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Reinforced concrete box culvert install


SH73 Arthurs Pass (Mingha Bluff realignment). Precast concrete box culvert installation beneath railway

Client: Hawkins Infrastructure

Stakeholder: Kiwirail

Principal Stakeholder: NZ Transport Agency - Highways and Network Operations

Duration: (Current)

Project Engineer (Hawkins): Julio Marti

Project Supervisor (Tru-Line): Andrew Powell

Consultant Engineer:  Peter Vandenende of Opus


Tru-Line Civil Ltd were engaged by Main Contractor Hawkins Infrastructure to provide a number of works packages, including drainage and culverts, on the $22 million NZTA Mingha Bluff Realignment contract on State Highway 73, just to the east of Arthurs Pass Village in the Southern Alps. This project is located at an altitude of 740m in a very remote, high precipitation alpine region of pristine native beech forest within a National Park. These conditions require careful planning and provision for a large number of contingencies including inclement weather, snow and ice, night work, sub-zero temperatures, the high conservation values of the National Park environment, and steep unforgiving terrain.

Excavating the 4.8m cut commences Included in the set of project works disciplines are a number of box culvert constructions beneath the Kiwi Rail Midland, or Tranzalpine railway line. To be undertaken at night, these installations involved rail line shutdowns, very tight installation and line reinstatement time windows. To date Truline Civil Ltd have completed 6 of these large box culvert structure installations, all within time frames that have set a new benchmark of speed, site safety, and satisfactory completion within budget.

Project stakeholders have congratulated Tru-Lne for the highly proficient, timely and safe delivery of these box culvert constructions. The Tru-Line Civil team have benefited from accumulating a mountain of valuable experience performing this Mingha Bluff series of six well synchronised Box Culvert constructions, the institutional knowledge that we have gained will be of significant benefit on similar projects in the future.

Precast reinforced concrete box culverts

Precast concrete box culverts are steel reinforced. There are several accepted ways to install the reinforcement and the overburden appropriately sizes the amount of steel reinforcement needed. 

Crane lowers box section culvert into positionThis precast concrete box culvert has specific loads associated with this site and conditions of use beneath the railway. These conditions include burial depth, rail traffic, waterway loading, and impact loadings. The reinforcement in the precast concrete box culvert is significant and that same reinforcing along with quality concrete and installation ensures at least a 100 year life cycle.

Excavation and preparation of the bedding foundation

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 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 and the water went to ground. The water was not allowed to run straight into the 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 subgrade was proof compacted.

Bedding under the box culvert beneath the railway has been designed and formed 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. 

The thickness of the bedding material was a minimum of 15 cm, 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.

Installing the precast box culvert

The 1.5 m box culvert sections were crane lifted in and laid on the compacted granular backfill to the precise, g.p.s. specified line and laser-proven grade and depth. The bedding design for the box culvert was designed by the Opus Engineer who is knowledgeable about the Arthur's Pass ground conditions, on the River Bealey riverbed, which thereby ensured a successful installation.
Reinforced concrete box culvertThe proper installation of precast concrete box culvert typically starts at the downstream end and works back up stream with the culvert spigot end facing downstream and the bell end facing the upstream side. Variance from this norm was required to ensure the centre section of the culvert conformed to the precise depth beneath the rails specified. Once the centre section was accurately placed, installation proceeded first downstream, then upstream of the centre. These installation practices were consistent with the engineer’s design assumptions.

Installation of 10.5 m culvert beneath rails took place between 4:55 P.M—10:55 P.M., a total of 6 hours.

Box culvert sealing 

The best method of sealing the compression joint between the two mating concrete sections was the hydraulic application of a force of 80 tonne longitudinally to the 10.5 m culvert in order to optimally compress the sealant material. The sealant used was designed to resist hydrostatic forces at the box section joints to provide a watertight seal. The sealant provides a reasonable level of assurance for performance in a hydrostatic conditions.  

Shape: Sealants work best at higher rates of compression. As the sealant is compressed, the resistance force for further compression increases as a function of the percentage of compression.

Size: The size of sealant applied was determined by the volume needed to fill enough of the void to prevent a hydrostatic force being capable of pushing the sealant.  
Placement: Best practice was followed, the sealant was placed in the annular space. The sealant material was caulked around the bottom and sides of each interior joint.
Adhesion: The sealant adhered well to the clean, dense, well cured concrete surface.

Tension the six threaded rebars

Immediately after the final upstream section of culvert was placed at 9:10 pm, the 6 x high-tension steel threaded rebars were completely pushed through the 10.5m culvert. The multi-thread 80 ton post-tensioning of the culvert was executed using a Hydraulic Steel Rebar Tensioner. The task of tensioning the first tierod got underway just before 9:30 pm.

Placement and affixing of the two downstream wing walls

At the same time as the multi-threads were being post-tensioned, the balance of the TruLine crew completed the placement, joining and reinforced concreting in place of the two wing walls at the northern end of the culvert. This task got underway at 9:28 pm and was completed with the affixing of the two wing wall’s temporary prop at 10:45 pm. This event marked the end of the culvert install task.


Backfilling started 30 minutes after the box culvert had been installed, the reinstatement to match the original ground conditions and of the rails took place between 11:30 P.M–4:10 A.M., a total of 4 hours 40 minutes. The trench was filled evenly on each side up to the level of the top of the culvert, care was taken to not disturb the box culvert alignment. Fill was done in well-compacted layers of AP65 with the layers not exceeding 30 cm. The compacted haunching extended 1 m either side of the culvert at the invert, and this fanned out progressively wider as we came up to 1.2 m over the the 2.4 m square box culvert. Then to finish, 600 mm of railway ballast was laid up to the rail level, at which point the trench was completely backfilled. The backfill was matched to existing ground condtions and extended out to the end of the North Side wing walls. The box culvert installation job was complete when the rail was lifted, levelled and reinstated. 

Download the print ready PDF of the Mingha Bluffs case study.

Precast concrete box culvert installation slideshow

Box culvert installation slideshow 4:54 p.m–10:53 p.m. documents the 6 hour install process. Click to view the Mingha Bluff box culvert install slideshow >>

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TruLine Civil is determined to incrementally improve the service we deliver our clients, and the quality and range of civil engineering services that we offer to owners and stakeholders who expect excellence.

Excellence in Infrastructure.

TruLine Civil.

Interested in our services? You can contact us +64 (21) 228-5090 and ask for Geoff Powell or e-mail me at 

Christchurch Address 

Greywacke Road extension,
New Zealand 

Slideshow controls: [1] Double-click to "pause" any slide, [2] Double click to "play"/continue the show. [3] Jump to any slide in the show by clicking on the relevant thumbnail. [4] To escape at any time click "x" top right.

Credits: Photos and case study documentation by Shaun G. Waugh


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Reinforced concrete box culvert install

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