Brunel’s suspension bridge secrets unveiled

For more than 150 years the Clifton Suspension Bridge had a secret – one that Isambard Kingdom Brunel never thought would be discovered.

The engineer who designed the world famous bridge had baffled experts with the construction of the Leigh Woods abutment, the structures at the end of the bridge.

The abutment was the first part of the bridge to be constructed between 1836 and 1846 and rises 33.5m out of the Avon Gorge. For more than a century the stone structure, which supports the bridge tower, was believed to be solid.

Then, just 14 years ago, a series of huge cavernous vaults underneath the abutment were discovered. The vaults have given engineers and historians a unique insight into how the bridge was constructed and the very humble beginnings of such a famous structure.

“All they would have done is hack out some relatively flat shelves out of the rock and built the structure straight up,” said bridge master David Anderson.

This somewhat underestimates the scale of the task as, once you are inside the chambers, the size is astonishing. There are 12 masonry arched vaults in all, rising up out of the Avon Gorge itself supporting the road and the footways 30m above.

At the first level there are seven chambers up to 11m (36ft) high, beneath that are a lower series of five more chambers of the same size

But Brunel left no clues about the existence of the vaults and for decades what lay beneath the abutment was left to speculation.

“We never knew for sure if there were vaults there or not,” said Anderson. “We thought over the years that there probably would be vaults within the structure as it would make sense to build it that way, but Brunel did not leave us with any access”.

Breakthrough

The vaults were accidentally discovered during some routine repairs on the bridge in November 1999 when a workman discovered a narrow shaft into the structure.

Foreman Ray Brown explained what they discovered: “We did find an opening and it was covered by two pieces of what appeared to be railway sleepers, cut to fit into each other perfectly and they were still in perfect condition, When I lifted them off it revealed that it was covering over what looked like a well shaft, on dropping a tape down it was over five metres deep.

“My biggest regret is that the battery on my video camera was flat and so I never lowered it down the hole. That would have revealed that it was a chamber of one of the vaults and would have been a  massive discovery at that point.”

It would be two more years before the shafts were explored again and this time specialist cavers were called in.

John Mitchell was bridge master at the time of the explorations. He was reported as saying that it was “amazing to think we are exploring these chambers… which have not been seen since completion”. 

The task of exploring the tunnel was left to the cavers, who carefully made their way down more than 36 metres before finding narrow side tunnels which led them into the enormous vaults.

Exploring the vaults

Today it is still a scramble down to reach the entrance of the vaults. Cave spiders congregate around the door frame, they are the only living thing found in the vaults and act as guardians to the entrance.

Inside, water steadily drips through the masonry roof creating a spectacular stalactite and stalagmite display. You can see the rock of the gorge itself and the walls built of sandstone rising out of it. Huge piles of rubble from the construction lay at our feet and access from one vault to another is by squeezing through tiny passageways and shafts – not for the claustrophobic.

The vault may be cathedral-like in size, but it was built in a relatively rough and ready fashion.

“They are not pointed or finished off with any particular care because Brunel didn’t think anyone would come here to look at them,” said Mr Anderson. “He left no access points so I guess the thought it would look after itself for centuries to come, and it has.”

However, despite the apparent lack of aesthetic care, the skills of the craftsmen and designer are all too apparent in the condition of the caverns themselves: “We’ve had to do very little masonry work since we discovered them. If we hadn’t discovered them and they’d remained secret for another 100 years I don’t think there would have been any particular problems,” said Anderson.

Explorations of the vaults continue, but they are not giving up their secrets that easily. “We have only found artefact. We recently found a pick head when we were clearing out rubble in one of the access shafts a few weeks ago. It is 180 years old and still in good condition. That is the only thing, no bones or treasure, not so far anyway.”