Railway 200: Brunel's timber viaducts of Cornwall

In his latest tribute to Britain’s railway infrastructure, as part of Railway 200, Dr Joseph Brennan travels to Cornwall to honour the great Isambard Kingdom Brunel.

In this article:

• Isambard Kingdom Brunel championed timber viaducts for the Cornwall Railway, valuing their cost-efficiency and ease of maintenance.
• Though timber was seen as temporary by others, Brunel designed viaducts with replaceable components to ensure longevity.
• Most timber viaducts were eventually replaced by stone and iron, but many original piers still stand as lasting tributes.

Did you know that Isambard Kingdom Brunel’s most famous railway bridge - the Grade 1 Listed Royal Albert Bridge in Saltash, Cornwall - was almost the wooden wonder of the world?

Of all the railway bridges to stand the test of time, those constructed of iron, steel and stone seem the surest to prevail.

But timber was used in bridging structures. And it was a favourite for one great engineer of the age, being both lightweight and (initially) cost-efficient.

Brunel was a key advocate, harbouring what turned out to be a healthy distrust of cast iron.

“I avoid the use of cast iron whenever I can,” Brunel said in 1849.

“In all cases, where it is beyond a certain span of from about 35 to 40 feet, I should prefer using timber or wrought iron, or the two combined.”

As Dr L. G. Booth (a timber engineering lecturer at Imperial College, London, at the time) writes in his 1976 treatise on the subject: “It can be argued that Brunel was the greatest timber engineer this country has ever known. The coming of the railways presented challenges of a scale never before experienced by engineers, and of all the railway’s engineers he was the one who turned to timber most frequently and used it with the greatest flair and confidence.”

Wrought iron’s high cost, together with Brunel’s mistrust of it, is probably what led him to embrace timber.

Certainly, the lack of capital available was a key driver for timber’s selection on the Cornwall Railway (CR), which opened in 1859 as a broad-gauge single line (although most bridges were built to eventually accommodate broad-gauge double track). Brunel’s use of timber on the CR is probably his most famous - and is our focus here.

Using timber-engineering expert Booth as our guide, together with the listing authority and contemporary sources, this story surveys the standouts of Brunel’s CR timber viaducts (there were 42 in all), as well as the structures built to replace them.

Many of Brunel’s originals were constructed of masonry piers with timber bracing and deck spans, and a number of Brunel’s masonry piers remain today - either alongside or incorporated into current structures.

These offer a tangible connection to the origins of this stretch of the Cornish main line, as well as a means of remembering structures too special to be forgotten.

The virtues of timber

Timber viaducts came with higher maintenance costs than structures built from more durable materials. But they reduced the initial cost of a line, which could then open and earn revenue.

Some companies even used timber viaducts as a temporary measure and budgeted in their replacements by iron girders or masonry - to be erected when sufficient finance was available.

However, Booth says there is no evidence that Brunel considered his timber viaducts temporary.

As will come as no surprise, Brunel conducted rigorous testing on the strength of timber and (for its use externally) on methods of preventing decay. He paid particular attention to how his timber viaducts would be maintained, and he built this maintenance programme into their design.

As individual parts of the yellow pine used decayed, these could be rapidly replaced without the need to close the structure to traffic. And this worked in practice, too.

The only maintenance problems experienced were at St Germans (replaced 1908), although even here these problems were overcome by use of a temporary truss.

That’s not to say that the long-term sureness of these viaducts was never in doubt. Those built on a curve and with timber piers needed reinforcing, and eventual replacement of all the CR viaducts by structures constructed with more durable materials was probably inevitable. Indeed, experts suggest that Brunel himself foresaw this.

Brunel took a site-specific approach - for example, using stone piers for many of the longer and loftier spans, but timber piles for muddier environs that called for lighter-weight structures.

Of Brunel’s CR viaducts, Booth lists 34 on the Plymouth to Truro (P-T) stretch, opened in 1859, and eight on the Truro to Falmouth (T-F) stretch, opened in 1863.

We embark on a selective tour of the replacement programme, focusing on those replacements with elements of Brunel’s originals that have been granted listed status.

1871-1882 replacements

The first to be replaced was at Probus (P-T, opened 1859, replaced 1871), after only 12 years in service. Evidence of Brunel’s original can be found here, though covered by overgrowth and unlisted.

Draw Wood and West Largin followed in 1875. The latter’s replacement is Grade 2 listed.

A further nine were then replaced by 1883 (together with five on the South Devon main line). These (all on the P-T stretch) were Moorswater in 1881 and Cartuther, Bolitho and St Pinnock in 1882.

Reports that are broadly contemporary with these replacements offer insight into why it was deemed ‘time’s up’ for Brunel’s viaducts.

On August 25 1877, The Railway News - reporting on a meeting of the British Association at Plymouth, where the CR’s engineer P. J. Margary had presented a paper - states simply: “When the line was made, timber was less costly than now.”

