95 Repair and maintenance of historic bridges Lindsay Farquharson

Introduction Courant of 17 November 1725 as ‘hardly possible for a single man’, would not have been fit for General Bridging ’s Past was a three-year project Wade’s purpose which was the rapid movement of delivered by Perth and Heritage Trust (PKHT) large numbers of troops and wheeled vehicles such and supported by the Heritage Lottery Fund (HLF). as gun- and baggage- trains to and from isolated The project ran between October 2008 and September forts and barracks. 2011 and focused on the preservation and promotion of The first road Wade constructed and improved the 18th-century military roads, bridges and associated was that along the Great Glen, effectively connecting monuments in Perthshire. planned and existing forts (Fort William, Killiwhimen The project had three key objectives: to deliver a and Inverness). Wade then turned his attention to creating programme of outreach and education, to improve access and improving two major lines of north–south communi- and interpretation and to carry out a programme of repair cation so that those troops stationed in major strongholds and enhancement at seven bridges that comprised an such as Stirling and Edinburgh and barracks such as essential part of the 18th-century infrastructure. This Perth could be rapidly deployed to the Highlands should paper addresses the last aspect of the project and outlines trouble arise. Finally he improved communication to his some of the considerations involved in the repair of new fort at Killiwhimen by branching off the historic bridges. to Inverness route at (Illus 1), (NLS GB233/ MS.7187). General Wade left in 1740 but before he left he entrusted his road-building programme to Historic background of the bridges Major Caulfeild who had been in service with him from at least 1729 (NAS GD1/369/55). It was a position In 1725 General George Wade, Commander in Chief, Caulfeild was to keep until his death in 1767. North Britain initiated a massive overhaul of the military As with the roads, the construction of bridges before infrastructure in Scotland by constructing and strength- the military investment into Scotland’s infrastructure ening forts and barracks and connecting them with would have been a matter for the Commissioners of a network of roads across the Highlands. It was an Supply or individual landowners and, as such, their ambitious and expensive undertaking designed to construction would have been intermittent. Without a impose Hanoverian control on a population, many bridge the crossing of rivers, especially by foot, could of whom were dissatisfied with the ruling regime be treacherous and there would have been long waits and had therefore allied themselves with the Jacobite until the water had fallen to a potentially manageable cause. Much-needed repairs were carried out at level. Ferries were rare except on the wider rivers and major strongholds, such as Edinburgh Castle and even then their usefulness could be limited depending Fort William, and two new forts were constructed at on the size of the boat. In many cases, especially in the Inverness and Killiwhimen (Fort Augustus), both of Highlands, the boats could only carry three or four which commanded key passes, thereby preventing the people with no room for horses (Burt 1822, 287). In rebels descending into the Lowlands. Scotland had short, river crossings could severely hamper the quick been garrisoned many times before, most recently after and easy passage of troops and therefore a programme the 1715 rebellion but the Highlands remained largely of bridge- building became essential for the security impenetrable to the British forces and their forts and of the Highlands. barracks isolated. Wade, however, had spent time in The masons working on the bridges would have been Minorca and had witnessed how a good road could civilians and appear to have largely used traditional improve military effectiveness. As early as 1724 Wade Scottish construction methods and techniques. In the notes in his report to King George the disadvantageous Tummel contract, Wade gives general dimensions for the situation in which the regular soldiers found themselves bridge and requests that it be well made, of good quality due to the mountainous terrain of the Highlands which materials and easily passable for wheeled carriage or was made ‘still more impractical for the want of roads cannon (NAS, GD1/53/97). He does not, however, give and bridges’ (NAS GD112/47/1). any further direction as to building methods or design. There were of course roads in the Highlands prior A more distinctive style of bridge is generally associated to General Wade arriving in Scotland and maps such with those built under the supervision of Caulfeild as Herman Moll’s A Pocket Companion of ye Roads and his four engineers and may be exemplified by the of ye North Part of called Scotland 1718 bridge at the Spittal of Glenshee. It has a gently sloping show them to be fairly extensive. However, many of parapet rising to a peak above the crown, generally the existing roads, described by the Edinburgh Evening with lower arches to provide a wider span (Curtis

Tayside and Fife Archaeological Journal vol 18 2012, 95–104 96 Lindsay Farquharson

Illus 1 Scotland’s military roads and strongholds. (© Heritage Trust) Repair and maintenance of historic bridges 97

