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Stream Culvert under Embankment, Magdala Link


New Horizontal Civil Infrastructure: Twin 1600mm Reinforced Concrete Culverts, Stream Culvert under Embankment, part of the Wigram – Magdala Link Highway upgrade, Christchurch. A joint initiative from Christchurch City Council and NZTA.

A key component of the large Drainage Package for the Wigram Magdala upgrade project.


Client: Christchurch City Council Capital Programme Group

Principal Stakeholder: Christchurch City Council 

Duration: May 2015 

Consultant: Opus Consultants Limited

Project Overview

Project Engineer Luke McCarthy explains: “The large drainage package within the Wigram–Magdala Link job includes twin 900mm and twin 1600mm culverts. The twin 1600 mm culverts are laid to a radius of 107m for pipe 1 and 110m for pipe 2. That radius is at the limits that the rubber O rings are specified to reach so we have really got to pull each of the 2.5 m straight pipes around horizontally to conform to that specification. Then there’s actually an arching vertical displacement, or hog, in the pipeline where it passes beneath the bridge abutment, so the pipes are not laid on one gradient. This is to allow for settlement so as they drop down under pressure from the mass of the 6m tall Reinforced Earth® abutment above the twin pipelines, they will settle to be in alignment with the specified target gradient. So it is a particularly challenging aspect, especially with the work site alongside the motorway with lampposts and live services within the vicinity.” The large drainage package, in particular the lay of the twin 1600s, is one of the key features of the Magdala Link job, which is the largest job that TruLine have taken on to date. 

Magdala–Wigram Link dual 1600mm dual pipeline mapThe dual pipeline is being laid on a 200 mm hog, arching up and over, so that when the mass of the 6m tall Reinforced Earth® abutment that is going to be constructed above it bears down on the pipeline during the 6 month settlement period post construction, the engineers have calculated that the pipeline should settle 200 mm to the targeted gradient that they want the dual pipeline to come to in the finish.


Pipelaying Methodology in a nutshell

The common pattern of events, tasks and sub-tasks for the laying of culverts 1 and 2 conforms to the following iterative sequence; 

Twin 1600 culverts eastern headwall endHaunching is laid and compacted up to 1/3 of the pipe height anchors the pipeline. Once both pipelines are laid through their 107m and 110m diameters and they comply with the arching vertical displacement or hog then cover gravels are applied up and over the pipelines, compacting every 250mm of gravel depth. The Geotech cloth envelope is closed on completion of the pipeline backfilling.


Pipelaying Methodology

Adherence to specified gradient marked up by surveyor

Consistent with the plans orange pegs have been precisely placed to GPS coordinates, marked on each peg is the specific vertical heights and horizontal offset for each of the two pipes. Site foreman Edward explains:
The surveyor positions the pegs every 5m“The surveyor has precisely positioned the pegs every 5m, every second 2.5m pipe length joint, all along the pipeline. Each marker peg is marked up with the specific height and the offset from the peg where the two pipes should be laid. So we have a black horizontal benchmark on the peg and the pipe invert, or interior low-point, is 2.5m down from that horizontal plane. The distance down to the pipe inverts are 2.5m down from that horizontal plane marked on the peg. That's where we get our height from.”

The dumpy laser

“We've got the laser, the dumpy laser on the work site, that's the optical instrument we use to establish and check points are in the same horizontal plane. We use it to check that each 2.5m length of pipeline is offset, that the horizontal offset from the peg and depth to the invert is to specification.”

The depth of the pipe invert is measured using a staff“We measure this with a vertical staff to measure and set the heights consistent with the engineer's specifications for the sequence of height measures required to achieve the correct horizontal offset from the marker peg and gradient of the paired pipelines. The mark-up on the pegs, the dumpy laser setting the benchmark horizontal plane and our checking of each pipe length laid by measuring the depth from the horizontal benchmark down to the invert of the pipe with the vertical staff, down to the inside of the bottom of the pipe where the water will run, this method ensures we achieve the specified gradient.” 

Excavation, soil Quality Assurance test, foundation gravel and compaction testing and pipelaying Q.A. measurements

Site foreman Edward explains:

“The soil for the pipeline is excavated and the entire foundation aggregates, haunches and backfiill over the top of the pipeline is wrapped in a geocloth jacket. The foundation bedding aggregates are 300mm–600mm deep, depending on how suitable the underlying soil is, this is compacted with a 400kg plate compactor, the backfilled haunchings up to a depth of the first third of the pipe, these are manually compacted with a hand-rammer immediately adjacent to the pipe and with the “jumper” plate compactor.

Above that first third we have to compact in 250mm layers right to the top, with appropriate compaction testing carried out by TruLine with an approved measuring device. The results of these compaction tests are all duly recorded by us in the log to assure the gravels are compacted to specification. The minimum compaction density of the Drainage 40 aggregate, specified for stormwater concrete pipes by the City Council, must be achieved and recorded by us. The Drainage 40 contains no fine sand in it so it doesn’t compact as densely as say an AP65 or AP20 for example.”  

Settling of the pipeline over the 6 months after laying

Compacting the 1600mm culvert foundation gravelEdward explains that prior to the Wigram–Magdala Link overbridge abutments being constructed alongside Curletts Road and the Reinforced Earth® retaining wall being constructed along Magdala Place: “The settling of the pipeline is monitored for the next 6 months, a GPS position is set on every collar on every pipe and the settlement is monitored before they build the Reinforced Earth® abutment. The engineers have allowed a 200ml settlement with a 100ml tolerance. In a worst case scenario if the settlement after six months is outside of spec we would have to come back, dig the pipeline up and re-lay it to specifications.”

Dual pipeline lay worksite viewed from Curletts Road


01 work preparation slideshow >>

02 surveyors’ invert setout slideshow >>

03 precise trench excavation slideshow >>

04 ground testing and resulting foundation gravel depth excavation slideshow >>

05 Geotech cloth envelope and foundation gravel placement slideshow >>

06 foundation gravel placement and compaction slideshow >>

07 foundation gravel measurement and depth fine tuning slideshow >>

08 Pipe 1 pipe lay slideshow >>

09 Pipe 1 Quality Assurance invert depth measurement slideshow >>

10 the haunching placement and compaction slideshow >>

11 the haunching gravel resupply and trench shield advance slideshow >>


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.


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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 


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


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Stream Culvert under Embankment, Magdala Link

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