Geology and the Tunnels of Gibraltar (Late Tunnels)

The first contingent of military tunnellers (Royal Engineers 108 Tunnelling company) arrived in Gibraltar in October 1940. Three more companies of engineers subsequently arrived to join them.

Although activities first started at the north end of the Rock, providing accommodation for the rapidly increasing garrison became paramount. The existing tunnels were made use of to fulfil the accommodation requirements for the influx of troops. After 150 years of silence and inactivity in the gun embrasures of Willis's, Queen's and the Upper Union Galleries they were converted into barrack rooms fitted with bunk beds. And a new tunnel was cut above Willis's Gallery linked to a reservoir which had been converted into a stonehouse and barracks occupied by the 4th Battalion Black Watch. It was later extended south to become the Great North Road, and again continued further south to the sea area at the other end of the Rock. The motive behind it was the realisation that the main base area should be in the south of Gibraltar where it was well protected from Spain as well as facing the Mediterranean.

Development drives (tunnel entrances) were cut to a 7 feet by 7 feet section and chambers leading off them were further cut to accommodate Nissan and Iris Huts, standard hutting of easy construction and for war economy. As tunnelling activities continued and tunnelling techniques improved, all communication tunnels were made wider for vehicular traffic. This also gave rise to arching roofs on practically every extended or altered tunnel. Early tunnels were cut with flat soffits (roofs) which suffered from stress relief and were prone to rock falls. During tunnelling activity in the nearby vicinity, chambers exceeding a 12 feet span were subsequently driven with arched soffits (roofs) ( the rise being 1/5 the span). The largest chamber ever cut in the Gibraltar limestone is the REME Chamber, which has a 50 feet span by 238 feet long and 11 feet high.

By the end of the war about 25 miles of tunnels were cut in the Rock, and about 35 million cubic feet (approx one million cubic meters) of rock removed.

Post-war tunnelling continued, but at a lesser scale. Apart from military projects two more water reservoirs were constructed. Each with a capacity of 1 million gallons which also required the construction of 10 acres of water catchment area and auxiliary channels. The project started in 1958 and completed in 1961.

The last surface tunnel to be cut in Gibraltar was the Keightley Way Tunnel in 1960, although military tunnelling continued in the years 1965-7.