Civil Engineering Reference
In-Depth Information
(a)
Weight impact force
absorb energy by compression, and disperse the
point impact energy so that the energy trans-
mitted into the structure occurs over a large
area. Furthermore, the cushion should remain
intact after impact so it does not need to be
replaced. The effectiveness of the material can be
expressed as the difference between the “weight
impact force” induced by boulder impact, and
the “transmitted force” that is absorbed by the
structure (Figure 12.26(a)). Gravel is the most
commonly used cushioning material because it is
inexpensive and widely available. However, the
disadvantage of gravel is its weight, and there
is a point where the gravel layer is so thick
that its weight exceeds the rock fall impact load-
ing. Rubber tires have also been used, but it
is found that they are highly compressible with
little energy absorption. A viable alternative to
gravel is reinforced Styrofoam that is an effective
energy absorbing material with low unit weight,
which allows for some saving in the dimen-
sions of the structure (Mamaghani
et al
., 1999).
The disadvantage of Styrofoam is its cost com-
pared to gravel, so the cost benefit of its use
should be carefully evaluated. Figure 12.26(b)
shows typical force-deformation characteristics
of gravel, Styrofoam and rubber tires (Yoshida,
2000).
In locations at which it is impractical to con-
struct a rock shed or stabilize the slope by other
means, it may be necessary to drive a tunnel to
bypass the hazard zone. For example, a railway
in British Columbia drove a 1200 m long tunnel
to avoid a section of track located on a nar-
row bench between a steep, unstable rock cliff
above a 400 m deep lake. Major rock falls were a
hazard to train operations and had caused track
closures lasting as long as two weeks (Leighton,
1990).
Rock
mass
Cushion
material
Rock shed roof
Transmitted force (integration
of the transmitted pressure
on distributed area)
(b)
Sand
Styrofoam
Rubber tires
Deformation
Figure 12.26
Characteristics of cushioning materials:
(a) definition of weight impact force and transmitted
impact force due to rock fall impact on cushion;
(b) relationship between force and deformation for
impact loading of gravel, styrofoam and rubber tires
(Yoshida
et al
., 1991).
various cushioning materials in absorbing and
dispersing the impact energy, and to assess the
influence of the flexibility of the structure on the
maximum impact that can be sustained without
damage.
A critical feature of shed design is the weight
and energy absorption characteristics of the cush-
ioning material. Ideally the cushion should both
