Environmental Engineering Reference
In-Depth Information
FIGURE 9.37
Stereo-pair of aerial photos of the Portuguese Bend area of the Palos Verdes Hills (January 14, 1973, scale 1:24,000).
Slide history: The area has been identified as one with ancient slide activity (see Figure
9.36). Using radiometric techniques, colluvium older than 250,000 years has been dated,
and intermediate activity dated at 95,000 years ago. In recent times some block movement
was noted in 1929, but during the 1950s, when housing development began on the pres-
ent slide surface, the slide was considered as inactive. Significant modern movement
began in 1956, apparently triggered by loading the headward area of the slide with con-
struction fill for a roadway.
Slide movements: The mass began moving initially during 1956 and 1957, at rates of 2 to 5
in./year, continuing at rates varying from 6 to 24 in./year during 1958, then 3 to 10 ft/year
during 1961 to 1968. After 1968, a significant increase in movement occurred. Eventually,
120 houses were destroyed over a 300 acre area. Studies have correlated acceleration in rate
of advance with earthquake activity, abnormally high tides, and rainfall (Easton, 1973). The
average movement in 1973 ranged from about 3 in./day during the dry season, to 4 in./day
 
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