3107
Rockfall-protection embankments – design concept and construction details
Merlons de protection contre les chutes de pierres - modèle de conception et d’exécution
Hofmann R.
Ziviltechnikerbüro Dr. Hofmann, Perchtoldsdorf, Austria
Vollmert L.
BBG Bauberatung Geokunststoffe GmbH & Co. KG, Espelkamp, Germany
Mölk M.
Geologische Stelle des Forsttechnischen Dienstes für Wildbach- und Lawinenverbauung, Innsbruck, Austria
ABSTRACT: In the past, protection embankments were often erected in situations in which high design energies were anticipated; it was
assumed that these embankments, if constructed appropriately, would provide adequate protection for such load cases. Rockfall-
protection embankments are favoured in cases where the slope geometry and the space available allow their construction. In comparison
with rockfall-protection nets, whose capacity to absorb energy is currently limited to 8000 kJ, embankments have advantages in terms of
longevity, construction costs, and – depending on the construction – energy-absorbing capacity. To describe the failure mechanism
associated with a dynamic impact on such an embankment, and to develop a design approach, model testing was carried out on
conventional soil embankments, on reinforced embankments, and on embankments with stone facings. The objective of these model tests
was to investigate the effects of rock impact on built-up embankments of various types, and to develop a concept for their design.
RÉSUMÉ : Dans le passé, les merlons de protection étaient souvent construits à haute énergie en supposant que ces merlons aient une
capacité suffisante pour ces cas de chargement. Les merlons de protection contre les chutes de blocs sont construits, de préférence, dans
les cas où la géométrie du talus et l'espace disponible permettent un tel ouvrage. En comparaison avec des filets protection contre les
chutes de blocs, dont la capacité d'absorption d’énergie est actuellement limitée à 8000 kJ, les merlons ont des avantages, notamment en
termes de durabilité, de coût de construction et - selon l’ouvrage - de capacité d'absorption d'énergie. Afin de décrire les surfaces de
rupture des merlons causées par un impact dynamique et pour la mise en place d'une approche de conception, des modèles
expérimentaux ont été réalisés avec des merlons en sol pur, des merlons renforcés et merlons en sol avec un parement en pierres.
L'objectif du projet pilote était d'étudier les effets des éclats de blocs sur les différents types de merlons et de développer un
dimensionnement.
KEYWORDS: rockfall protection, embankments, impact, energy absorption, design, model test, prototype, geosynthetics
1 INTRODUCTION
In the virtual absence of design rules for dynamic actions on pure
soil structures, in the past 15 years designers have often resorted to
the use of concepts used in the design of rockfall-protection
galleries. The required parameters were derived from model
testing (and, in a few cases, from full-scale tests) carried out since
the late 1990s mainly in Switzerland, Austria, Italy and France
(Blovsky (2002), Labiouse & Heidenreich (2009), Lambert et. Al
(2011), Peila (2007) and Pichler et al. (2005)) In some cases,
numerical calculations (Peila (2007) and Plassiard & Donze
(2010)) were performed. In contrast to a rockfall-protection
embankment, a rockfall-protection gallery is a stiff, reinforced-
concrete construction overlain by a cushioning layer of various
materials with differing thicknesses.
The construction of protection embankments has increased
markedly in areas with a high risk of rockfall. No suitable design
models for soil embankments currently exist which enable a
geotechnical assessment of the stability of such structures.
1g model tests were therefore performed to obtain a picture of the
failure mechanism under impact forces, and to use this to devise a
design model.
A total of 150 tests with different structures (soil
embankments, reinforced embankments, embankments with rip
rap facing, embankments with cushioning elements), cross-
sectional profiles, impact angles, freeboards, and impact energies
were performed. The length of the embankment, and its height,
were kept constant in all the tests. Several significant parameters
have been varied, see 2.2. The test series were complemented with
model embankments with rip rap facing and cushioning elements,
and with geosynthetics. The model-scale geosynthetics were
manufactured and delivered by NAUE.
2 SCALED MODEL TESTS
2.1
Model tests
The tests serve as experimental model tests, and the questions to
be answered are limited to the shape of the failure body created in
an embankment by a dynamic impact. A model scale of 1:33 was
chosen to correspond to the energy involved in practice. The
objective of these qualitative model tests was to depict with model
embankments in the laboratory the failure mechanism generated in
embankments by rock impact, see Hofmann & Mölk (2012).
The sphere was impacted against the embankment at three
velocities (v1 = 4,5 m/s, v2 = 3,5 m/s, v3 = 6,0 m/s). Using the
model scale of 1:33, these correspond to velocities of:
v1 = 25,8 m/s, v2 = 20,1 m/s, v3 = 34,4 m/s in real life. The
plastic displacements in the embankment and the penetration
depth of the sphere were measured after each impact at two levels
using several model extensometers (Figure 1).
2.2
Structure types investigated
Different slope angles β
UPHILL
(4:5, 50°, 60° and 70°) and
β
DOWNHILL
(2:3, 50°, 60° und 70°) and crest widths (b
1
= 2,5 cm, b
2
= 5,0 cm, b
3
= 10 cm and b
4
= 20 cm) were investigated and the
impact height of the sphere (measured along the slope from the
lowest point of the sphere up to the crest h
1
= 8 cm, h
2
= 16 cm,
h
3
= 12 cm and h
4
= 20 cm) was varied.
