Precast concrete box culvert

BISON PRECAST
CAN ACCOMMODATE
A VARIETY OF SPECIALIST
MANUFACTURED COMPONENTS
TO SATISFY SCHEME
DESIGN REQUIREMENTS

DESIGN AND DETAILING
Bison Precast Box Culverts are designed in accordance with BS EN 14844, Eurocodes, PD6694, BS EN 13369, BS EN 206, BS 8500 and BRE Special Digest 1.

Exposure and cover requirements
Depending on depth of fill, proximity to a highway, and environmental conditions, units will be designed in accordance with Table 1. Minimum concrete grade used is C45/55 with a maximum aggregate size of 20mm. Please refer to BS8500 Table A.1 for further details.

Breaking and acceleration
In accordance with Eurocodes, the breaking and acceleration force generated from traffic loading needs to be considered.

Surface loading and fill depth
Loading applied at the ground surface and weight of fill material produce a combination of vertical and horizontal forces on the box culvert. Surface loading may be specified as a standard loading type, equivalent uniform loading or individual wheel loads. The critical load on a box culvert can occur at minimum or maximum fill. Each enquiry for a box culvert should state the minimum and maximum fill depth and the amount or type of surface loading. It is recommended that the minimum fill depth should be not less than 200mm or one-fifteenth of the internal width of the box culvert if this is greater.

1. The standard range of box culverts generally have flat inverts and 190mm corner splays up to 4800mm span, and 225mm splays from 5100mm to 6000mm span, and a maximum length of 2m.

2. Sizes other than those stated can be manufactured to customer requirements.

3. Special internal profiles, shaped inverts and dry weather flow channels can be produced on request.

4. Tapered units for bends, units with manhole openings and pipe access holes can be produced on request.

4. All box culverts are manufactured to order and to the specific required design criteria. The external loading conditions will govern the wall, roof and floor thicknesses, unit length and reinforcement content.

5. Joints are a standard rebate within the wall of the unit. Box culverts can be jointed using sealant strip to provide a seal and flexible joint if required.

6. Special insert pins are cast in to each box culvert to facilitate lifting. Please size by span x height. All dimensions are internal.

It is the contractor’s responsibility to offload the box culverts on delivery. A hard level access area that can be used safely by standard articulated delivery vehicles is required.

The contractor should provide a crane of adequate capacity for lifting the culvert.

For reasons of safety and economy, certain box culverts are delivered to site on end rather than as laid, and will require a safe method of turning during offloading.

A data sheet giving guidance of lifting and turning is available and is issued to clients prior to the first delivery.

The offloaded box culverts should be placed carefully on a firm, level base away from the edge of the trench. Any further movement should be done by lifting – the culvert should never be dragged or dropped.

Box culvert sections usually have rebated joints and can be laid open or sealed using preformed strips and/or pointing materials.

Reference should be made to the jointing material manufacturer’s specification and recommendation for use of the product.

A system using a preformed strip within the joint is most commonly used. When the strip is bitumen based, the joint faces should be cleaned, primed and allowed to dry. The strip is then applied to the internal corner of the socket just before the culvert is laid in the trench.

Joints are closed to a nominal gap by pulling against previously laid culverts, with an applied load of approximately one tonne per metre of strip plus about half of the weight of the culvert unit to overcome base friction, less if the unit is suspended from the crane whilst jointing. Heat may be required to soften the strip when working at low temperature.

When the box culvert is of sufficient size for access, it can be pointed internally with an elastomeric or bitumen-based material using a suitable primer. Not all methods of jointing, however, should be expected to be completely watertight.

Excavation can be kept to a minimum with only nominal working space required on each side of the box culvert. When working in trenches, normal requirements for health and safety must always be observed.

The base of the trench should be uniformly prepared before laying a 200mm bedding of compacted granular material over the full width of the trench. A surface blinding of the fine material will assist levelling. Local packings are subject to settlement and should not be used.

As an alternative to granular bedding, a concrete blinding layer is sometimes preferred to protect the formation or to allow a faster rate of laying the culverts. A layer of unreinforced concrete approximately 75mm thick on a trench bottom, which has been well prepared to provide a uniform support, is generally sufficient.

A culvert line is usually laid directly on the bedding, starting from the downstream end with sockets facing upstream to receive the next culvert.

The trench should be backfilled as soon as possible after the culvert has been laid and should be filled evenly on each side of the trench. Backfilling should continue in compacted layers of 200mm until the required depth of cover is reached.

Where loads from construction plant may exceed the design load of the box culvert, protective measures will be required. This is particularly relevant at shallow fill depths.