The design and construction of the A650 relief road around Bingley in Yorkshire, which opened three months ahead of schedule, provided main contractor AMEC plc and their design partner Ove Arup with several geotechnical challenges. The ground conditions along the route are highly variable. In particular, the north bog, which consists of highly organic peat, has moisture contents in the range of 350% and varies in depth from two to eleven metres.

The solution adopted for the settlement prone areas across the north bog was a low piled embankment, - a robust solution, which would have minimal impact on the adjacent railway line and provide cost and programme guarantees.

The main concern with low height embankments is to ensure that the deflection of the fill between the piles is controlled so that undulations at the base of the embankment are not reflected to the road surface. This was achieved using concrete piles and pile heads linked by Huesker's high strength, low creep Stabilenka® reinforcement geotextile.

Sandwiched largely between a railway and a canal, the new 4.7km long dual carriageway was constructed in three sections incorporating 28 major structures. It links the A650 at Cottingley Bar, south east of Bingley, to Crossflats to the north west of the town where it crosses the north bog adjacent to the railway to connect to an existing roundabout.

Piled Reinforced Embankment Design

Additional CPT testing, undertaken to confirm the exact extent of the peat deposit, revealed that it was underlain by glacial sands and gravels which increased in density one to two metres below the peat-glacial deposit interface.

The low piled embankment, approximately 440 metres long, was constructed using 350mm square, precast, driven concrete piles, varying in overall length from eight to twenty metres. These penetrated seven metres deep into the underlying dense gravels. In total, over 1800 piles were installed on a square 2.5 metre grid, with one metre wide square, precast, pile caps bedded onto the piles.

Controlling settlement was particularly important across the north bog since the height between the top of the pile cap and the road surface was only two metres. This problem was overcome by a combination of design and construction techniques. The design stress/strains in the reinforcement geotextile were limited; loose compressible soil was placed between the pile caps; tension was induced in the reinforcement by heavily compacting the initial thin layers of fill and any final settlement was induced prior to surfacing with a two metre high fill.

The design of the geosynthetic reinforcement was based on BS 8006, using ultimate and serviceability limit state criteria. However, at six per cent, the upper limit for serviceability strain was considered to be generous, which could lead to excessive settlement of the road surface. Instead, a lower design strain limit, of three to four per cent, was adopted which gave a maximum design mid-span deflection of the geosynthetic equivalent to 160-185mm.

This specification was achieved over an area of 68,000 sq.ms., using a combination of Huesker's high strength, low creep woven polyester geotextiles - Stabilenka® 600 longitudinally and Stabilenka® 800 transversely - which resulted in a very cost effective solution when compared with solid piled slabs.

Deflection was encouraged during the construction phase by placing loose compressible fill between the pile caps. Once the layers of geotextiles had been installed, they were covered with thin layers of fill and heavily compacted. This had the effect of inducing the undulations in the embankment thereby tensioning the supporting geosynthetic.

Finally, when the embankment reached full height, it was covered with an additional surcharge, equivalent to two metres of fill - greater than the expected traffic loading - to induce any final settlement prior to surfacing the road.

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