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New concept and technology for high energy rock fall protection fences

Final Report Summary - ABSORBNET (New concept and technology for high energy rock fall protection fences)

Executive summary

The aim of the ABSORBNET project is to fabricate a rockfall protection net to be used as the basis of a kit that meets the requirements of European Technical Agreement Guideline (ETAG) 27, a document that defines the technical performances that rockfall protection kits should satisfy including the relevant characteristics for the net and the set-up for a verification and assessment method.

Compared with other rockfall protection standards, ETAG 27 imposes the requirement that kits must be able to support two service launches. The net product should absorb energies up to class 8 (Maximum Energy Level (MEL) > 4500 kJ) in a temperature range between - 40 and + 50 °C.

The function of the protection kit is to absorb the energy of the rock and this is currently done in three steps using the three major system components.
1. The net intercepts the rock. Its function is to transmit almost 100 % of the energy to the brakes.
2. The brakes (energy-dissipating devices) absorb energy by plastic deformation or friction so as to minimise the strain transmitted to the anchorages.
3. The anchorages (foundations) transmit the remaining energy to the ground and hold the system.

All the systems on the market today are based on this process, which has two major disadvantages.
1. The net correctly transmits the whole energy to the brakes but in a short time, so the strain to be supported by the brakes and the anchorages is high.
2. The design of the majority of nets has not been modified over the last 50 years. The design used today by the vast majority of competitors is the same used for anti-submarine (ASM) nets during the Second World War.

In ABSORBNET we seek to overcome these two disadvantages and meet the ETAG 27 requirements by creating a net with absorbing properties. The absorbing net will be an inorganic reinforced polymeric composite having the dual characteristics of an energy-absorbing material that can deform to dissipate part of the rock energy as well as a high-strength material that will transmit the non-absorbed energy to ensure the net's integrity and solidity. The net will be designed to maximise the absorbing capability. As a consequence, the strain transmitted to the brakes will be lower than current products and more spread out over the time. That will in turn increase the global performance of the system, minimise the number of anchorages needed and so reduce installation costs. To create this solution, it is necessary to develop new technology in composite materials and dynamical-mechanics testing.

The most remarkable results achieved can be listed as follows.

1. A finite-element (FE) simulation methodology for the analysis of the high-performance materials and innovative net designs has been developed. The methodology is applied to a net-design demonstrator in order to check robustness and find necessities and problems.

2. Fibres of the most appropriate materials have been obtained by two processing methods: e-spinning and gel-spinning, from new designed materials based on composites from ultra-high-molecular-weight-polyethylene (UHMWPE) and nanoclays, for re-enforcement and for the mechanical properties of the corresponding yarns evaluated.

2. All the nanofibres produced with the electro-spinning process, both with the rotating disk or the charged rod as collecting system, showed to be aligned within the yarn. Regarding the process for the continuous production of fibres, the electro-spinning process is the one that presents some limitations. However, several common electrospinnable polymers (PCL, PAN, PVB, PVP, PS) have been tested using different proposed continuous collecting systems. Although not valid to meet ABSORBNET requirements, very promising results were obtained by the electrospinning of synthesised polyamides (PA-1, PA-2, PA-2’, PA-3, PA-3’, PA-4), polyamic acids (PAA-1, PAA-2) and an aromatic polyimide (PI-2).

3. The most outstanding effect on the enhancement of the mechanical properties of composites prepared by the addition of nanofillers to the polymer matrix is observed for the yarns processed by gel-spinning from APE-27 and APE-35. Indeed, all of the following composite materials, APE-15, APE-34, APE-19U and APE-21, presented enhanced mechanical properties when compared to reference APE-12 (UHMWPE = 'Dyneema')

4. The mechanical properties of the synthesised fibres and yarns have been evaluated and the stress-strength curves were used in the simulation model to help on the validation of the theoretical model.

According to the results on FE simulations, a possible reduction on the hole-border diameter would be possible with the use of these new materials based on UHMWPE.

5. To protect the high-performance fibres against UV radiation, several polymeric coatings have been identified in this first part of the project: silicones, polyurethanes, acrylic and fluoropolymers (PVF). Coated fibres were tested in order to evaluate the influence of weathering conditions.

The advantage of all of them is that they can be formulated as a waterborne coating satisfying Volatile Organic Compounds (VOC) legislation. Several commercially available coatings based on these resins have already been identified. The application method has also been identified according to Alterfil facilities.

