The Consolidation Of An 18^th Century Masonry Arch Bridge On The Old Military Road At Corgarff, Aberdeenshire, Scotland, UK.

John D. Addison, Peter Stephen & Partners, 15 Annandale Street, Edinburgh and Daniela Dobrescu-Parr, The Morrison Partnership, 242 Queensferry Road, Edinburgh

ABSTRACT

Conservation Engineering embraces knowledge of materials and engineering sciences, philosophy, art and archaeology. In projects involving the care of ancient and historic structures thinking about all these things must progress in parallel, producing a very demanding brief, which must be responded to with determination, courage, experience and sensitivity.

Many old structures, on initial inspection, appear to be beyond rescue and are subsequently demolished as the result of a lack of awareness about what can be done to seek to achieve stability so ensuring their survival.

An unusual conservation project to consolidate three ruinous military bridges dating from 1753 has illustrated how conservation engineering works and shows how even the most impossible looking tasks can be made to succeed.

The project involved jacking and correcting the alignment of a seriously distorted masonry arch before carrying out repairs to it in such a way as to leave the long-abandoned structure more or less as it would have been before its watercourse carried away most of one abutment.

Along the path of this project some interesting structural characteristics such as exceptional torsional strength and an unexpected degree of elastic behaviour during recovery were encountered. These should be of interest to all those involved in repairing and restoring masonry structures of all kinds and are worthy of further investigation and research. The most important message, however, is that old distorted masonry can often be pushed back into a stable geometry.

INTRODUCTION

Delavine, Tornahaish and Allt Damh are small masonry arch bridges built by General George Wade’s successor in Scotland, Major General Edward Caulfield in 1753 after ‘Bonnie’ Prince Charlie’s military campaign against the Hanoverian Crown halted at Derby and finally ended in defeat and massacre of his troops in a battle at Culloden Moor, near Inverness, in 1746. The bridges are three of the many masonry structures which were built along the vast network of military roads covering the Scottish Highlands in order to make it accessible for military control and trade purposes.

These three bridges are situated along part of the military road (Braemar to Grantown upon Spey) now an Estate track, which was not absorbed into the public roads system and lay neglected for many years. One had collapsed over three quarters of its width, another had lost three quarters of one abutment and had partially collapsed, and another remained in good condition but in need of consolidation.

Each bridge was researched, surveyed and assessed in terms of structural conservation (they are Scheduled Monuments) and proposals for their preservation, as consolidated and stable ruins, were approved by Historic Scotland’s Ancient Monuments Branch.

This paper covers the engineering challenges presented by the condition of the structure of Delavine Bridge.

Work commenced in 1997 after a worryingly prolonged period of shoring and emergency works to prevent collapse of the most seriously threatened structure at Delavine.  

The work to all three bridges was completed in October 2000 after three summers of working in Highland terrain and weather conditions. It was funded by: Historic Scotland, The Heritage Lottery Fund, Historic Scotland, European Regional Development Fund, Leader II, Aberdeenshire Council, Gordon Enterprise Trust and Landfill Funds donated by a local company, McIntosh (Aberdeen) Ltd. The project cost was £250,000.00, which was initiated and organised by the Client, Gordon Enterprise Trust, Inverurie, Aberdeenshire. The Trust has the three bridges in care, having leased them from Candacraig Estate for 99 years.

DELAVINE BRIDGE – APPROACH TO ITS CONSERVATION

A fundamental principle in conservation is to ‘conserve as found’. This leads to a careful approach to ancient monuments. ’Minimal intervention’ attempts to retain authenticity, use sympathetic materials, preserve important original fabric and apply least visual intrusion. Whether the structure is a ruinous medieval castle, monastery, or bridge such an approach will freeze it in time provided that there is not such a threat to overall stability that it demands new structural works alongside it.

The above stated principle had, in this case an environmental relevance. The environmental impact assessment on the bridge sites - indicated. for example, that the lime rich mortar in these structures, in a large area of mainly acid soils, acquired lime loving plants. Additionally where mortar had leached out between the slab like stones of the bridge, especially underneath it, deep cracks had developed, and these had become a bat roost that had to be protected during the work. (insert Drennan Watson)

The bridge at Delavine seemed at first to be beyond any hope of recovery with three quarters of an abutment washed away due to the meanderings of a watercourse. It was distorted and twisted to a frightening degree. At the same time, however, it displayed a dignified and stubborn resistance to the large overturning moments growing about its ever-changing thrust line brought about by progressive decay and collapse.

