07/12/2009
ROCKFALL MITIGATION. THE GIBRALTAR EXPERIENCE
In the gently rolling acres of Britain’s green and pleasant land, the issue of rockfall protection of roads railways and other vital facilities hardly registers on the public radar. But where flat turns to steep and steep turns to even steeper, the prevention of rockfall damage to property, vehicles and, even more importantly people, is becoming of greater and greater importance.
In Gibraltar, where the entire population lives on or close to the huge limestone rock that gives the nation its name, the issue of rockfall protection is taken very seriously. Here, a scheme to install a network of rockfall catchment fences has just been completed which will allow the re-opening of a critically important road at the south eastern end of the Rock, which was closed following a significant rockfall occurrence, seven years ago.
The tragic incident happened in 2002 when falling rock struck a vehicle travelling along the road leading to the Dudley Ward Tunnel. The accident led to the immediate closure of the road.
The potential dangers of rockfall in this and other areas of the Rock were already known and the Gibraltar Government had previously commissioned consultants, Golder Associates to advise them on rockfall protection methods, prior to the incident.
Their investigations as to the cause of the accident and wider survey work which included long range, laser scanning, led to the conclusion that the risk to human life posed by rockfall in the immediate area of the road was unacceptable.
Closure of the road, which formed part of an informal, southern ring road to the Rock, inevitably put huge pressure on other routes through the busy central area. The need to relieve the pressure on Gibraltar’s congested highway network put the re-opening of the road to the top of their “essential action list”.
Rockfall mitigation measures
Following their research, Golder Associates proposed a combination of rockfall mitigation measures to provide protection for the 500m long stretch of road leading up to the Dudley Ward Tunnel entrance. These included an extended rockfall canopy over the tunnel entrance, passive rockfall catch ditches cut into the lower slope, immediately above the road and a network of high resistance catch fences, positioned up-slope of the critical area.
The catch fence scheme, valued at some £1m, is being undertaken by contractor CAN Geotechnical Ltd. Three fences are being installed; firstly a 200m long section split into three lengths, plus a second 70m long and a third 90m long run. A degree of overlap has been included.
The element design and supply of the catch fences was put to Oxford based rockfall protection specialists Maccaferri, who proposed a network of the company’s 5m high, CTR 30-04-A Barriers, capable of withstanding 3000kJ impacts. This is one of their highest capacity barriers, the highest being capable of containing impacts of up to 5000kJ. – the equivalent of stopping a 16.5 tonne lorry travelling at 57mph.
Catch fence design is now a sophisticated, high tech process and the development of ever-more efficient systems, capable of absorbing huge amounts of kinetic energy caused by falling debris, [measured in kilojoules [kJ], has moved on enormously in recent years. Much of the development work is European lead and has resulted in the adoption of a new European testing methodology, ETAG 027
ETAG 027 – the European Technical Approval Guideline 027, sets out the minimum standards for the manufacture, performance and testing of rock-fall protection barrier systems used throughout the EU. ETAG 027 also forms the basis of the CE approval process so only rock-fall protection systems which pass ETAG 27 can gain CE approval. Compliance will become a legal requirement across Europe.
Catch-fence installation
The Maccaferri system, which is ETAG 027 compliant, comprises a network of continuous, high-strength steel wire, ring-panels suspended at 10m intervals from 5m high steel posts. The posts are an integral part of the system but also act as independent components so if one is struck and damaged, adjacent posts accommodate the additional loadings.
The fence panels are positioned down-slope with steel cables fixed to back-stay anchors, securely fixed in a reversed v-formation, up-slope of the fence run. The 5m high fence posts are fixed to articulating brackets which are attached to 600x600x600mm concrete head-blocks, set into the rock face. [See diagram]
During an impact, the system ensures that the energy of the falling rock is dissipated and the rock is prevented from moving any further down the slope. Impact forces are shared among spans so that the stresses on the individual system components are minimized.
Energy dissipaters fitted to the post heads, help absorb impact shock loads. These dissipaters work by absorbing the applied energy bydeformation and not by friction. They offer reliable, maintenance free performance as the units are formed in the factory, rather than with the old-fashioned friction brakes which require site assembly. This reduces the need for site based connections and the potential for errors in assembly.
Installation of the catch fence system high up on the steeply sloping site presented the CAN Geotechnical team with particular challenges. “We usually do most of our installation work by mechanical means” said Jerry Clefford, CAN site manager. “But access to the work area was so difficult, even getting materials in place would have been incredibly labour intensive”
CAN took the bold decision to bring in the services of a helicopter complete with a skilled pilot to transport and lower into place the majority of the posts together with the cables, anchors and rolls of catch-fence panels. “The biggest part of our costs is in labour” explained Clefford, “but the cost of using a helicopter was much lower than paying for labour to haul everything up the slope”
“We were able to fly in and fix four posts in 40 minutes using the helicopter, whereas it took five men, five days to install five posts manually” he added.
