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Wigram–Magdala Link Reinforced Earth® retaining wall


New Civil Structure: A Reinforced Earth® retaining wall, part of the Wigram–Magdala Link Highway upgrade, Christchurch.

Client: Christchurch City Council Capital Programme Group

Principal Stakeholder: Christchurch City Council 

Duration: 2015–16

Consultant: Opus


Project Overview

Project Engineer Luke McCarthy explains: “We have just over 2km of road to install with a large Reinforced Earth® retaining wall and one bridge abutment. We've got to build up 6m of ground conditions in order for a bridge to come up and over Curletts Road, which is classed as a motorway. TruLine Civil is responsible for pretty much everything on site other than the installation of the bridge and ground improvement works, so we have a full roading package, sealment, footpaths, streetlighting, road marks.

3D Google map of Magdala Place retaining wall siteThere is a new intersection to go down on Annex Road and Birmingham Drive with four sets of traffic lights and a whole new redesigned intersection down there. We've also got a large timber retaining wall to put in and a 6m abutment to build up on each side of the bridges. Other than that we have got to put in a drainage blanket area which is 7,000 sq/m, lay a high tensile reinforcement geotextile filter cloth then a 300mm drainage layer and re-wrap it over the top before we can build the abutment up.

On the West facing side of the abutment, due to the restrictions imposed by existing services and businesses in Magdala Place, we have got to put in this mechanically stabilised Reinforced Earth retaining wall, it goes up to about 6m high and has about 350 cruciform interlocking panels, they have horizontal reinforcing strips bolted to them which tie-back into the abutment which we will build up to the 6m height to hold the wall in place.

The abutment has got to sit for a 3 month settling period and the wall can actually sink anything up to 400mm, from design. After the settlement period has taken place then Concrete Structures will come in and install the bridge structure itself. The bridge has pre-fabricated components with it that will lifted into place over "live" lanes of traffic. All of that will be done as night works by Concrete Structures. Once the bridge is done and the installation is completed we will come back through, we've got guard rails to put in, we've got to surface the whole area and we have got to seal the whole 2km.

There's two new cyclepaths to put in and footpaths either side. We've also got a short roading package involved on Magdala Place and on Wigram Road. We've got some intersections to do associated with these. It is about a 60 week project altogether, so just over a year. The largest job that TruLine have taken on to date.

We also have a large drainage package within the job which has included twin 900mm culverts and twin 1600mm culverts.”

Maglala place Reinforced Earth® retaining wallBridge abutments are considered critical structures and the strength and the load distribution capabilities of Reinforced Earth® meet those needs in a structurally efficient way and the vertical retaining wall maintained the right of way for the businesses and road users of Magdala Place both during and after construction.

Don Asbey-Palmer, Reinforced Earth® construction advisor

Don Asbey-Palmer’s role was to assist TruLine in the implementation of correct construction procedures. Don explains:

“The face of the access ramp or bridge abutment on the Magdala Place side is a Reinforced Earth® wall, this is a technique that has been widely used since the 1960s for the construction of roads and motorways around the world. It was developed in France in the late 1960s, and was introduced into New Zealand in about 1979. The first structure in fact was a sea wall at the Port of Invercargill. 

Reinforced Earth® HA Ladder structure detailA Reinforced Earth® wall is, in principle, like reinforced concrete without the cement. The Wigram–Magdala Link highway bridge abutment Reinforced Earth® structure combines a select granular, engineered backfill of compacted river-run gravels with 5m–7m long horizontal galvanised steel “high adhesion” tensile reinforcements attached to a modular facing system, made of precast cruciform concrete panels, that face Magdala Place. This straightforward but unrivalled combination of materials creates a durable composite, a mass gravity retaining structure which in addition to supporting its own weight is able to support large dead and live loads imposed by associated structures such as the bridge, vehicles and the highway crash barrier guard rails.

Reinforced Earth® retaining wall half constructed Magdala PlacePresently, at the time of the photoshoot in August 2015, the panels are about 4m or so high, the fill is about 3m up and the construction of the wall is just over half done. The cruciform shape wall facing panels do not actually interlock, there is a system of filter cloth which is applied with adhesive to the backfill side of the panels to cover all the horizontal and vertical panel joints so you can't actually see through to the backfill. Elastomeric bearing pads are placed in the horizontal joints throughout the facing wall structure to prevent concrete to concrete contact. Rubber shims are also used as needed to adjust for minor variations in panel height. The back of the panels are wider than the front, you have to lower them into place with a light crane. The 5m–7m long reinforcing strips stabilize the facing panels which means they do not interlock, despite the appearance of the vertical wall facing.” 

