Environmental Engineering Reference
In-Depth Information
Glacial till
Original surface
Overconsolidated
plastic clay locally
varved, fissured
Excavation
Slip plane
W =30%
W
L
=55%
W
p
=23%
FIGURE 9.41
Failure surface in overconsolidated, fissured clays, undergoing progressive failure as determined by slope
inclinometer measurements along the Seattle Freeway. (From Bjerrum, L.,
Terzaghi Lectures 1963-1972, ASCE
[1974], 1966, pp. 139-189. With permission.)
Level from
piezometer
in till
April 15, 1971
Mottled brown
silty clay
300
−
stiff
Elevation of
gully seepage
Silty fine sand,
loose to compact
High-water level
April 15, 1971
South nation
river
200
c
Gray sensitive silty clay (sand lenses)
soft
−
FIGURE 9.42
Stratigraphy prior to the South Nation
River slide. An overconsolidated crust
over soft, sensitive clays. (From Eden,
W.J. et al.,
Can. Geotech. J.,
8, 1971.
Reprinted with permission of the
National Research Council of Canada.)
Silt and sand till with boulders
100
Limestone
0
0
100
200
300
400
ft
Geology:
Glaciomarine soils in general are described in
Section 7.6.5.
The stratigraphy at
the South Nation River slide prior to failure consisted of 6 to 23 ft of stratified silty fine
sands overlying the Champlain clay (Leda clay) as shown in Figure 9.42. The undrained
strength of the clay was about 0.5 tsf, its sensitivity ranged from 10 to 100, the average
plastic limit was 30%, and liquid limit 70%, and the natural water content was at the liq-
uid limit.
Slide history:
An all-time record snowfall of 170 in. (432 cm) occurred during the
1970-1971 winter and gradual melting resulted in saturation of the upper clays. The slide
occurred at the end of the snow-melting season during a heavy rainstorm. A contributing
factor was the river level at the slide toe area. It had risen as much as 30 ft during spring
floods, remained at that level for a week, then dropped back rapidly to preflood levels. At
the time of the slide, groundwater at the lower part of the slope was observed to be nearly
coincident with the surface. From the appearance of the ground after failure, it appears
that the slide retrogressed as a series of slumps as shown in
Figure 9.43.
Glaciomarine Soils: Turnagain Heights Slide
Event
: Much of the damage to the Anchorage, Alaska, area from the March 1964 earth-
quake was caused by landslides induced by seismic forces. The slides occurred in the city
in the Ship Creek area and along the waterfront formed by the Knik Arm of Cook Inlet.
The largest slide occurred at Turnagain Heights, a bluff some 60 ft high overlooking Knik
Arm. Many homes were destroyed in the slide area of 125 acres as illustrated in
Figure
9.44.
The slide at Turnagain Heights is an example of sliding along horizontal strata. It was