Work on the barrier installation started in June 2009 with a programme of de-vegetation of the immediate site. CAN then used Dywidag self-drilling anchors, drilled through the thick overburden to provide support for the catch fence posts and anchoring points. These anchors ranged in capacity from 200kN to in excess of 300kN and were up to 10m in length with a hollow stem. Each had integral, 76mm diameter sacrificial drill bits with grout pumped through the anchor stem to fill the void between the bit and the surrounding rock, fixing them permanently in position.
The catch fence components were supplied to site in prefabricated kits comprising hot-dip galvanised steel posts, post base plates, cables, energy dissipaters, mesh and high energy absorption panels for on-site assembly. The kits come with the majority of connections, cables and related components factory-fitted to minimise installation variations.
Special post base plates, designed to be bolted to their respective foundations, are attached to the posts via articulating hinge mechanisms which allow the barrier to be set at any predetermined angle to the rock face.
Special flexible head units, developed for this project by Maccaferri, were used to connect the back-stay anchors to the heads of the ground anchors. Full scale testing of the units was required to confirm sufficient capacity to transfer the loads from the fence to the anchors.
Installation of the rockfall catch fencing was completed in November with work on the second phase, including the cutting of catchment ditches and preparatory work for the construction of the protective canopy over the tunnel entrance following on. Completion of the project and the reopening of the road are expected during summer of 2010.
Conclusion
Although rockfall protection measures adopted in Gibraltar may at first appear of little relevance to the UK, the reality is that rockfall mitigation is an issue with enormous importance to the public and private sector bodies that manage road, rail and waterway infrastructure. As well as these are the industrial and commercial landlords with responsibility for literally thousands of buildings throughout the country, many of which may be sited in areas of potential risk.
In the highlands of Scotland a rock-fall catch fence system from Maccaferri, has been installed to protect the exposed visitor-centre at Knockan Crag, one of the most important geological sites in the British Isles.
Road widening projects and the introduction of steep embankment cuts also highlight the issue of slope instability. Holiday, leisure and educational facilities in mountainous areas can also benefit from the protective curtain provided by catch fence installations.
Barrier testing methodology
Industry testing methodologies for rock fall barriers fall into two camps: ETAG 027 from the European Union and the Swiss methodology, produced by the Swiss Agency for the Environment, Forests and Landscape [SAEFL], sometimes referred to as BAFU. Both guidelines offer methodologies for testing rockfall protection kits but the tests are different and the results should therefore not be directly compared.
Swiss Method [Testing is divided into three stages]
Test 1: Preliminary test with small ‘debris’ particles
Test 2: 50% energy level test.
[Replacement/full repair of the barrier may then take place. The time to repair and the cost to repair is recorded].
Test 3: 100% energy test.
Deflections are measured on the fence and residual height of the barrier is noted giving a performance Class of the barrier.
ETAG 027 Method [Testing is divided into two stages]
Test 1: Two no. consecutive Serviceability Energy Level [SEL] level tests at 33% design energy with no repairs or no replacement of parts allowed between the two tests.
Test 2: One no. Maximum Energy Level [MEL] test at 100% design energy.
Deflection and residual height are measured and assessed to ascertain the performance Class of the barrier.
In the development of their test, ETAG took the view that rock falls are commonly not isolated events and that a series of significant impacts could occur without time for maintenance to the barrier. The ETAG testing therefore makes no allowance for repairs between the consecutive tests and consequently, the ETAG testing methodology is extremely onerous.
Specification guidance to engineers
Clearly, with complex testing methods, great care has to be taken in the preparation of project documents. For example, is the barrier to be designed for a single 1000kJ event, or two consecutive 1000kJ events, without repair between the two events? In the double impact scenario, an Engineer would specify an ETAG compliant 3000kJ barrier [because SEL energy = 33% MEL/design energy]. However it would be difficult to prove a barrier tested in accordance with the Swiss methodology only, could accommodate two consecutive impacts as it is not tested in this manner.
Category A performance standard
The Maccaferri’s CTR system was designed and developed in conjunction with its Italian parent company Officine Maccaferri. Independent tests undertaken at the University of Bologna, Italy, proved that the Maccaferri barriers performed to “Category A” standard – the highest standard, as defined in this EU document.
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