Rock Fall Protection I—
Barriers, Nets, and Fences
This chapter describes a variety of commonly used rock fall protection structures, ranging
from inexpensive ditches to high-energy-capacity wire-rope fences. Design principles for
reinforced concrete rock fall sheds and wire-rope canopies are discussed in Chapter 11.
The selection of a protection structure that is suitable for a particular site will depend on
the following three conditions:
a. Impact kinetic energy—The mass, m, and velocity, V, of the rock that may impact the
structure denes the impact kinetic energy (KE = ½ m · V
). The mass of the rock fall
depends on the discontinuity spacing and the dimensions of the blocks that can be
formed. The rock strength also inuences the impact energy because blocks of weak
rock tend to break into smaller fragments as they impact the slope, while blocks of
strong rock retain their mass during the fall. The velocity of the fall depends on the
fall height, the slope angle, and the effective friction coefcient of the slope surface
as dened by Equation (3.13). The mass and velocity data are then combined to select
the service and ultimate limit states design energies (see Section 8.2).
If appropriate, the rotational energy (RE = ½ I · ω
) can be included in the total
impact energy. For example, Worked Examples 6B and 6C discuss the energy changes
at impact point #A26 at the Tornado Mountain rock fall site where the rotational
energy is about 18% of the total impact energy.
b. Site geometry—The main slope geometry parameters relevant to the design of protec-
tion structures are the fall height and the slope angle. The site geometry also dictates
the trajectory path and the required height of the structure. The distance between the
base of the slope and the facility to be protected dictates the space available to con-
struct protection measures. It is usually preferable to locate the protection structure
such as a barrier or fence at the same level as the road or railway, for example, since
this facilitates both construction and maintenance. Where the available space at the
base of the slope is limited or cannot be created by excavating the base of the slope, it
will be necessary that the protection structure, such as an anchored fence, be located
on the slope.
One factor to consider in the fence location is the deection that will occur during
impact, with sufcient clearance being provided that the house, for example, is not
struck when the fence is deformed by the impact.
c. Cost-benets—The construction cost of a protection structure should be consistent
with the expected cost of a rock fall damaging the facility below the slope. As dis-
cussed in Section 8.5.5 on decision analysis, protection options can be compared in
terms of their total expected cost, which is the sum of the construction cost of the pro-
tection structure, and the expected cost of a rock fall that may include delays to trafc,
damage to equipment and possible injuries, and required slope stabilization. Expected
162 Rock Fall Engineering
costs are the product of the cost of an event and the probability of its occurrence. This
probabilistic approach accounts for the uncertainty in the size and frequency of rock
fall events and their consequences. In summary, a costly protection structure such as
a reinforced concrete shed is usually only justied where large falls are possible and
the consequences are severe, such as disruption to a high-speed railway or high-trafc-
Figure 10.1 shows a variety of protection structures and ranges of their impact energy
capacity. With respect to their approximate construction cost, ditches are usually the least
expensive and concrete rock shed the most expensive.
The protection measures described in this chapter are divided into two categories as follows:
• Ditches and barriers—These structures, which are usually constructed at the base of
the slope where space permits, are often a reliable, low-cost, and low-maintenance
• Net and fences—These structures, which can be constructed either on the slope or at
the base of the slope, require reliable information on impact energies and trajectories
in order to prepare dependable designs.
Diagonal and ring wire nets
Retained ﬂexible barriers
Figure 10.1 Ranges of impact energy capacity for a variety of protection structures. (After Vogel, T. et al.
. Rock fall protection as an integral task. Structural Engineering International, SEI Vol. 19,
304–12, IABSE, Zurich, Switzerland, www.iabse.org.)