The tunnels of Gibraltar were constructed over the course of nearly 200 years, principally by the British Army.Fa & Finlayson, p. 6 Within a land area of only , Gibraltar has around of tunnels, nearly twice the length of its entire road network. The first tunnels, excavated in the late 18th century, served as communication passages between artillery positions and housed guns within embrasures cut into the North Face of the Rock. More tunnels were constructed in the 19th century to allow easier access to remote areas of Gibraltar and accommodate stores and reservoirs to deliver the water supply of Gibraltar.
The 20th century saw by far the greatest extent of tunnelling when the Rock was turned into a huge underground fortress capable of accommodating 16,000 men along with all the supplies, ammunition and equipment needed to withstand a prolonged siege. The tunnelling finally ceased in 1968 when the British Army's last specialist tunnelling unit was disbanded. Since then, the tunnels have progressively been turned over to the civilian Government of Gibraltar, although a number are still owned by the Ministry of Defence and some have been sealed off entirely as they are now too dangerous to enter.
Geology of Gibraltar
The Rock of Gibraltar is a klippe of Jurassic dolomitic limestone with series of 'shale' formations underlying and in part overlaying the limestone.Rose (2000), p. 244 It consists primarily of a series of carbonate deposits some thick, varying from dark grey bituminous dolomite at the base through to a very thick, apparently homogeneous sequence of light or medium grey fine-grained limestones. The formation appears to have been laid down in a tropical environment somewhat similar to the Bahamas today, and on the basis of fossil evidence an Early Jurassic (Sinemurian) age has been proposed for the Gibraltar Limestone, though in appearance it has a strong resemblance to the Carboniferous Limestone that underlies large parts of England and Wales.
The formation's thick bedding, extensive cementation and dolomitisation have made it a generally stable engineering material.Rose (2001), pp. 99–100 Tunnelling has been possible almost throughout the Rock, with the exception of one region where it has been precluded due to high ground water pressures. In some areas, zones of weak rock or intrusions of the underlying or overlying 'shales' have caused tunnel instability.Rose (2000), p. 259
Phases of development The tunnels of Gibraltar were constructed between 1782 and 1968, in five distinct phases.Rose (2000), p. 255
1782–99
After the capture of Gibraltar in 1704, the British began constructing fortified defensive lines on the north-west flank of the Rock of Gibraltar – the King's Lines, Queen's Lines and Prince's Lines. These consisted of trenches cut into the solid rock or natural ledges which were made more defensible by building masonry walls and scarping the cliffs below. The Lines were constructed in stages, building on earlier defences constructed by the Moors and Spanish; the King's Lines reused Spanish defensive walls built prior to 1704, the Prince's Lines were excavated between then and the Thirteenth Siege of Gibraltar in 1727 and the Queen's Lines were constructed during the Great Siege of Gibraltar (1779–83).
The first man-made tunnels in Gibraltar were created during the Great Siege when members of the British Army's Soldier Artificer Company dug out a tunnel behind the North Face of the Rock. It was intended to reach a position called the Notch, which was inaccessible by any other route, where the British garrison sought to install a new artillery battery to cover a blind angle.Connolly, p. 14 The tunnel was the initiative of the company's Sergeant-Major, Henry Ince, who began the task on 25 May 1782. As the works progressed, the tunnellers decided to create an opening in the cliff face to provide them with ventilation. It was immediately realised that this would offer an excellent firing position. By the end of the siege, the newly created Upper Gallery housed four guns, mounted on specially developed "depressing carriages" to allow them to fire downwards into Spanish positions on the isthmus to the north. The Notch was not reached until after the siege had ended; instead of mounting a gun above it, the outcrop was hollowed out to create a broad firing position called St George's Hall.Fa & Finlayson, p. 30
The tunnelling continued after the siege to build a series of connecting galleries and communication tunnels to link them together with the Lines. By the end of the 18th century, nearly of tunnels had been dug.Hughes & Migos, p. 248 These comprised:
Windsor Gallery – Queen's Union Gallery – Upper Union Gallery – Lower Union Gallery – Prince's Gallery – King's Gallery – Queen's Gallery –
1880–1915
The second phase of tunnelling was driven by a number of disparate requirements towards the end of the 19th century as Gibraltar's naval base became a key strategic asset for the United Kingdom. Two short tunnels were constructed in Camp Bay on the southwest end of Gibraltar in 1880 to provide access to a quarry. In 1895, the Army dug out two natural caves under Windmill Hill to create an underground ammunition store, the Beefsteak Magazine.
Major new construction works in the harbour at the end of the century prompted the Admiralty to construct the Dockyard Tunnel between 1898 and 1899, driving through the entire width of the Rock in an east–west direction to reach Sandy Bay on the east side, where quarries were being worked to provide stone for the construction of the dry docks. In 1901, the Admiralty followed up by extending the natural Ragged Staff Cave to form a tunnelled underground magazine adjoining the harbour. Gibraltar's long-running water supply problems were tackled between 1898 and 1900 with the construction of four underground reservoirs. They were supplied with rainwater gathered from catchment areas on the Great Sand Dune on the east side of the Rock and conveyed to the reservoirs by a new east–west tunnel. A fifth reservoir was constructed between 1911 and 1915.