1980, 485). However, earlier and later examples of Repair, maintenance and enhancement bridges being built to this pattern in Scotland are not of military bridges unknown. As with Wade it is likely that Caulfeild and his engineers would have provided the masons with Over the years 2005–06, Perth and Kinross Heritage general dimensions and instructions based on their Trust carried out a low-level condition survey of the earlier survey results rather than any detailed written bridges on the line of the military road, using as a specifications (Ruddock 2008; Farquharson 2011). baseline the historic data held in the Perth and Kinross The bridges that survive are of masonry arch Heritage Trust’s Historic Environment Record. The construction; although records of General Wade’s information held in this record largely comprised brief expenditure show that at least two of the 35 bridges notes compiled mid-20th century giving a general mentioned were of timber with stone abutments. The description and condition assessment, sometimes masonry bridges built around this period tend to supplemented by photographs and engravings. The comprise a single arch made up of flat and irregularly 2005–06 survey looked primarily at continued shaped stones. Spandrel walls are of rubble masonry, survival of the bridges, current condition and an most commonly roughly dressed boulders and field initial identification of maintenance and repair needs. gatherings bedded in lime mortar. Between the spandrel The final selection of bridges was informed by the walls there would be a fill of earth and gravel, most survey but also by more prosaic considerations such as likely from the excavation of the bridge foundations. landowner permissions, accessibility and the best result The main purpose of the fill would be to provide a that could be achieved with the budget at hand. Also, solid foundation for the carriageway. There may have on the whole, any bridge that still carried a significant been a waterproof clay layer separating the carriageway amount of traffic was not considered for repair by the and arch, although many historic bridges relied on Trust as, in many instances, this would have involved the structure’s permeability to allow the free passage the consideration of larger-scale engineering works, of water. Gutters in the carriageway are likely to which was outwith the scope of the project. Rather the have been provided, although this can be difficult project focused on maintenance and repair designed to establish. The cobbled surface of the carriageway to retard future deterioration, works to enhance the has often been resurfaced with asphalt to provide a cultural resource and very low-level engineering works wearing surface for modern farm vehicles or have under the advice of a structural engineer. Only one of grassed over. However, gutters were reported during the bridges, that at the Spittal of Glenshee, still carried the consolidation of a military bridge at Corgarff, traffic but was included as the works were minimal Aberdeenshire (Addison 2001). The use of lime-based and not intrusive. A list of the project bridges, their mortars would also have provided the bridge with location and brief descriptions of repair, maintenance a pathway for the movement of moisture through and enhancement works are presented in Appendix 1. the structure and allowed the structure to dry out However, most of the bridges showed a series of similar (McKibbens et al 2006, 33). Finally, a lime harling challenges and considerations – the presence of bats, would be applied to provide a weatherproof coating vegetation growth, deterioration of materials and scour – to protect the joints and pointing, particularly from which are discussed in more detail below. wind-driven rain. Traditional harling was a mixture of aggregate and lime binder. As with all lime-based Dealing with bats materials, it is permeable and worked by impeding the passage of water to the face of the walls by absorption Bats are known to roost in old bridges, partially in and allowing rapid evaporation from its large surface the soffit. Any repair works such as re-pointing and area (ibid). repair to masonry can disturb bats and result in the Records of expenditure show an ongoing programme loss of cracks and holes necessary for roosting and of maintenance during the period of military investment hibernation. Bats and their roosts are protected under of the infrastructure, particularly under Caulfeild, with the Conservation (Natural habitats, &c) Amendment the most common including re-pinning, re-pointing, (Scotland) Regulations 2007. Under this law it is replacement of copes and re-harling. In other instances, illegal to intentionally or recklessly kill, injure a bat, bridges had to be rebuilt, the most common cause for disturb a roosting bat or to damage, destroy or obstruct their collapse being the fast flowing torrents of rivers access to a bat roost. Any action that may disturb or (NLS GB233/MS.7187). That the bridges continued to damage a bat roost requires a licence from the Scottish be repaired, rebuilt and adapted is evident from visual Government. inspection and changes can be recorded in pictorial, A degree of forward planning is usually necessary to photographic and documentary records. Often modern ensure bats receive the correct protection but this does construction techniques and materials have been not affect work schedules, especially when working unsympathetic and have exacerbated deterioration. with traditional materials. It is generally recommended However, it can also be argued that their continued use that work with lime-based mortars is undertaken during as road or, more recently, pedestrian bridges have also the more clement summer months (April to September) ensured their survival. as lower temperatures can cause carbonation times to increase, leaving mortar vulnerable to later frost attack. 98 Lindsay Farquharson

