News

December 2006

Safety Catch

Andrea Roth and David Gibson look at new rockfall barrier installation guidelines and a world record test.

It won't come as a surprise to many that rockfall barriers have been designed and installed alongside roads and railways and above buildings for many decades in the UK. What is surprising, however, is that there has been no definitive set of guidelines for their installation. Rockfall barriers are, after all, critical safety devices.

Research body CIRIA recently published a guidance document titled Soil Nailing Best Practice Guidance, which has been welcomed by the industry and is now used by clients, consultants and contractors for this increasingly used slope stabilisation solution. But more encompassing guidelines are in the pipeline.

The European Organisation for Technical Approvals (EOTA) has produced a draft document: Guideline for European Technical Approval of Falling Rock Protection Kits. It is currently with the member countries for approval and will become the definitive set of guidelines for barrier kits in the European Union.

UK clients, designers and installers will need to specify and install barriers designed and tested in accordance with these guidelines or possibly face a rockfall with inadequate protection. As a result, untested barrier installation will no longer be acceptable.

The Swiss have had a proven set of guidelines since 2001 and several specialist companies are manufacturing flexible barriers and testing them in accordance with these, with certified barriers routinely installed throughout Europe.

The Swiss guidelines are more onerous than the draft European guidelines and barriers installed using them will exceed the forthcoming European requirements.

The secret of the Swiss guidelines lies in the rigorous testing procedures: the vertical freefall test procedure, introduced by the Swiss Federal Office for the Environment (FOEN) in 2001, is the world's most stringent.

The Swiss Federal Research Institute WSL operates the test site in Walenstadt, and WSL instrumentation includes 15 heavy duty load cells with a maximum load of 500kN and a recording frequency of 2000Hz. It also has high speed cameras with a recording rate of 250 frames per second.

It was here that Swiss manufacturer Geobrugg (the natural hazards protection subsidiary of Fatzer AG) recently set a world record rockfall barrier test for a 5000kJ impact. To produce this energy a reinforced concrete cube with an edge length of 1.9m and weight of 16t was released in free fall from 32m above the barrier, striking it at 90km/h.

The main part of this tested rockfall barrier is the ring nets, made by Geobrugg from high tensile steel wire. These are notable for their outstanding energy absorption properties. The ring nets are supported by steel wire ropes, which are guided through brake rings pipes bent into loops. In larger rockfalls these dissipate the residual energy from the ring net. Steel posts support the barrier, fastened to ground plates with hinge pins and held in position by retaining ropes.

The tested barrier was 6m high with a10m post spacing. The main test, with 100% of the targeted impact energy of 5000 kJ into the middle section of the test fence, was the last in a series of different tests spread over two weeks.

The entire braking process of the falling 16t test body hitting the barrier at 90 km/h took hardly more than half a second and the measured braking distance of approximately 9m was well below the maximum permissible value of 15m. This deflection is important to determine the proximity of the barrier relative to the object to be protected. The residual useful barrier height in the impact section, required to obtain certification, requires it to be above 50%. A first visual inspection revealed the RXI 500 which will now replace the Geobrugg RX-300 rockfall barrier achieved this and that the useful height in the neighbouring fields after the impact was still almost 100%.

Measurements of maximum rope forces were about 240kN; which is very low considering the high energy classification of the RX-¬500. This is explained by the impact force being dissipated over large areas of the barrier as well as recent developments in the braking process elements.

Higher load capacity anchors are unnecessary for the RXI 500 barrier, owing to the energy dissipation design, which means anchor installations remain the same as for the lighter RX 300 barrier, so there is no increase in installation cost.

The main applications for the RXI-¬500 will be in cases of large rock sizes, high falling heights or high bouncing heights. This alternative to protective concrete structures will now be available since the energy absorption capacity of the RXI-500 surpasses the capacity of most concrete galleries currently in use as well as those being planned.

While the UK does not have an alpine topography, there are requirements for the protection of roads, railway lines, buildings and quarries below unstable faces and cliffs. Consequently, Geobrugg has provided four barrier kits to the UK in the last 12 months of which Ritchies, Geobrugg's certified product installer in the UK and Republic of Ireland, has installed three.

In addition, there are several other potential sites requiring barriers including one in north Wales that may require a 5000kJ barrier.

Other innovative solutions mean debris flows like those at Boscastle in Cornwall, Dooncarton in Ireland and Lochgilphead in Scotland in recent years can now be arrested by debris flow barriers. These are similar to rockfall barriers but cope with higher overall loadings at lower impact speeds.

Geobrugg has recently launched its Spider Rock Protection System. This is suited to rock slopes and overhangs where there is an irregular surface or clearly defined failure mechanisms.

The Spider comprises a spiral rope net which is attached to anchors or bolts at the top of the area to be treated and then unrolled downwards and secured around the perimeter. Its high strength enables it to provide active support to the face or feature such as an overhang or isolated loose block.

Andrea Roth is head of Geobrugg's technical department. David Gibson is business development manager at Ritchies.

Article courtesy of Ground Engineering (December 2006).

Further Info

For further information, please contact:

David Gibson
Business Development Manager
Ritchies, Nailsea Wall, Kenn Pier
Clevedon, Somerset. BS21 6UE
Tel: 01275 875338
Fax:01275 870076
E-Mail: david.gibson@edmund-nuttall.co.uk

Peter Bishop
Public Relations Manager
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk