Environmental Engineering Reference
In-Depth Information
0
20
40 feet
Badly disturbed zone
Badly disturbed zone
70
60
7
60
50
40
5
40
30
20
3
20
Badly disturbed samples
70
0
10
0
−
10
−
−
10
20
−
20
0
0.50 100
0 0.50 100
0
0.50 100
0
0.50 100
Shear strength (tsf)
Shear strength (tsf)
Shear strength (tsf)
(b)
0 20 4060 feet
Silt
Clay
60
60
Sand and gravel
40
40
20
20
0
0
−
20
−
20
(a)
FIGURE 9.45
Soil profiles through east end of slide area at Turnagain Heights (a) before and (b) after failure. (From Seed,
H.B. and Wilson, S.D.,
Proceedings of ASCE, J. Soil Mechanics Foundation Engineering Division,
Vol. 93, 1967, pp.
325-353. With permission.)
in lateral spreading of the mass which came to rest with a profile more or less as illustrated
in Figure 9.45b. The movement continued for the duration of the earthquake (more than 3
min), but essentially stopped once strong ground motion ceased.
Conclusions:
The magnitude of the 1964 event was 8.5 (Richter) with an epicenter 80 mi
east of Anchorage. Previous earthquakes of slightly lower magnitudes but closer epicen-
ters had occurred, but the Turnagain Heights area had not been affected (see
Section
11.3.4).
Seed and Wilson (1967) concluded that, in light of previous earthquake history, the
slide was the result of a continuous increase in pore pressures caused by the long duration
of the 1964 event; and, that it is extremely unlikely that any analysis would have antici-
pated the extent of inland transgression of the failure. Considering the local stratigraphy
and seismic activity, however, the area certainly should be considered as one with a high
slope failure hazard.
9.2.7
Debris Slides
General
Debris slides involve a mass of soil, or soil and rock fragments, moving as a unit or a num-
ber of units along a steeply dipping planar surface. They often occur progressively and can
develop into avalanches or flows. Major causes are increased seepage forces and slope
inclination, and the incidence is increased substantially by stripping vegetation. Very large
masses can be involved, with gradually developing progressive movements, but at times
total failure of a single block can occur suddenly.
Recognition
Occurrence is common in colluvial or residual soils overlying a relatively shallow, dipping
rock surface. During the initial stages of development, tension cracks are commonly
formed. After partial failure, the tension cracks widen and the complete dislodgment of
one or more blocks may occur, often leaving a clean rock surface and an elliptical failure
scar as shown in
Figure 9.46.
Total failure can be said to have occurred when the failure