However, bats spend October to May in hibernation and, since the status of bridges with regard to bat roosting can change with time, each survey could only be taken two to three months before the work was scheduled to start. This meant an optimal date of mid to late May to ensure the findings would still be relevant when work began but would also allow time to apply for a protected species development licence if necessary without delaying work. Each bridge was initially subject to a daylight survey and an assessment made of its suitability for bat roosting. If signs of bats were found, or no signs were found but the bridge was considered suitable for bats, a further dusk survey was carried out. The criteria for suitability included the height of the arch, presence of cracks or crevices and habitat, for example, Illus 2 Errochty Water Bridge, north-east elevation, the availability and access to productive foraging showing vegetation growth and parapet collapse. areas (Swift 2011). The bridges were surveyed from (© Perth and Kinross Heritage Trust) below as roosts tend to be in the soffit. Any deep holes were checked using a visual optics endoscope and an ultrasonic bat detector was used during the dusk survey. Species identification was made using wave analysis software (ibid). Bats or signs of bats were recorded at only two of the bridges: Dalnamein and Moulinearn. A suitable mitigation strategy was devised by Dr Sue Swift, consultant zoologist, who had undertaken the survey and this was accepted by Scottish Natural Heritage on behalf of the Scottish Government. Since neither of the bridges held a maternity roost, only two or three males, the timing of the work was of less importance (ibid). The males being itinerant would readily move to alternative sites in response to the disturbance. This being the case, it was important to avoid trapping the bats by careful positioning of the scaffold, along with Illus 3 Errochty Water Bridge, north-east elevation, the distance set between the scaffold poles. Concrete after repair works. (© Perth and Kinross Heritage Trust) bat bricks used as a form of replacement roost were not thought suitable for the bridges (ibid). On a practical level, they can be difficult to attach to shallow spans such as that at Moulinearn and their inclusion would be of the raking out process. Tree growth, however, had visually intrusive and would detract from the heritage caused extensive damage to the fabric of Errochty value of both bridges. Instead, crevices and holes in Water Bridge (Illus 2). A tree growing out of the bridge the soffit indentified as being used by bats, or those had caused the collapse of 5m of the parapet and two which were deemed to be potential roost sites (usually further trees were found to be growing out of the those deeper than 100mm) were left unfilled. This had NE elevation of the bridge disrupting and dislodging little impact on the conservation of the bridges. At masonry and encouraging mortar loss. Dalnamein, the re-pinning, pointing and replacement In the first instance, it was possible to dismantle the of missing masonry was localised, focusing largely parapet, remove the tree and rebuild to the original on the area above the springing (see below for further profile. However, the root systems of the other two description of works). Much of the soffit was left trees were established deep within the structure. As untouched. Similarly, the soffit of Moulinearn Bridge removal of the root system would have caused further is relatively sound and the majority of pinning and substantial damage to the bridge, the trees were cut pointing was concentrated around the exposed back as close as possible to the elevation and the stumps arch ring. treated with herbicide to retard future growth (Illus 3).

Vegetation Deterioration of materials

Vegetation growth was observed in most of the bridges, The condition of the existing joints and pointing on although in general, plant growth was confined to the bridges was variable with areas of missing and within open bedding joints and was removed as part cracked pointing. Apart from that at Newton Burn Repair and maintenance of historic bridges 99