Initially, therefore, it seemed that the conservation-led policy of minimal intervention could not be applied to this structure. For this lonely marker of history to remain it seemed that it would have to be completely re-constructed. From economic and philosophical points of view, however, such re-construction would be completely out of the question. Alternatively, any re-building of the missing parts would only produce a bizarre looking bridge, albeit stable.

After much deliberation, therefore, it was decided that first of all the distortion should be almost fully eliminated by jacking the surviving structure back to the position it occupied in 1753. New stone foundations would then be built to replace those eroded away and the missing part of the arch would be built-up in natural stone sympathetic to the original. This became the ‘Design Brief’ after agreeing the approach, technically and philosophically, with Historic Scotland and the Client.

Minimal intervention policy could, therefore, still be respected by avoiding taking down any part of the arch to correct its misalignment. Once the jacking was completed, normal conservation processes could be applied in the usual way, carefully matching all new stones and mortars, retaining as much as possible of the original, yet not confusing old and new from the perspective of future archaeology.

DELAVINE BRIDGE – CONSTRUCTION

The Delavine Bridge spans some 10 metres over a small watercourse. It is roughly 4.6 metres wide with a rise to the crown of the arch of approximately 3 metres. It had small parapets and a cobbled roadway.

The voussoirs are of large flat fragments of schist, which has a slate-like bedding; the thickness of the arch is about 800 mm.

Lime mortar was used throughout the construction leaving an imprint of the original timber formwork beneath most of the arch except where it had been leaching out as stalactites.

The spandrel walls and parapets were constructed using ‘field gatherings’, i.e. stones found locally.

The foundation strata is compact glacial gravels and sands.

DELAVINE BRIDGE – STRUCTURAL CONDITION

A first inspection was made in 1996. The structure appeared to be on the point of collapse. It seemed almost ready to overturn although there was no way that its factor of safety could be calculated or even guessed at. About three quarters of the west abutment had been washed away over the north side of the bridge. Half of the north side of the arch had collapsed leaving it supported almost on a ‘chisel’ point on the west abutment. This is illustrated on the section and plan below.

The structure was twisted in a grotesque manner and rotating towards the missing part of the arch due to the large overhanging mass of masonry left on the north side of the ‘new’ arch centreline. There were significant cracks in the soffit and in the voissours due to the torsional moments being induced.

Altogether, about 200 tonnes of monument were about to collapse into the streambed. It was therefore decided to mount an initial rescue operation. This involved the installation of some underpinning using bagged lime concrete and several 300 mm diameter timber struts orientated to give lateral stability and to create an additional line of thrust through the bridge into temporary foundations.

Apart from the gross distortions and the ‘bite’ taken out of the arch, the condition of the rest of the masonry was surprisingly good given its long exposure to extreme weather conditions. There was slight bulging of the spandrel walls and parapets, and in some areas a leaching out of original mortars.

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The compressive stress in the masonry at the narrowest part of the arch was calculated to be around 0.6 – 0.8 N/ mm². Viewed from the south, the structure looked whole, elegant and robust with only some cracking visible but from the north-west the impression was of a structure disastrously beyond recovery!

DELAVINE BRIDGE – IMPLEMENTATION OF APPROACH

To ensure that the structure did not break into large pieces during the lifting procedure, a strong steel and fully timbered soffit formwork support was built. This also included a layer, or ‘cushion’, of dense fine sand and a soft membrane to minimise cracking and loss of the original board mark finish by reducing frictional ‘drag’ effects as the masonry moved across the formwork. The soffit support comprised 200 mm x 200 mm baulks of timber supported by a pair of steel trusses designed by the Engineer. The trusses were braced together and seated on spreader beams placed on timber mats on consolidated hardcore placed under each end of the bridge.

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As it was necessary to rotate the bridge and its failed west abutment at the same time, the arch steelwork was extended with a nosing under the narrowest part of the arch. This was strengthened by the addition of steel ropes to take any outward spread of the stones as the weight was transferred to the formwork.

Finally, a hydraulic jack was positioned under the north side of the west line of support together with a series of steel props for lateral restraint to the sound parts of the arch.

Under the direction of the Engineer one end of the arch was lifted and rotated in a carefully monitored operation, which lasted one whole day. Lines and levels were recorded at regular intervals.

