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Underneath the arches

Mouchel (01932) 337000)

Telford Arches, a Grade II listed, brick built, seven-arch bridge which spans the River Colne in Hertfordshire, has been upgraded to meet the 40t vehicle limit while being restored to its original appearance.

Working for Hertfordshire County Council, which funded the project, consultant Mouchel decided to strengthen the structure internally without altering the arches. Support for traffic loads was achieved by inserting reinforced concrete leaf piers and suspending a deck slab above the weakened arch barrels. To minimise impact on the original structure, the new concrete elements were designed to fit under the carriageway with minimum vertical realignment.

Strengthening work was carried out in two stages with east and west halves of the bridge excavated sequentially to construct the relieving slab. Footway extensions added in the 1950's were demolished and cantilevered reinforced concrete footways constructed, integral with the new relieving slab necessary to retain the width of highway.

The reinstatement of the brick parapets, plus the use of Portland stone on copings and cutwaters, helped return the 24m long bridge to its 18th century elevations while also satisfying current safety regulations.

Main contractor for the £300,000 project was Wrekin Construction. Brickwork repairs took 20 weeks using handmade bricks similar in appearance and weathering properties to the originals. Areas ranging from a single brick to several square metres were removed and replaced, mainly on the arch barrels, abutments and inverts. Where stonework had to be repaired, this was done by cutting in new stone wherever possible. In total, the contract took seven months to complete.

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