Where a pipe is required as part of an embankment construction, it may be installed by compacting layers of fill uniformly on either side. It is important to bring the layers up uniformly on either side of the pipe. After a sufficient layer is compacted over the top of the pipe, ordinary embankment construction may proceed. Alternatively, some agencies require that the embankment be constructed first, then a trench dug for the installation of the pipe.
The open-trench method is commonly used for culvert construction. It is more cost — effective than tunneling except when a pipe must be constructed in an existing high fill. Shoring may be necessary, particularly if the installation is under a traveled way. This will keep the limits of excavation to a minimum and, by the use of steel cover plates, allow the roadway to remain open during nonworking hours. Where it is necessary to use an open-trench method of construction in urban areas, it is wise for the designer to make available to the contractor options for the type of structure to be placed. For example, if a box culvert is deemed necessary by the engineer because of hydraulic considerations and physical constraints, a precast concrete or a prefabricated metal box, as alternatives to cast-in-place construction, should be permitted. In this manner, the traveling public experiences a minimum of disruption of service when open-trench construction is used. AASHTO recommends a trench width equal to 1.25 times the outside diameter of the pipe plus 1 ft (300 mm) for concrete pipe and a width to provide for 2 ft (600 mm) minimum on each side of the pipe for flexible culverts. However, some states simply recommend a constant clearance between the outside of the pipe and the trench wall to ensure that there is room for compaction and compaction-testing equipment.
