Abstract : Bridges are amongst man's most important constructions. They are of necessity practical, but can also have wonderful aesthetic qualities. For centuries, bridge building depended almost wholly on the use of the arch, and while this traditional method of construction has tended to be superceded during the twentieth century, there is a vast legacy of arcuated bridges, many of which are of architectural and historic interest, on which our road network depends. Currently, new regulations relating to loading, and in many cases, lack of maintenance and resources, and understanding of how the structures worked,have conspired to put a large number of them under threat.
Terms used include: arch - segmental/semi-elliptical, arcuated, ashlar, boss, buttress, cobble, concrete, coping, cramps, cutwaters, formwork, drip, gargoyles, grout, indent, intrados, key stone, lime/hydraulic lime mortar, iron, joints, masons mark, mechanical damage, mortar, pier, parapet, pointing, portland cement, pre-cast, puddled clay, refuge, render, rubble, slurry, solum, spandrel, springing line, stone, string course, tamp, tradition, voussoirs.
This bridge, some eighty metres in length, and orientated north/south spans the River Earn approximately six miles SW of Perth. Built around 1776, it is listed category B. It has four segmental river arches (illustration), and two semi-elliptical, or basket handled, flood arches (illustration). The arches are formed in long, narrow voussoirs, with no noticeable key-stones. Constructed on ashlar piers with rubble work above the springing lines of the arches, the bridge has V-shaped cutwaters which carry up its full height to form refuges in the parapets. The parapet has a cope formed by flush fitted, rectangular stones, held together by iron cramps. One of these stones on the western parapet and above the south bank of the river, has a magnificent mason's mark.
The bridge, when first built, would have been a traditional "hump backed" construction, designed to allow water to run freely to either end of the bridge, where it could drain away, and not penetrate the structure. The carriageway would have been surfaced in cobbles, hand set into puddled clay. The core of the bridge, would probably have been clay, beaten earth, gravel and stone, with another membrane of puddled clay above the arches. All of the stone used in the build would have been bedded and pointed in lime mortar, which would probably have varied between a hydraulic lime used below and immediately above the water to feebly hydraulic used higher up in the structure. While most of the water landing on the bridge would have been shed, it was recognised by the original designer, that the structure could never be entirely waterproof. Instead, the materials used were selected to interact naturally with each other, accommodating movement and allowing for evaporation of moisture from the structure through the softer mortar of the joints.
While it is probably true to say that this bridge is in better condition than many similar constructions, typically, the 20th century, with its growing dependance on motor transport and steady increase in vehicle size, has lead to a national programme of repair and strengthening, which has not always been successfull. Work has usually been carried out in modern materials, and with little understanding of how the structure worked. Current regulations demand that 40 ton articulated lorrys should be able to roam freely over all of the highways and byways of Europe.
At some point, probably when motor transport became more important than the horse, the carriagway was tarmacadamed and its levels were altered at the north end, where, because of a high river bank, the carriageway dropped before climbing to the centre of the bridge. This dip would have been awkward for long vehicles, and it was levelled out with approximately half a metre of fill (illustration). The parapet upstand varies considerably along its length, which gives a clue to the new levels of the road surface. The south bank is much lower, and the road surface had to be inclined over a distance of some thirty metres to rise to meet the superstructure of the bridge. The resulting mass of material settled at some early stage and substantial buttresses were added to both sides. (illustration)
While the original cobbles may survive under the tarmacadam, its very even surface indicates that it is more than likely they have been removed in an effort to achieve a waterproof seal. In all probability the puddled clay will also have been removed, to be replaced with a layer of concrete. It is unlikely that all of this will prevent at least some water entering the structure because tarmac is not impervious. In addition, narrow concrete haunches and concrete kerbs have been added at the foot of the parapets, probably with the intention of offering protection, but also to channel water into drains which exit at plastic pipes which have been forced through the stonework of the refuges. Intended to throw water clear of the structure, they are in effect, dreadful modern gargoyles and will quickly degrade. In an agricultural area, there is inevitably earth on the bridge which continuously blocks these drains, something which would not have originally been a problem, when the steeper levels would have allowed the material to wash off.
