Environmental Engineering Reference
In-Depth Information
heavily jointed or steeply dipping stratified rock on steep slopes, and in saturated fine-
grained soils even on shallow slopes. Loess or other deposits of fine sands and silts, and
clays with seams and lenses of fine sand or silt are all highly susceptible.
Debris Slides and Avalanches
Shallow debris slides are probably the most common form of slope failure during earth-
quakes and can be extremely numerous in hilly or mountainous terrain. Very large mass
movements occur on high, steep slopes such as in the avalanche that buried most of the
cities of Yungay and Ranrahirca during the 1970 Peruvian event (see
Section 9.2.8).
An earthquake-induced debris avalanche in relatively strong materials occurred at
Hebgen Lake, Montana, during the August 17, 1959 event (
M
7.1). Approximately 43 mil-
lion yd
3
of rock and soil debris broke loose and slid down the mountainside, attaining
speeds estimated at 100 mi/h when it crossed the valley. Its momentum carried it 400 ft
up the opposite side of the valley, and the material remaining in the valley formed a nat-
ural dam and new lake.
Lateral Spreading
Common in lowlands along water bodies, lateral spreading results in considerable dam-
age, especially to bridges and pipelines. During the Alaska quake of 1964, 266 bridges
were severely damaged as a consequence of lateral spreading of floodplain deposits
toward stream channels. During the San Francisco event of 1906, every major pipeline
break occurred where fills overlay the soft bay muds (Youd, 1978).
The Turnagain Heights failure that occurred during the 1964 Alaska quake is described
in detail in
Section 9.2.6
(see
Figure 9.44).
Of interest is the previous earthquake history for
the area without the incidence of major sliding including
M
7.3 (1943) with an epicentral
distance of 60 km,
M
6.3 (1951) with an epicentral distance of 80 km, and
M
7 (1954)
with an epicentral distance of 100 km. The 1964 event had
M
8.3 with an epicentral dis-
tance 120 km, but a duration of about 3 min, which appears to have been the cause of
many large slope failures.
Flows
Flows can be enormous in extent under certain conditions. During the 1920 earthquake in
Kansu, China, formations of loess failed, burying entire cities. Apparently, the cause was
the development of high pore-air pressures. The flow debris, which extends for a distance
of 25 km down the valley of the Rio La Paz near La Paz, Bolivia, is considered to be the
result of an ancient earthquake (see
Section 9.2.11).
Offshore
Flows or “turbidity currents”
offshore can also reach tremendous proportions. An earth-
quake during November 1929 is considered to be the cause of the enormous “turbidity
current” off the coast of Newfoundland. It is speculated that a section of the Continental
Shelf broke loose from the Grand Banks, mixed with seawater and formed a flow that
moved downslope along the continental rise to the lower ocean floor for a distance of
about 600 mi. Its movements were plotted from the sequential breaking of a dozen marine
cables in about 13 h, which yielded an average velocity of 45 mi/h (Hodgson, 1964).
Large submarine flows
occurred during the Alaskan event of 1964, carrying away much of
the port facilities of Seward, Whittier, and Valdez. At Valdez, 75 million yd
3
of deltaic sed-
iments moved by lateral spreading, resulting in displacements in the city behind the port
as large as 20 ft (Youd, 1978).
