1033-B2
Reto Baumann 1
Rockfall protection structures have been evolving and being field tested in Switzerland since 1988. Today their capacity for taking up energy is a multiple of what it was then. Four manufacturers have tested their products at seven different locations. Over 350 field tests have infused new knowledge into the development of rockfall protection structures and into the testing procedure itself. High quality products have emerged out of the different test conditions, but they cannot be compared. This always leads to situations of conflict in public tenders. For this reason the Swiss Agency for Environment, Forests and Landscape (SAEFL), together with the Federal Research Institute WSL, has introduced an objective and standardized approval procedure for flexible rockfall protection nets. For this test the protection nets will be evaluated on different criteria in an energy range from 100 - 5,000 kJ.
Switzerland has been fighting natural hazards for centuries
Switzerland is famous for being in the middle of the Alps. They have their beautiful side, such as the world famous Matterhorn. They also have a very threatening side, however, in the form of fearful natural hazards. Avalanches, floods, slides, rock avalanches and rockfalls arouse anxiety and fright in the populace. The Swiss have learned, however, to live with them and to protect themselves effectively. Evidence of this centuries long battle for survival can be seen everywhere today in the many protection structures. Avalanche structures, deflection dams, road and railway galleries stand witness, as do rockfall protection nets.
Experts call for better rockfall protection nets
The protection structures built have not always performed as well as hoped. For this reason a group of experts and practitioners had already called for the development of better rockfall protection nets back in 1974, with their functionality to be tested by an independent authority. W. Heierli presented in 1985 the first research study about protection against rockfall.
Research and industry carried out more than 350 tests
Due to an intensive collaboration between scientists and manufacturers in the design of new products it was possible to increase the capacity for absorbing energy, which is the most important criterion for the nets, from ca. 250 kJ to around 3000 kJ - a factor of 12!
Right from the beginning it was clear that no further help was to be had from calculations. The dynamic behavior of the nets cannot be calculated (yet). Tests on a 1:1 scale were required. In the period between 1988 and 2001, over 350 tests were carried out with rockfall protection nets! Development was based mainly on realistic field tests, in which the nets were tested with moving rocks. Not only were the protection nets evolving, the testing methods also continued to improve. The experience thus gained pointed up the weaknesses in the test arrangements chosen.
Figure 2: Schematic representation of different test arrangements
Test series 1 Test series 2 Test series 3
In the first test series we let rocks roll down the mountain. In the second test series we let the rocks ride down on an oblique cable-crane setup and crash onto the ground just in front of the net. Both of these types had the big disadvantage that there were extreme variations in the velocity of the rocks, and we could not determine in advance where they would strike the net. Sometimes the rocks even bounced over the net. Finally, in the third test series we let rocks fall directly into the net from the cable-crane. However, even this arrangement was not satisfactory. The impact was not accurate enough an large rocks could no longer be handled well. They represented a hazard for personnel and the test setup. None of the three test series could meet the high requirements for an approval test. Above all, it was impossible to reproduce test results.
Practitioners call for approval
The purchasers of rockfall protection nets during a period of such rapid development find themselves constantly confronted with new or changed products. The different works plans do not lend themselves to comparisons. New products of unsatisfactory quality are constantly appearing on the market. Those responsible for safety are very insecure and cannot compare products in public tenders. Surveys of practitioners in 1997 and 1998 revealed an urgent need for action. Again there was a call for neutral and objective testing of the nets in Switzerland.
What are the requirements for an approval?
Separate the wheat from the chaff. The approval process must be able to separate the "wheat from the chaff". That means that it must be able to ascertain which products are not suitable for rockfall protection. These must not pass the test.
Provide relevant information. The approval must provide the information that planners and purchasers need for the design and dimensioning of the rockfall protection structure. Much information is interesting but not important for the practitioner. The information provided by the test should simplify product choice, not complicate it.
Standardize. The test procedure must be strictly standardized and reproducible at all times. The test procedure must be structured so that the test conditions are always exactly the same. Only if this constancy can be guaranteed in the test procedure will the manufacturers of protection nets be prepared to accept the results.
Model reality. As much as possible, the test should reproduce the real-life situations it is attempting to model.
Be reliable and independent. The manufacturers, planners and purchasers must be able to have confidence in the test. For this reason it must be carried out by an independent institution, and it must have an official character. Only a government authority can meet this need.
Be technically and economically feasible. The most beautiful ideas for a test are worthless if they are not technically feasible or are so expensive that they are unaffordable. Approval testing should not make the products offered on the market more expensive. That would not be in the public interest! The products for sale should be getting better, not more expensive.
The new test standard in Switzerland
A strict but fair approval was 1999 designed with the participation of industry, research and various authorities. The test facility in Steinbruch Lochezen near Walenstadt was officially opened on May 31, 2001. This was the birth of the first worldwide and, up to now, the only official approval for rockfall protection nets!