Meanwhile, a September 5 1879 report in The Building News paints a more urgent fiscal need by the company: “The timber viaducts on the Cornwall Railway Company, which form so picturesque a feature of the combe scenery from the vicinity of the line, but which have occasioned much anxiety and expense to the company, are being gradually replaced by stone structures under the supervision of Mr. P. J. Margary, the engineer.”

St Pinnock is one of two Brunel viaducts that Booth marks out as having been particularly impressive, rising 151 feet high and being 633 feet in length.

It’s a fitting tribute to Brunel today, given that its original piers are part of the replacement structure. It has seven piers of roughly-dressed, coursed slatestone from Westwood quarry. In the redesign, the piers were heightened and timber trestles replaced by lattice iron girders.

Brunel’s original St Pinnock viaduct dates from 1854-55, its piers consisting of eight buttresses with weatherings rising to form five stages, with pointed openings piercing the four upper stages.

The 1882 redesign is easy to spot, for the heightened portions of the piers have a slightly cruder tapering, pierced sixth stage. The viaduct is now listed at Grade 2 and has the distinction of being Cornwall’s tallest.

Leading Victorian sanitary engineer A. P. I. Cotterell offers some detail on key CR replacements (up to that date) in his 1890 paper on the subject, his account having been enriched with details provided by then resident engineer, T. H. Gibbons.

Of St Pinnock’s replacement, Cotterell writes: “The superstructure was completely transformed in a most successful manner without endangering the safety of a single train.

“The traffic was not disturbed for a single day, although the very decking upon which the permanent way was laid was taken up and replaced.”

Quite the feat - and something that I’m sure Brunel would have appreciated.

Carrying out the replacement works without disruption to traffic was achieved, in part, through the construction of temporary ways on which hand cranes worked, keeping the main line clear.

By 1890, nearly all of the higher viaducts had been rebuilt. And in the case of the masonry viaducts, these differed little from each other, thereby saving on labour and calculations.

Of more than special significance - indeed, the most significant of the replacements on the line - is Moorswater, listed at 2*.

An 1881 edition of The Railway Engineer, while noting that the CR has many “fine examples of timber structures”, cites renewal and maintenance as “so costly that it has been determined gradually to replace them by more permanent structures in stone and iron”.

Moorswater is drawn out as an illustration of one of the “fine engineering works” to result from the replacement programme.

Moorswater is another masonry replacement, consisting of seven tapering piers of rock-faced slatestone with eight semi-circular arches with granite long and short voussoirs.

The remains of Brunel’s earlier piers are to the south and are similar to those at St Pinnock, though not incorporated into the replacement.

While many of Brunel’s masonry piers were “admirably built”, Cotterell says that those at Moorswater were “rather poor”.

That’s understandable given that the “handy steam derrick cranes” available at the time of the replacement’s construction had not been around when the piers for the timber viaduct were built.

These “handy” means of hauling materials, however, were a source of tragedy in this case.

As Cotterell explains, Moorswater - in fact, all the early replacements - was designed by H. J. Cole, who was acting as resident engineer under CR’s chief engineer, Margary.

Cotterell recounts that a small travelling crane was being used to transport the stone. The piers, at this stage, had not progressed very far when, with the driver unloading stone slowly, “the resident engineer became impatient, and jumped upon the crane to show the driver how he wanted it done. In lifting a heavy stone, however, he upset the crane which, falling upon him, crushed and scalded him to death instantly, and so injured the driver that the poor fellow died a few days after.”

It was ultimately on Margary’s recommendation that the belief that all of Brunel’s timber viaducts should be reconstructed in a more durable material was carried forward – although this replacement programme ended up being more protracted than sources at the time might have expected.

1884-1908

After Moorswater, Fal was replaced in 1884, followed by Combe St Stephens and Largin in 1886.

Combe-by-Saltash and Liskeard were replaced in 1894, Milltown in 1896, Tresulgan, Coldrennick and Treviddo in 1897, and St Austell and Gover in 1898.

At the dawn of a new century, Keyham was replaced in 1900, followed by Tregarne and Tregeagle in 1902, Weston Mill and Carvedras in 1903, and Truro in 1904.

There were then six replacements made in 1908 - Stonehouse Pool, Forder, Wivelscombe, Grove, Nottar, and St Germans.

Several of these replacements are Grade 2 Listed, while those whose listings also protect original Brunel piers include Combe St Stephens (running parallel on the south side, of rectangular plan, battered) and Gover (running parallel, north in this case).

Gover is a particular treat, having (according to the listing authority) “great landscape value”.

Carvedras (over the River Kenwyn) also has five surviving Brunel piers.

But it’s Truro that is the highlight here, where there are 15 surviving Brunel piers of dressed granite. This is the second of Brunel’s viaducts that Booth singles out as impressive - for its length (1,329 feet).

Meanwhile at (East) Largin, we find another replacement that incorporates Brunel’s original (as at St Pinnock), with the top stage added in larger masonry and now spanned by lattice girders.

