Selection of Protection Structures
The selection of rock fall protection structures that are appropriate for the site conditions
depends on a combination of factors that include design impact energy, topography, slope
geometry, and the type of facility that is to be protected. This chapter discusses rational meth-
ods to select protection structures suitable for site conditions based on the relationship between
the return period for rock falls and their mass, and the application of risk management and
decision analysis to match the level of protection with the consequences of a rock fall.
Figure8.1 shows a rock fall that has broken through the roof of a reinforced concrete
shed; this impact energy has clearly exceeded the design capacity of the shed.
8.1 IMPACT ENERGY—DETERMINISTIC AND
PROBABILISTIC DESIGN VALUES
A primary design parameter required for rock fall protection structures is the impact energy
that the structure is required to withstand. The design energy is the total of the kinetic and
rotational energies, for which the components are the mass and shape of the rock fall, and
the translational and rotational velocities; the rotational energy is usually a small portion
(about 10% to 20%) of the total energy.
The design energy can be expressed using either deterministic or probabilistic methods,
depending on site conditions. The following two examples illustrate typical project condi-
tions and how they inuence the selection of design energies.
Deterministic design energy example, single hazard location—a 10 km length of highway,
with high trafc volumes and very low tolerance for trafc disruptions, is located in
a steep-sided canyon and has a single rock fall hazard at a tunnel portal. It has been
decided that a reinforced concrete rock shed, which will have a long design life and
require little maintenance, is appropriate to provide highly reliable protection against
rock falls. The structural designers for the shed require that the design energies be spe-
cic (deterministic) values that they can use to prepare a design that can withstand these
impacts. In accordance with structural design procedures, the design energy is expressed
as a service limit state and an ultimate limit state, as dened in Section 8.2 below.
Probabilistic design energy example, multiple hazard locations—a 25 km (15.5 miles)
length of railway with high trafc volumes located in the same canyon as the high-
way discussed in the previous paragraph, has 18 rock fall hazard locations. At each
of these hazard locations, mitigation measures have been implemented that include
ditches, slide detector fences (see Section 10.3), and a number of wire-mesh fences.
The mitigation measures provide a level of protection that is signicantly better than
no protection, but the railway accepts that an occasional, large rock fall will damage
116 Rock Fall Engineering
the protection structures and that repairs will be necessary. It has been decided that
this is an economical approach to rock fall protection because the cost is prohibitive
to provide highly reliable protection such as concrete sheds at all 18 locations. Under
these conditions, probabilistic methods, as described in Section 8.3 below, are used
to determine the design energy that the structure is designed to withstand without
damage, with the understanding that the occasional fall with energies greater than
the design energy will cause damage and service disruptions. The cost of both damage
and service disruptions would be considered in selecting the design energy appropriate
for the site.
Using probabilistic design methods, the design would provide protection for 96%,
for example, of all fall energies, but that damage to the structure will occur for 4%
of the fall energies. Section 8.4 describes how the relationship between return periods
and rock fall mass can be determined, with large falls occurring much less frequently
than small falls. Complete (100%) protection would require relocating the railway in a
tunnel, whereas protection for 96% of the falls can be achieved by installing rock fall
fences. The return period/rock fall mass relationship would show if the substantially
greater cost of driving a tunnel, compared to installing fences, is justied.
8.2 IMPACT ENERGY—SERVICE AND ULTIMATE STATES ENERGIES
The design of rock fall protection structures is analogous to the seismic design of bridges
and buildings where the impact energy and the level of ground shaking that the structures
must resist are uncertain. One method of addressing this uncertainty is to use Limit States
Design (LSD) methods in which the service limit state (SLS) and ultimate limit state (ULS)
loads are applied (Canadian Geotechnical Society, 2006).
Figure 8.1 Rock fall impact that exceeded ultimate design capacity of concrete shed.