed over the access opening of the manholes and bituminous materials are placed on its top to construct the pavement.
(ii) 2nd Stage: The bituminous material around the rigid cover is removed and several courses of brickwork are normally placed on the access shaft to adjust the level of manhole cover and frame to fit with the final height of the pavement. Finally, manhole cover and frame are installed in position as shown in the sketch below.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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