Culverts

Box Culverts are generally designed as a bespoke product based on the following minimum parameters;

- Minimum & maximum fill over the culvert.
- Traffic loading conditions.
- Exposure Classification.
- Internal span and internal height.

The factors above are used to establish (for example) cover to reinforcement and reinforcement content, which in turn allows the final geometry of the culvert to be established. So a variation of wall/base/roof thickness's need to be accommodated based on such design parameters.

Box culverts can be designed to satisfy a wide variety of situations which mainly relate to the depth of fill over the units and vehicular traffic. Some culverts can take LM1/LM2 bridge loadings at 0.25m of cover. Box culverts can be specially made to higher loading requirements. This document explains this in more detail.

The minimum allowable ground bearing pressure will be dependent on the self-weight of the culvert, the traffic load applied, any construction loads, e.g. plant equipment trafficking over the culvert, the depth and density of the fill material placed on top of the culvert.

MPA Precast Drainage members can provide data for the predicted bearing pressure due to the culvert self-weight and fill material information supplied by the customer and the traffic load when disbursed through the fill material. However, the decision regarding suitability of the ground conditions is the responsibility of the main contractor and their geotechnical engineer.

The minimum classification is XC1 and this increases to XD3 for the most onerous condition. The actual exposure condition depends on the position of the culvert in relation to the carriageway in terms of horizontal distance and depth below, i.e. depth of fill. There could be some relaxation if it is confirmed that the carriageway will not be gritted with de-icing salts.

No protective layer is normally required. However, there may be a requirement based on the culvert location and design specification. The designer and installer need to consider the specific details for each project and discuss these with the box culvert manufacturer.

This can vary depending on the location of the installation and the client’s requirements. In most situations the design and specification of the backfill will be based on the Specification for Highway Works or defined by the checking engineers/authority.

When using traditional types of joint material, e.g. bitumen based sealants, it is important to clean the surface of both faces of the joint and apply a coat of primer, if it is required by the manufacture. With these types of materials it is important to heat the material e.g. with a blow torch, to ensure that the jointing material is pliable and that it adheres to the surface of the box culvert. In cold weather it may be necessary to continue to apply heat to the material whilst the units are being pulled together. We would recommend that the material is stored in a warm environment, e.g. the welfare facility, so that a minimum amount of heat is required.

Suppliers such as Piletec, MGF and Mabey can usually supply suitable equipment - generally of the internal puller type.

Specific finishes can be provided but will incur a cost premium. A standard finish culvert would be offered as struck from a steel mould or F2.

Yes holes can be provided within box culverts. However, it may be necessary to increase the roof or wall thickness to accommodate these. For large openings it may be necessary to split the opening over two units which may require some re-alignment of the incoming service/pipe so that it is located at a joint position between the two units.

This will depend on the moulds that MPA Precast Drainage members have. As a guide a 6m wide x 3m high are the maximum dimensions. Larger culverts are generally cast in smaller lengths, e.g. 1m as opposed to 2m, to meet the crane requirements of the manufacturer / site and to utilise standard haulage options. It should be noted that smaller length units have reduced capacity to accommodate holes. An alternative to a single unit would be to provide two smaller units that can be placed side by side. The range of sizes available from each MPA Precast Drainage member can be found here.

It is recommended that the external face of the joint is filled with mortar to protect the sealant material. It may be advisable to also fill the inside face to improve the hydraulic performance of the culvert run and to enhance the joints performance above the minimum requirements.

This will be determined by the clients requirements.

Yes there are various options available which can be provided by the manufacturer and added at the time of manufacture.

No it is not normally a requirement to fill in the lifting points.

Culverts are designed to clients specifications whereby exposure conditions and subsequent concrete mix design / cover requirements are in accordance with BS8500-1. The options are for a 50 or 100 year working life. In practice most designs are for a 100 year working life.