Protection from Landslides and High Speed Rockfall Events

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Protection from Landslides and High Speed Rockfall Events
–
Reconstruction of Chapman’s Peak Drive
Axel VOLKWEIN
Civil Engineer
WSL Swiss Fed. Inst. of Snow
and Avalanche Research SLF
Davos, Switzerland
[email protected]
Louis MELIS
Geotechnical Engineer
Melis & Du Plessis
Consulting Engineers
Capetown, South Africa
[email protected]
Bruno HALLER
Senior Engineer
Fatzer AG Geobrugg
Protection Systems
Romanshorn, Switzerland
[email protected]
Robert PFEIFER
Geographer
Geotest AG
Zollikofen, Switzerland
[email protected]
Summary
Chapman's Peak Drive (CPD) is a famous 10km scenic road in South Africa that has been closed to
the public for more than four years due to heavy rockfall and landslides. Extensive reconstruction
works allowed the reopening in the beginning of 2004. The paper presents the assessment of the
occurring hazards using innovative numerical and geotechnical methods. This includes a special
GIS-based mapping of all potential rockfalls together with a corresponding 3D-trajectory analysis,
an additional probabilistic analysis of the expected rockfalls and the design of the protective
structures. These consist of different arrangements, e.g. a special half tunnel, canopies and flexible
protection barriers. The latter have been simulated using special FE software. New contributions are
the concept of the half tunnel, a new statistical approach of rockfall hazards and the computational
assessment of protection systems for high speed rockfall events.
Keywords: Chapman’s Peak Drive, high speed rockfall, landslides, reconstruction, flexible rockfall
protection barriers, numerical simulations, half-tunnel, canopy.
1. Introduction
Chapman’s Peak Drive (CPD) is located close to Cape Town in South Africa and is one of the most
spectacular coast roads in the world (see fig. 1). It has a length of about 10 km and leads from Hout
Bay around its name giving mountain to Noordhoek. The street with a maximum height of ca.
600 m above sea level has been built into sheer rock during six years after 1916 by WW I prisoners
and other convicts although it was commonly called a ‘mission impossible’ at that time. Nowadays,
the road is frequently being used by 3’500 – 8’700 tourism and domestic vehicles and transports per
day.
After heavy rainfalls, particularly in winter, the road had always been closed because of the danger
of falling rocks and landslides due to the instability of the cliff-face and the roadway. After too
many fatal accidents the Southern Peninsula Municipality made the inevitable decision to close
Chapman's Peak Drive in September 1999. The closure became official in January 2000.
It then took more than four years to reconstruct CPD till its reopening in December 2003 as toll
road. The first 1.5 km could be reopened for tourism in summer 2000 after this part of the road has
been freed from threatening rockfalls by abseilers pushing the unstable rocks off the cliff-face. The
planning, funding and realisation of the final solution for the protection of the remaining road
including necessary repairs of road damages took the rest of the time. An overview of the complete
project organisation and the overall construction procedures is given in [1]. The technical part of the
rockfall protecting process is described in this paper.
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CAPE TOWN
Fig. 1: Map and view of Chapman’s Peak drive
2.
Assessment of rockfall hazard
The rock above CPD presents itself in a poor condition due to its horizontal layering and the strong
weathered sedimentary front face. The analysis of the rockfall danger has to be extensively done. At
first, the terrain has to be modelled on a very detailed level. For this purpose the landscape along
CPD is photogrammeticly recorded by specially equipped airplanes. Through this, a digital
elevation model (DEM) with a 2 m resolution can be retrieved (see fig. 2), where official DEMs
normally resolve 12 – 25 m. For the use of the terrain profile within the special Swiss rockfall
analysing software the longitude and latitude position of CPD had to be shifted into the Swiss
coordinate grid CH-1903.
For an efficient use of the rockfall simulation software, the terrain properties are additionally
integrated into a Geographical Information System (GIS). From the photogrammetic data on the
one hand and local inspections on the other, the terrain is classified considering the rockfall
trajectory influencing parameters (see fig. 2). This information is then integrated into additional GIS
layers. 20 different terrain classes are defined and allocated according to the input parameter
specification of the trajectory computing software. The surface roughness is divided into 5 classes
from more or less even ground to very blocky rock fan. The damping and absorption properties of
the ground classes are also described with values from 1 to 5 [2].
In the next step the rockfall hazard has to be evaluated. Only the trajectories which reach the road
area are interesting. Therefore, all areas with an average slope inclination between the release point
and the road of less than 28° can be excluded as potential release areas [3]. The remaining start
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points are obtained in two ways, an automatically generated grid and manually recorded blocks.
