Environmental Engineering Reference
In-Depth Information
FIGURE 9.38
Panorama of the Portuguese Bend landslide looking south. The highway on the left is continually moving, and
the old abandoned road appears in the photo center. The broken ground on the right is the head scarp of
rotational slides in the frontal lobe of the unstable mass (photo taken in 1973).
Tank hill
0
500 1000 0 2000 ft
Horizontal and vertical scale
Alluvial fill
Peacock hill
1000
Shaly
rocks
800
Portuguese bend complex
600
Portuguese tuff
400
Basalt
200
Portuguese bend
F
0
E
FIGURE 9.39
Geologic section through the Portuguese Bend landslide. For location see
Figure 9.36
,
section along lines e-f.
(From Jahns, R.H. and Vonder Linden, C.,
Geology, Seismicity and Environmental Impact
, Special Publication
Association Engineering Geology, Los Angeles, 1973, pp. 123-138. With permission.)
during the rainy season, to peaks of 6 in./day during heavy rains. Rainfall penetrating
deeply into the mass through the many large tension cracks builds up considerable hydro-
static head to act as a driving force on the unstable blocks supported by material undoubt-
edly at residual strength. The maximum horizontal displacement between 1968 and 1970
was about 130 ft and the maximum vertical displacement about 40 ft. An interesting feature
of the slide is the gradual and continuous movement without the event of total collapse.
Stabilization:
Because of the large area involved and the geologic and other natural con-
ditions, there appears to be no practical method of arresting slide movements. The cracks
on the surface are too extensive to consider sealing to prevent rainwater infiltration, and
the strength of the tuff layer is now inadequate to restrain gravity movement even during
the dry season. A possible solution to provide stability might be to increase the shearing
resistance of the tuff by chemical injection. Since this would be extremely costly, it appears
prudent to leave the unstable area as open space although continuous maintenance of the
roadway in
Figure 9.38
has been necessary.
Glaciolacustrine Soils
Glaciolacustrine soils composing slopes above river valleys normally are heavily overcon-
solidated (see
Section 7.6.4).
Shear strengths, as measured in the laboratory, are often high,
with cohesion ranging from 1 to 4 tsf. Therefore, these soils would not usually be expected
to be slide-prone on moderately shallow slopes, and normal stability analysis would yield
an adequate factor of safety against sliding (Bjerrum, 1966). Sliding is common, however,
and often large in scale, even on shallow slopes.
