Huesker Fortrac® polyester geogrids have been used to control differential settlement as part of the design and construction of the approach embankments to the new Welham road bridge on a newly aligned section of the A614 near Goole in East Yorkshire.
Under the ECC target cost contract, Mowlem Northern Civil Engineering was responsible for the design of the scheme which included value engineering during the design process. The client, East Riding of Yorkshire Council, had specified very tight limits on the allowable surface and differential settlement on the bridge structure and approach embankments.

The ground conditions in the immediate vicinity of the river consisted of five metres of peat and between one and two metres of soft silts and sands overlaying two to three metres of firm laminated clays. Sands and gravels, with mudstone, underly these superficial deposits at approximately twelve metres below existing ground level.

Due to the tight serviceability criteria and poor ground conditions, Mowlem, along with their designer Pell Frischmann, opted for a piled solution very early on in the design process. As Huesker geogrids had been used successfully on the recently opened A63 Selby Bypass, just fifteen kms west of the Welham bridge scheme, Graham Horgan, Huesker's northern application engineer, was asked for advice prior to detailed design commencing.

Construction Techniques

The embankment was constructed using driven cast in-situ, (DCIS), piles with flared heads to depths of between twelve and thirteen metres. The core of the main embankment was designed using PFA fill with a mine stone shoulder. These lightweight locally available fills reduced the loadings on the piles allowing the pile spacings to be optimised.

The original indicative design was based on a square pile arrangement with a reduced pile spacing beneath the bank seats and the geosynthetic reinforcement design based on the British Standard BS8006 approach. This approach details two orthogonal layers of high strength reinforcement. However, increasing the pile spacings slightly along the length of the embankment and decreasing the spacings across the embankment enabled the same grade of geosynthetic reinforcement, Fortrac® R800, to be used in both directions.

Huesker provided this reinforcement in custom made lengths to eliminate wastage and reduce handling on site. In addition, they were produced and delivered in five weeks so as not to affect the tight construction programme.

Limiting Differential Settlement

The longitudinal geosynthetic reinforcement was also extended thirty metres into the transition zone beneath the traditional embankments, beyond the first and last piles. Not only did this provide anchorage but, additionally, it helped to limit the potential for differential settlement between piled and un-piled embankments.

The PFA fill, which tends to exhibit a pozzolanic reaction, was omitted from the zone immediately above the edge piles and replaced with a granular wedge of a more dilatant fill. This also helped to compensate for differential settlements at the base of the embankment.

In addition, loose compressible soil was placed between the first six rows of piles at the low ends of the embankment to encourage the geosynthetic reinforcement to develop its tension whilst placing the first few layers of overlying embankment fill. This technique was originally used on the Bingley By-Pass to assist in reducing any surface undulations on thin embankments.

Improved Alignment

The 40 week design & construct contract for the A614 realignment was awarded to Mowlem Northern Civil Engineering in December last year and forms part of Mowlem's three year framework agreement with the East Riding of Yorkshire Council.

The scheme involved the construction of a new one km section of 9.3metre wide carriageway and a new bridge over the River Foulness. The existing bridge had been assessed as inadequate in previous inspections and a "temporary" bailey bridge installed to alleviate traffic loadings. As the bridge was on a relatively tight bend in the road, the decision was taken to build the new bridge on a new improved alignment with the old road being maintained to provide access to local farms.

The bridge and new road alignment opened to traffic on 1^st October 2004.

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