HAL Id: hal-01745499
https://hal.archives-ouvertes.fr/hal-01745499
Submitted on 28 Mar 2018
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
based on the current state of knowledge
Stéphane Lambert, B. Kister, B. Loup
To cite this version:
Stéphane Lambert, B. Kister, B. Loup. Evaluating existing rockfall protection embankments based on the current state of knowledge. Rocexs 2017, May 2017, Barcelone, Spain. pp.130-133. �hal-01745499�
R o c e x s : R o c k f a l l E x p e r t N e t w o r k Page 1
EVALUATING EXISTING ROCKFALL PROTECTION EMBANK- MENTS BASED ON THE CURRENT STATE OF KNOWLEDGE
Stéphane Lambert1, Bernd Kister2, Bernard Loup3
This article deals with the evaluation of large rockfall protection embankment parks, as ob- served in some countries. In this purpose, an expedient criterion for assessing the impact re- sistance of these structures and based on results from the literature is proposed. It is then ap- plied to the Swiss park consisting in more than 250 structures.
Keywords: embankment, assessment, design, impact,
INTRODUCTION
Rockfall protection embankments (RPEs) have been widely used for now more than 30 years for protecting elements at risk against events with energies up to 150 MJ. As a result, large structure parks exist in some countries of the Alpine arch as for instance in Switzerland and France. Such structure parks are heterogeneous in terms of construction date, structure tech- nology, constitutive materials, dimensions, and designed capacity. The vast majority of RPEs are owned by public owners.
In parallel, the improvement of the design of RPEs with respect to block impact has motivated various research works, based on experimental and numerical developments (see a review in [1]). In particular, real-scale impact experiments provide reliable data on the real response of RPEs to impact. Nevertheless, none of these works really benefited to commonly used design methods, except the rather recent Austrian standard (ONR 24810) that is based on small-scale experiments [2]. Besides, most of existing analytical methods, and thus easy to use for design- ing RPEs, have been shown to be unreliable [3].
In such a context, questions concerning the efficiency of existing RPEs occur, in particular due to the high heterogeneity of the parks. Such questions may rise when dealing with risk management as for instance revising natural risk prevention plans.
In order to get a better view of the Swiss RPE park, a survey was conducted under the super- vision of the Federal Office for the Environment (FOEN) in the frame of the AERES project (Analysis of Existing Rockfall Embankments of Switzerland). The expected results were a quan- tified inventory of existing RPEs and a global vision of the design approaches in use in Swit- zerland. Based on the collected data, a global evaluation of the RPE park was conducted. In this purpose, an expedient criterion was developed in order to evaluate the ability of existing
1 Irstea, 2 rue de la papeterie, 38402 Saint Martin d’h7res cedex, France, + 33 4 76 76 27 94, [email protected]
2 Lucerne University of Applied Sciences and Arts, Technikumstrasse 21, CH – 6048 Horw, Switzerland, since 2017: kister - geotechnical engineering & research, Neckarsteinacher Str. 4 B, D – Neckargemünd, +49 6223 71363, [email protected]
3 Federal Office for the Environment (FOEN), 3003 Bern, Switzerland, +41 58 465 50 98, ber- [email protected]
R o c e x s : R o c k f a l l E x p e r t N e t w o r k Page 2 RPEs in withstanding the impact by the design block. This criterion was developed based on results from real-scale experiments provided by the literature.
This article gives a general overview of the structure park, introduces the expedient criterion, and applies it to the Swiss rockfall protection embankment park.
PARK DESCRIPTION
The inventory revealed that the total number of RPEs in Switzerland by far exceeds 250 units.
The study focused on 53 RPEs, very well documented and less than 20 years old and thus providing indications on the currently used design methods. The vast majority of these RPEs is made of compacted soil, with a rockery facing at the uphill slope. The dimensions range between 15 and 700 m in length and 1.5 and 13 m in height. Approximately 64% of the em- bankments have a height of 4 m or less, but only approximately 6% have a height larger than 7 m. The average values are 155 m in length and 4.3 m in height respectively.
These RPEs were designed considering reference blocks with a weight and a kinetic energy in very wide ranges: 15 to 1600 kN and 160 kJ to 50 MJ, respectively. About 40% and 64% of the embankments have been designed for stopping blocks with a kinetic energy less than or equal to 2000 and 4000 kJ respectively. 18% of the RPEs were designed for kinetic energies higher than 10 MJ.
When block impact resistance was considered, the design was based on equivalent static force methods as described in ONR 24810, FEDRO guideline for rock sheds, etc.. Obviously, such methods were only employed for recently built structures.
