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Underground Plotting Room
Robben Island, Western Cape

Date:1941
Type:Military structure
Status:Extant

 


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Coordinates:
33°48'43.68" S 18°22'29.04" E Alt: 23m

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Introduction

The underground Plotting Room facility co-ordinated the fire control system of the two coastal artillery batteries on Robben Island. The Plotting Rooms received data on ship position and direction of motion - via underground communication cables - from the fire control instruments in the Battery Observation Posts and the Fortress Observation Posts, on Robben Island and on the mainland. The information would be sent via the Battery Plotting Room Switch to the receivers in the Fire Direction Table in the Plotting Rooms. The Plotting Room operators then determined and transmitted - again via the Battery Plotting Room Switch - to each of the Batteries, the range and bearing that the guns would have to fire.

Of particular interest is that the Plotting Rooms were largely staffed during WWII with members of the Artillery Specialist Women’s Auxiliary Army Service (ASWAAS) . This was over the period 1941 to 1945.

The walls floors and ceilings of the underground spaces are all remarkably well preserved with limited water penetration and the absence of associated moisture damage. This suggests that the structure was well detailed, constructed and protected from water ingress.

Architectural Description

The underground Plotting Room facility comprises a rectangular, underground, robustly constructed concrete bunker, sub-divided into four separate but interconnected spaces with a stepped access shaft. A separate escape passage and escape shaft was provided as an emergency escape route. The top of both entrance and escape shafts project above ground level and each shaft was provided with a pair of air ventilators fabricated from plate steel.

All four of the air ventilators have a square base with a fabricated transition piece terminating in a circular pipe at the top of the ventilator. The fresh air intake ventilators are shorter than the exhaust ventilators. The circular pipe of each ventilator was in turn capped with a larger heavy steel circular cap. Both pairs of ventilators were provided with a fine steel mesh to prevent the ingress of birds or insects.

A feature of the construction method used for the Underground Plotting Room facility is that the ‘witness marks’ of the original timber shuttering planks are clearly visible on the faces of the concrete work. Chamfered corners, a typical detail for in-situ concrete work, were generally used at external corners.

Stepped Access Shaft

Entry into the underground Plotting Room facility was via a square, hinged metal trapdoor set into the northern side of a pair of steel chequer plate covers at the top of the stepped access shaft. The two steel plate covers were seated on a perimeter steel plate forming an up-stand mounted on the inner top edge of the concrete shaft. The trapdoor provided for easy personnel access while the larger cover plates could be lifted to provide equipment access. The stepped shaft was presumably so designed to minimise the distance in the case of an accidental fall down the shaft.

A simple removable pipe balustrade was fixed to the external exposed vertical faces of the concrete shaft using robust u-brackets fabricated from steel plate The balustrade protected the perimeter of the shaft in case the two plate covers were removed. The balustrade was lowered and returned on either side of the trapdoor to facilitate access.

A vertical steel ladder was originally mounted below the trapdoor into the concrete face of the shaft. The ladder comprised two heavy flat bar vertical stringers and presumably had rungs of solid round bar and gave access to the upper landing of the stepped access shaft. The ladder has since been removed but the ‘witness marks’ of the cut off curved returns of the stringers are still visible on the face of the concrete shaft.

A makeshift intermediate platform of corrugated iron floor on a timber frame has however been installed into the shaft - above the first landing - onto which one descends via an informal timber ladder and from which one descends through a square opening onto a second informal timber ladder - at right angles to the first - to arrive at the upper landing of the stepped access shaft. This upper landing forms the concrete roof structure over the Fan Room directly below it.

The ceiling of the lower portion of the stepped shaft accommodated a pair of air vents, one of which is open and visible as a square aperture in the concrete soffit. The other opening is concealed by a sheet metal duct. Both apertures continue up to ground level as independent square shafts within a single concrete structure. The open aperture was a fresh air intake and the other was connected to a mechanically vented ducted system, extracting stale air from the lower level spaces.