Straightforward Principle

Reinforced Earth® is based on a simple principle. As originally conceived by its inventor, French architect and engineer Henri Vidal, the interplacing of soil and reinforcements develops friction at the points of contact between the two, resulting in a permanent and predictable bond and creating a unique composite construction material. In the case of the Magdala Place Reinforced Earth® wall, it is a composite material formed by the interaction between a granular frictional soil and the galvanised steel High Adherence (HA) reinforcing strips. In concept, the mass of granular river-run aggregate and reinforcement behaves like reinforced concrete; that is, Reinforced Earth® is an economical means of improving the mechanical properties of basic material, earth, by reinforcing it with another, steel. 

The Reinforced Earth® wall is built up from the back, from the building up of the reinforced volume, to the front. The reinforcing strips or HA strips are 50mm wide ribbed galvanised steel strips in this case, the composite mass resists stresses produced within the abutment’s 6m tall aggregate “soil” mass behind the wall; soil stresses are transferred to the strips via friction. 

Placing a reinforced concrete facing panelA Reinforced Earth® structure constructed using this technique is the “reinforced volume” behind the facing panels (above). The cruciform Precast Concrete panels are used at the face of the reinforced volume of the retaining wall to prevent erosion of the backfill and provide an attractive, finished appearance.

Risk mitigation

The Magdala Place Reinforced Earth® structure has been built to protect lives and properties against natural disasters. Great strength and ductility under ground acceleration are two essential features of the technique when the structures are exposed to earthquakes. Analysis of structures which have been actually affected by earthquakes, such as the one constructed at the eastern end of Blenheim Road in 2007, has demonstrated that Reinforced Earth® performed extremely well as an arterial highway construction material in the Christchurch earthquake events of 2011–12 and the ongoing subsequent aftershocks. The performance of that wall has confirmed that the safety level and the design were particularly well adapted despite the fact the active faultlines beneath Christchurch came as a surprise to seismologists and civil engineers in 2011–12.

Don Asbey-Palmer explains: “What happens when you are building up the wall is that in the strips tension develops as you put the backfill on it and compact it. So the fill settles and it tries to stretch the steel. If you put stress measurement instruments on these facing panels you'll find that there is a line where the tension is at the maximum, and it is not at the face, it is behind the face. What would happen if you didn't have the strips is that this vertical face would fail on about the shape you see in when a slip sloughs out, as it happens on a hillside, this is what we call the active zone. Another way to think of it is that the active zone is actually held in place by the mass of the passive zone. To design the retaining wall you need to understand what the driving force is for the active zone to fail, and then work out how much force you need to apply to hold it in place. 

In the event of an earthquake, the sum of all the minor movements occurring within the mass of the Reinforced Earth wall when the ground accelerates amounts to the wall having the capacity for a significant amount of energy absorption within it, so that the Reinforced Earth structure will attenuate the ground acceleration, the acceleration at the top of the wall being less than down at the base. This is what provides the energy absorption, or damping effect of the structure in an earthquake. 

Reinforced Earth walls in New Zealand are designed to the New Zealand Bridge Manual, what we do to analyse their structural strength is in keeping with what has been learned from the proving structures in Europe, which were basically rectangular structures or Reinforced Earth blocks. In New Zealand when Reinforced Earth blocks are subject to quite high shaking, we proportion the blocks, we will make the rectangular Reinforced Earth blocks say wider than they do in Europe or Australia. When they are carrying bridge loads, such as the Barrington Street, or Blenheim Road abutments, where the bridge beams are just sitting on top of the Reinforced Earth structure, they are not piled.”

Construction of the retaining wall

Place panel triptych

The construction methodology of the vertical Magdala Reinforced Earth® wall and bridge abutment structure required neither scaffolding nor heavy machinery. In addition to the dump trucks and earth-moving equipment used to place and compact the backfill, excavators were used as the light cranes required to enable 3 men to stack ready for use, move and install the concrete facing panels. Another team of 3 men were tasked with laying out and affixing the HA Laddder structure to the back of the facing panels on top of the compacted reinforced mass as it was built up, they then carefully placed, spread and compacted the gravels over the HA strips using an excavator, a walk-behind vibratory rolling compactor and a small plate compactor or “jumper” close to the vertical plane of facing panels. Great care is taken to prohibit all heavy earthmoving machinery and compacting rollers near the vertical wall face to prevent the reinforced mass being over-stressed. This precaution ensures precise alignment of the wall facing to the smooth vertical plane specified.

