ITM-Ltd, the sister company of Soil Instruments were awarded a subcontract to install instrumentation on the Water Catchment Slope in Gibraltar. The instrumentation was required to monitor the slope during de-commissioning of the slope.

The British Ministry of Defence constructed the water catchment system in the early 1900’s to supply water for the island from rainwater. Rainfall ran down corrugated sheeting to troughs, which fed holding tanks constructed in the Rock.

Modern desalination plants now supply the water, so the Gibraltar Government commissioned Ritches (the Geotechnical division of Nuttalls) to decommission the slopes. This operation involved the drilling and installation of several thousand soil nails, removal of the corrugated sheeting, covering the exposed sand with biodegradable, geo-textile membrane and seeding. Directly underneath the sheeting were 2 to 3 metres of dry friable sand, overlying cemented sands. To monitor the slope, ITM were contracted to install Inclinometer casings with In-place Inclinometers and Rod Extensometers installed to a depth of 8 metres at 8 locations. All the instruments were wired back to a central datalogger, which linked via a short-haul modem to a PC in the site offices, enabling the engineers to constantly monitor the slope.

Due to the difficult nature of the site, rope access trained engineers carried out all the installations using rope access equipment.

Points of interest:

• A Solar Panel was used for powering the datalogger
• To get from the site offices to the upper catchment meant climbing 1200 steps
• Rock-falls were a frequent hazard; you could often hear a shout of “ROCK” and watch a boulder bouncing down the slopes.
• Barbary Apes occasionally wandered down the slopes to have a nose around.
• Seagulls were a constant problem; everyone on the site got hit occasionally by their droppings.