This slideshow requires JavaScript.

6:00–8:00pm, excavation and preparation of the box culvert bedding foundation, placement of the first three sections

Tru-Line Civil Logo. Brands for New Zealand companies, Greymouth, New Zealand.The successful installation of precast box culverts begins with special site preparation for the foundation, typically referred to as bedding, on which the culvert will rest. TruLine Civil Is an experienced Box Culvert installation contractor who understands the importance of bedding the culvert structure properly. The excavation cut was down to 4.8m below the rails. To alleviate the problem with groundwater below the culvert bedding foundation, a drain was dug down to a half a meter beneath the culvert to drain away the the silted ground water. A settling pond was excavated for the dirty ground water to drain into. From the settling pond the water went to ground. The water was not allowed to run straight into the Bealey river. With the surface and subsurface water controlled the site preparation conditions for the bedding gravels were dry. During installation provision of dewatering methods were available on the remote Mingha Bluff site in case they needed to be used to ensure the subgrade was dry. Unsuitable, unstable materials below the plan foundation were removed. After the appropriate excavation was performed the sub grade of river-run gravels was proof compacted.


Each of the 7 x 1.5m box culvert sections was craned into position. Negotiating the overhead rail and fragile underground services without interrupting the flow of the installation takes skill and expertise.

Bedding under the box culvert beneath the railway has been designed and formed so as to be able to support the full load of the installed pre-cast box culvert, its contents, and the loading of the overburden and rail traffic above.

Detail of caulking on the compression joint between the two mating concrete sections.

Detail of caulking on the compression joint between the two mating concrete sections.

The thickness of the bedding material was a minimum of 15cm, as required by the project specifications. The bedding comprised of medium granular crushed AP20, it was installed and compacted and precisely levelled to provide uniform support for the full length and width of each box culvert section. This granular base material was sufficiently fine to achieve a level bedding surface. The final grading for the bedding was achieved with a laser. For the final grading, the granular material was screeded. The final grading allowed an easy crane-lift installation to set the box culvert sections precisely to marked targets on the foundation. The time and care taken ensuring the box culvert bedding preparation was done correctly enabled for very swift and accurate placement and fitting of the 7 x 1.5 m individual culvert segments.