R.A.F. Maintenance Unit 28 – Harpur Hill
In the late 1930s, as the threat of war again developed over Europe, the Army, Navy and Royal Air Force were each looking for additional sites for the storage of weapons, ammunition and bombs. The preferred criteria for sites for ammunition and bombs were in disused mines (to give protection against aerial attack) and north of a line from the Bristol Channel to the Humber to increase the distance from German air fields. The three branches of the U K armed services were in competition with each other in this search and the RAF was slower off the mark than its competitors. It became apparent that there was a distinct shortage of suitable disused mine sites and a further approach was required. For this reason, the Frith site at Harpur Hill was again selected as a key location in the nation's defence system, as a site for a new design of major bomb store.
The design concept for the Harpur Hill store was a series of reinforced concrete galleries that would be covered by 60 feet of overburden, to give a similar level of protection against a bombing raid as would be provided by subterranean tunnels. As with the WW1 artillery range, the proximity of the site to Harpur Hill quarry, with its rail connection and other services, was favourable to the choice of site and an additional feature was the large quantity of quarry waste immediately available for covering the galleries on their completion. Furthermore, the close proximity of the large quarry meant that the construction work close by could easily be mistaken for quarry related activities in aerial reconnaissance.
The site was selected in 1938 and construction work began shorttly afterwards, with the RAF taking over in December 1939. The site was known formally as RAF Maintenance Unit 28 and it continued in operation until December 1960, with important effects for the town.
The design of the site was regarded as a success and a second site, to a similar design, was built in the large slate quarry at Llanberis in North Wales. However, during the adding of the overburden at the Llanberis site, large cracks were noticed in the concrete walls of the galleries and construction work ceased. Checks were then made at the Harpur Hill site and similar cracks were observed. The bombs were removed temporarily, in case of collapse, and a large part of the overburden was removed before the bombs were replaced in the repaired galleries.