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Gibraltar: Rescuing the rock

A vital road in Gibraltar has remained closed since an accident over seven years ago. Despite unforgiving conditions a scheme is underway that will see the road soon reopen. Alexandra Wynne reports

As a nation almost entirely made up of the huge limestone monolith that gives it its name, it is perhaps unsurprising that Gibraltar is associated with the risk of rockfall. Equally unsurprising is that considerable work has already taken place to secure sections of the Rock’s face and protect dwellers and road users below.

But in February 2002 tragedy struck when falling rock crashed down onto a vehicle killing the driver. It happened just outside an area protected by a shelter leading to the Dudley Ward tunnel on the south east of the Rock. The road was closed immediately while investigations took place.

The danger on the rock was already known: before the accident, consultant Golder Associates had been working elsewhere for the Government of Gibraltar, advising on rockfall protection, and quickly became involved in investigating what triggered the event.

Its work, including the use of airborne laser surveys from Lidar to predict the sources and trajectories of any rockfall, concluded that there was an unacceptable risk to human life.

Increased pressure

However, the road is a critical pathway around Gibraltar and diversions have increased pressure on roads through its busy central areas so the need to reopen the road has become more pressing.

As a result, Golder developed a design for its client – one that combines an extended rockfall canopy, rockfall catch fences with a rock catch ditch into an embankment to provide protection for the 500m stretch of road leading to the tunnel.

Now, a bevy of site workers are braving the elements across a stretch of the face to install a heavy duty rockfall barrier, also known as catch fencing. The first purpose of this fence is as a temporary protection while work to build a rock canopy and improvements to the highway are carried out. But, because it will remain functional beyond that, it will provide long-term protection for the highway.

Contractor Can Geotechnical is carrying out this phase of the project, worth about £1M. It has responsibility for putting in the high resistance barrier supplied by Maccaferri – among some of its highest capacity fencing, with the ability to withstand up to 3,000kJ kinetic energy.

Advanced design

Design of catch fences has come a long way from early versions, which were rigid mesh systems and were much more susceptible to damage.

The system being used here absorbs higher kinetic energy from debris by dissipating energy so the impact on the fence is lower. This happens because the retaining layer – a continuous fence made from steel ring panels – has the flexibility to move when hit, while still containing debris and stopping anything of substantial size hitting the road, or even worse, vehicles, below.

Three fences are being installed in the critical areas defined by Golder – the first 200m (split into three sections), second 70m and third 90m (including a small overlapping section) in length.

“More and more these days, it’s usually the case that work is carried out by mechanical means. But here that’s just not the case.”

Jerry Clelford, Can Geotechnical

The ring panels are placed downslope of the barrier and kept in place by 5m tall posts at 10m spacings. These posts are a part of the catch fence system but also act independently of it – if one post is hit and damaged adjacent posts take the additional forces.

However, there is another crucial element in the system – the foundations beneath each post.

A vertical post anchor and angled raking anchor are being installed into the rock, which lock into the 600mm by 600mm by 600mm concrete headblock that supports each of the posts.

In addition, in between each post and at either end of each fence, back stay anchors are installed upslope of the barrier and connected by cables to the fence at the top of each post.

All of this has to be carried out at height on a steep rock face – factors that make for an interesting environment in which to work. Even getting materials in place is labour intensive.

“More and more these days, it’s usually the case that work is carried out by mechanical means,” says Can Geotechnical site manager Jerry Clelford. “But here that’s just not the case.”

It is a typical scene with rock face work that machinery seems limited to modest looking A-frame drilling rigs capable of working in compromising conditions, and outnumbered by rope access workers.

However, this was one occasion when the contractor opted to bring out the big guns in the form of a helicopter and talented pilot able to transport and lower into place the majority of the posts, as well as bringing up anchors and rolls of ring panels.

“Most of our costs are labour costs,” says Clelford. “But the costs of the helicopter were less compared with the cost of man days spent dragging everything up the slope.”

To illustrate the point he adds that one helicopter took 40 minutes to get four posts in position whereas five men needed five days to install five posts – understandable when air temperatures often hit 34ºC.

Natural challenges

However, there are some natural challenges on the job that Clelford beat. Although working through a Gibraltan summer – Clelford and the team started work on 1 June – the difficulties of heat have been supplemented by high risk working conditions caused by frequent high winds as well as low cloud, fog and rain.

When the winds picks up, which it did rapidly in a few minutes during NCE’s visit, the risk of loose material coming free from the face, and its shrubbery and caves, means immediate work stoppages and a quick clamber away from danger.

