Devon's Totnes Castle is one of the largest examples of a Norman motte and bailey castle in the UK, with an almost intact keep sitting on top of a 15m high motte - a man-made mound of soil covered with slabs of rock. For 1000 years the castle remained stable, emerging unscathed from the chaotic Middle Ages.
It might have remained that way had it not been for the Victorians, who filled in the moat on one side of the castle and built houses right up to the toe of the motte. Home owners then extended their back gardens by building retaining walls to terrace the slope. Since then, new houses have also been built as close as 1.5m from the site boundary.
In January 1999 there was a sudden catastrophic failure of two of the Victorian terrace walls, crushing cars and threatening homes and businesses. The cause of the failure is thought to have been a slip plane progressively developed in the clay motte material connected with a tension crack which developed behind one of the terrace walls. A period of heavy rainfall tipped the balance.
Emergency work was carried out to make the slope safe.
Collapsed material was excavated and removed by hand to expose the slip surface, which was covered with plastic sheeting to protect it from water. This work was carried out under the watchful eye of archaeologists, who have had a constant presence on the project.
Permanent stabilisation began in August 1999, with a structural survey by Babtie Group and a site investigation by the Exeter office of Geotechnics. This involved boreholes, window sampling and trial pitting at strategic locations along the terracing.As became the pattern with most of the materials and equipment used on the project, cable tool rigs had to be dismantled and hoisted to the top of the slope to sink the boreholes.
Investigations revealed that the Normans used two types of local material to build the motte - a sandy, gravelly clay with sandstone fragments in the vicinity of the failure and predominantly granular material elsewhere. Bedrock outcrops occasionally at the motte base.
The six-month remediation contract, which began in May 2000, was carried out under a partnering contract between the castle's guardian English Heritage and local contractor E Thomas Construction.
The remediation strategy was developed by Babtie's geoengineering division, which has a geotechnical term commission for English Heritage. After assessing the stability of the slope, the design had to consider not only the engineering needs but access and height restrictions and environmental and archaeological implications. A specific requirement was to avoid penetrating the ground under the keep or the castle walls, explains Babtie engineer Sally Wyatt.
Work consists of three sections - the lower, middle and upper walls. The new 14m long lower wall is supported by 14, 4m high galvanised steel columns at 1m centres. These are concreted into a trench dug into the competent bedrock, typically 400mm deep, although an old basement found in one area meant it had to go down to 1m.Each column is tied down with two 4.5m long 36mm diameter double corrosion protected Dywidag rock anchors with working loads of 400kN.
Twelve concrete planks sit between each pair of columns, grouted in place and strengthened with stiffners behind. Two levels of weep holes, connected to a Polyfelt geocomposite between the wall and the compacted granular backfill wedge behind, will prevent build-up of water pressure behind the wall. A layer of 150mm seeded topsoil has been secured with a layer of Greenfix to prevent any soil wash occurring while the grass gets established.
The facade of the new wall matches the Victorian terraces, using new and original stone with lime mortar cement. It is tied into existing walls at each end but derives no structural support from this link, Wyatt explains.
The 10m long middle wall comprises seven stainless steel joists embedded in 200mm diameter Odex-drilled holes at 1.5m intervals.
Odex drilling was used because of the difficult ground - a lightweight power auger could not penetrate the hard, compact and 'stony' material and there was no access for heavy plant.
The drill rig was broken down to less than the 300kg hoist limit. The joists were grouted typically 2m to 3m into the motte material, and support a stainless steel waling beam.This beam in turn supports the stone facade, in effect suspending it above ground level. Behind the joists, 6mm thick stainless steel sheeting has been fixed, with a geotextile and gravel wedge behind that. Six, 4.5m long stainless steel 'self-drilling' soil nails were installed using a pneumatic jackhammer to pin the new structure back into the slope.
Although it showed no sign of movement, the original upper wall has been replaced. Eleven 6m long galvanised steel columns have been placed in cased Odex holes at 1m centres and grouted in. A front waling beam was attached to these and a masonry facade and a buttress built.
English Heritage wanted the finished slope to match the appearance of the Victorian terraces rather than the Norman original. All the new structures have a 120-year design life, so Totnes Castle and its surrounding area should remain safe well into another century.