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.

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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.

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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.

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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:

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(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.

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Basically, a culvert means a covered hydraulic structure which conveys fluid. Therefore in a broad sense, pipe culverts in a small scale represent normal pipes like precast concrete pipes.

In terms of hydraulic performance, circular section is the best geometrical sections among all. Therefore, for relative small discharge, precast concrete pipes and ductile iron pipes are normally used which are circular in shape. But for applications of very large flow, precast concrete pipes and ductile iron pipes may not be available in current market. In this connection, cast-in-situ construction has to be employed. It is beyond doubt that the fabrication of formwork for circular shape is difficult when compared with normal box culvert structures. However, circular shape is the most hydraulic efficient structure which means for a given discharge, the area of flow is minimum. Therefore, it helps to save the cost of extra linings required for the choice of box culverts.

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Screens are provided at culvert inlet owing to one of the following reasons:

(i) To trap trash or debris which might otherwise accumulate in the culvert and block the flow subsequently;

(ii) To prevent access into the culvert by children.

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The bevel is sometimes introduced at the inlet of box culvert to decrease the flow contraction at the inlet. In fact, the outlook of bevel is similar to a chamfer except that a chamfer is smaller in size and it is mainly used to prevent damage to sharp edges of concrete during construction. The bevels can be designed as plane or round edges.

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The use of double-cell box culverts is preferred to single-cell box culverts for cross-sectional area larger than about 5m²
owing to the following reasons:

(i) Where there is tight headroom requirement, the use of double-cell box culvert can shorten the height of culverts by having a wider base so that the same design flow can be accommodated.

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In the hydraulic design of box culvert, there are two flow controls, namely inlet control and outlet control. In inlet control, the entrance characteristics of the box culvert (e.g. headwater depth and entrance configuration) determine the hydraulic capacity of the box culvert, and the culvert is actually capable of conveying a greater flow than the inlet would allow. Barrel shapes and tailwater depth are of no significance in determining the hydraulic capacity. Inlet control usually takes place for culverts lying on steep slopes.

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