The solum of the refuges have been cemented over, again, in an effort to achieve a waterproof seal. The facework of the bridge was,some years ago, repointed in portland cement, with the mortar smeared over the face of the stone to the extent that it has almost formed a render in places. It has been lined out to echo the stones. The intrados or arched extent of the river arches have become coated with a slurry of calciferous material which has leached out of the structure. The cutwaters have been reinforced up to the level of the springing lines of the arches by approximately a foot of shuttered concrete (illustration). Some of the parapet copes have been replaced in long lengths of concrete and a number of the remaining copes are showing signs of distress where the iron cramps have rusted, and despite being set into lead, have expanded to split the stone (illustration). They are no longer preventing water from penetrating into the core of the parapets, a situation which is not helped by the fact that they are flush fitted and there is no projection in which to form a drip.
As with all bridge parapets, they are prone to mechanical damage, and there is evidence that the west end, with its slightly awkward bend is particularly vulnerable. With the raising of the road level it is not as substantial as it once was. So far there has been no attempt to reinforce or replace the parapets, and recent research has in fact demonstrated that the majority of stone parapets are capable of absorbing considerable impact, well in excess of current safety levels.
Several voussoirs have been replaced in the dry arch at the south end of the bridge. Where eleven stones previously existed, three pre-cast units, tinted, lined out and stugged to match the originals have been inserted (illustration). They are already showing signs of distress, being incapable of accommodating the movement which happens in the structure. Worse still, this lack of movement is causing cracking in the surrounding voussoirs. The concrete applied to the cutwaters is also showing signs of deterioration and virtually all of the ashlar work which can still be seen is severely eroded. Two of the blocks on the string course which defines the springing line have been replaced in pre-cast units, presumably at the same time the voussoirs were replaced. Consideration should be given to removing the concrete and indenting new ashlar blocks.
The arches are still very true, a good indication that the structure of the bridge is still sound. It is however worrying that past repairs have not been sympathetic, stonework is starting to deteriorate badly on the cutwaters and large areas of the spandrels and parapet, due to the unsympathetic pointing. Most of this pointing is boss, but will still be holding moisture within the structure, the long term effects of which will clearly be damaging. While the replacement voussoirs mentioned above are unfortunate, there are signs that circumstances are improving. Recent pointing on the inner face of the west parapet, looks to be in a more appropriate mortar mix and is confined to the joints, although, judging by the vegetation that is appearing in these joints, most of this newer work is very superficial, there must have been very little thorough raking out and tamping. It is to be hoped however that this is an indication of a move to properly investigate and understand the structure of the bridge, and to remedy the more unfortunate modern repairs, particularly the overpointing, and the damage that has occurred to the copestones. Trees at the north end of the bridge are very close to the structure, and should be cut back. They are sheltering the stonework from the efects of sun and wind, as a result of which, moisture is being retained in the structure. Deterioration in the facework at this part of the structure is generally, more pronounced. (illustration)
Most of the newer, damaging work, the cement pointing, the build up of the carriageway, and the reinforcement of the cutwaters happened many years ago, and the bridge does at least appear to have been spared the commonly used, massively invasive technique, of replacing all the core material in cement, a process which reduces the masonry to acting as formwork. An alternative process where voids exist, of pressure grouting the structure can be particularly disfiguring. Current regulations require a full inspection every six years, with a level of access which makes it physically possible to touch every part of the structure, with interim general inspections every two years. The more recent repairs undertaken to the bridge are probably a result of an interim, general inspection.
It will be interesting to follow what happens to the bridge over the next few years.The understanding of how these structures work, and they are a great lesson in traditional building skills, is improving all the time. However, many local authority roads engineers who are responsible for maintaining the majority of these structures, almost inevitably find their efforts to maintain and improve, constrained by parsimony on the part of their employers, which, when the alternative is considered, has to be severely misguided.