Branch President Mark Holliday, Prof MICME, welcomed members and guests to the evenings lecture and introduced the speaker for the evening, Dominic Grosvenor of Barr and Grosvenor Ltd.
Prior to 2006 Barr and Grosvenor were invited by English Heritage to undertake trials for conservation work on the Abraham Darby bridge in Ironbridge. The issue with this project was how to cost a project when the true extent of the work could not be known until it was started. However, the tender was accepted and work began.
Although the foundations had been substantially strengthened some years previously little conservation work on the bridge had been carried out in its lifetime. However despite this the bridge had stood for more than 200 years and survived massive pressure put on the structure when the river had been in flood.
The bridge is constructed with five cast iron half arcs, the pressure exerted by its weight being passed down to the foundations on the river banks. The arcs are held in place with full width cast deck plates which in turn are secured with cast wedges. The wedges had not been replaced since they were originally fitted 240+ years ago and examination showed that due to corrosion there was little holding the upstream deck plates in place. The bridge was in urgent need of work to assistance. Work to replace the cast wedges was considered as a priority.
Replacement of the wedges required access from above which meant the removal of the tarmac surface. Once this surface had been removed it revealed the wedges but above them was a water pipe running along on one side and a gas pipe on the other so not only was removal of the old wedges difficult due to corrosion but access to them was hindered by the pipes, and there were around 300 wedges needing to be replaced.
Trials to determine the best way to proceed began with drilling out the wedges. This was a slow process due in part to the hardness of the wedges and their location under the pipes. Drills quickly burned out and had to be sharpened time and again so removal of only 5 wedges in a day was not untypical. Then English Heritage asked that the replacement wedges be made from wrought iron, a material which today is very difficult to source, pushing costs of the project ever higher. This was a major consideration as the project was undertaken for a fixed sum. A readily available material was sourced,pure iron and it was forged into shape on site. With everyone working extended days it still took more than 3 weeks to remove and fit 40 new wedges. This was not a practical method to complete the work.
The main contract to conserve the bridge began in 2017 and in order to continue more effectively the water and gas pipes were removed and that then gave easy access to the wedges. Trials found that hitting the wedges with hammers would loosen the rust and then some of the old wedges could be removed with relative ease. To produce new wedges a wooden pattern was made but the holes for the wedges were not uniform. A better solution was to cast the wedges in SG iron and to form the pattern from air set sand and then rub the sand pattern to shape for each wedge position so once cast the new wedges would fit perfectly and a light tap with a hammer would have them secured. Three hundred wedges later and the bridge was once more stabilized. Other work on the project consisted of casting and fitting new parts for those that had fallen off and brace repairs to historic breaks.
Throughout the work visitor numbers steadily increased as news of the project spread not just in the UK but around the world. People came from a variety of countries and it was often those that had travelled from abroad that showed the most interest. One German conservation professional visited and donated €1M to the project and knowledge of this led to further donations.
This work has stabilised the bridge but it still needs many more millions to fully conserve it. Surely such an iconic piece of history has to be worth preserving?
David Hall, Prof MICME, gave the vote of thanks for a talk that kept all present totally enthralled and then members and guests enjoyed an excellent buffet sponsored by Barr and Grosvenor Ltd.