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Sweat on the Rock

Stabilising a slipped section of Gibraltar's cliffs is difficult and potentially dangerous work. While locals retreat for siestas, the contractors work on through the heat.

Mention Gibraltar and most people immediately conjure up a picture of the eponymous rock at Spain's southern tip.

So representative is the 452m high block of limestone of the tiny British Crown Colony, that when the British press reported two years ago that the rock was crumbling into the sea, local reaction was extreme.

In fact the story was almost completely wrong, but like most stories that provoke widespread outrage and reaction, it contained a grain of truth. Heavy rainfall in January 1997 had caused local cliff instability at Camp Bay on the south west of the 3km by 2km peninsula. However, the collapse did not affect the famous rock, and in hindsight was not that exceptional.

To describe this as evidence that the rock would soon be lost to the sea is like a doctor saying a patient with a broken toenail needs a leg amputation.

The Gibraltar government was already looking at the stability of three sections of cliff at the time of the collapse, explains Stewart Lightbody, engineer's representative with geotechnical consultant Golder Associates.

After the slip, the government commissioned Golder to immediately focus on the Camp Bay area, with a brief to design remedial works to the collapsed section and advise on the stability of adjacent cliffs.

The landslide affected a 45m section of the cliff and contained about 15,000m3 of debris. The slipped area was heavily faulted and so the collapsed material was predominantly fault-brecciated limestone, with strong competent limestone either side forming solid rock 'abutments' to the collapse. An old military building at the top of cliff was left teetering precariously and had to be demolished, while collapsed debris blocked the coast road and access to one of the few beaches.

Work to stabilise the landslide is being carried out by main contractor Costain Civil Engineering, which faces a number of logistical challenges in processing the landslip material.

Golders did not consider stabilising the landslipped mass insitu a feasible option as it would have meant realigning the coast road around the western abutment, significantly reducing the beach area. The general approach has been to remove the landslide debris, form a new pre-split slope behind the backscar and use the landslipped material to construct a rock bund up against the new slope.

Controlled blasting was used to form the pre-split face and to process blocks of collapsed limestone up to 500t. Trial blasting was carried out to ensure nearby buildings were not damaged.

Quality of the landslip debris is variable, and Costain is having to do much work to make it suitable as a grade 400 rockfill. The toe of the bund is defined by the original road alignment and working at a slope of 1:1.2 will provide support to the lower part of the new 30m high face. Above the bund there will be some regrading together with further stabilisation and building of surface water drainage.

Anchoring in this area is extremely demanding because of access difficulty and work is being carried out by UK roped access specialist CAN, subcontractor to Costain.

CAN is also carrying out rock stabilisation work further along the cliff as part of the same contract, to increase stability and reduce the risk of further landslides.

The problem here is an irregular rock face, up to 70m high with significant overhangs. Steeply inclined open joints striking almost parallel to the cliff face mean there is potential instability. CAN has nearly finished installing 162 anchors up to 32m long in this section.

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