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Highway innovations: Wall-to-wall blocks

Two variants on the reinforced earth wall, one with a “green” vegetated facing, and one with stone-textured blocks, have been proving their worth in Ireland, reports Adrian Greeman.

New variants of reinforced ground embankment are part of the new Wicklow road project south of Dublin. They are relatively simple to construct, tolerant of poor ground and have an aesthetically pleasing finish.

Both main types used are built around a geotextile grid reinforcement from Italian maker Maccaferri, which, combined with a narrow outer layer of well graded fill Type 6N fill material, forms the outer facing of an embankment with a standard fill core. Outer facings used on the project vary.

Sturdy and green solutions

There are also block walls, supplied as Macwall Landmark and using interlocked facing blocks, with a patented groove system which helps position and lock them together while locking in the geogrid reinforcement behind.

This enables construction of a vertical retaining wall which is just four degrees from the vertical. More conventional gabions with geogrid reinforced ground behind are also in evidence.

There is a “green” solution which uses stamped down seeded topsoil which rapidly grows a plant facing, and uses plant roots as part of its structure.

“That gives a much better connection than pure friction which many other retaining wall solutions use.”

Craig Douglas, Maccaferri

The embankments are needed as part of a major relief road construction programme for the Irish Sea port town of Wicklow, some 70km south of Dublin (see box below). Wicklow is no giant container terminal but the 3,000t coasters which it does handle can generate significant truck traffic.

For the narrow streets of the town the big articulated lorries are major contributor to jams and congestion.

The Macwall Landmark system is used on a section of route where land take has to be minimised as the road curves past the lorry park for a local logistics firm. Here the contractor has used the specially made blocks to create a near vertical facing for a reinforced ground retaining wall using Maccaferri’s soil reinforcement geogrids. Engineering design for this section of the work is done by the supplier, an option open to the contractor, although most of the contract is done to a conventional design from Arup.

The embankment side wall is built up in two sections, a back part of reinforced ground using granular fill compacted in layers over a geogrid. The geogrid connects at the front into a simple dry built block facing, assembled using shaped blocks which each have a tongue and groove locking system. The grooves help position the blocks as they are assembled, and help to hold them together without the need for mortar.

Keeping in position

Tie bars inserted into the groove also push through the geogrid, helping to keep it in position. “That gives a much better connection than pure friction which many other retaining wall solutions use,” says Craig Douglas, from Maccaferri’s Belfast office. He says the polymer is expected to last 120 years.

The granular fill tapers with height, starting with a 6m wide layer at the base and finishing about 2m wide. The block facing has a slight inward slope of 4°. Fill placed in layers to match the 320mm height of the blocks, although higher up the grid one layer is the height of two blocks.

For the contractor, the wall was a good solution since it allowed a flexibility and ease of construction. It also allows the simultaneous construction of the main width of the embankment behind, where a normal class one clay type fill could be used. On most projects this would probably come from elsewhere on site although some fill was imported for the Wicklow job.

Bearing capacity is important for a structure like this wherea reinforced concrete structure might have been too heavy. The 290m long wall, reaching a 7.2m height sits on loose fine gravels. Another embankment on the project uses a different solution.

The scope of the project

Map

The first 1.8km section of the project crosses a river estuary, the Broad Lough before passing over a single track railway line.The second part of the project is to provide a new 2.7km back road into the town to service new housing estates on the surrounding hillsides overlooking the coast.

The two new road sections are separated by a single track railway. A third, 800m section of road, will link the other two sections via a new bridge over the line.

“Much of the project is relatively conventional road layout,” says Arup resident engineer Duncan Cole. Arup is design consultant for client Wicklow County Council’s €33M (£28M) project.

“The project also includes provision for substantial service routing, including two 1500mm diameter tunnels through a railway embankment” says Cole.

Four structures are needed, two bridges for the railway which loops back from the town, a small service culvert, and a crossing of the Broad Lough.

“The project also includes provision for substantial service routing, including two 1500mm diameter tunnels through a railway embankment.”

Duncan Cole, Arup

“That is the biggest,” says Colin Cleary, project manager for the SM Morris-Coffey joint venture contractor which is carrying out the scheme. It is a 154m long post-tensioned box girder with a central span of 58m.

Fortunately the river and its marine estuary lough is only a maximum 2.5m depth and with limited small vessel traffic, so it was relatively straightforward to extend cofferdams from either bank to enable contractors to cast all but the 11m central section of the deck in situ.

The bridge is founded on 900mm diameter bored friction piles through loose gravels, installed to a maximum 28m by Bauer Foundations.

Just beyond the Lough a conventional embankment keeps the road at height for the short intermediate section until it reaches the 62m long concrete box bridge across a railway line.

The embankment continues beyond the railway and here the Landmark block anchor wall has been used to reduce land take.

 

It is being built up to raise the road by 10.5m as it approaches a new railway bridge. This embankment sits on peats and clay layers. It is not vertically faced at this point but a reinforced ground solution allows it to have a relatively a steep face of 60° enabling the contractor to save on earth fill and space. It is also less likely to overload the ground, which a concrete structure might have done and thereby eliminates the need for piling. Some four months of surcharging has been used for the whole embankment width to take out the worst of the expected settlement.

The contractor chose a “green” option for this area, using Maccaferri’s’ Green Terramesh. The structure is again built up with a tapering reinforced granular fill at the edge of the embankment with a normal site fill making up the internal bulk of the embankment.

Twisted wire Terramesh panels form the outer edge of the embankment. They rest on facing panels which are placed onto the granular fill at the required angle. The very front 300mm depth, however, is filled with a topsoil which is “compacted” much in the way a gardener tramps in plants, with boot pressure.

The topsoil is seeded and perhaps unsurprisingly in the lush rainy climate of Ireland, the facing on Wicklow’s embankment was already greening when the embankment was only half way up.

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