Bedding, laying & backfilling
Excavation can be kept to a minimum with only nominal working space required on each side of the box culvert. When working in trenches the 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 the sockets facing upstream, to receive the next culvert.
The trench should be backfilled as soon as possible after the culvert has been laid and it should be filled evenly on each side of the trench. Backfilling should continue in 200mm compacted layers to reach the required depth of cover.
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.
The culvert sections generally have rebated joints and can be laid open or sealed using pre formed 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 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.