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CRACK TEAM

GEOSYNTHETICS

High strength geosynthetics are spanning fissured and fractured bedrock beneath a section of the realigned A5 in Staffordshire.

The last single-lane section of the A5, between the M42 and the A38 in Staffordshire, is about to be replaced by a new 5km stretch of dual carriageway.

The £40M Highways Agency scheme, between Weeford and Fazeley near Tamworth, is due to open to traf'c later this summer.

A section of the new road passes through a deep cutting in Hints Hill to the north of Hints village. Past tectonic uplift at Hints Hill created parallel tension faults, resulting in fissures and fractures in the underlying bedrock. Sands and gravels next to the route have been extensively quarried, providing anecdotal evidence of crown holes developing in the Hopwas Breccia Sandstone of the quarry floor.

Borehole logs produced during the site investigation provided further evidence of voids, often recording 'loss of drilling fluid' and 'dropping of drill casings'.

During construction additional geophysical investigation and visual inspection, combined with high pressure water jetting, con'rmed the presence of fissures along a section of the carriageway. The revealed bedrock was lightly fractured with narrow fissures, generally less than 100mm wide (some are much wider - over 1m) but several metres deep.

Typically, they run perpendicular to the alignment of the carriageway.

Geologists believe there are two possible reasons for the fissuring.

The first is that tectonic uplift may have caused cambering of the sandstone, resulting in tension cracks, or gulls, along joints in the bedrock.

The second theory is that a process similar to Karst erosion in limestone or chalk may have created cavities in the sandstone when the water table was higher.

It was decided that a layer of high strength reinforcement was required to span the voids created by the ' ssures. The geosynthetic used is Huesker Stabilenka woven polyester reinforcement. With an ultimate tensile strength of 300kN in the warp direction and 45kN in the weft, it provides the strength and strain characteristics to span the voids for the design life of the road.

The installation method was to over-excavate to 1m below formation level along the entire 650m length of the affected carriageway. Generally, the fissures were left exposed before placement of the woven geosynthetic. However, revealed fissures greater than 1m wide were excavated and the sides of the rock cleaned with pressure jetting.

Site workers then backfilled with single size stone to within 1m of the formation level, with the top metre filled with a mass concrete plug.

Reinforcement has been installed across the width of the carriageway, with the principal strength direction laid parallel to the centre line. The design was carried out in accordance with the BS 8006: 1995 Code of Practice for Strengthened/Reinforced Soils and other Fills.

This contains strict surface settlement serviceability criteria which were used to determine the allowable design strain in the supporting geosynthetic layer.

A similar design was used for the carriageway drainage runs. Here a 150kN/45kN Stabilenka is preventing loss of bedding and back'll materials into the voids and providing support to prevent localised differential settlements of the drains.

The Highways Agency awarded the early contractor involvement target cost contract to a joint venture between Interserve Project Services and Hanson, with Parsons Brinckerhoff as design consultant.

Building began in April 2004 and by last month more than 3km of the new road surface had been laid with installation of white lines, safety barrier, drainage channels and traffic signs to follow.

Over 60% of the proposed hedgerows and woodland areas have already been planted, a year earlier than originally planned.

The new carriageway has been surfaced using stone mastic asphalt which creates less noise than traditional surfacing materials. When it rains, the surface texture allows water to pass through it, reducing spray and making the road safer in the wet.

Traffic is scheduled to switch on to the new A5 later this summer.

This will allow completion of the tie-in works at the eastern end of the scheme.

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