Bridge, all the bridges showed signs of having been repaired at various stages over the years with cement- based mortars. Where extant, the bedding mortars appeared to be lime based. Localised loss of pinnings and masonry units were visible, particularly in the soffits of Dalnamein and Errochty Water Bridges and to a lesser extent at Newton Burn Bridge and Moulinearn. This was most likely caused by water ingress and the consequent decay of the pointing and bedding mortars, and the extent of leachate deposits on the underside of the arch appeared to support this analysis. One of the many functions of mortar is support even load transfer which can be a particular problem in rubble masonry constructions where the masonry units have few uniform contact surfaces (McKibbens et al 2006). The loss of mortar in the joints can affect the ability of the structure to transfer and evenly dis- tribute loading forces, leading to localised stresses and loosening masonry. This problem is exacerbated by leaching where water passes through and washes out salts and minerals in suspension, leaving the mortar weak and friable. Mortar samples were taken from a number of the bridges by Craig Frew, LTM Consultancy and a specification devised to achieve technical and visual compatibility with the surrounding structure. It was acknowledged that the re-pointing work would have different requirements from when the bridges were first constructed. The specification was therefore informed by the samples but took into account the current condition of the bridge so that the mortar produced Illus 4 Dalnamein Bridge showing pinning, pointing would be fit for purpose. The masonry fabric of the and replacement of missing masonry in widened part larger and more exposed bridges was damp – saturated of the bridge. (© Perth and Kinross Heritage Trust) in some places – and unlikely to dry out even in clement weather. In such damp conditions, there is a risk that any newly placed mortars may be unable to carbonate, by masonry loss (Sinclair 2009). Work comprised leaving them vulnerable to frost damage should temp- resetting or replacement of missing masonry units and eratures drop below freezing (Frew 2010, 2011). It was pinnings (Illus 4). To ensure the stonework was in therefore recommended that a binder with low free lime keeping with the surrounding structure and had similar content be used as this would increase the element of physical characteristics including appearance, reclaimed hydraulicity. The set would then have a greater reliance local stone was used as far as possible. Where conditions on chemical reaction with water, allowing the mortar to allowed, the new stonework was very slightly recessed so adequately harden to provide longer term durability. A that it was readily identifiable from the original fabric. natural hydraulic lime (NHL) 3.5 (grey) Roundtower, which has a relatively low free lime content but sufficient Scour workability and not an excessive compressive strength, was chosen for rebuilding and consolidation of the Scour has been cited as one of the foremost causes of parapet and spandrel walls and abutments (ibid). For failure and collapse of bridges (McKibbens et al 2006). work to the soffit, an NHL5 Cimpor was used as it Damage usually occurs when the flow of water removes has the lowest proportion of available free lime of any material around the structural elements, causing loss of NHL5 currently on the market in the foundation support (Ackers et al 2002). This is likely and as such its use would minimise the risk of leaching to have been a key contributory factor in the collapse from the new mortars (ibid). of most of the upstream spandrel wall and parapet at Cement pointing was removed where possible, Moulinearn Bridge. Localised scour around the NE along with decayed lime mortars and replaced with abutment was continuing to cause erosion of the NE lime-based bedding and pointing mortars, ensuring embankment and had further displaced the base stone that the wide joints typical in rubble masonry structures of the arch barrel. To provide adequate bank protection were packed with pinnings. The underside of the arches which would withstand the force of the water flow of at Errochty Water and Dalnamein Bridges required Lochbroom Burn (prone to flash flooding) but which localised rebuilding work to improve load paths affected would also minimise any adverse environmental effects, 100 Lindsay Farquharson un-mortared rip-rap was placed around the exposed NE embankment and abutment of the bridge. This also provided protection to the base of the arch barrel that had been repaired by indenting. The un-mortared rip-rap falls under the Scottish Environment Protection Agency’s (SEPA) preferred option of greener bank protection and comprised local angular whinstone of minimum 800mm size with flat faces suitable for interlocking to ensure durability (Sinclair 2011). All work was contained within the footprint of the bridge which had been identified after clearance of vegetation and accumulated debris had established the lower base courses of parapet and spandrel wall.

Summary

The purpose of conservation work is to retard the decay process and prolong the life of the monument. The least damaging and most cost effective way of preserving bridges is through continual small-scale maintenance and repair. Ongoing maintenance is often overlooked, especially in structures that have largely become redundant; few of the bridges are still used for traffic and are in relatively isolated locations. Over the three years, the project has attempted to address this through promotion of military infrastructure in order to raise awareness of its significance within British history, Partnership Agreements and the establishment of a volunteer monitoring group. It is hoped that by these means the bridges, along with the roads, can be Illus 5 Moulinearn Bridge, NW elevation showing preserved for future generations. effects of scour. (© Perth and Kinross Heritage Trust)

Illus 7 Moulineran Bridge, NW elevation after works showing rip and, consolidation of broken arch barrel and tensioned post and wire safety guard. (© Perth and Kinross Heritage Trust) Repair and maintenance of historic bridges 101 Illus 6 Illus Drawing showing works to Moulinearn Bridge, by Andrew Driver. (© Perth and Kinross Heritage Trust) Heritage Kinross and Driver. (© Perth Andrew by Bridge, Moulinearn to works showing Drawing 102 Lindsay Farquharson

