Summary

A series of below ground transport infrastructure comprising two railway platforms, a conveyor control room, conveyors, a slope shaft, a power station, and a locomotive shed. These are contained within the worked-out chambers of a Bath Stone quarry which was converted into a 50 acre ammunition depot: Central Government War Headquarters (CGWHQ).

Source: Historic England

Details

PRINCIPAL ELEMENTS: this sub-depot of CAD Corsham is an approximately 50 acre site within Tunnel Quarry. The area under consideration for designation includes the core infrastructure of the facility, and covers 5,385 square metres. It comprises two parallel train platforms with a conveyor control box, the District 10/11 conveyor system including No. 4 Incline Shaft with conveyor, the Power Station and the Locomotive Shed.

DESCRIPTION: the platforms stand in parallel with a 3m thick wall dividing them. They are constructed of concrete and stand on the levelled tunnel surface. The north platform (Platform 1) is 204m long, and the south platform (Platform 2) is 173m long. Platform 1 has a conveyor running for most of its length by the rear wall. The belt has been removed and a section to the east, that formerly led down a ramp to a double-level conveyor serving the districts, has also been removed. The conveyor on Platform 2 has mostly been removed. Fixed concrete steps stand to the side of the north conveyor at intervals, and there are access points between the two platforms as well as two toilet facilities. At first-storey level, the Conveyor Control Room has been inserted at roughly the centre of the platforms. It overhangs both platforms, supported on steel beams. It is accessed by opposing metal stairways on Platform 1. It contains controls for the conveyor system and has metal-framed windows overlooking the platforms.

The District 10/11 conveyor is approximately 264m long and runs from the edge of Platform 1 to an area by the Northern Service Tunnel. The conveyor is approximately 1m wide and constructed of steel on concrete supports. The belt is intact and made of rubber. At intervals there are emergency stop panels set in the side of the conveyor, below belt level. Concrete steps stand opposite each other on either side of the conveyor at intervals, with a steel handrail straddling the conveyor. Light units in steel cages are fixed to the roof at intervals. Approximately 73m from its south end, the District 10 conveyor adjoins another conveyor that diverts up No. 4 Incline Shaft, to the gated loading bay that leads to the Main Surface Loading Platform, above ground. No. 4 Incline shaft has a central flight of concrete stairs, with handrails to either side, and a winch ramp to the right. A variety of electrical switch units and an alarm bell are fixed to the wall beside the bottom of the shaft. There is also a metal sign detailing the maximum loads for the conveyors. To the immediate south of the conveyor junction, is a winch engine set in the floor. A winch track continues south to Platform 1. Towards the south end of District 10 conveyor is a lobby created by large steel doors to the left, and steel surrounds with square openings to the right. The openings have hinged steel covers. At the south end of the conveyor is a raised platform under a barrel-vaulted roof, beside the track by Platform 1. There are steps down to track level from either side, and a steel handrail. The concrete render surrounding the District 10 entrance is blown, revealing red brick structure.

The Power Station stands in rooms on the north side of the railway tracks, to the right of the District 10 entrance. Steps lead down from the trackside into the main generator rooms that have two five-cylinder Ruston Hornsby 5VLB diesel engines for use in the event of the failure of the national grid. Many of the components of No.1 engine have been removed. No. 2 engine is largely complete. The room retains peripheral equipment, lighting and signage from the Second World War era. To the rear is the electricity switching room. Although this room was not inspected is it reported to contain the original monitor panels, and transformers and switchgear for reducing incoming electricity from 11,000V to 440V/240V. In the room to the west of the main generator room are two Broomwade air compressors of 1938, originally used for the ventilation of the depot.

The Locomotive Shed is a large chamber with coarsely-worked stone walls. Large double doors seal the entrance, with narrow gauge tracks embedded in the floor. The chamber has a central linear section with a railway turntable and an inspection pit set in the floor. The ceiling is reinforced by framed, vertical steel beams set in concrete plinths. Five sub-chambers for storage lead off it, three to the left and two to the right. The first sub-chamber on the right contains six low, hexagonal, concrete plinths set in the floor. Large iron tanks are resting on three of them. There is some original lighting in the sub-chambers.

Across the site many areas of roof are reinforced with steel beams, notably at the train platforms. Most of the stone pillars were reinforced with concrete corsets and some walls were altered with red brick skins overlaid by concrete render. The scheduled area extends to 1m around the designated boundary, and above and below. No above ground structures are included in the designation.

Source: Historic England

Reasons for Scheduling

Tunnel Quarry, part of Central Ammunition Depot Corsham, in CGWHQ, below MoD Corsham is designated as a Scheduled Monument for the following principal reasons:
* Period: the peril from the threat of enemy aerial attack at the outbreak of the Second World War is inherent in the need to provide this facility to store ammunition and supply it to British forces.
* Rarity: only 13 quarries were requisitioned or bought in the 1930s/40s for this purpose, and this is the most extensive and fully fitted-out example of an underground ammunition depot.
* Survival: this is the most complete and best surviving underground ammunition depot from the Second World War.
* Group Value: the location within an extensive mine complex that also includes extant C19 quarry areas, a former aircraft factory, and the CGWHQ site during the Cold War confers added national importance as a highly rare and historically significant site.

Source: Historic England

Sources

Books and journals
McCamley, N J, Second World War Secret Bunkers, (2010)
McCamley, N J, Secret Underground Cities, (2000)

Source: Historic England