The automatic procedure assumes a rockfall release between every 3 x 3 m up to 6 x 6 m depending
on the average number of rocks per area unit, ground class and steepness of the area. Comparable
rockfall analysis for other areas with not this much potential rockfalls usually use only a 5 – 10 m
grid. For every start point, several block sizes are used to guarantee the most possible variety of
rockfall events. The assumed sizes are 0.3, 0.5 and 1.0 m3. The manually obtained additional
rockfall release points are added manually with the necessary data obtained in the field: The size of
the rock is measured and the location is determined via GPS. Altogether about 38’000 rockfalls are
finally considered. The experience obtained at CPD shows that rockfall triggering does not happen
below 31-34° slope inclination. Therefore, only the steeper sections have to be checked which
results in a remaining amount of rockfall trajectories of 32’000.
Fig. 2: 3D digital elevation model (DEM) of CPD (left) and 2D terrain classification (right)
A 3D analysis of those potential rockfalls provides information about their trajectories projected on
the 2D map (see fig. 3). This presents their distribution and/or concentration and the resulting
maximum energies. For the consideration of rockfall protection measures, additional 2D
simulations have to be performed. The corresponding slope profile is obtained through the DEM
along the projected 3D trajectory (see fig. 3). This simulation also delivers the bouncing heights of
the rocks and enables the planning and design of protection measures corresponding to the rock
energies that have been calculated in the 3D analysis.
Fig. 3: Rresulting rockfall energy distribution(left) and 2D rockfall trajectory analysis including
barriers (right).
3.
Rockfall protection measures
Main attention when selecting the right protection measure has been drawn to the condition that the
impact on the landscape at CPD must stay at a minimum level. Massive or well visible
constructions are not wished to keep CPD in its most natural state. For this reason, structures like
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large-area slope stabilisations by nets or shotcrete as well as massive rockfall protection galleries
drop out. The preferred option are therefore flexible and almost translucent rockfall fences, which
are able to absorb rockfall energies of up to 3000 kJ. This corresponds to a mass of 10 tons moving
at 25 m/s and 90 km/h or 56 mph respectively.
The calculated rockfall trajectories threatening the road are classified in to the energy classes
<500 kJ, <750 kJ, <1’500 kJ, <2’000 kJ and <3’000 kJ that correspond to the Swiss rockfall
protection guidelines [4] and also the maximum fence capacities of the different rockfall protection
devices. Finally, altogether 1’615 m of fences are installed. Determined higher energies have to be
handled separately and suitable protection measures have to be planned as described in the
following.
3.1 Solution for high energy levels > 3’000 kJ
The rockfall energies too high to be dealt using fences usually occur on only extremely exposed
locations concentrated mostly within small parts of the road. Overall around 6’000 of them have to
be considered. They can be get under control by either reducing the energy through measures
further above the road or by stronger structural measures remaining small in their extensions
compared to the road length. The control variant can be solved by getting rid of the large blocks
causing this high energy. This procedure is done by abseilers pushing the blocks of the cliffs using
lever and blasting tools. But this way is only suitable, if the area is reachable by the workers.
Another way on some locations is a second protection barrier high above the road that stops the
most highly located rocks before they can build up too high velocities and energies respectively as
they would normally do until reaching the road level. The overall rockfall analysis described in
chapter 2 is the so-called basic modelling where all potential rockfalls are analysed according their
danger to the road. For the consideration of those additionally highly above installed barriers two
case scenarios are modelled. One, assuming a rather short installation at the place the most
trajectories would pass by and another, where a longer barrier would retain still more potential
rockfalls.
Structural measures are chosen in areas with very unstable rock that cannot be controlled by the
application of abseilers and areas being too steep to install additional barriers above. The chosen
method is a so called half tunnel, a 150 m long tunnel blasted into the cliff with an open side to the
sea (see fig. 4). So, the road is completely removed from the endangered zone. Because of the high
instability the section directly above the tunnel has to be secured. This is done by the installation of
over 200 cable strand anchors and rock bolts, each 16 m long and partially prestressed to 95 tons [1].
Afterwards the half tunnel can be excavated.
Fig. 4: Half tunnel and adjecting rockfall canopy
Adjacent to the half tunnel the road still has to be secured from high energy rockfalls. This can not
be done by using fences, because on the one hand the rockfalls would be too large for fences and on
the other hand the maximum barrier height would not be large enough to cover all potential rockfall
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due to their large bouncing heights when falling down almost vertically from up to 400 m.
