Site Records


SiteName: Llanberis RAF Reserve Depot (Bomb Store)

FFord Clegir
Llanberis, Caernarvonshire
OS Grid Ref: SH566610

Sub Brit site visit July 2000

[Source: Nick McCamley]

On 18th August 1939, the Air Ministry sought approval to acquire the disused Glynrhonwy Isaf slate quarry which had closed in 1930; the quarry, near Llanberis in North Wales, was deemed suitable for the storage of 18,000 tons of bombs. It consisted of a number of deep open pits, linked together by tunnels. Following the apparent success of the design employed at Harpur Hill in Derbyshire, the air ministry decided to use the same technique at Llanberis, converting the eastern pit into an underground depot, but because of the great depth of the quarry the design was adapted to produce a structure with two floors throughout. The lower level and a conventional flat reinforced concrete ceiling which also formed the floor of the upper level which had an arched roof like that at Harpur Hill. Standard and narrow gauge railway lines entered the lower level of the depot through the original quarry access tunnels, while three electric lifts transported bombs to the upper floor. The deep pits to the west of the depot were later used for burning and dumping redundant and dismantled ordnance.

Overhead protection was given by forty feet of broken slate. In response to pressure from the treasury efforts were made to cheapen and accelerate the construction of Llanberis, but unfortunately the cost cutting had disastrous consequences only six months after the depot was opened.

Photo:The cleared collapsed area looking towards the surviving galleries
Photo by Nick Catford

On 25th January 1942, two-thirds of the structure collapsed within seconds under the weight of the overlaying backfill, completely engulfing a train of twenty seven wagons which was in the process of unloading. The collapse buried over 14,000 tons of bombs which at the time represented 14% of the total RAF stock. A court of inquiry concluded that faulty design was the principal cause of the failure; cracks were noticed in the structure as the building neared completion but these were attributed to minor defects rather than to a major and fatal miscalculation.


Clearing the bombs after the collapse

Most of the bombs were recovered and although the remaining tunnels were eventually cleared of debris, no ammunition was ever stored underground at Llanberis again. The surviving underground galleries were abandoned and the collapsed section was used for open storage of incendiaries and as a demolition area with ordnance being dumped in the pits to the rear of the depot.

The depot remained in use after the war but all functional stock was removed by March 1955 and the depot closed in July 1956.

This wasn't, however, the end of the story. After the war there was long term activity at Llanberis, in the form of a small RAF detachment of bomb disposal people, patiently clearing dumped weapons. After the war large quantities of incendiaries were dumped into the pits, some water filled, to the rear of the main storage area.

Photo:Plan of the depot after the collapse
Drawn by Nick Catford

The following text was taken from the book 'Designed to Kill' by Major Arthur Hogben,

The demolition and burning of explosives within the quarry area started in June 1943 when the Royal Air Force School of Explosives moved to the site. The school curriculum included the destruction of explosives, so large quantities of bombs and pyrotechnics were brought in for demolition or burning. The destruction of explosives continued until July 1956 when the site was closed. Included in the destruction programme was virtually every type of explosive item on the Royal Air Force wartime inventory. Unfortunately, as sometimes happens with mass demolition, a proportion of the items were not completely destroyed. Thus this large and practically inaccessible complex of quarries was known to contain quantities of explosive items. As such the site solicited a certain amount of indiscriminate dumping of unwanted or recently recovered explosive items. The shape of the site was such that much of the explosive material dumped ended up on ledges and slate outcrops, never reaching the quarry bottoms. More still had been dumped or fallen into the lakes, which had formed in the quarries during the years of inactivity between 1956 and 1969.

Photo:The railway loading platform
Photo by Nick Catford

In 1969 a decision was taken to clear the entire site of explosives and explosive debris. The task was given to 71 Maintenance Unit EOD Flight from Royal Air Force Bicester. By the time the task was completed in October 1975 the personnel of the Flight had become expert in lifting tons of explosives from the quarry pits and lakes and in the handling of special mechanical equipment. They had also learnt the arts of tunneling and rock climbing, which in the earlier days had been the only ways of gaining access to some of the pits and their surrounding ledges. This must have been one of the few bomb disposal tasks carried out by any Service where members of the unit had first to be instructed by a Mountain Rescue Team. The various rock climbing techniques and rescue procedures taught were essential to enable members of the unit to reach much of the explosive ordnance with which they had to deal.

From 1969 onwards, the various pits and tunnels were progressively cleared. Members of the EOD Flight burrowed further and deeper into the debris and slate rubble to uncover such items as incendiary bombs and high explosive bomb detonators. The latter, together with the numerous bomb fuses, which were uncovered, were in an extremely hazardous condition and required careful handling. With the help of the Royal Engineers, roads were constructed into the more difficult pits.

Royal Navy divers were co-opted to investigate the contents of a large lake in one of the pits as it was suspected that it might contain some explosive items. The divers reported that the bed of the lake was littered with explosive items including a number of large bombs. Subsequently, over 20,000,000 gallons of water and sludge were pumped out. By April 1973 the lake was emptied revealing everyone's worst fears; it took a further two years to recover and dispose of the explosive items revealed. Fortunately, this pit was one of those to which 38 Engineer Regiment, RE, had constructed a road, otherwise the task would have been impossible.

On completion of the task, 71 Maintenance Unit EOD Flight had moved approximately 85,000 tons of slate and debris, recovered and disposed of 352 tons of explosive items together with 1,420 tons of non-explosive ordnance debris.

Further information and pictures about this site continues here

[Source: Nick McCamley & Major Arthur Hogben]

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