Acknowledgements

The author would like to thank the following organisations and individuals: Perth and Kinross Heritage Trust; Heritage Lottery Fund; Historic Scotland (support at Moulinearn Bridge); Little and Davis Construction (Newton Burn Bridge and Alt Ach’Mhenich Bridge); LTM Group (Dalanmein Old Bridge, bridge at Spittal of Glenshee, Errochty Water Bridge and Moulinearn); John Sinclair (Allen, Gordon & Co, consulting civil and structural engineer); Dr Sue Swift (consultant zoologist); Northern Steeplejacks Ltd (Tummel Bridge plaque). Also, to all owners who allowed us to carry out work on the bridges and special thanks to David Strachan (PKHT Manager) and Andrew Driver (PKHT Conservation Architect) for all their advice and guidance over the three-year project. Repair and maintenance of historic bridges 103

Appendix 1 Contributions by David Strachan and Andrew Driver

Newton Burn Bridge / Lurg Burn Bridge Errochty Water Bridge NGR NN 8892 31833 NGR NN 7250 647 Status: none Status: Grade C(s) listed building Notes: Bridge had been widened but, unlike Description of works: Soffit: rake, re-point and pin. Dalnamein, an inflexible cement joint had been N approach: installation of timber-lined cut-off drain provided which does not allow each part of the bridge across the N approach to divert excess hillwash away to move independently of each other as it should. from carriageway of the bridge. Drain comprised larch Description of works: soffit: rake, point and pin and boards with non-ferrous nails to form box channel, set replacement of missing masonry above springing to flush with surface of road and fixed with stakes. improve load paths. NE elevation: cut back trees x 2 Date of work: 2009 growing out of elevation as flush with face as possible without causing further damage to masonry. Poison Alt Ach’Mhenich Bridge to retard future growth. Removal of failing cement NGR NN 8763 4738 and lime-based mortars and selectively point and pin. SW elevation: removal of failing cement and lime- Status: none based mortars and selectively point and pin. Parapet: Description of works: E elevation: selective rake, pin controlled down-taking of NNE section, removal of and re-point; indent SE springing stone. W elevation: tree root and rebuilt to original profile. controlled down-taking of partially collapsed spandrel Date of work: 2010 and re-instate. Date of work: 2009 Moulinearn Bridge NGR NN 9694 5474 Dalnamein Old Bridge NGR NN 7548 6966 Status: scheduled monument (Works carried out with grant assistance from Historic Scotland). Status: Grade C(s) listed building Description of works: soffit: selective pointing and Notes: S elevation: bridge was widened, most likely in pinning; SW elevation: selective re-pointing and the early 20th-century to accommodate modern farm pinning; collapsed NE elevation: Consolidate arch vehicles. The bridge was then tied with steel anchors. ring by pinning and pointing, replace missing stone Works described to soffit apply to ‘additional’ arch. at base of arch barrel to prevent further undercutting Description of works: soffit: rake, selective point and by scour. Broken edge of NW parapet: consolidate pin. Replacement of missing masonry above spring surviving base source by pointing filling voids in the to improve load path. Stone selected for indenting of masonry and by pinning up with stones bedded in compatible composition to existing stonework and an appropriate lime mortar. NNW abutment and recessed back by 25mm to ensure new stonework embankment: un-mortared stone rip-rap protection identifiable and in keeping with surrounding structure. using 800minnimum size angular whinstone with flat Parapets: remove failing cement, selective rake and re- faces suitable for interlocking. Handrail: installation of point to secure loose copes. visually unobtrusive and reversible tensioned post and Date of work: 2010 wire fencing with stays. Posts to be fabricated in 76mm diameter CHS MS hot dip galvanized with a buried 450 Spittal of Glenshee Bridge x 450mm base plate. NGR NO 1089 7010 Date of work: 2011

Status: scheduled monument and Grade B listed Tummel Bridge building NGR NN 7624 5918 Notes: bridge is still in use for vehicular traffic. Carriageway is hot-rolled asphalt. The decay of mortars Status: Grade A listed building at the case of the parapets (internal) is likely to have Description of works: brass plaque with 1730, the date been accelerated by de-icing salts. of construction, engraved and black painted, mounted Description of works: parapets: selective point and pin on 9mm thick marine ply backing, the whole fixed base courses (internal). Indent of delaminated stone x 1. using non-ferrous fittings in a recess near the apex of Stone selected for indenting of compatible composition the bridge of the SE elevation. The plaque covers graffiti to existing. etched into a concrete surface. Date of work: 2010 Date of work: 2011 104 Lindsay Farquharson

References

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