Therefore, galleries or canopies are more suitable and have to be constructed to withstand an
expected maximum energy of 7’000 kJ. They are conducted on one tunnel end as cantilever
structure to keep the ocean view clear (see fig. 4) and on the other end using additional front-side
columns due to the poor rock quality, which does not allow a firm anchorage. To reduce and to
damp the high impact loads the gallery roofs are loaded with 3 m of earth and small rocks. At the
front, gabions are used to build up a front wall where the poured material can be stored behind. The
impact of this measure on the landscape is being reduced by a suitable coat of paint using the colour
of the surroundings. This almost hides the gallery load in this area if looking from the ocean.
In the remaining part of the road protected only by fence structures high energy rockfall events stil
occur. But such events happen very seldom. The manual survey over several thousands potential
rockfalls described above ends up in a statistical distribution of the rockfall events (see fig. 5). From
this, the high energy rockfall events with fast and heavy rocks occur much more rarely than events
with smaller rocks at the same speed. Therefore, the rockfall events can be classified into different
categories depending on the volume of the rocks:
0.001 - 0.03 m3, 0.03 - 0.1 m3, 0.1 - 0.5 m3, 0.5 - 1.0 m3 and 1.0 - 3.5 m3.
Because of the very rare occurrence of rockfall events within the last two categories (<2%), those
categories are not considered for the dimensioning of the rockfall barriers because the insurance
policy says that the protection measures must be able to cover at least 95% of all rockfall events.
Here, every single event no matter the rock’s size counts the same. Therefore, it can be accepted
that the remaining few high energy events break through the barriers and require corresponding
repairs, as they are covered by the insurance policy.
Fig. 5: Statistical analysis of rockfall volumes at CPD
3.2 Simulation and design of the flexible protection fences
The fencelike structures used at CPD (see fig. 6) consist of steel posts, which are kept in position by
restraining and lateral steel ropes. Suspension ropes are strained between the posts and span the ring
nets. The net itself consists of loosely connected rings with a diameter of 300 mm and are made of
3 mm-high-strength steel wire bent to 5 – 19 windings. Special brake elements are integrated in the
ropes to guarantee a clearly defined location of energy absorption. The described system acts very
flexible with high deformations in the order of the barrier size. This guarantees an extended braking
time for the falling rock and therefore results in a peak-load reduction of the involved components.
After drilling and completion of the anchorage works the barriers can easily be installed by crane or
helicopter. The barriers can also used against landslides. The barrier dimensions usually countervail
against those masses because their maximum velocities stay much behind the ones reached during
rockfall. This and the fact that a landslide does not load the net punctually but as an area load
reduce the maximum utilization of the components compared to rockfall.
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Fig. 6: Example of flexible ring net barrier during installation and a rockfall event.
The rockfalls at CPD are classified in terms of their maximum kinetic energy at road level or the
location of the protection fences respectively. The suitable fence structure is chosen according to
their maximum energy capacity. Different rockfall events are usually distinguished through their
kinetic energy content, which results from mass and velocity to Ek = ½ mv2. Previous research in
the Alpine region has shown, that most rockfalls reach a maximum velocity of v = 25 m/s that is
also described in the Swiss guidelines [4]. This is the limit velocity all barriers are tested for.
However, the rockfall events at CPD show a different mass-velocity-fragmentation.
Due to the landscape shape with free fall heights of up to 250 m right above CPD, the rockfalls can
reach up to v = 68 m/s (= 245 km/h or 109 mph). With an assumed falling mass of maximum
1’300 kg this results in a rockfall energy of 3’000 kJ. The corresponding test specimen in
Switzerland weighs 9’640 kg at the above described speed of 25 m/s. For a proper use of the
barriers, they have therefore to be checked according to these high speed rockfall events, which
might load the barrier in a different way. In contradiction to the common Alpine rockfall events
which can be experimentally tested [4] a full-scale test with a final velocity of 68 m/s will not be
possible. This would require a free fall height of about 230 m (air resistance neglected) and one
won’t exactly hit a barrier from this height. Therefore, the influence of high speed events on
protection fences has to be numerically evaluated. This is done using the Finite Element software
FARO a specially developed code for simulating flexible ring net barriers [5, 6].
As described above, only the high speed rockfall events are simulated that are expected most at
CPD. The simulations consider a minimum coefficient of friction µ = 0.25 between rock and net
and a special aspect ratio of the rock. It is the aim of these simulations to achieve a high, but still
realistic loading of the ring net within each category. Therefore as a realistic assumption, an aspect
ratio of length/width = 2.5 is used as worst load case. The simulation itself still uses a sphere as this
was the test specimen shape used for the calibration and verification of the software [7]. For the
CPD simulations the rock radius is adjusted to the small face of the longitudinal rock together with
a higher density. This resulted in following simulated rocks described in Tab. 1.
