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Nat. Hazards Earth Syst. Sci., 9, 1291–1298, 2009www.nat-hazards-earth-syst-sci.net/9/1291/2009/© Author(s) 2009. This work is distributed underthe Creative Commons Attribution 3.0 License.

Natural Hazardsand Earth

System Sciences

Technical Note: Design of rockfall net fences and the new ETAG 027European guideline

D. Peila and C. Ronco

Department of Land, Environment and Geo-technology, Politecnico di Torino, Turin, Italy

Received: 30 April 2009 – Revised: 24 July 2009 – Accepted: 27 July 2009 – Published: 29 July 2009

Abstract. The need for protection against rockfall has ledto the development of different types of technological solu-tions that are able to both prevent blocks from detaching fromrock walls and to control, intercept or deviate the blocks dur-ing movement. Of the many devices that are able to inter-cept and stop a block, one of the most frequently used is netfence. Many different types of full-scale tests have been car-ried out, with different test site geometries and proceduresto study their behaviour and to certify these devices. Thishas led to a series of data and information that are not easyto compare. The recent endorsement, by the European Or-ganization for Technical Approvals (EOTA), of a EuropeanTechnical Approval Guideline (ETAG), which defines howto test and assess the performance of a net fence, is there-fore a great innovation that will change both the market andthe design procedures of these devices. The most importantinnovations introduced by this new guideline are here pre-sented and discussed and a net fence design procedure forprotection against rockfall is provided.

1 Introduction

Many mountain side roads and railways are often at risk torockfall and inhabited areas are often involved (Bunce et al.,1997; Guzzetti et al., 2003; Jaboyedoff et al., 2005; Lo-catelli, 2005; Peila and Guardini, 2008). Rockfall can be de-scribed as the quick bouncing, rolling and sliding movementof one (or several) boulders down a slope which can reachsignificant kinetic energy as it (they) travels. The speed val-

Correspondence to:D. Peila([email protected])

ues range from a few metres per second to up to 25÷30 m/s(Broili, 1973; Giani, 1992).

The need for protection against this phenomenon has ledto the development and use of different types of technolog-ical solutions. These solutions can either prevent the blocksfrom breaking off the rock walls, thus reducing the frequencyof the collapses, or control, intercept or deviate the blocksduring their movement. These latter devices include ditches,rockfall shelters, ground embankments and net fences madeof metallic meshes (Peckover and Kerr, 1977; Peila et al.,2007).

Analytical procedures for the mathematical descriptionof rockfall phenomena have been established by several re-searchers and these procedures have led to the developmentof various types of statistical forecasting procedures (De-scoudres and Zimmermann, 1987; Giani, 1992; Azzoni etal., 1995; Agliardi and Crosta, 2003; Yang et al., 2004).When the best position with reference to the interceptionpercentage of the trajectories, the corresponding maximumbouncing height and the kinetic energy have been estab-lished, a suitable device can be chosen. In the past, manytests were carried out by manufacturers and universities todefine the maximum energy that can be safely absorbed bynet fences (Smith and Duffy, 1990; Duffy and Wade, 1996;Gerber, 1999; Grassl et al., 2003; Peila et al., 2006), butthese tests were developed using different standards and pro-cedures and the results were not easily comparable. For thisreason, the European Organization for Technical Approvals(EOTA) (http://www.eota.eu) has endorsed a new EuropeanTechnical Approval Guideline “Falling rock protection kits”(ETAG 027), where a testing procedure for CE marking of anet fence (which has been called falling rock protection kitin the guideline) has been defined.

EOTA was founded after the drawing up of ConstructionProducts Directive 89/106/EEC (CPD) and it has the goal

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1292 D. Peila and C. Ronco: Design of rockfall net fences and the new ETAG 027 European guideline


Fig. 1. Example of the inclined test site in Meano (TN), Italy (Peila et al., 2006).

of removing technical obstacles, in the manufacturing sector,for the European falling rock protection kit market through:

– the compliance of trade marks with six essential re-quirements: (1) mechanical resistance and stability; (2)safety in the case of fire; (3) hygiene, health and envi-ronment; (4) safety in use; (5) protection against noise;(6) energy economy and heat retention, which globallydefine the suitability of use of a product;

– the transformation of these six requirements to productrequirements;

– the determination of the performances of products usedin net fence construction by means of technical specifi-cations;

– the attestation of conformity of the products;

– CE marking of the products.