Once the leveling was completed the missing part of the arch was built on the formwork using techniques very similar to those used in the original work. The new mortars were matched after laboratory analysis of samples of the original.

Repairs were after carried out to the parapets and cobbled surface of the roadway. The watercourse was re-aligned to prevent scour of the foundations; drainage around the bridge was also introduced.

DISCOVERIES DURING THE JACKING OPERATIONS

It was anticipated that some large cracks might open up the arch during the lifting process given that the structure had to be twisted as well as lifted to gain an acceptable re-alignment.

Those experienced in dealing with many types of old stonemasonry constructed in lime mortar have learned that it has a significant amount of ‘forgiveness’ to all sorts of stimuli including settlements, and lateral movement. Stability is often maintained through all sorts of grotesque distortions albeit at the expense of open joints, consequent loss of mortar and loosening of individual stones in the matrix. Notwithstanding this, its inherent ability to absorb massive movements has allowed many a ruin to survive. It was hoped, therefore, that some of this ability of the masonry to flow back ‘ elastically’ would occur here thus minimising the amount of work to repair the arch.

This in fact turned out to be the case. Little damage actually occurred and the arch was reinstated without major follow-up work.

Although the operation went relatively smoothly and generally as predicted by the Engineer, it was punctuated by the sound of some stones grinding and moving relative to their neighbours. After 250 years of history it was on the move again! An occasional sound was heard reflecting the re-aligning of steel and timber as it squared up the structure. The jacking was carried out using standard hydraulic hand-pumped equipment with the support being transferred on to steel packs after each lift so the operation was not continuous.

Towards the end of the sequence, as the structure neared its final planned position, there was a sudden, almost explosive release of upward and lateral movement which bent the lateral restraint props on the south side (the ‘good’ side) of the bridge and lifted the masonry off the formwork by about 50 mm. After a cautious examination of the structure, including a check for any settlement of the steelwork, it was concluded that the original masonry had decided to restore itself to its original alignment without further effort on the part of the jacking team! The stored strain energy to achieve this must have been considerable.

Removal of the off-centre loading on the intact part of the bridge had thus rejuvenated the structure. It also demonstrated the stiffness of the foundations at both ends.

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COMMENT ON CONTRACTUAL ISSUES

The project, although complex and unpredictable in many areas, was completed within the budget and to the approval of Historic Scotland’s Ancient Monuments Branch.

The site was remote with difficult terrain and conditions of access. This included the construction of a ‘bothy’, a ford and making improvements to the existing track. Much of the budget was spent on overcoming logistical problems relative to delivering labour, materials and welfare facilities. A ‘time and materials’ approach was applied to all the contracts the success of which relied on the dedication, trust and goodwill of all involved in this small but challenging project.

CONCLUSIONS

Many masonry bridges from the military roads building period in Scotland are imperilled. These still in regular public use have a better chance of surviving but others are under threat even when Scheduled and Listed as being of historical importance. Ownership problems, lack of interest, inadequate funds, politics, ignorance, vandalism and plain neglect, all conspire to deplete this important heritage resource.

The Corgarff project, with particular reference in this paper to the operations to save the Delavine Bridge, illustrates that there are practical ways of rescuing these important markers of history even when the odds appear very slim.

Conservation Engineering involves many skills demanding at a fundamental level the ability to see structures for real and not solely through interpretation of often very crude mathematical models to develop solutions. These need to be innovative, yet practical, buildable and economically viable. The solutions need to connect the ancient and the modern respecting the past and reflecting the needs of today. The Conservation Engineer needs the inspiration of Brunel and sometimes the brawn of a Bricklayer in order to communicate at all levels in this special area of creativity.

Equally the Conservation Architect has to grapple with the many difficult and often conflicting priorities involved in running such complex work including an enthusiasm for helping the Engineers to solve difficult structural problems by appreciating the issues and sharing the pressures, constraints and unique demands of the project.

The project produced many ideas of interest and in concept, also for further research including the resistance of masonry to torsional effects and its capacity to be recovered from extreme distress and vulnerability without destruction of significances.

