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Durham Castle underpinning
Palace Green, Durham, County Durham, UK
Durham Castle underpinning
associated engineer
Oscar Faber
date  1927 - 1945
era  Modern  |  category  Castle  |  reference  NZ272423
photo  Jane Joyce
Durham Castle is part of a UNESCO World Heritage site, along with Durham Cathedral. Both are sited on a loop in the River Wear, now the centre of the city of Durham. The site is dramatic, as the river has cut itself a deep ravine at this point. The view across the Wear Valley to cathedral and castle is memorable and likely unchanged for centuries.
The castle was started by the Normans after 1066 and sits some 35m above the river, dominating the surrounding area. It consists of a keep and a series of buildings built on sandy clay brought to the site. In 1927, when engineer Oscar Faber was first contacted, its south battlement wall was leaning outwards. However, it was the west side of the complex that required more-urgent repairs. It was in perilous condition and slipping ever-closer to the river below.
Not only were the battlement walls unstable but major buildings in the complex were also sinking towards the river, following the walls. The Norman Gallery and the west wall of the Great Hall were the worst affected and localized shoring had been in position there for many years.
The whole west side of the castle had been constructed on made ground. The closest stable rock is more than 21m below the level of its courtyard. The first solution Faber considered for the supporting walls was the use of raking shoring members with lateral ties. As these would have to be over 45m long and good foundations were not available because of the proximity of the river, this idea was soon rejected. Alternative solutions were needed.
The first stage in the remedial programme decided upon was the strengthening of the battlements by injecting grout under pressure at regular intervals. Holes were drilled through the walls in a grid pattern, and some 193,000 litres of fluid grout injected. This returned to the wall to basic integrity, making the next stages more predictable. There was still on-going failure: "a desperately dangerous condition" was Faber's description of the battlements at the time. However, the walls were now ready for underpinning.
Eleven pairs of 60mm diameter steel ties set in two tiers were introduced. Each tie is enclosed in a 150mm lining tube. They are set out in a radial manner and extend 45m back into the ground to anchorage points in the courtyard. They are located at two levels. The first is at 4m deep, and these ties resist the overturning moment. The second is at 9.3m deep and is designed to resist slip. In order to position them, a combination of horizontal drilling (a comparatively new technique for 1934) and tunneling was used. The ties had to pass under the Norman Gallery and Great Hall, so tunneling was carried out by experienced local miners.
The ties are secured within the walls using reinforced concrete bedstones, which are cased with stone to blend with the existing materials. At the other end, a series of pits was excavated, each 12m deep and 2.5m x 1.2m in plan. These were filled with reinforced concrete into which the ties were anchored.
The underpinning of the battlement walls was then completed by keying into the rock base 21m below courtyard level. This required sequential digging-out to a depth of 12m and the construction of underpinning works with a profile similar to that of a gravity dam. The 'dam' varies in thickness between 4m at the highest point and 7m at the base where it keys into the rock.
With the battlement walls stabilized and underpinned, works to the castle buildings could begin. Conventional methods were used sequential shoring of the walls and underpinning using reinforced concrete foundations. However, there were challenges.
Most of the building walls had no footings. As the walls had moved considerably out of vertical and there was some bowing, to ensure uniformity of future settlement, Faber assessed each wall and set a new notional centre line for each. The new foundations were constructed set symmetrically about this new line. Together, the new foundations also act as a ring beam, preventing further outward movement.
Today there is little evidence of the scale of the works undertaken to save the castle. Faber's inspired interventions were not only effective but managed to be pretty well invisible too. The castle buildings have been occupied since 1840 by University College, Durham, and continues to be a working building complex, thronging with students.
Research: ND
bibliography
"The Underpinning of Durham Castle" by Oscar Faber
in The Structural Engineer, November 1934, p.438, London
"Oscar Faber, his work, his firm, & afterwards" by John Faber
Quiller Press, London 1989
www.dur.ac.uk
Location

Durham Castle underpinning