The Consolidation Of An 18th
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
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
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
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.
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
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
|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
The voussoirs are of large flat fragments of
schist, which has a slate-like bedding; the thickness of the arch is about
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.
click on thumb for larger image
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
click on thumb for larger image
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
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
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.
click on thumb for larger image
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.
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
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.
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!