Reinforced soil, discrete panel faced walls

Jon Lawson

One of the first reinforced soil, discrete panel faced walls (MacRes) supporting live railway tracks constructed in the UK is in Berkshire, The Reading Viaduct.

The Reading Viaduct carried its first train in 2015 and cost £45 million with a total length of 2000m. It was built to ease the bottleneck which troubled the railway system for years.

The construction

Balfour Beatty constructed long approach ramps to raise the line. Working within 15 metres of the existing operational rail line meant that conventional earthwork ramps were not feasible, so Maccaferri were brought in to design and construct reinforced soil walls compliant with the tender from Network Rail Consulting Engineers, Atkins.

The tender outlined the suitable solution would be a vertical, mechanical reinforced soil wall (MSEW) with discrete concrete panels to one side of the ramps with a 1:2 slope on the other. This solution was built around the tight space constraints.

To provide a suitable foundation for the train loaded ramp/MSEW structures, ground improvement was necessary due to the variable nature of the existing soils.

Atkins proposed vibro concrete columns (VCC), with the ultra-high strength Paralink geogrid reinforced Load Transfer Platform (LTP) to transfer the embankment loads and to ensure settlement would be kept within the acceptable limits for live rail traffic.

Coffey Geotechnics Ltd, designed under the direction of Maccaferri, the LTPs using ParaLink. These geogrids are utilised in the LTP design to absorb, spread and dissipate applied loads vertically downwards into the piled ground, increasing the capacity of the soil to span or arch over the inter-pile space.

Coffey also designed the ramp soil reinforced walls, using MacRes with the polymer geocomposite strapping Paraweb, for the wall reinforcement.

This system using Paraweb was first used in the UK in the late 1970s.

The polymer reinforcement was used over traditional steel strapping. Rather than increasing or reducing the strap spacing to change the reinforcement strength as would be required with steel, different strengths of polymer strapping can be incorporated to change the capacity. At the Reading site, the strapping strengths vary from 27kN to 100kN.

Paraweb’s additional advantage is that it could be used in adverse soil conditions. This has enabled recycled material (such as PFA or crushed concrete) to be used as the structural fill, in the reinforced soil block.

Approx. 2,600sqm of MacRes panels system was installed.

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