The approval rests on two pillars. First, the new test facility permits 1:1 tests and the necessary comparative measurements. Second, the test procedure, minimum requirements and measurement methods were developed. All this is set out in the federal guideline for the approval of rockfall protection nets, issue 2001.
The following criteria, important for the purchaser, are tested:
Administration of the approval is mainly in the care of the SAEFL. Manufacturers send their applications for approval to this agency, along with all necessary construction plans, detailed drawings and data on the material qualities. The agency orients the involved parties on the further course of the procedure, the rough schedule and, for the manufacturer, also the costs that will be involved.
Based on the documentation, the WSL verifies whether the construction meets the basic requirements for protection structures and clarifies how the structure can be installed in the test site. The WSL organizes the test, generates the measurement and observation plan and carries out the approval; it records the results and produces a comprehensive report.
The Federal Expert Commission on Avalanches and Rockfall (FECAR) evaluates the results and determines to what extent the structure fulfills the conditions for approval. It gives the SAEFL a recommendation as to whether the test was passed.
The SAEFL writes the test certificate and forwards it to the manufacturer along with the test report.
It goes without saying that the approval is open to all manufacturers and not just intended for Swiss products!
The test facility in Walenstadt.
Experience from the first three test series led to the idea of building a test facility for testing vertical fall with test bodies. This way all of the disadvantages of the oblique fall setups could be avoided. The test site where the structures are tested now consists mainly of an almost perpendicular rock face and a level installation area. The structures are installed on the rock face at a height of 15 m; the structure's posts are fastened to a 32 m long foundation adaptation unit and anchored to the rock face by ropes on the post head.
Figure 3: Frontal view of the Walenstadt test setup during a test
The geometry of the test setup is shown in Figure 4.
Figure 4: Test setup mainly used in recent years (left) and new arrangement at Walenstadt (right).
The works are divided lengthwise into three section widths of equal lengths. Thus for a standard structure 4 posts are installed, with 3 net sections between them. The minimum height of the structure being tested, on the other hand, depends on the particular energy category (Table1).
Figure 5: Arrangement and technical data for the rockfall test setup
Legend: | |||
1 |
Crane |
Hoisting force |
160 kN (16 tons) |
2 |
Tested nets |
Maximum length (horizontal) |
30 m |
3 |
High speed video camera |
Number |
2 |
4 |
Tensile force sensors |
Number |
15 |
5 |
Test body |
Type |
Steel concrete cube in ashlar shape with flattened corners and edges |
Requirements for the protection nets
In principle the task of a protection structure is to stop moving rocks and blocks. The protection structures to be tested in accordance with this guideline should be able to absorb the full kinetic energy of a falling rock. Certain additional requirements will be imposed on the maximum deformation of the structure. On one hand the braking distance should not exceed a certain mass after an event, and on the other hand the structure should still retain a certain residual useful height.
The effective surface of a structure should also remain as large as possible after an event and be impaired as far as possible only in its height, but not in its width. In recent years structures have been developed for which the effective width has been interrupted after an event. In such a case it is possible that rocks and blocks coming after can roll on unimpeded through the structure and cause damage further below.
The 4 test parts
The protection structures are divided into 9 energy categories of 100 kJ to 5,000 kJ. A structure must pass the test parts a) through d) described below in its energy category. For tests a) through c) the individual tests take place on the structure with rocks, and test d) includes a qualitative assessment of the structure and an overall evaluation of tests a) through c).
Concrete blocks are used for testing the structures. They are dropped vertically and must be decelerated by the structure. The velocity used is always 25 m/s, and the mass of the blocks depends on the energy category.
a) Preliminary tests with low energies: (boundary section)
These preliminary tests serve on one hand to test the bearing and deformation capacities of the laid-on mesh and on the other hand to load individual ropes or rings in the net with smaller rocks. Rocks as listed below are dropped together by size category into a boundary section.
For this test all of the rocks must be stopped by the structure. Deformations of the individual ropes and mesh elements are permitted. They are recorded. No repair work is allowed.