6. A prototype of ABSORBNET has been developed. A net has been weaved and manufactured in order to be adapted to the support structure and perform the validation tests.

7. It was found not possible, in practical terms, to produce enough quantities of the HPM materials synthesised in the project at the laboratory scale (i.e.: APE35) to build a full-scale net prototype (10 x 5 m = 50 square m) within the schedule of the project. Therefore, it has been decided to build the prototype of the net with commercial HPM material: Dyneema.

8. Furthermore, it has been demonstrated according to the obtained FE simulation results that, with a given configuration of HPM e-fibre and weaving, ETAG 27 requirements are fulfilled: 5 MJ impacting energy, 50 % of which is absorbed by the APE35 net itself.

9. Therefore, according to FE simulation results, it is feasible to use the new e-fibres (APE35) as new HPM to, in the future, build a net able to stop the rock at a MEL energy level under the previously commented requirements.

10. Three additional handmade net prototypes have been proposed: Knot net, Splice net and Ring net.

11. The candidate that was proposed for the final ABSORBNET full-scale prototype was the Splice net. This prototype is not the most energy-absorbing one, but the most feasible in terms of patentability and for the time available to the project.

12. A full-scale test was performed on the Splice net (6m x 3m) specimen and afterwards, from the results obtained, the Finite Element (FE) analysis of the experimental full-scale test was carried out.

12. Results comparison showed good agreement in quantitative and qualitative terms at both levels of numerical predictions and experimental measurements.

13. A successful numerical-experimental validation of the results from FE simulations, used to predict energy absorption performance of the new HPM fibres and to provide recommendations to weaving design of the net itself, has been achieved.

14. Regarding fulfilment of both the ETAG27 requirement (5 MJ absorbed by the barrier) and the ABSORBNET objectives (2.5 MJ of which is absorbed by the polymeric net itself, on the basis of using HPM materials and innovative weaving designs), it has already been reported in technical deliverables that the best Dyneema weaving design was able to theoretically fulfil those requirements and objectives with the net configuration labelled 'orientation 45° square-shaped net' with hole-border (HB) braided-rope diameter of 20 mm.

Project context and objectives

The ABSORBNET project aims at fabricating a rockfall protection net to be used as basis of a kit that meets the requirements of ETAG 27 (European Technical Agreement Guidelines).

The ETAG 27 was endorsed in January 2008 by the European Organistion for Technical Approvals (EOTA). This document defines the technical performances a rockfall protection kit has to meet to get CE marking on the product. The rockfall protection market is growing rapidly (+ 17 % p.a.) due to new awareness of safety, not only for existing infrastructures, but also for future ones for which safety is considered at the conception stage. ETAG 27 will be fully applied from 2014. At this time only certified rockfall protection kits will be authorised for retail. ETAG 27 raises a technical issue in the case of high-energy rocks. Only two models for low-energy rockfall protection were approved by EATO in November 2009. Such a time gap proves the extent of the technical challenge issued by ETAG 27.

Compared with other rockfall protection standards, ETAG 27 imposes the kits to support two service launches. The net product should absorb energies up to class 8 and 9 (MEL > 4500kJ) in a temperature range between - 40 and + 50 °C.

This difference is fundamental and it is the reason why at the beginning of the project, no product on the market met ETAG 27 requirements in the case of high-energy rockfall. Only three companies (Isofer, Geobrugg and Stahlton-AVT) provide protection kits approved by the Swiss norm, which was the most rigorous after ETAG 27.

Current high-energy protection barriers come from the Second World War anti-submarine net design and need efficient braking devices with huge supporting structures and foundations to fulfil standards like ETAG. The elaboration of a protection system which meets the ETAG 27 requirements for high-energy levels will allow the protection of new areas but also the replacement of obsolete protection kits already installed, especially if this system is able to better dissipate and diffuse impact.

The function of the protection kit is to absorb the energy of the rock and this is currently done in three steps where the net catches the rock and this net transmits most of the energy to the brakes, which absorb energy to minimise the strain transmitted to the anchorages.

All the systems on the market today are based on this process which has got two major disadvantages: the net correctly transmits all the energy to the brakes but in a short time, so the strain to be supported (by the brakes and the anchorages) is high. Also, the design of the majority of nets is obsolete and has not been modified in the last 50 years.

