Fort George Bridge Investigations

H+R were instructed to undertake a detailed timber condition investigation of the Principal and Ravelin Bridges at Fort George near Inverness following on from a preliminary survey undertaken 16 years previously, in order to provide cost-effective and sympathetic remedial advice, using environmental means where applicable.

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Work undertaken:

The investigation involved detailed visual and physical examination combined with micro-bore decay detection drilling of all components of the large-section supporting timber piers to determine the extent of any wet rot decay. Deep moisture content readings were taken from each hole drilled in order to profile the weight-for-weight (w/w) percentage of moisture within individual piers, and also to identify which pier elements were most affected and where, ultimately to identify root causes of moisture penetration. Detailed examination of the balustrading of both bridges was also undertaken in order to identify areas requiring repair or replacement.

How we solved the problem:

Recommendations provided by H+R in the previous survey had mostly been implemented and this had rectified many of the defects and issues highlighted; however, further decay detection revealed several pockets of decay within those piers not included in the previous preliminary survey, and deep moisture contents were found to be generally above 20% w/w, especially within the main pier support posts and bearers. Detailed drawings of each pier were drawn up in AutoCAD® showing moisture contents, and any decay or structural defects noted. This ‘mapping’ of the decay and moisture within the piers, combined with decay detection drilling results, will enable the client, Historic Environment Scotland, to produce a schedule of repair aimed at conserving the historic structure, whilst ensuring that the bridges are structurally sound for pedestrian and vehicular access.

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