ASSESMENT CRITERION
The criterion was developed based on results from real-scale experiments proposed in the literature and with the aim of finding a simple relation between the downhill face displace- ment and the block kinetic energy. The downhill displacement is deemed relevant as it is re- lated to the post-impact RPE stability. It is proposed to normalize both these parameters with respect to the structure dimensions, in order to allow comparison from one case to the other.
As an initial approach, the kinetic energy is divided by the cross section of the structure. In- deed, the larger the cross section, the higher the energy dissipation capacity of the RPE. As for the downhill displacement, it is proposed to normalize this value by the mid-height struc- ture width, which is representative of the structure dimension in the impact direction, irre- spective of the cross sectional structure shape.
Fig. 1 plots the results presented in the literature in terms of these two normalized parameters.
Data concerns various types of structures impacted under different conditions (see [1] for more details). It can be seen that there exists a limit in terms of kinetic energy beyond which the downhill displacement exceeds 25% of the structure width. This limit equals 250 kJ/m².
Considering this finding, the RPE is considered impact resistant if:
ܥଶହ = ܭܧ
250 ∗ ܣ< 1
R o c e x s : R o c k f a l l E x p e r t N e t w o r k Page 3 where KE is the block kinetic energy (kJ), A is the structure cross section area along the verti- cal axis calculated from the ditch elevation (m²). The subscript 25 in C25 refers to the maxi- mum allowable downhill deformation with respect to the structure width (here, 25%).
The validity domain of this expedient criterion is related to the experimental conditions:
- Reinforced structure;
- RPE with a height in the 3-4.2 m range, and a mid-height width in the 3-4.3 m range;
- Block with a 30° approx. downward incident trajectory;
- Impact point located at a significant distance from the crest (at least ¼ of the structure height).
Fig. 1 Comparison of real-scale impact experiments on embankments (for precise references, see [1])
Fig. 2 Embankment Swiss park evaluation
R o c e x s : R o c k f a l l E x p e r t N e t w o r k Page 4 Also, this expedient criterion was compared to data presented in the literature and concerning existing structures designed considering various complex approaches. None of the structure failed the criterion, suggesting the criterion to be consistent with these methods.
PARK EVALUATION
As most of the RPEs of the park are non-reinforced structures, and considering results from the literature, it was decided to divide the acceptable limit of C25 by 2.
When applied to the 53 RPEs, the efficiency criteria reveals that more than 50% of the RPEs have a C25 value less than 0.5 and thus may be considered able to withstand the impact by the block (Fig. 2). On the other hand, the C25 exceeds the value of 1 in 17 cases, among which 7 cases exceed the value of 2. It is worth highlighting that the criterion is used out of its validity domain for the 4 tallest ones out of these 17 cases, due to the RPE height and impact point location.
This expedient evaluation thus draws the attention on possibly critical structures. For these, complementary analysis may be required, depending on the protected elements at risk in par- ticular. This concerns first, structures with a C25 higher than 2, and second structures with a C25 higher than 0.5. But prior to any detailed analysis, the relevance of using the C25 should be checked depending on the impact case vs. the experimental conditions. Also, the accepta- bility of the destruction of the RPE should be checked versus the return period of the event considered: destruction may be tolerated in case of a 300-year return period event but not for a 30-year one.
This park was also evaluated via criteria based on small and half-scale tests with rotating blocks [4]. This second evaluation complements the C25 approach and it is not detailed here.
CONCLUSION
Based on the AERES inventory more than 250 rockfall protection embankment structures exist in Switzerland. The vast majority are unreinforced structures with a rockery facing. Only a limited number of these structures were designed accounting for their block impact re- sistance. In such a context, an expedient criterion was proposed to allow the identification of possibly critical structures with respect to this design facet. When applied to the structure park, this criterion appeared to be fulfilled for the majority of the structures. On the opposite, this criterion draws the attention on about 20% of the structures for which complementary analysis could be conducted to assess their impact resistance.
REFERENCES
[1] LAMBERT, S, BOURRIER, F (2013) Design of rockfall protection embankments: a review. Engineering geology 154 (28), 77-88
[2] ONR (2013) ÖNORM 24810: Technischer Steinschlagschutz – Begriffe, Einwirkungen,
Bemessung und konstruktive Durchbildung, Überwachung und Instandhaltung, Austrian Standards Institute [3] KISTER, B, FONTANA, O (2011). On the evaluation of rockfall parameters and the design of
protection embankments – a case study. Proc.of Rocexs 2011, Innsbruck, Austria, 31-32
[4] KISTER, B (2015) Development of basics for dimensioning rock fall protection embankments in experiment and theory (in German). FEDRO report 1524.