On the transitory nature of timber in railway bridges, those with timber piers as well as superstructures were lighter propositions that suited the soft ground of tidal regions - including at Weston Mill, Forder, Wivelscombe and Nottar.

In the case of the last three, these were demolished and bypassed when the line was diverted to a more inland alignment in 1908.

In its service life, Wivelscombe was the site of a mail train derailment (in 1889). The train ran off the rails while passing over the viaduct. No one was injured.

Grove was also the site of an accident, this time with fatal consequences.

In 1859, a few days after the opening of the line, a train from Plymouth collided with the end of the framework forming the right parapet of the viaduct. The two leading carriages plunged off the viaduct nearly 30 feet into the creek below.

The driver and the fireman were suffocated or drowned, while a guard also died. The passengers in the two carriages (believed to be more than 14) escaped, some with severe cuts and bruises. No fault was attributed to the viaduct.

1923-1934

The 1908 realignment of the line marked the completion of replacements on the Plymouth-Truro stretch.

But on the Truro-Falmouth stretch, the programme was yet to begin. Several of these originals gave many more years of service - it was only after the First World War that their maintenance costs became uneconomical.

One of the nails in the coffins of Brunel’s Cornish viaducts was the increasing scarcity of yellow pine, and that its replacements (pitch and Oregon pine) had shorter lifespans. Oregon pine gave a life of around eight years, whereas the average life for yellow pine was 30 years, and even up to 60.

Pascoe and Penryn were first to be replaced in 1923, followed by Penwithers in 1926 and Perran in 1927. Ponsanooth was then replaced in 1930, followed by Ringwell and Carnon in 1933.

Collegewood was the very last to be replaced - in 1934, having had a life of 71 years. In the words of Booth, “an era had ended”.

Collegewood viaduct in Penryn is a highlight of these replacements. The longest of viaduct along this stretch (at 954 feet), here Brunel’s 12 remaining piers are individually listed at Grade 2.

The piers are of rock-faced granite and a tapered H-plan, and although somewhat obscured by trees, are dramatic to look upon when crossing the parallel replacement.

A legacy in stone

On the CR, Brunel was confronted with challenging topography and considerable fiscal restraint, hurdles he rose above through timber.

As Dr Nicholas A. Bill, a practicing structural engineer with a PhD from Cambridge in 19th century railway bridges, notes: “Brunel oversaw the construction of nearly a quarter of all known timber structures built, including some 601 bridges and viaducts for over 22 different railways.”

Using timber when many of his peers would have considered it suitable only for house roofs and floors, and while cast iron was being embraced by many as a hero material, Brunel tackled some truly daunting spans, often through “fan viaducts”.

But Brunel’s achievement could have been so much greater. For, in Booth’s words, while “the Cornish viaducts can be considered as the triumphant culmination of Brunel’s work in timber they would have been surpassed by a bridge which, sadly, he never built”.

This was for crossing the River Tamar - the major obstacle for the railway’s push into Cornwall.

Given Admiralty requirements for a bridge with a large central span with adequate head room, Brunel had proposed to make the crossing at Saltash, where the river was 1,100 feet wide, with one span of 255 feet and six of 105 feet. This bridge was to have superstructures of timber trussed arches.

It is perhaps for the best that it was never to be, because if the rest of Brunel’s CR viaducts are anything to go by, it would probably no longer be with us.

And a world without that great tribute to the man, the Royal Albert Bridge, is difficult to wish away.

The legacy of Brunel’s great experiments in timber must be judged (on balance) a success, if ultimately uneconomical.

A 1930 discussion among members of the Institution of Civil Engineers (ICE), when the replacement of Brunel’s original viaducts had an end in view, offers insight into the legacy of (and admiration for) these structures, together with just how novel they were for many.

A Mr Smith, “who had been responsible for the maintenance of a number of those timber structures for many years”, so “probably there was no one more intimately acquainted with their constructional details”, had his views relayed by the ICE president.

“He was sorry in one sense that so few of them were left. It was only a question of time till they disappeared altogether. But no doubt they would always be remembered as examples of the boldness and simplicity of design favoured by Brunel.

“The President had certainly considered them bold when he first, as Chief Engineer, became responsible for their maintenance. In fact, he had regarded them with some suspicion, and it was well known that some years ago, when a fair number were still in existence on the main lines, quite a number of the travelling public, visiting Devon and Cornwall for the first time, were glad to reach their journey’s end.

“But, as a matter of fact, the structures had served their purpose admirably, and no serious mishap had ever occurred.”

The history of cast iron in railway bridges is much more grimly chequered. Brunel’s misgivings were proved prudent, tragically, in the 1879 failure of Thomas Bouch’s Tay bridge (once a monument to cast iron, now a memorial to many lives lost).

By that time, replacement of Brunel’s wooden viaducts was already under way. But the legacy of the piers left alongside their replacements - unlike Bouch’s stumps alongside the second Tay bridge - serve as just some of many tributes to Brunel’s genius.

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