A matching - albeit longer - wall mounted vertical steel ladder originally gave access from the upper landing down to the bottom landing of the stepped shaft. This steel ladder has also been removed, with the same witness marks remaining. However a steep, robustly constructed, timber ladder stair was installed in its stead. This has two heavy timber stringers, thick timber treads (carefully let into the stringers) and a simple pipe balustrade.

The floor level of the bottom landing was 6” (150 mm) below the general floor level of the Plotting Room facility. The bottom landing gave access into a perimeter service and ventilation corridor and into the Battery Plotting Room.

Service and Ventilation Duct

A small scale service and ventilation duct ran around the entire perimeter of the underground Plotting Room facility and was accessed from the bottom of the stepped access shaft. Entrance into the duct for inspection and cleaning was through a low rectangular opening. A second smaller square aperture - at the end point of the service duct - opened into the stepped shaft directly adjacent to the entrance.

The duct functioned to allow the movement of fresh air into each of the adjoining spaces. The duct also provided a route for electrical cables between the different adjoining rooms. The flat concrete roof of the service duct was penetrated at intervals by sets of three narrow pre-cast concrete pipes cast into the concrete roof structure. The upper openings of these pipes were overlaid with a layer of broken stones which formed an 'agricultural drain’ and allowed any accumulated ground-water to drain into the duct.

The service duct had a gently dished floor which sloped gradually from the end point down to the lowest point at the entrance to the duct. The dished floor terminated in a square sump in the floor at the entrance to the service corridor. A semi-circular floor drain was recessed into the floor of the stepped shaft and also drained to the sump.

Battery Plotting Room

Entry into the Battery Plotting Room from the bottom landing of the stepped shaft was controlled with a heavy steel plate door, rivetted to an angle iron frame set in a 3’ (914 mm) thick concrete wall. The door was provided with six separate heavy duty rotating catches, all with double-sided handles to enable operation from either side of the door. A rectangular horizontal sliding panel set into the door allowed views through the door. The sliding panel could be secured in a closed position from within the Battery Plotting Room.

A sheet metal duct mounted to the underside of the Battery Plotting Room ceiling served to extract stale air. Fresh air intakes - from the perimeter service corridor - were located at low level around the external sides of the room, some of which intakes were controlled with a moveable louvred panel. The ceiling of the Battery Plotting Room was formed with Rolled Steel Joists (RSJ) set at close centres with permanent shutters supported between the adjacent bottom flanges. The RSJ’s were cast into the concrete and formed both the steel reinforcement of the roof slab and the underside of the ceiling.

The Battery Plotting Room in turn gave access to the Fan Room, the Fortress Plotting Room and the Emergency Escape route.

Fan Room

The Fan Room was accessed via a heavy steel plate door - with a reduced width and head height - through a 3’ (914 mm) thick concrete wall. An electric motor - mounted on a pair of steel C-channels fixed into the wall - originally provided the motive power for the extract fan. The fan motor has however been removed. The extract fan was in turn connected into a complex arrangement of galvanised sheet metal ducts. One duct extracted stale air from the Fortress Plotting Room; another duct extracted stale air from the Battery Plotting Room. These two stale air ducts joined together before the fan, then split after the fan, with one duct exiting through the stepped entrance shaft and the other duct exiting through the escape route.

Both the stale air ducts exhausted at ground level via the heavy steel plate ventilators with circular caps. Adjacent to each exhaust ventilator was a shorter ,similarly detailed steel plate fresh air intake ventilator.

Fortress Plotting Room

The Fortress Plotting Room was accessed from the Battery Plotting Room via a heavy steel plate door through a 3’ (914 mm) thick concrete wall. The ceiling was also formed with heavy RSJ’s set at close centres, cast into the concrete as the steel reinforcement of the roof slab and as the underside of the ceiling. Fresh air intakes - from the service duct - were again located at low level around the external sides of the Fortress Plotting Room.

