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Preservation of 12th century foundations

Preservation of 12th century foundations was a high priority during work on Durham's Millennium Hall, which is taking shape on a split-level site involving substantial reclamation and heavy foundation work.

Durham has two reasons to celebrate this year: as well as welcoming in the new millennium, it's also the city's 1000th anniversary.

Commemorating both occasions is a £25m scheme, supported by the Millennium Commission, devised to echo the scale and importance of Durham's world- famous cathedral and transform it into the renaissance city of the north.

The project will regenerate a former industrial site and heal severance created by the A690 dual carriageway cutting through the centre of the city. It's centrepiece is a 500-seat Millennium Hall surrounded by an equally impressive square, creating a new cultural heart for Durham.

Other elements include a centre for life-long learning and a community resource centre as well as a series of pedestrian piazzas and paths leading down to the River Wear and a riverside park.

As consultant for the scheme Mott MacDonald is responsible for all civil and structural engineering for these elements, plus a bridge-link building over the A690 and a footbridge over the river.

Site conditions are complex with a lot of made ground - up to 6m in places. This lies over sands and gravels, mixed fluvial materials including peat and organic clay, coal measures and sandstone bedrock at 25 to 30m below ground level.

'The made ground actually represents 1000 years of the history of Durham,' explains Mott MacDonald project director David Young. 'And in the early stages of the project we were very involved with the archaeological investigations which not only provided valuable historical material but also guided the design of pile foundations to avoid further obstacles.'

A number of excavation schemes with various means of temporary support were investigated in an attempt to maximise the archaeological activity without restricting construction work. Ultimately a scheme was developed which allowed the full 6m depth to be excavated revealing foundations dating back to the12th century.

The project site lies between Claypath and Walkergate and falls some 12m between the two areas, requiring a major retaining wall to enable development to proceed.

Location of the wall varied as the planning of the project evolved and the height of the retained material and the imposed loads on the wall changed. During design many schemes were explored but the one finally adopted was a contiguous CFA bored pile retaining wall with a double row of ground anchors. This was extended with a 4m high reinforced concrete wall and small relieving platform at the top of the contiguous piles.

Design of the anchors was complicated by proposed pile foundations for the main hall and foundations from the historic development. The performance spaces and the old development are built on an irregular grid and are not well aligned with the retaining wall, which made co-ordination of piles and ground anchors extremely complicated.

Foundation engineering sub-contractor Kvaerner Cementation Foundations proposed an alternative scheme for taking the ground anchors to the bedrock. Initial installation proceeded without a separate trial anchor, but testing of initial production anchors showed inadequate anchorage in the rock. It was therefore necessary to redesign the later anchors and to develop an alternative scheme to supplement the initial anchors. Close collaboration between Kvaerner Cementation Foundations and Mott MacDonald allowed a rapid response and minimised impact on the construction programme.

All ground anchors are stressed to an initial interim load pending completion of back filling behind the reinforced concrete extension to the wall. Monitoring of the wall is under way as the back filling continues. The loads in the anchors have been predicted and anchor forces will be adjusted on completion of back filling if necessary. In addition the anchors have been designed to be restressable during the life of the building. The internal layout of the hall and satellite facilities takes this into account and will allow restressing to take place without interfering with the day to day working of the buildings.

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