Operation Chastise, or the Dambustersâ Raid as it is better known, was one of the great technical achievements of the Second World War.
The final outcome of the raids in May 1943 is well documented, and has been portrayed in film and on TV many times. But the part played by BRE at the very beginning of the story was omitted from many of the history books, owing to the secrecy surrounding it. We tell that story here, from material in BREâs archives.
Even before the declaration of World War II, ways were being considered to disable the vast hydro-electric power stations of the Ruhr valley that were known to power the munitions factories in the area. Amongst these was the idea by engineer and aviation designer, Barnes Wallis, to deploy a ten-tonne bomb from 40,000 feet which would bury itself in the ground on impact, with the resulting subterranean explosion causing an âearthquakeâ that would shatter the vast Mohne and Eder dams. The Air Ministry dismissed this plan, since, at the time, no plane could carry an armament of such weight, and no bomb of such a size could be made.
Undeterred, Barnes Wallis turned his thoughts to attempting to identify the smallest charge that would successfully breach the dams, and to identify the best position to detonate this, relative to the dam itself.
Early days at BRE: Davey and Glanville
Norman Davey was one of the first staff of the Building Research Station (BRS, the forerunner to BRE as we know it now), joining as a research assistant in November 1921. William Glanville joined the fledgling BRS as an engineering assistant in November 1922.
Glanville left BRS in 1936, to become a Director of the newly-formed Road Research Laboratory, RRL, at Harmondsworth in Middlesex.
Glanvilleâs expertise was in structures, and Davey specialised in cement and concrete. They became good friends while studying together at Queen Mary College in 1919: âBillâ Glanville was best man at Daveyâs wedding in 1924, and they remained friends until Glanvilleâs death in June 1976, aged 76. Dr Davey retired from BRE as Head of Engineering in 1960.
Wartime explosive testing
Shortly before WWII, a team of engineering staff from BRS and their work studying the effects of explosives on structures, and, crucially, on models of structures, was transferred to the Road Research Laboratory, RRL (now TRL).
Barnes Wallis approached RRL in October 1940, to discuss the possibility of attacking enemy dams, including the Mohne Dam. A team was set up at RRL under Dr A R Collins.
In a document written by Davey in 1993, he recalls that in late 1940, William Glanville brought Barnes Wallis to the BRS site at Garston for a secret meeting where Wallisâ plans to attack the Mohne and Eder dams were outlined. At that meeting, it was decided that the most effective way to determine the weight of explosive needed, and optimum location to detonate it to breach the dam, was to construct and test a scale model. Davey agreed to build this at BRS, and work began in November 1940.
Building the model
The Mohne Dam across the Ruhr valley in Germany was opened by Kaiser Wilhelm II in 1913, and comprehensive, dimensioned details of its construction were published in technical papers at the time. Using data from these papers, Dr Davey designed aÂ 1/50th scale model of the dam, to be built across a small stream in secluded woodland at the edge of the BRS site.
The model, which survives today at the centre of the now enlarged BRE site, was built in seven weeks between November 1940 and January 1941. Temperature records from the time show that the winter was very cold, with daytime temperatures close to or below freezing over much of this period, and photographs show snow on the ground.
To conceal its identity, the model was referred to as âWeir No. 1.â in the records. These show that work at the site started on Monday 25 November 1940, when the area was excavated to widen and deepen the stream, and to prepare an area for the concrete base of the dam. The foundation concrete was poured on 29 November, and the two towers of the dam were cast in situ the following Monday. The side wings completed shortly after this. To allow the model to be built across the stream, a pipe was placed in the foundation to carry the water beneath the centre section during its construction. The work was carried out by Davey, A J Newman, who was in charge of the BRS concrete laboratory, and two of his assistants, A B Stapleton and A Smith.
Millions of tiny blocks
The centre section of the model took over a month to construct, as Davey decided it should be built to mimic the âcoursedâ blockwork of the real dam, as seen in the engineering drawings. Miniature blocks were made by producing long strips of mortar in moulds, âsnapped upâ to form individual pieces, each roughly 0.4 inches long x 0.3 inches wide and 0.2 inches deep (10.2 x 7.6 x 5.1mm). After being allowed to cure for three days in the controlled environment of the BRS concrete laboratory, these were carried by the bucketful to the work site. Laying the blocks into mortar in freezing conditions must have been a real challenge. Strength tests were carried out on sample batches of the blocks, and additional tests carried out on the mortar used to lay the blocks are also detailed in the concrete laboratory records.