Table 1: The test parameters for the 50% and 100% test parts
Test part (50%) |
Test part (100%) | ||||||||
Cate-gory |
Post length (m) |
Energy (kJ) |
Mass of test body (kg) |
Edge length (m) |
Energy (kJ) |
Mass of test body (kg) |
Edge length (m) |
Max. perm. stopping distance (m) |
Min. net height (m) |
1 |
1.5 |
50 |
160 |
0.41 |
100 |
320 |
0.52 |
4.0 |
0.90 |
2 |
2.0 |
125 |
400 |
0.56 |
250 |
800 |
0.70 |
5.0 |
1.20 |
3 |
3.0 |
250 |
800 |
0.70 |
500 |
1,600 |
0.88 |
6.0 |
1.80 |
4 |
3.0 |
375 |
1,200 |
0.80 |
750 |
2,400 |
1.01 |
7.0 |
1.80 |
5 |
4.0 |
500 |
1,600 |
0.88 |
1,000 |
3,200 |
1.11 |
8.0 |
2.40 |
6 |
4.0 |
750 |
2,400 |
1.01 |
1,500 |
4,800 |
1.27 |
9.0 |
2.40 |
7 |
5.0 |
1,000 |
3,200 |
1.11 |
2,000 |
6,400 |
1.40 |
10.0 |
3.00 |
8 |
6.0 |
1,500 |
4,800 |
1.27 |
3,000 |
9,600 |
1.60 |
12.0 |
3.60 |
9 |
7.0 |
2,500 |
8,000 |
1.51 |
5,000 |
16,000 |
1.90 |
15.0 |
4.20 |
b) Preliminary test with 50 % energy: (middle section)
This test primarily serves to test the required repair effort and the service-friendliness of a structure. It also tests the excursion at half energy. The block used for this test is dropped into the middle section. After the test the following data are recorded:
c) Main test with 100 % energy: (middle section)
For the main test the full kinetic energy of the rock is to be taken up by the structure; the bearing capacity and the deformability are tested. The block envisaged for this energy is dropped with a velocity of 25 m/s into the repaired middle section. After the test the following data are recorded:
The residual useful height of the structure may not be less than the value given in Table 3.
The maximum excursion of the structure with respect to the braking distance of the block is then determined from the video films. This may not exceed the value given in Table1.
d) Assessment of the structure
During installation of the structure and tests a) through c) the structure will be evaluated for simplicity of construction. The structure should also be able to be installed as efficiently as possible in difficult terrain. If it is damaged, it should be as easy as possible to replace individual elements.
For test b) and c) the tensile and compressive forces are measured and recorded at a maximum of 12 locations during deceleration of the block. Maximum forces up to 500 kN can be measured with tension load sensors, whereby the scanning frequency is normally 500 Hz. The motion of the block and individual bearing elements of the structure are filmed by video cameras from two directions (Figure 3). The effective forces acting on the rock can be calculated from the motion of the block (path/time records), and these can be compared with the measured forces and the bearing ropes.
The initial knowledge gained from the approval process
The approvals carried out thus far show that the test procedure meets the requirements. It is suited to differentiating between unsatisfactory products and those that meet the strict quality requirements in Switzerland. The information provided and the test results are objective and allow a good assessment of the protection net tested.
The effort involved for an approval is considerable, despite limitation to the most important test parameters. This is also reflected in the cost, which is about $ 40,000 per construction tested.
Where can I find out more about the protection nets tested?
The SAEFL publishes the certificates on the Internet. They can be downloaded from the following address: www.schutzwald-schweiz.ch
Gerber W. (2001): "Guideline for the approval of rockfall protection nets", Environment in practice, Swiss Agency for the Environment, Forests and Landscape (SAEFL) and the Federal Research Institute WSL, Bern, 39 pages
Gerber W. (1999): "Highly Flexible Wire Net Rockfall Barriers". In: H. Masuya, V. Labiouse (eds) Proceedings of the Joint Japan-Swiss Scientific Seminar. KKR Hotel Kanazawa, 4-7 October 1999: Impact Load by Rock Falls and Design of Protection Structures. 61-66. Kanazawa University, Kanazawa, Japan.
Gerber, W.; Baumann R.; Böll A.; Ammann W. (2000) Rockfall tests with protection structures: testing and certification - a new guideline for Switzerland. In: Internationales Symposium Interpraevent 2000 [International Symposium Interpraevent 2000] - Villach. Conference publication, Interpraevent, Klagenfurt, vol. 1, 327-337.
Gerber, W.; Grassl, H.; Böll, A.; Ammann, W. (2001) Flexible Rockfall Barriers - Development, Standardisation and Type-Testing in Switzerland. In: Kühne, M.; Einstein, H.H.; Krauter, E.; Klaperich, H.; Pöttler, R. (eds) Proceedings of the International Conference on Landslides - Causes, Impacts and Countermeasures. Davos, Switzerland / 17-21 June 2001. United Engineering Foundation Inc. Glückauf, Essen, 515-524.
Heierli W., Merk A. Temperli A. (1985) Schutz gegen Steinschlag (2. Auflage) Bundesamt für Strassenbau Bern 100 Seiten
1 Forestry Engineer (ETH), President FECAR, Swiss Agency for Environment, Forests and Landscape SAEFL, CH-3003 Bern, Switzerland.
E-mail: [email protected]