Civil Engineering Reference
In-Depth Information
Figure 3.3
Stereo pair aerial photographs of Hope Slide, British Columbia; slide had volume of 47 million m
3
of rock and buried a 3.2 km length of highway.
table, or the degree of fracturing, porosity and
saturation of the rock. Seismic velocities of a vari-
ety of rock types have also been correlated with
their rippability, which is a useful guideline in
the selection of rock excavation methods (Cater-
pillar, 2001). The seismic method is effective to
depths in the range of tens of meters to a max-
imum of a few hundred meters. Discontinuities
will not be detected by seismic methods unless
there is shear displacement and a distinct eleva-
tion change of a layer with a particular density as
a result of fault movement.
Seismic surveys measure the relative arrival
times, and thus the velocity of propagation, of
elastic waves traveling between a shallow energy
source and a number of transducers set out in a
straight line along the required profile. The energy
source may be a hammer blow, an explosion of
a propane-oxygen mixture in a heavy chamber
(gas-gun), or a light explosive charge. In elast-
ically homogeneous ground subject to a sudden
stress near its surface, three elastic pulses travel
outward at different speeds. Two are body waves
that are propagated as spherical fronts affected
to only a minor extent by the free surface of the
ground. The third wave is a surface wave (Raleigh
wave) that is confined to the region near the sur-
face; its amplitude falling off rapidly with depth.
The two body waves, namely the primary or “
P
”
wave and the secondary or “
S
” wave, differ in
both their direction of motion and speed. The
P
wave is a longitudinal compressive wave in
the direction of propagation, while the
S
wave
induces shear stresses in the medium. The velocit-
ies of the primary
(V
p
)
and secondary
(V
s
)
waves
are related to the elastic constants and density of
the medium by the equations:
(K
1
/
2
+
4
G/
3
)
γ
r
V
p
=
(3.1)
G
γ
r
1
/
2
V
s
=
(3.2)