Over the years, various documents have speculated about the number of blocks used in the model. Numbers as high as 5 million have been reported. However, photographs taken at the time (above), which came to light in the 1990s, clearly show four rows of blocks on each face of the part-built dam, with an infill of concrete. Based on the dimensions of the blocks and the size of the model, over half a million would have been needed if it was all built this way. The final figure is likely to be much higher as the upper sections were in fact constructed entirely from blockwork. Davey suggests in his 1993 report that around 2 million blocks were used in total.
Construction of the centre section started on 2 December 1940, and was completed on 15 January 1941. A photograph of the completed model shows that an inner wall of clay was placed on the upstream side. This mirrors an earth embankment seen in the technical illustrations from 1913, and also served to seal the pipe that diverted flow beneath the model as it was being built. Six days later, the basin behind the dam was filled with water, and the very first explosive test was carried out the following day, 22 January 1941, eight weeks after construction had begun.
Explosive testing of the model
The model at BRE was subjected to explosive testing, carried out by staff from RRL. These tests were to determine the distance from the dam wall that gave the greatest effect. Ten charges were detonated, at distances of three feet, two feet and one foot from the dam, on the bottom of the pool. A test probe placed against the dry side of the dam measured the deflections that resulted. Photographs from the time show that after the sixth charge was detonated, sufficient damage was caused to make water leak through the dam, and part of the parapet had been blown away. By the time the tenth charge was fired, the model was badly damaged.
Further tests on simplified models
The model at BRS was only used for this very first set of test explosions, and was badly damaged by them. A series of subsequent tests were carried out at RRL at Harmondsworth (close to what is now London Heathrow airport), using âsimplifiedâ versions of the model dam. These were entirely cast concrete rather than built from individual blocks. From the results of these later tests, RRLâs Dr A R Collins concluded that to breach the full-scale Mohne dam, 7,000 lbs (3,400 kg) of explosive would be required, detonated 30 feet (9 m) below the water level and against the inside face of the dam.
Secret until 1954
The first public mention of the 1941 testwork at BRE was in a news release from the Air Ministry in 1954. This started with the line ” ‘Dig-for-Victory’ allotment holders at Garston, near Watford, were bewildered and annoyed, early in 1941, when a mysterious and sudden onrush of water swept down a nearby hill and inundated their plots”
It went on to explain that “The flooding at the Hertfordshire allotments came from the breaching of the first detailed scale model of the Mohne Dam which was tested… at the Building Research Station”.
In reality, the dam was never breached, and it is very unlikely that a large amount of water rushed down the stream at any point. This news release was issued as the final shots for the film The Dambusters were being recorded, and means that the very first tests at BRE were not reported in the film.
The last remaining piece
BRE’s involvement with the development ended with the testing of that first model. But staff at RRL, and later from the National Physical Laboratory (NPL) at Teddington, continued to work to scale up the tests and to develop the ‘bouncing’ principle. All of the test models at Harmondsworth were destroyed by testing. The Nant-y-gro dam in Wales, used to validate the model tests at a much larger scale, was blown apart. And the ship testing tanks at NPL were broken up in 1996 (see more from NPL’s website). Only the model at BRE, from the very beginning of the story, remains in tact (the damage caused by testing was repaired at some point, possibly in the 1960s). It wasÂ scheduled by English Heritage (now Historic England) in 2002 as being a historic monument of “not only of national but also international importance“.
BRE: still using models, 70 years on…
BRE continues to use models, both physical and software-based, to solve complex challenges in the 21st century. From wind tunnel testing (see our video) to prediction of fire spread, we are still at the forefront of innovative research across all sectors of the built environment.
Sources: edited by Peter White, from two unpublished internal documents in the BRE archive:
N154/93 Part (b), Some reflections on the Mohne Dam, Dr Norman Davey, Watford, 1993
N101/96 Operation Chastise: documents surviving at the Building Research Establishment, P S J Buller, Watford, 1996
Elsewhere on the web
Visit BRE and see a piece of history
The BRE site (including access to the dam and much more) has previously been open for Heritage Open days. We are hoping to open the site again once Covid-19 procedures allow.
See photos from the previous Open Days on Flickr