A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
can cope with large flow situation where headroom is limited because the
height of box culverts can be reduced while the size of pipe culverts is fixed.
Secondly, for some difficult site conditions, e.g. excavation of structure in
rock, for the same equivalent cross-sectional area, the width of box
culverts can be designed to be smaller than that of pipe culverts and this
enhances smaller amount of excavation and backfilling.
136
Fig. Small spatial
requirement of box
culver than pipes.
9. What are the functions of the following features observed in a
typical manhole? (i) groove near benching, (ii) R.S.J. (iii) double seal
manhole cover and (iv) u-trap with rodding arm. (M1)
(i) The groove is used to facilitate the maintenance of manholes and
sewer/drain pipes. Shutoff boards are erected on the grooves during
maintenance operation so that water flow coming from upstream is
terminated in the manhole and backwater from downstream is also
blocked. In addition, the groove also facilitates water flow diversion for
routine maintenance operation.
(ii) R.S.J. is a small-scale size of universal beams and is used for
resisting the high stresses incurred by heavy traffic loads acting
directly on the upper narrow projected section of manholes.
(iii) Double seal terminal manhole covers are used for sealing off gases
emitted inside sewer/drains and prevent them from releasing out of
the manhole.
(iv) U-trap with rodding arms is also used for sealing off unpleasant gas
smell by the trapped u-shaped water columns. Rodding arm is
normally closed with rubber rings during normal operation. However,
during maintenance operation, the rubber ring is removed and
rodding can be carried out through the rodding arm.