Our proposed solution was aimed at clearing these two disadvantages and meeting the ETAG 27 requirements by creating a net with absorbing properties. The absorbing net will be an inorganic reinforced polymeric composite having the dual characteristics of being an energy-absorbing material, which will deform to dissipate part of the rock energy, and a high-strength material that will transmit the non-absorbed energy to ensure net integrity and solidity. The net will be designed to maximise the absorbing capability. As a consequence, the strain that will be transmitted to the brakes will be lower than current products, and spread out over the time. That will in turn increase the global performance of the system, minimise the number of anchorages needed and so reduce installation costs. To create this solution, it is necessary to develop new technology in composite materials and dynamical mechanics testing.

The objective of the ABSORBNET net was not only to meet ETAG 27 requirements but also to minimise the remaining stress on anchorages below a level that allows installation of rockfall protection kits in every ground type and/or for higher energy impact.

The project concept is to develop new knowledge, an integrated solution and technology development that will enable efficient securing of areas from high-energy rock threats. The safety of roads, highways, railways, beaches, hotels, etc., will be guaranteed by a European standard. Moreover the load on anchorages will be limited, which will enable the construction of protection barriers wherever needed.

The overall objective is to develop an energy absorbing net that does represent an improvement on the existing systems in terms of materials and set-up. This radical improvement will be achieved by implementing novel techniques, all aimed at increasing the performance of the system. To enable this innovative technology, new knowledge must be acquired concerning the following points:

1. 1. identifying the most adequate material to absorb energy and slow down force propagation in the appropriated conditions (temperature range = – 40/+ 50 °C, speed = 25 m/s, energy > 4500 kJ);2. replace the current material (steel), which gives strength, with an electro-spun material that is lighter, more cost-effective and easier to process;
3. Selecting, developing and validating the best system/set-up to include the absorbing materials in the panel;
4. selecting, developing and validating the best weaving (2D, 3D, Ring, Lacrosse, etc.) for the net using dynamical mechanics simulation, lab-scale tests and finally, full-scale testing.

The aforementioned points represent our scientific objectives. The specific scientific and technical objectives of our work, which need to be met to allow for the project aim, are:
1. to halve-widen the peak of strain versus time curve measured on the anchorages;
2. to keep the strain applied to the anchorages below 200 kN;
3. to produce a rockfall system in which there is a high degree of confidence, that will meet ETAG 27 requirements for class 8 products (MEL = 4500 kJ).
Project results
The actual developments obtained for each result after the execution of the ABSORBNET project have been the following.
Result 1
The expected results of Result 1 according to the DOW are as follows:'new material and a process for the manufacture of oriented high-performance fibres', in particular, according to section B 3.1.2 'which is the best material to absorb energy in the use conditions imposed by ETAG 27'.

With the results obtained by ITAV, APE-27 showed an elastic Young's modulus of about 11.66 GPa and tensile strength of about 0.88 GPa, which is higher than UHMWPE without nanotube fillers. The square-shaped net oriented by 45° indeed absorbed 100% of 2.5 MJ energy and was made of Dyneema fibres (UHMWPE without fillers).

It can be concluded that the ABSORBNET project came out with a novel material (UHMWPE 95wt % + modified carbon nanotubes 5wt %) that can be processed by gel spinning to manufacture a high performance fibre (APE-27) that can absorb the 100% of 2.5 MJ energy for the objectives of ABSORBNET. There exists no legal obstacle to patent the use of carbon nanotubes as a filler to increase the mechanical properties of the UHMWPE fibres. Therefore, APE-27 gel-spun fibres are novel and susceptible to being protected by a patent.

Additionally, PA1, PAA-2 and PI-2 are novel polymeric materials made of aromatic polyamides susceptible to being protected by patents, also with an economic potential to be exploited for the production of fibre yarn for applications other than ABSORBNET, such as mats and gas filters.

Result 2

In the DOW, the expectations of Result 2 read as follows: 'threading process to achieve an optimum yarn and external protective coating', in other words, 'establish how to process and set-up the material to optimise energy compression as well as providing protective housing to ensure end-product resistance to field-based operative conditions'.

With the results obtained from CENTROCOT and ITA (rope) and ITAV and CENTROCOT (coating), the tested double 12-strand braided rope was successfully used to build up a splice net that indeed passed the MEL test. In comparison with the uncoated yarns, it was concluded that the coating, a Lumiflon®-based fluoropolymer, can improve the ageing of the yarns. Using these coatings the stiffness (tensile modulus) and elongation at break are more or less maintained after all the different ageing tests (heat, weathering, light and environmental effects).