Flexible head units

Site workers installed flexible head units between the head of the lateral and back stay anchors and the cables from the barrier posts − these allow for an element of flexibility.

The 25kg galvanised steel heads are screwed to the top of the anchor protruding from the ground. The head then has a length of chain with a shackle on the end, to which the cables from the barrier posts are attached. The heads are coated with an epoxy based paint for durability.

 

Although work can begin again as soon as the winds die down or when fog clears, heavy rainfall means a 24 hour wait because the water potentially leads to even more unstable ground.

These were all predicted in the planned schedule, but unforeseen ground conditions have caused further issues and created about a two week delay.

“The costs of the helicopter were less compared with the cost of man days spent dragging everything up the slope.”

Jerry Clelford, Can Geotechnical

Relatively little was known about the ground until Can Geotechnical began de-vegetating the site. Some ground investigations had been carried out but extensive investigations in this area would simply have been too costly.

When drilling for the anchors began, using the contractor’s own A-frame drilling rigs, things became tricky. Initial plans involved drilling through the 2m to 3m of overburden using casing to support the bore until the auger reached sounder ground.

However, rig operators discovered that the combination of sand, scree, breccia and karstic limestone collapsed down the length of the bore – meaning a significant number of bores had to be abandoned.

Taking stock of the situation, the team opted for a change of approach and came up with a plan to use self-drilling anchors, with air flush. The anchors range in capacity from 200kN to 300kN and a 76mm diameter sacrificial bit on the end of each hollow steel bar – supplied by Dywidag – enables the up to 10m long anchors to reach depth. Grout is pumped under pressure through the anchor stem, coming out through the base and filling the annulus between the anchor and surrounding ground. This creates a bond but also serves to improve the unstable ground.

Timely progress

Even with the added programme time, Can Geotechnical is still expecting to finish installing the catch fence by the beginning of next month. Meanwhile, August saw the start of the second phase of work to prepare the site for the rock canopy and highway improvements.

This is due to finish around November/December. Local firm Sharrock Shand is main contractor for this next £2M worth of work.

As well as the catch fence, the first 100m stretch leading from the tunnel will be protected by a new cast insitu concrete canopy – replacing an old, smaller, damaged one. The new canopy is “much more substantial”, according to Golder Associates project director and associate Stewart Lightbody. This is important as the rock face nearer the tunnel portal becomes increasingly steep, causing any falling debris to reach a higher velocity.

Barrier testing

Gibraltar

These Maccaferri rockfall barriers are 3,000kJ Maximum Energy Absorption capacity.

These are tested and certified by the University of Bologna and are in accordance with the guidelines of European Technical Approval Guideline (ETAG) 027. ETAG imposes a series of tests − a Maximum Energy Level (MEL) test and two consecutive Serviceability Energy Level (SEL) tests.

In the MEL test, the test mass is dropped into the assembled barrier and deflections/forces etcetera are measured. In this case 3,000kJ maximum load, although Maccaferri also produces 5,000KJ barriers. The barrier is then repaired and reset ready for the SEL impact test.

The SEL tests are consecutive. A 33% (1,000kJ) impact is carried out on the barrier and deflections/forces recorded. Straight afterwards, a second impact is carried out. No repairs are allowed between these two SEL impacts.

This is the crux of the difference between the new ETAG testing and the older ‘Swiss method’ of testing barriers. The Swiss method allows repairs between impacts.

 

This means the canopy will have its own built in rockfall protection – a triple layer system above the canopy involving a 750mm-thick layer of polystyrene blocks, 300mm of reinforced concrete and between 1m and 2m of granular fill. “It’s a much more efficient energy dissipater than just granular fill,” says Lightbody.

Realignment work is also planned to remove some of the tightest bends in the old road.

But that does result in the road getting closer to the rock face, despite the slopes being more gradual. Therefore, beyond the canopy, embankments will be re-profiled, retaining walls installed and rock catch ditches beside the road where required.

The third phase of the project, including building the canopy, retaining walls, rock catch ditch and highway improvement is due to start in November. If all goes to plan, the entire scheme should be finished by the end of next summer and the road is expected to reopen – allowing locals, businesses and tourists to benefit form a more connected network bypassing central Gibraltar once again.

Who’s who

Client Government of Gibraltar

Main contractor (phase one) Can Geotechnical

Main contractor (phase two) Sharrock Shand

Consultant Golder Associates

Structural consultant Clark Smith Partnership

Main supplier Maccaferri (catch fence), Dywidag (steel)

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