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Take me to the river

Strain gauges have been used for the first time to monitor the performance of a geotextile strip reinforcement system for new approach embankments to a bridge in North Yorkshire.

Refurbishment and strengthening of Carlton Bridge over the River Aire in North Yorkshire is due to finish this month. As well as strengthening the steel truss structure, the project has included replacement of the concrete approach viaducts which have reached the end of their design life.

Two new approach bridges and two new embankments take their place. The earth structures, constructed by main contractor Costain, are supported on piles and a basal reinforcement platform built using a single layer of geogrid, instead of a more conventional multi-layered geogrid system.

The ParaLink strip system was developed by geosynthetics manufacturer and supplier Maccaferri. It consists of a single layer of high strength, high modulus polyester-based geogrid strips from Terram, placed at piling cap level, to carry the load between adjacent caps.

Maccaferri says the system is more economical and can be installed faster than conventional systems. At Carlton Bridge, for the first time, the system's performance is being confirmed by strain gauges fitted to the strips.

The northern embankment is on a curve, and it is the first time ParaLink has been used on such an alignment.

The A1041 crosses the River Aire between the villages of Snaith and Carlton, south of Selby, over the Carlton Bridge. The Highways Agency, Area 18, awarded the refurbishment contract to Costain, with consultant Halcrow supervising site works that started earlier this year.

Work on the embankments was let on a design and build basis. Costain's original plan was to reinforce the embankments using a multi-layered load transfer structure but after consultation with Maccaferri, it chose the ParaLink system.

The system replaces designs based on BS8006 for reinforced earth structures that have been the norm for the last decade, says Maccaferri. These are typically based on a system of multiple layers of blanket geogrid within a 800mm deep granular layer.

The firm, together with manufacturer Terram, has sponsored research into alternatives to these designs. This has been carried out over the last three years by Darren Russell and Nick Pierpoint at Mott MacDonald and has compared conventional designs and strip systems using numerical analysis techniques.

Results showed traditional design assumptions were over-conservative in some areas and inadequate in others (Ground Engineering, November 1997).

Additional research concluded that the Guido method of analysis favoured by some geogrid engineers was not applicable to threedimensional issues. It was argued that using a two-dimensional approach to solve a threedimensional problem was inappropriate, and the problem should be addressed using 3D numerical analysis.

The ParaLink system was developed from this research. The strips provide high unidirectional strength, are supplied in a range of strengths from 100kN/m up to 1250kN/m and are custom-made for individual projects.

Three-dimensional analysis was used to specify materials for the Carlton Bridge embankments, and verified using the VIPS finite element analysis package from software house Visage.

Ground conditions are soft clay over peat and clay with bedrock at about 12m depth, which meant the embankments had to be piled. To reduce the size of the piles, Costain decided to use lightweight PFA fill from local power stations to build the structures.

Stent Foundations designed and installed the precast concrete piles for the embankment.

These were put down at 2. 75m centres and were fitted with 800mm square pile caps to comply with the needs of the reinforcement design. The ParaLink strips are laid directly on to and fixed to the pile caps before being covered with Terram geotextile and a layer of recycled railway ballast.

Consultant Halcrow wanted instrumentation of the subsoil and strain readings from the geogrids to establish that performance specification was met.

Instrumentation manufacturer and supplier Soil Instruments installed profile gauges, piezometers, extensometers and inclinometers under each embankment and fitted strain gauges to the ParaLink strips. The strain gauges, normally used to measure strain in aircraft bodies, are set to the anticipated range of strains in the geogrid.

Performance specifications for this type of structure have not been verified before, other than ultimate surface settlement. Maccaferri says monitoring will provide valuable feedback on the true mechanics of the embankment and its effect on the surrounding subsoil.

Another geosynthetic, ParaGrid, was used to reinforce and steepen the sides of the embankments, reducing the footprint, the amount of piling and the amount of material needed. The material has bi-directional strength and is available in strengths from 30kN/m to 150kN/m in the main direction and 15kN/m to 100kN/m in the secondary direction.

Maccaferri says the system is designed for structural applications with a 60-120-year design life.

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