A sheet metal duct was mounted centrally on the underside of the ceiling to extract stale air.. A short recessed floor duct gave access - presumably for cables - into the Fan Room. A metal frame mounted on the north wall presumably supported the electrical control and distribution boards. The wall opposite the Fortress Plotting Room entrance carried a communications patch panel

Emergency Escape

The Emergency Escape route from the Battery Plotting Room had to bypass the low level perimeter service duct. The entrance into the escape route was therefore through a square metal plate door set into the wall at a position well above the floor level of the Plotting Room. Sets of metal rungs of round bar were let into the concrete wall adjacent to the door to facilitate easier access through the door into the narrow raised escape passage with low ceiling. Similar metal rungs were mounted on the adjacent wall within the escape passage for the same purpose.

A sheet metal exhaust duct was mounted on the underside of the escape passage ceiling. A series of communication and power cables was originally mounted on the walls of the escape passage. A narrow slot was provided in the floor of the escape passage directly over the Service Duct and a curved vertical slot provided at high level in the escape passage wall into the Battery Plotting Room to facilitate routing the cables through the facility.

The escape route terminated at a narrow vertical escape shaft with a vertical ladder mounted onto the east wall - now removed. Communication and power cables were mounted on the west wall of the escape shaft and exited through the wall, a short distance below the top of the shaft.

The escape shaft was capped with a heavy steel plate cover with two triangular brackets fixed to a cable and pulley system in turn connected to two suspended weights in protective tubes at the base of the shaft to provide mechanical assistance in lifting the heavy cover. Presumably the cover was secured from within the shaft with a simple sliding catch.

Heritage Value

The Underground Plotting Room facility's heritage value lies in its historical associations with World War II and in particular with the Artillery Specialist Women’s Auxiliary Army Service (ASWAAS). It is an example of an unusual, well preserved underground building designed for a particular purpose and specific function and is a significant heritage building on Robben Island. There are only three other similar facilities in South Africa, namely at Apostle Battery (Llandudno), Scala Battery (Simon’s Town) and Da Gama Battery (Bluff, Durban).

Statement of Significance

The underground Plotting Room facility was closely associated with the defence of Table Bay (and Cape Town) during World War II (1939-45) and during the immediate post-war period when the Island was used as a Military Training Base. The structure accommodated and protected the Battery Plotting Room and the Fortress Plotting Room which enabled the proper functioning and operation of the two Robben Island Batteries and communication with other mainland batteries that could support, or be supported by, Robben Island.

The underground Plotting Room facility is a good example of specialised coastal defence architecture. The scale and depth of the underground structure and the in-situ off-shutter cast concrete finish all create a distinctive structure.

The Underground Plotting Room facility reinforces the character of the historically important WWII layer present on Robben Island. The choice of construction materials and the robust detailing relate closely to the other concrete structures built for the military on Robben Island.

Character Defining Elements

The heritage character of the Underground Plotting Room facility resides in the following character-defining elements: The underground bunker of in-situ concrete construction, the pair of heavy duty steel intake and extract ventilators at the top of the two ventilation shafts, the hinged steel access panels, the counter-balanced weight-assisted opening of the escape hatch, the memory of the significant sets of communication and power cables, the witness marks of interior fittings (now removed), the concrete slabs reinforced with RSJ’s, the sheet metal ducts for exhausting stale air and the heavy steel plate internal doors.

Acknowledgements:

The assistance of Chris Dooner (ex SA Navy) in documenting the Plotting Room facility is acknowledged with grateful thanks. Chris identified the site of the underground Plotting Rooms and willingly assisted with the preparation and interpretation of the as-built drawings and shared his understanding of the different spaces. Jens Kuhn assisted with the measuring-up process.

William Martinson, Osmond Lange Architects, September 2019

Reference:
Martinson, William & Kuhn, Jens. Robben Island Museum: Built Environment Conservation Manual. Osmond Lange Architects + Planners, Cape Town, 2019.

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These notes were last edited on 2020 08 05


Writings about this entry

Crook, Lionel. 2013. Island at War : Robben Island, 1939-45. Simon's Town: Naval Heritage Trust. pg