Installation of the facing panels along Magdala Place

After the first row of panels was placed on the well levelled, cast-in-place, smooth finish concrete pad that ensured the correct initial positioning or placing panels, then the upper rows of panels were gradually installed as the HA strip installation, backfilling, spreading and compaction operations proceeded. Elastomeric bearing pads were installed inside the horizontal joints between the concrete panels to provide enhanced flexibility and a degree of compressibility to the facing, as well as preventing damage to the panel edges by preventing the possibility of concrete on concrete friction along the edges.

>> Click to view the facing panel placement slideshow >>

To brace the initial course of panels a system of clamps, one per vertical joint for the length of structure under construction, and wooden wedges in a quantity sufficient to provide at least 4 to 6 wedges per vertical joint for the length of structure under construction is required. Clamps are used throughout the erection of the wall to brace the area under construction for each subsequent course of panels. Rubber shims, wooden spacers and wedges are used to ensure exact alignment of the wall to specification as it is being erected and to preserve the smooth aesthetic finish of the reinforced panel faces and joint profiles throughout construction.

>> Click to view the panel clamping and bracing slideshow >>

Fastening the HA reinforcing strip layers and backfilling

Fastening the reinforcing strips to the back of a facing panelDuring construction the reinforcing strips were fastened to tie strip connections embedded in the back of each facing panel using 12mm diameter bolts, washers and nuts made of galvanized structural steel.

The reinforcing strips are 50mm wide ribbed galvanised steel strips. The reinforcement HA strip layers attached to the facing panels are spaced 70–80cm apart vertically, this corresponds to twice the thickness of the backfilling layers of aggregate that are carefully spread and compacted in stratum 35–40cm deep. The steel HA strip reinforcements are installed on the compacted backfill layer and bolted to the facing panels. The fill around the steel is pH neutral to minimise corrosion. The backfill over the HA strips was placed in stratum 30 to 40 cm thick and compacted with various small walk-behind vibrating rollers or flat-plate compactors within 1m of the wall. The compacting rate at any point of the Reinforced Earth® structure was regularly Quality Assurance tested to ensure 95% of the Normal Proctor optimum for road backfills was achieved.

>> Click to view the HA reinforcement strip placement slideshow >>

Don Asbey-Palmer explains: “The reinforcing strips go back from the face of the wall, depending on the load and the height of the wall. So at the 6m high part of the wall, nearest the top, because there is a TL5 traffic barrier right on the top of the wall which imparts a high impact load into the top of the wall, the length of the layers of strips at the top is 7m and there are 5.8m strips in the top two layers immediately underneath that to take the impact loads. While at the eastern, low end of the abutment slope the strips are 4m long.”

Constructing the non-reinforced backfill mass

The backfill adjacent to the reinforced mass was placed in stratum 30 to 40 cm thick, spread with combinations of earthmoving equipment and compacted with various rolling vibratory compactors. The compacting rate at any point of the non-reinforced mass was regularly Quality Assurance tested to ensure 95% of the Normal Proctor optimum for road backfills was achieved. The river-run aggregate is dry, containing just the 3% moisture required to achieve compaction. The backfill conforms to Contract Specifications within both the reinforced and the non-reinforced volume.

>> Click to view the construction of the non-reinforced backfill slideshow >>

Retaining wall construction completed

Completed retaining wall, Magdala PlaceThese shots taken beside and on top of the completed Reinforced Earth® retaining wall 3 weeks later depict the smooth panel joints and clean vertical face of the Magdala Place retaining wall.

>> Click to view the completed retaining wall slideshow >>



Reinforced Earth® retaining wall Slideshows

>> The facing panel placement slideshow >>

>> The panel clamping and bracing slideshow >>

>> The HA reinforcement strips placement slideshow >>

>> The construction of the non-reinforced backfill slideshow >>

>> The completed retaining wall 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 

Christchurch Address 

Greywacke Road extension,
New Zealand 

Credits: Photos, infographic and case study documentation by Shaun Waugh


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Wigram–Magdala Link Reinforced Earth® retaining wall

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