ACKNOWLEDGEMENTS

The authors organised, designed, directed and ran the project with the Clients, Gordon Enterprise Trust and Grampian Military Roads Ltd. Both authors gratefully acknowledge the support and enthusiasm of Mr Bill Marshall, Mr Ron Reid and Mr Michael Crossland of Gordon Enterprise Trust, and of Mr Sean Norman of Aberdeenshire Council. Special thanks are also given to Major General John A.J.P. Barr, Trustee, Grampian Military Roads Ltd., for his advice and encouragement. The second author also wishes to thank Mr George Morrison, her employer, for the time given during normal office hours to prepare all the illustrations and co-ordinate the text. Thanks are also given to Edwards Engineering (Perth) Ltd. for the use of their drawings for the steel shoring, and to Dr. M.C. Forde for asking the first author for a glimpse of his daily work!

PROJECT TEAM

• The Morrison Partnership – Architects and Planning Supervisors (Edinburgh and Glasgow)
• Peter Stephen & Partners – Consulting Engineers (Edinburgh)
• Alistair G. Urquhart – Stonemason (Aboyne, Aberdeenshire)
• Cumming & Co. Ltd. – Conservation Contractors (Perth, Perthshire)
• Edwards Engineering (Perth) Ltd. – Steelwork Contractors (Perth, Perthshire)
• McIntosh (Aberdeen) Ltd. – Civil Engineering Contractor, Aberdeen
• Tom Addyman – Archaeologist
• Nick Bridgland, Michael Pendery and Lothian Webster – Historic Scotland

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REFERENCES USED IN HISTORICAL RESEARCH

• Highland Highways, Old Roads in Atholl, by John Kerr, 1991.
• Jacobite Risings, 1644-1746, and The Old Military Roads, A Historical Guide for Scots Youth, by W. K. R. Neilson, printed by Milne, Tannahill & Methven Ltd., 12-14 Mill Street, Perth, Scotland.
• Proceedings of the Society of Antiquaries of Scotland, Vol. 110 (1978-80), National Museum of Antiquaries of Scotland, Queen Street, Edinburgh, Editor: D V Clarke, assisted by: Fiona Ashmore, Ian G Shepherd, printed by Clark Constable Ltd, Edinburgh, 1981.
• The Loch Lomondside Military Road by James Chirrey.
• Thesis – ‘The history of roads in the Highlands of Scotland in the 18^th and 19^th Centuries contrasting the military roads (i.e. those of Wade and Caulfield) with the parliamentary roads (i.e. those by Telford), by James Souter Stephen, 1934 (Aberdeen).

STRUCTURAL FAULTS + REPAIR – 2001

AUTHORS BIOGRAPHICAL DETAILS

TITLE OF PAPER:

‘THE CONSOLIDATION OF AN 18^TH CENTURY MASONRY ARCH BRIDGE ON THE OLD MILITARY ROAD AT CORGARFF, ABERDEENSHIRE, SCOTLAND, UK’.

JOHN DOUGLAS ADDISON

QUALIFICATIONS:

BSc (Eng) – First Class Honours University of Aberdeen (1970), Member of Institution of Civil Engineers, Fellow of Faculty of Building.

CURRENT EMPLOYER/ ORGANISATION AND POSITION:

Partner in Peter Stephen & Partners, Consulting Engineers, Edinburgh.

BRIEF CAREER SUMMARY:

• 1970 – 73Site Engineer with Taylor Woodrow on Hartlepool and Heysham Power Stations.
• 1973 – 82Project Engineer with several firms of Consulting Engineers, in Edinburgh.
• 1982 – 2001Partner with Peter Stephen & Partners (merged with Aspen Consulting Group in May 2000).

SPECIAL AWARDS / PRIZES, DECORATIONS, ETC.:

• Medal for Structural Engineering Design –  University of Aberdeen (1970).
• Awards (1997 & 1998) from The Association for The Protection of Rural Scotland for footbridge design (with Daniela Dobrescu-Parr).

MEMBERSHIP OF TECHNICAL COMMITTEES:

• Affiliate of The Royal Incorporation of Architects in Scotland (RIAS).
• Vice-Convenor of The Society for The Protection of Ancient Buildings in Scotland (SPABiS).
• Member of RIAS Conservation Committee.

PUBLICATIONS:

None – but has lectured to the National Trust for Scotland, SPABiS and The Royal Incorporation of Chartered Surveyors on Conservation work.

 

| Introduction | The Drovers of Cattle | The Military Roads | The Whisky Road | The Walkers Roads | The Road Today | Tornahaish Gallery | Delavine Gallery | Allt Damh Gallery | Delavine Technical Paper | Links and Credits | Bibliography |