For pipe size < 300mm, rodding is usually adopted in which workers place about 1m long rods through the pipes to the location of blockage and manually operate the rod to clear the blockage.

For pipe size < 700mm, water-jetting is normally employed in which water is supplied from nearby fire hydrants and pressurized water jet is used for clearing blockage.

Winching method is adopted for all sizes of pipes.

For instance, for pipe size exceeding 1050mm, it is stated in Stormwater Drainage Manual that maximum intervals between manholes along straight lengths should be 120m. This is because for sizes over 1050m, the main method of pipe maintenance is by winching whose maximum length of operation is 120m. Similarly, the maximum intervals of manholes for other straight pipes are derived from their corresponding maintenance methods.

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.

Traditionally, manholes covers are made of cast iron. However, in the viewpoint of pipe maintenance, frequent opening of manhole covers has to be carried out. Therefore, it poses potential safety hazard to the workers during the lifting-up process of manhole covers because cast iron manhole covers are very heavy to normal workers. Consequently, research has been conducted and ductile iron is considered as a better choice than cast iron because it can resist the same traffic loads with lower self-weight. Moreover, as ductile iron is less brittle than cast iron, the traditional cast iron manhole covers are more susceptible to damage and thus requires higher maintenance cost.

However, ductile iron manhole covers do suffer from some demerits. For instance, owing to their relative low self-weight, vehicles passing over these manhole covers would lead to the movement of covers and generate unpleasant noises. To solve this problem, instead of increasing the self-weight of ductile iron manhole covers which similarly causes safety problems to workers during regular maintenance, the covers can be designed to be attached to the manhole frames which hold them in firm position.

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.

In some countries, manhole covers are designed into split triangular shapes to reduce the effect of rocking from traffic. However, the two pieces of triangular covers should be bolted together. As for a piece of triangular cover, it is easily dropped into the rectangular hole of manhole during routine maintenance. Therefore, from maintenance point of view, some countries prefer another geometrical shape i.e. circular, as this is the only shape that the cover could hardly be accidentally dropped into the manhole.

On the other hand, for other geometrical shapes such as rectangle or square, they could still be dropped into their formed hole when inclined into proper angles.

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.

Manhole covers are generally made up of two pieces of triangular plates to form a square cover. One may wonder why two rectangular halves are used for a rectangular cover. To understand this, one should note that a planar surface is usually in contact with three support points. For a triangular plate, the centre of gravity (CG) normally lies within the zone bounded by three support points even when loaded, thus creating a stable support system for the manhole cover. For a rectangular plate, the CG of a loaded manhole cover may lie outside the three support points, causing it to rock. Hence, the potential problem of rocking produced by vehicular traffic by rectangular traffic could be eliminated by using two triangular halves.

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.

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.