In order to establish whether a product is “fit for its intendeduse”, it must comply with a European harmonized standardor European Technical Approval (ETA), if no correspond-ing standard exists. According to the Construction Products

Directive, an ETA document sets the basis for the certifica-tion procedure, which must be carried out by an ApprovedBody that has been recognized by the European Commission(Nando – New Approach Notified and Designated Organi-zation – Information System,http://ec.europa.eu/enterprise/newapproach/nando). Since no relevant European harmo-nized standard existed for rockfall net fences, the EuropeanCommission gave a mandate to EOTA members to develop aguideline (ETAG) in order to:

– identify the relevant and regulatory characteristics of thenet fences;

– establish a method for the verification and assessmentof these characteristics;

– identify the threshold values that have to be respectedfor technical reasons;

– define the relevant identification tests for the kit compo-nents.

The guideline was therefore drawn up by a working group,which consisted of members nominated by EOTA, who took

Nat. Hazards Earth Syst. Sci., 9, 1–9, 2009 www.nat-hazards-earth-syst-sci.net/9/1/2009/

Fig. 1. Example of the inclined test site in Meano (TN), Italy (Peilaet al., 2006).

of removing technical obstacles, in the manufacturing sector,for the European falling rock protection kit market through:

– the compliance of trade marks with six essential re-quirements: (1) mechanical resistance and stability; (2)safety in the case of fire; (3) hygiene, health and envi-ronment; (4) safety in use; (5) protection against noise;(6) energy economy and heat retention, which globallydefine the suitability of use of a product;

– the transformation of these six requirements to productrequirements;

– the determination of the performances of products usedin net fence construction by means of technical specifi-cations;

– the attestation of conformity of the products;

– CE marking of the products.

In order to establish whether a product is “fit for its intendeduse”, it must comply with a European harmonized standardor European Technical Approval (ETA), if no correspond-ing standard exists. According to the Construction ProductsDirective, an ETA document sets the basis for the certifica-tion procedure, which must be carried out by an ApprovedBody that has been recognized by the European Commission(Nando – New Approach Notified and Designated Organi-zation – Information System,http://ec.europa.eu/enterprise/

newapproach/nando). Since no relevant European harmo-nized standard existed for rockfall net fences, the EuropeanCommission gave a mandate to EOTA members to develop aguideline (ETAG) in order to:

– identify the relevant and regulatory characteristics of thenet fences;

– establish a method for the verification and assessmentof these characteristics;

– identify the threshold values that have to be respectedfor technical reasons;

– define the relevant identification tests for the kit compo-nents.

The guideline was therefore drawn up by a working group,which consisted of members nominated by EOTA, who tookinto account that, when the work was started, no guide-lines or standards were available in the EU concerning netfences and only a Swiss guideline had been issued (Gerber,2001; Guideline for the approval of rockfall protection kits –Amendment 2006). The working group also considered thatin Italy, France and Austria tenders are usually based on themaximum energy that a net fence can sustain, which is mea-sured using full-scale tests carried out on a prototype, at bothinclined and vertical test sites (Figs. 1 and 2). Furthermore,some dynamic numerical calculations had been developed tostudy the global behaviour of net fences (Nicot et al., 2001;Cazzani et al., 2002; Grassl et al., 2002; Volkwein, 2005;Valfre, 2006).

2 Product characterization using full-scale tests

Net fences consist of a sequence of functional modules madeup of an interception structure, a support structure and con-nection components. They are linked to the foundationswhich in turn are anchored in the ground. The foundationsare not subject to ETA document and are therefore on thediscretion of the designer (Table 1). As suggested by theguideline, the certified kit has a minimum of three functionalmodules (Fig. 3). The energy that can safely be absorbed bya kit during block impact is one of the key points. ETAG 027defines two energy levels as reference values: SEL “ServiceEnergy Level” and MEL “Maximum Energy Level”. SELis defined as 1/3 of MEL and the kit should be able to re-tain such a SEL event twice. These energy levels are usedto classify the kits (Tables 2 and 3). The tests involve thelaunching of a plain or reinforced polyhedral concrete blockagainst the central module of the tested kit (Fig. 4). The max-imum size of the block must be at least three times smallerthan the nominal height of the kit and, in the last metre ofthe trajectory before the impact, the block must move withan average speed that is greater or equal to 25 m/s (Fig. 5).


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D. Peila and C. Ronco: Design of rockfall net fences and the new ETAG 027 European guideline 1293

Table 1. Net fences main components.

Main parts Components Function

Interceptionstructure

Principal net: made up of metallic cables, wires and/orbars of different types and materials (for example cable netsjoined by clamps, submarine nets and ring nets. In the lasttwo cases the rings forming the net are connected to eachother). Additional layers: usually with a finer meshworkthan the principal net made up of cables and/or wires orother materials.

To bear the direct impact of the mass, deform elasticallyand/or plastically and transmit the stresses to the connectioncomponents, the support structure and the foundations.

Supportstructure

Posts made of different materials, geometries and lengths(for example, pipes, structural metallic elements) having ahinge at the bottom.

To keep the erected interception structure. It can be directlyconnected to the interception structure or through the con-nection components.

