After more than 900 years, Chepstow Castle's Norman stonework is beginning to crumble.
Most of the masonry is undergoing long term care from Welsh heritage body CADW's team of trained stonemasons - but the projecting bulk of the 'garderobe' which cantilevers out from the north eastern outer wall is a different matter.
Access contractor SGB has managed to install strong stable platforms across most of the face of the cliff beneath the castle.
On these scaffolding has been erected, allowing CADW's masons to carry out major renovations.
Scaffolding the garde robe, however, will tax the ingenuity of the project team even further.
Access for the masons had been a pressing need for some time. Rising sheer from the edge of a limestone crag above the swirling waters of the River Wye, the outer face of the castle was out of bounds for centuries, its condition and stability an unknown quantity.
'CADW's initial concerns focused on the cliffs themselves, ' says Arup Geotechnics project manager Matthew Skuse. 'Even from the other side of the river it was obvious the limestone was crumbling and threatening to undermine the walls. Stabilising the cliff face was the first priority.' To investigate the oolitic limestone cliff face, Arup engineers borrowed a trick from the 17th century masons who swung down on ropes to repair damage wrought by Cromwell's cannon during the English Civil War (see box).
Arup was awarded the inspection commission in 1997, and the results, Skuse reports, were far from reassuring.
'Fracture planes were at awkward angles, there was cracking and flaking, slabs and buttresses were unstable, and some rocks were very loose, ' says Skuse.
'We spotted lots of problems on the castle walls as well, but the cliff face had to be prioritised as bits were already falling off.' Seven seasons of remediation work followed, all carried out by rope access. A combination of rock bolting and anchoring, with rock mesh used sparingly in the most weathered areas, eventually stabilised the face. Attention then turned to the local sandstone masonry above.
Skuse says continuing with rope access was briefly considered - then rejected.
'It would have been tricky anyway, working at up to 60m down an irregular face. But the key factor was that CADW needed to get its trained masons down there to carry out effective repairs. The only realistic option was to find a way of supporting conventional scaffolding up against the masonry.' This is where SGB came in.
SGB designer Chris Rogers devised a special fixing which could be rock-anchored and bolted to the irregular cliff face by Arup's rope access technicians (see diagram). This made possible the construction of the triangulated base platforms, sturdy enough to carry scaffolding up to 12m high.
'Above the base platform the scaffolding is standard, ' Rogers says. 'But getting the base platforms along the cliff face was far from easy.
'There is an old inlet in the cliff face which was once used for boat access. We actually had to drop scaffolding down here to gain access to some arches that helped the wall span the inlet.' With the wall far from vertical, tying in the scaffold became a matter of seizing every possible opportunity. The infrequent windows were the main option, but in places temporary ring bolts had to be fixed to the walls.
These will be removed in the end, Skuse insists.
But some areas in need of attention remained out of reach of SGB's scaffolding. 'There were a couple of areas that badly needed some minor attention which would have been very expensive to access via scaffolding, ' says Skuse. 'The only realistic option was to train rope access technicians in the art of repointing with lime mortar and get them to do the job as best they could.' With 80% of the restoration now complete, planning is under way for the garderobe repairs.
'This is a real challenge, ' says SGB contracts manager Tom McElhinney. 'We have to cantilever out much further from the rock face and support up to nine, 2m lifts of scaffolding.
'And we'll have to take advantage of every available opportunity for tying the scaffolding in - there aren't many windows in this section, so every crack and crevice will be utilised, ' he says.
Getting the base platform into position should take around five days. Again, initial anchoring work will be carried out by rope access specialists under the eye of SGB operatives. Once the first tubes are locked in place SGB will take over to assemble the double triangulated platform structure.