Tab. 1: Rock sizes used for simulations of high speed rockfall events for an aspect ratio l/w = 2.5
Volume category
0.001-0.03
0.03 - 0.1
0.1 - 0.5
Chosen characteristic volume V [m3]
0.03
0.1
0.5
2.7
2.7
2.7
81
270
1350
Volume of sphere representing the longitudinal rock V’=V/2.5 [m ]
0.01
0.04
0.19
Rock radius r=(3V’/4π)1/3 [m]
0.13
0.21
0.36
3
Density ρ [tons/m ]
Mass m=ρ*V [kg]
3
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Simulation results
Some selected simulation results are now presented . Fig. 7 illustrates the degree of utilization of a
barrier due to a high speed and a energy equivalent standard rockfall event. The single components
are coloured corresponding their load in relation to their maximum load capacity. Tab. 2 shows the
corresponding relative change numerically.
Fig. 7: Different loading percentage of a rockfall barrier due to a ‘slow’ and large Swiss standard
test specimen (top) and a small high speed rockfall event (bottom). Details of impact areas on the
right.
Tab. 2: Influence of high speed rockfall events on the loading of the barrier components compared
to the Swiss standard test conditions (100 %).
Barrier component
Change of maximum load for
high speed rockfall events
Ring net
190 - 230 %
Ropes and brake elements
70 – 85 %
The main results obtained from the simulations can be summarized as followed:
a. The barrier as a whole is being loaded less through a high speed events compared to a
normal energy equivalent event. This is explainable through the different rockfall impulse
I = mv (The energy relation contains also the velocity but squared). Therefore the high speed
impulse is reduced by the factor vhigh speed/vnormalspeed.
b. Because of the smaller size if the high speed projectile and the resulting higher energy
density when impacting the net, the net load at the impact location increases.
The simulation results now can be used to recommend a barrier for the high speed application or to
suggest necessary barrier enhancements. For example, the barrier RX-300 is being equipped with an
additional ring net layer Rocco 16/3/300 to resist the punctual loading through high speed rockfalls.
On top of the ring net barriers usually a normal wire-mesh is used to prevent small rocks from
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penetrating the ring net. The high velocities require its strengthening, too. For this purpose, the socalled special wire-mesh Tecco made from high strength steel is installed instead. This enhancement
is not simulated, but field events proove, that the mesh withstands this loads as shown in fig. 8.
Fig. 8: Retain rockfall events: Left: ^Small event captured by high-strength wire-mesh on top of the
ring net. Right: Several events caught by one barrier on the right.
5.
Conclusions
The paper presents an impressive engineering project that deals with natural hazards of unusually
high dimensions. Although it was initially estimated to be almost impossible to realize, a
consequent analysis of the natural hazards paired with innovative problem solutions now enables
the road use as safe as never before. The main success contribution are a half tunnel securing a high
risk area of the road by hiding the road in the rock without loosing the ocean view, the use of the
insurance policy conditions to dimension the necessary barriers and a specially developed Finite
Element tool, which allows the simulation of load cases that would never be testable in practise.
6.
References
[1]
WHITTAKER B., “Chapman’s Peak – Dynamic barrier systems absorb impact energy”,
IMIESA, Vol. 28, Issue 11, Nov/Dec 2003, pp. 29-39, ISSN 02571978.
Dienstleistungsübersicht Steinschlag, Naturgefahren, Geotest AG, Bern.
GERBER W., “Beurteilung des Prozesses Steinschlag“, Swiss Task Force Natural Hazards
(FAN), Course from 20-22. October 1994, Poschiavo
GERBER W., ”Guideline for the approval of rockfall protection kits”, BUWAL, 2001, Bern.
VOLKWEIN A., “Numerische Simulation von flexiblen Steinschlagschutzsystemen“, Diss.
ETH Zürich, 2004, 134pp, IBK-Bericht 289, vdf Hochschulverlag AG, ISBN 3-7281-2986-0,
VOLKWEIN A., “Numerical Simulation of flexible rockfall protection systems”, Proc.
Computing in civil engineering, ASCE Cancun, 2005
Grassl H. G., “Experimentelle und numerische Modellierung des dynamischen Trag- und
Verformungsverhaltens von hochflexiblen Schutzsystemen gegen Steinschlag“, Diss ETH
Zürich, 2002, 202pp.
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