Technical Case Study
A two phase plan
A conservation report on the monument was drawn up in 2002, and this recommended that the repairs were carried out in two phases. Phase one was the emergency works which consisted of making a temporary access route to the column for the crane and other equipment. Next the breach where the lightning had torn the stone apart was bricked up. The column was strengthened with supporting scaffolding and the capital and a number of sections of the shaft were dismantled.
Given the height of the column and the strength of the winds in this exposed area, the restoration couldn’t be carried out during the winter, so the work was divided over two separate summers. The second phase commenced in June 2003 and involved the cutting and carving of replacement stone, as well as the straightening and rebuilding of the damaged part of the column itself. This involved the rebuilding of the top twelve courses of the shaft and all of the capital using stones that had been carefully salvaged during the dismantling, and new stone where needed.
Capital Movements
Works to the capital involved the construction of a temporary masonry infill into the breach in the column structure, to enable more substantial straps to be installed to consolidate the panels of loose masonry. The masonry infill panel consisted of soft red bricks and clay tile slips, bedded in a soft, fat lime mortar that can be easily removed at a later date. The bricking up of the breach was a very difficult and delicate operation, as the main structure of the column remained unstable during this work. On completion of the masonry infill a series of nylon tyfores were placed around the damaged part of the shaft and capital to make them stable.
The removal of the capital was a delicate operation. It required the careful propping of every underlying stone, prior to the removal of each top course, as the stability of many of the projecting stones from the acanthus and volute courses, depends on the weight of the stones above. The stones of the capital are all through stones, projecting from the outer to the inner pace of the capital, and some weigh over a ton. Another feature of the stones making up the capital is that many are bedded with straight, rather than staggered vertical joints. This is a result of the complex shapes of the exterior faces of many of the stones containing the acanthus enrichments, and which would be difficult to construct with staggered joints. Therefore every stone is tied horizontally, with wrought iron cramps to its neighbour. Ironically, despite the weaker bonding of the masonry in the capital, it is likely that the mass of the through stones, plus the lateral restraint created by the cramps, are the reason the top three courses remained intact despite the enormity of the force that struck just below them.
Steeplejack skills
Once the capital had been safely removed and lowered to the ground with each stone carefully numbered on its top bed, it was loosely reassembled and the missing stones were re-carved.
The greatest difficulty when rebuilding the capital was in the raising and accurate positioning of the individual stones, some of which weigh more than a ton. The deeply projecting stones that form the volutes and the third rank acanthus leaves were the most challenging to replace. They all include significant portions acting as cantilevers, which required temporary propping until the dead load of subsequent courses had been created.
The process of realigning the misplaced masonry panels of the shaft involved innovative engineering and experienced steeplejack skills. All of the rubble stone backing skin along with the remaining fragments of the damaged staircase were removed down to the lowest point of the damage. This reduced weight and made the realignment of the disturbed masonry easier. All of the cracks were flushed out using high-pressure jets of water and compressed air. A system of vertical steel channels with continuous softwood packing was installed around the displaced panels of masonry to act as splints. These are held in place by curved steel collars made up in two of three sections with adjustable connection bolts were tightened and slackened to realign each panel. Some internal bracing was also needed.
In total eighteen new facing stones were required for the rebuilding of the shaft, four new stones required for the capital and twenty-nine new stone steps were cut for the internal staircase.
Ceiling coffer with the accumulation of two hundred years of dirt and coffer after cleaning
Browne Clayton column covered in scaffolding during the repairs
On the rocks
The outcrop of rock upon which the column was built made it impossible to bring up a normal sized crane close enough, so an outsized crane had to be specially imported. The difficulties didn’t end there. The nine separate sections of the capital were very heavy - most weighed well over a ton which made moving them difficult. Additionally many of its projecting sections were dangerously fragile.
Looking down the interior of the column with the capital removed
Technical diagram showing the column before, during and after the lightning strike