It can be concluded that ABSORBNET successfully used a rope configuration (double 12-strand braided rope, each strand made of 30 yarns of 1760 D-tex) that could be effectively used for the later manufacture of a splice net passing the MEL test, that is, absorbing 100 % of 2.5MJ energy. Also, ABSORBNET successfully developed a polyfluoroethylene vinyl ether coating formula (containing Lumiflon 4400®, Bayhydur 305® DMM and H2O) to provide weather protection and cut resistance to the yarns required for the final high-energy rockfall protection net. This coating formula can be protected as a trade secret to be exploited for the protection of commercially produced yarn.

It may be worth considering the development of a new rope configuration or testing alternative braiding configurations if ever there is the time, the will and the need to eventually improve the double 12-strand.

With the selected polyfluoroethylene vinyl ether coatings, the tensile strength is slightly decreased with the weathering test. It seems that the spraying of water every 30 minutes for 14 days in the presence of UV radiation (weathering test) diminishes the strength of the samples. In this sense, the coating cannot assure the complete stability of the yarns under a full-weathering ageing test. Therefore, one of the lines of action during the time to market should be to improve the weathering stability of the coating.

Also, the stage at which the material is coated should be further investigated. For example, individual filaments, or the subsequent yarn made of different filaments, or the strand of the final rope, composed of 30 yarns, could all be coated. Coating at filament, yarn, strand or rope level has consequences on the amount of coating used, the technology and machinery to coat each different element, the thickness of the coating, the total effective diameter of the coated element, the diameter of the next level and the mechanical properties of the next level. Therefore, another line of action should be to select the most appropriate level (filament, yarn, strand of rope) to coat the material after a techno-economic evaluation of the alternatives.

Result 3

In the DOW, the expectations of Result 3 read as follows:'a net weaving pattern and design optimised for the project', and in particular, 'which is the best pattern with which to build an energy absorbing net',

With the results obtained by CENTROCOT and ITA, the consortium selected a hand-made splice net because it presents the best compromise between the mechanical performance and innovative design. The best net design of the splice net is a square shaped mesh.

It can be concluded that the ABSORBNET project developed an innovate net made of Result 2 ropes in the form of a splice net with a square-shaped mesh oriented at 45° that allows an absorption of 100% of 2.5 MJ, which allows it to pass the MEL test and therefore it can be protected as a patent to be commercially exploited as a high-energy rockfall protection net.

Potential Impact

The three results in the DOW are consistent with the repartition of the patents agreed by the consortium and owned collectively by all SMEs according to deliverable 6.8 (the preliminary plan for patent applications in the view of exploitation agreements between partners, issued by month 9) which is the following:

1. result 1 – Esfil-55%, Alterfil-15%, T.A.S-15%, Engineerisk-15%;
2. result 2 – Alterfil-55%, Esfil-15%, T.A.S-15%, Engineerisk-15%
3. result 3 – T.A.S-55%, Esfil-15%, Alterfil-15%, Engineerisk-15%.

The costs of patenting will be proportional to the part that is owned by each of the partners. The list of countries will be chosen by a majority of SMEs (at least three).

Concerning the patents on the results, it is stated that every major owner of the patent on each result has entire rights to use the corresponding result on his own (i.e. without royalties) but only in the way that will not violate the rights of other partners. Especially for results on which others depend (result 1 for result 2, results 1 and 2 for result 3), that means that they cannot be used to enter in competitors’ products.

For example, ESFIL will have rights to manufacture oriented fibres, to use these fibres or to sell or give them to anybody it wishes. The only limit is for the needs of manufacturing nets for the protection of human beings for which fibres cannot be sold to competitors TAS. In case of one of the partners deciding to sell or give away its part of one patent, other partners have the priority right to purchase this part. Partners have one month to decide on purchasing the part from the moment they became aware of other partner wishes to sell or give away its part. If two or more partners want to buy the same part, the one with the already-larger part has priority. If the partners already have the same part, they can buy half of the new part.Composite materials (APE-27) can be protected by a patent to be exploited by the consortium SMEs for the production of filaments. These filaments or subsequent multifilament yarns and ropes can be coated with the developed formula (polyfluoroethylene vinyl ether coating) and then be used to produce rockfall protection nets using the novel splice hand-made net and best weaving design.

Some dissemination activities took place during the project period, namely the start-up and up-dating of the webpage of the project.

Due to the novelty of the project, participation on conferences and presentation of the results as scientific papers or brochures and flyers have not been considered and it was preferred to keep results secret till the very end of the project.

Project website: http://absorbnet.groupemnd.com