Connectioncomponents

Connecting ropes, steel cables, wires and/or bars of differenttypes and materials, junctions, clamps, energy dissipatingdevices (elements which are able to dissipate energy and/orallow a controlled displacement when stressed).

To transmit the stresses to the foundation structure duringimpact and/or keep the interception structure in position.

Foundations Cables or bars anchored in the ground with adequate grout(not part of the ETAG).

To transmit the forces derived from the block impact to theground.

Table 2. Falling rock protection kit classes (as used in ETAG 027).

Energy level classification 0 1 2 3 4 5 6 7 8

SEL [kJ] – 85 170 330 500 660 1000 1500>1500MEL [kJ]≥ 100 250 500 1000 1500 2000 3000 4500>4500

SEL is defined as the kinetic energy of a block that impactsthe kit twice and which allows the following constraints to befulfilled:

– the kit should stop the block during two impacts withthe same kinetic energy without maintenance after thefirst impact. The first block must be removed before thesecond impact;

– the block should not touch the ground until the kitreaches maximum elongation during both the first andthe second impacts; this is necessary to avoid uncon-trolled energy dissipation due to ground-block contactand to reduce the energy level absorbed by the barrier;

– after the first impact, there should be no ruptures in theconnection components and the mesh openings shouldbe smaller than twice the initial size of the mesh itself;the residual height of the kit (that is, the minimum dis-tance between the lower rope and the upper one mea-sured orthogonally to the reference slope without re-moving the impacted block) should be higher or equalto 70% of the kit height (Fig. 6).

The residual height value was chosen as it was considered areasonable height for an impacted net fence that would per-mit an already impacted modulus to intercept a new falling

Table 3. Falling rock protection kit categories derived by evaluationof MEL residual height (as used in ETAG 027).

Category Residual height

A ≥50% nominal heightB 30<nominal height<50%C ≤30% nominal height

block. This value is also high enough to prevent the modulesnext to the impacted one from being slightly perturbed as aconsequence of the impact.

MEL is defined as the kinetic energy of an impacting blockthat fulfills the following constraints:

– MEL>three times SEL;

– the barrier stops the block during the impact;

– the block does not touch the ground until the kit reachesthe maximum elongation (Fig. 7).

A SEL value, assumed equal to 1/3 of the MEL value, wasadopted to give information about the threshold energy levelthat does not require any practical repairs after the impact.

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Fig. 2. Example of the vertical test sites: Forzaso (BL), Italy (left),and Walenstadt, Switzerland (right).

The maximum deformation of the structure and the forceson the foundations are measured during the MEL test andreported in the ETA document.

The development of the ETA document not only requiresthe previously described full-scale tests, but also a complete

Fig. 3. Front view and picture of the falling rock protection kit withthe definition of the most important geometrical properties.

evaluation of the factory production control and an analy-sis of the accompanying documents (i.e. installation manual,maintenance and handling procedures).

The Approved Bodies have to check that the factory pro-duction control, which has been activated by the manufac-turer and the ETA-holder, maintains continuous surveillanceand that a traceable documentation of all the production pro-cesses is available.

The standard conditions for the installation of falling rockprotection kits in real sites should be clearly described in themanufacturer’s installation guide, where the producer, on hisown discretion, should provide the technical specifications ofall the components and the admissible geometric tolerances,particularly concerning the spacing of the posts and the dipof the main ropes. It is very important to highlight that theheight of the barrier cannot be reduced, with reference to thetested kit, and cannot be raised by more than 1 m for a testedheight of no less than 4 m and 0.5 m for a tested height oflower than 4 m.

3 Design procedure for rockfall restraining nets

The design of protection devices in an area prone to rockfallis a complex task that requires the designer to take into ac-count many different kinds of data from the site (i.e. geolog-ical, geotechnical and topographical) and from the trajectoryforecast, as summarized in Fig. 8. When the best positionof the protection devices and the impact energy have been



Fig. 4. Shape of the block (as foreseen in ETAG 027).

Fig. 5. Photogram of the video of a full-scale test in a vertical testsite for a 500 kJ net fence when the maximum elongation is reached(courtesy SAFE s.r.l.).

evaluated, on the basis of the trajectory forecast, it is neces-sary to design and choose the correct net fence on the basisof the design laws in force (for example Eurocode 7 – EN1997–1:2004).

The first design step is to choose whether to design usinga MEL or SEL approach:

– in the case of a forecast of low frequency rockfall eventsand with different fall directions, in other words, notinvolving the same modulus, it is possible to adopt theMEL approach;

– if the barrier has to be installed in positions in which itis difficult to carry out maintenance work and it is there-fore preferable not to repair it after each block impact,the SEL design approach should be chosen (consideringthat the design safety factor is three);

Fig. 6. Scheme of the test conditions and of the measurement of thebarrier height as defined in ETAG 027.

Fig. 7. Cross section of the kit with the definition of the maximumelongation as defined in ETAG 027.

– where the same modulus could be impacted severaltimes, that is, in the same direction, the designer’schoice could be: installation of two net fences with thealignment defined at a MEL level or one net fence withthe alignment designed at a SEL level.

The designer should then verify that:

– the energy that can be dissipated by the net fences isgreater than the computed energy of the block:

Edesign−EETA,netfences

γE

≤ 0 (1)

where: γE is a safety factor, which is suggested as 1.30, ifthe barrier is designed taking into account the MEL energylevel (as used in Eurocode 7 in Design Approach 2), and 1.00



Fig. 8. Flow chart for the design of a rockfall protection device in an area prone to rockfall (modified from Peila et al., 2006).

if it is designed taking into account the SEL energy level (asused in Eurocode 7);EETA,netfencesis the energy certified byETA (MEL or SEL);

– the interception height (hi) of the net fence is greaterthan the design interception height (hp), which is deter-mined from the trajectory forecast, taking into accountthe computed block passage height relative to the slope(95% percentile) in the design position plus a clearance(f ) that is not less than half the average size of theblock:

hi ≥ hp + f (2)

– the maximum barrier elongation towards the valley (da),multiplied by a safety factor (γE), must be smaller thanthe design distance between the net fence alignment andthe area that has to be protected (dp):

daγE ≤ dp (3)

This means that the barrier should always be installed at asufficiently safe distance from the protected area.

The computation of the kinetic energy of the blockduring the impact is usually carried out using the computedblock speed and the design block mass, applying the usual



classical physics formulations and the concept of the partialsafety factor, as indicated in Eurocode 7 (point 2.4.7.3.3).

The design speed of the block (vp) is obtained taking thecalculated 95% percentile (vt ) of the computed speeds of thefalling trajectories as the characteristic value, multiplied by asafety factor (γF ):

vp = vtγF (4)

where γF =γT rγDp (EN 1997–1:2004 Annex B; EN1990:2002), with:

γT r , the reliability coefficient of the trajectory compu-tation, which takes into account the possibility of devia-tion of the action values from their characteristic values,for which the following values are suggested:

=1.00 in the case of statistical computations derivedfrom a back analysis;

=1.10 in the case of computations based on restitutioncoefficients obtained from bibliographic information;

γDp, the partial factor linked to the slope discretizationquality that takes into account the uncertainties involvedin modelling the actions, for which the following valuesare suggested:

=1.05 for a carefully discretized slope characterized bya precise topographic survey;

=1.10 for a slope modelled with a simplified 2-D cross-section derived from a large scale map.

These partial factors should be considered as suggestions andmust be defined and evaluated from case to case for eachproblem.

The design block mass (mp) is equal to the product of thedesign block volume (Volb) and the rock unit weight (γ ) mul-tiplied by a specific safety factor (γ m):

mp = (Volbγ ) γm (5)

where γm=γVolF1γy (EN 1997-1: 2004 Annex B; EN1990:2002), with:γy , the coefficient derived from the rockunit weight evaluation, which can be assumed equal to 1.00;

γVolF1, the coefficient derived from the volumetric sur-vey accuracy of the “design block” which should be chosenon the basis of the “engineering judgement” of the designer,for which the following values are suggested:

=1.05 for a precise survey of the rock slope, for exam-ple obtained from a detailed reconstruction of the rockslope using laser scanning, photogrammetric methodsor several direct geological surveys of the slope;

=1.10 for a survey of the rock slope carried out withonly a limited number of site investigations.

4 Conclusions

The new ETAG 027 is playing an important role in the designof falling rock protection kits as it is a credited guideline totest these protection devices. For the first time, a testing stan-dard (that is utilized in all UE Countries) makes it possibleto compare products on the basis of their absorbable energylevel.

Other significant information for designers, such as themaximum elongation of the net fences and the forces appliedto the foundations, can be obtained during full scale tests andthis will lead to an improvement in the design quality of aprotection device, as shown by the proposed design proce-dure.

These data can be combined for a robust design which in-cludes the systematic use of partial safety factors, such asprescribed by the geotechnical Eurocode design approach,which represents the official standard in force in Europe forgeotechnical work design.

Acknowledgements.This study was financially supported by theNational Research Project PRIN 2007, Application of advancedtechnologies to the safety, environmental protection, optimizationof management and yield in quarries: geomining characterizationand monitoring methodologies (National coordinator: Prof. PaoloBerry; Local coordinator of the DITAG Research unit, Politecnicodi Torino: Daniele Peila).

Edited by: K. SchellenbergReviewed by: A. Roth and another anonymous referee

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