1933–38
The third phase of tunnelling was prompted by the rise of Nazi Germany and the outbreak of the Spanish Civil War. Air-raid shelters and underground hospitals were constructed from 1936, while improvements to Gibraltar's water supply were made between 1933 and 1938 with the construction of four new underground reservoirs. An additional reservoir was begun but was used, in its incomplete state, as a storehouse and barracks for 4th Battalion the Black Watch during the Second World War. The improvements made during this phase increased the total length of the tunnels from to .Rose (2000), p. 256
1939–45 The most intensive phase of tunnelling in Gibraltar's history came during the Second World War, when the territory played a vital role in the North Atlantic and Mediterranean theatres. The total length of the tunnels increased greatly during the war, from to .Brayley, p. 41
At the start of the war, the civilian population was evacuated and the garrison was greatly increased in size. Numerous new tunnels were excavated to create accommodation for the expanded garrison and to store huge quantities of food, equipment and ammunition. The tunnelling was carried out by four specialised tunnelling companies from the Royal Engineers and the Canadian Army. A new Main Base Area was established in the south-eastern part of Gibraltar on the peninsula's Mediterranean coast, shielded from the potentially hostile Spanish mainland, and new connecting tunnels were created to link this with the established military bases on the west side. A pair of tunnels, the Great North Road and the Foss Way, were excavated running nearly the full length of the Rock to interconnect the bulk of the wartime tunnels.
The tunnels accommodated what amounted to an underground city. The entire 16,000-strong garrison could be housed there along with enough food to last them for 16 months. Within the tunnels there were also an underground telephone exchange, a power generating station, a water distillation plant, a hospital, a bakery, ammunition magazines and a vehicle maintenance workshop.Rose (2000), p. 266 They also housed one of Gibraltar's most secret places – Stay Behind Cave, built for Operation Tracer, a plan to maintain a secret observation post manned by six men within the Rock if it had fallen to a German invasion. It was not rediscovered until as recently as 1997.Fa & Finlayson, p. 46
Among the major tunnels and systems built during the Second World War were:
AROW Street Beefsteak Magazine Brewery Magazine Calpe Hospital Flat Bastion Hospital Fordham's Accommodation Fosse Way Magazine Gort's Hospital Great North Road Green Lane Magazine Harley Street Levant Accommodation Liddell Union Maida Vale MacFarlane's Gallery Power's Drive REME Chambers Upper Sandy System Williams Way
1956–68 The final phase of tunnelling took place during the Cold War to meet a number of new military and civilian requirements. Storage facilities were constructed under the Rock to hold bulk fuel, and two more reservoirs were also constructed. Several more connecting tunnels were excavated to improve road access across the south of Gibraltar. The final tunnel, Molesend Way, was completed by 1967. Tunnelling ceased for good in April 1968 and the last Tunnelling Troop was disbanded and its members dispersed to other units.Rose (2000), p. 257
Unfortunately during the construction of this tunnel the last Royal Engineer to be killed digging tunnels in Gibraltar died during an accidental explosion. Bill Pointon was working with Harry Calligan on the tunnel face when they drilled into a pill of powder that had not been "fired" in the previous shift. Calligan was badly injured but Pointon died of his injuries. When the tunnel was completed the Squadron, following a long established convention, wanted the tunnel named Pointon's Way. That was not accepted and the tunnel was instead named Molesend Way.
Excavation methods
Gibraltar's first tunnels were constructed by hand in a slow, laborious but durable method of excavation. The first task was to fragment the limestone rock. This was done by using a variety of methods, including gunpowder blasting, fire setting (building a fire against the face of the rock to heat it, then quenching it with cold water to cause it to shatter), quicklime (used to fill boreholes which were then slaked with water, causing it to expand and so shattering the surrounding rock) and hammering in wooden wedges which were expanded by soaking them with water, again causing the rock to shatter. The fragments were then removed with crowbars and sledgehammers.
The tunnels thus created were about to in diameter with smooth-sided walls. Excavation was slow, at a rate of only about per year, but the methods used by the original tunnellers had the advantage of causing only minimal damage to the surrounding rock. This has resulted in very stable tunnels which are still readily accessible today.
The later tunnels were constructed at a far faster speed using explosives and machinery. By 1942, the tunnelling had reached a peak rate of advance of of tunnel per week. However, the use of explosives produced large quantities of high-pressure gas which penetrated the surrounding rock and weakened it, and it appears that too much explosive may have been used. This has led to the surrounding rock fragmenting on a large scale, causing the Second World War tunnels to suffer from fracturing requiring them to be scaled periodically to clear away loose rock. Some of the tunnels have had to be sealed permanently as they are now too unsafe to enter. Others have needed to be supported by rock bolts, weld mesh, props, arches or tunnel linings, and their integrity is monitored by carrying out scanline surveys.Rose (2001), p. 112