Precast concrete box culverts have traditionally been used as channels to divert watercourses where new construction creates a barrier to the natural pathway of flow. They are strong, durable and are available in a wide range of sizes. This makes box culverts extremely versatile and has resulted in them being used for other applications including: surface water storage, attenuation tanks, pedestrian subways, access shafts, service tunnels, sea outfalls and crossings beneath roads.
Unlike structures manufactured from other materials such as steel, precast concrete box culverts do not require additional protective treatments to prolong service life or improve performance in normal conditions of use. They do not rust and the smooth internal finish of the concrete ensures efficient flow characteristics.
The most common loads applied to the design of a box culvert are:
- The unit’s self-weight
- Where a culvert is buried, the weight of fill above the culvert
- Where the culvert is installed under a road, the weight of road surface material (in addition to that of the fill)
- Vertical live loads, such as those from vehicular wheel loads or pedestrian traffic
- Horizontal live loads from vehicles braking or accelerating
- Where buried, horizontal earth pressure loading applied to the culvert’s side walls
- There will be a ground bearing pressure on the bottom of the box culvert
- There might also be water pressure from inside, where the culvert is carrying a river for example, or from hydrostatic pressure from outside where the culvert is in a high water table of flooded ground
Members of the British Precast Drainage Association (BPDA) design and manufacture box culverts specifically to meet particular applications and defined loading conditions. Box culverts which are to carry highway or railway loading are designed to the standards and specifications as stipulated by the client. Box culverts can be specially made to higher loading requirements. Some box culverts can take LM1/LM2 bridge loadings at 0.25m of cover.
BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk