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.
However, box culverts do possess some advantages.
For example, they 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.