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GROUND IMPROVEMENT CONTRACTS

Dynamic compaction and preloading have been used to treat old landfills and silt lagoons on the route of the new A6 Clapham Bypass in Bedfordshire.

The £26M contract for the 5km dual carriageway was awarded to joint venture main contractors Edmond Nuttall and Norwest Holst Construction by the the Highways Agency in March 2001. Design is being carried out by consultant the Symonds Group. URS Thorburn Colquhoun is the Agency's site agent.

The road runs to the west of the village of Clapham. From its northern end it swings west from the A6 in shallow cut and fill. It then runs south, climbing the northern flank of Oakley Hill where it enters a cutting which is up to 12m deep.

From here it heads south east on a 3m high embankment, then in cutting to a grade separated junction before going down and and over the Great Ouse River. The road then runs over the valley floor, east of a railway.

Precautions are being taken to ensure there are no obstructions to the flow of water if the river floods.

Within a broad river meander to the east, the route passes on a shallow embankment over refuse tips and silt lagoons. After crossing the river again it rises on embankment through a former water abstraction wellfield and joins the existing A6. The wellfield was infilled under a separate contract.

Solid geology formations in the area are broadly parallel with each other and generally horizontal. The Great Oolite Limestone and the Cornbrash form the main scouring platform within the valley floor. The Kellaways Beds and the Oxford Clay are found on the relatively high ground at each end of the route and at Oakley Hill. Beneath the Upper Estuarine Series in the meander of the Great Ouse is a variable sequence of siltstones, sandstones and limestones interbedded with clays.

Glacial Till occurs on the high ground. At lower elevations within the valley itself there are spreads of Terrace Deposits and more recent tracts of River Alluvium. Economic deposits in the area include Oxford Clay, formerly used for local brick making, and the Terrace Deposits, a local source of sand and gravel.

Dynamic compaction was used to treat most of the 100,000m3 of landfill on the route, although after trials preloading was used immediately next to the railway to limit vibrations in sensitive embankment structures. Preloading was used on the silt lagoons because they are relatively freedraining and thin. This avoided offsite material disposal.

Structures are generally supported on spread foundations and work involves 600,000m3 of earthworks. The two-year contract is expected to be finished in spring 2003.

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