Environmental Engineering Reference
In-Depth Information
During initial stages flows may begin by slump failure followed by lateral spreading in
the case of sensitive clays. During final failure, a tongue-shaped lobe of low profile extends
back to a bottleneck-shaped source area with a small opening at the toe of the flow as
shown in
Figure 9.4f,
and a distinct scarp remains at the head. Flowing masses can extend
for great distances, at times measured in kilometers.
Examples of Major Failures
Achocallo Mudflow, La Paz, Bolivia
Believed triggered by an earthquake some thousands of years ago, an enormous portion
of the rim of the Bolivian altiplano (elevation 4000 m), roughly 9 km across, slipped loose
and flowed down the valley of the Rio Achocallo into the Rio La Paz at an elevation
approximately 1500 m lower. The flow remnants extend downstream today for a distance
of about 25 km, part of which are shown on the aerial oblique panorama given in
Figure
9.55.
The head scarp is shown on the photo
(Figure 9.56).
The altiplano is the remains of an ancient lake bed, probably an extension of Lake
Titicaca, underlain by a thick stratum of sand and gravel beneath which are at least sev-
eral hundred meters of lacustrine clays and silts, interbedded with clays of volcanic ori-
gin. The photo (see
Figure 10.36)
of a large piping tunnel was taken in the bowl-shaped
valley about 3 km downslope from the rim.
Province of Quebec, Canada
Event
: In Saint Jean Vianney, on May 4, 1971, a mass of glaciomarine clays completely liq-
uefied, destroying numerous homes and taking 31 lives (Tavenas et al., 1971).
Description
: The flow began in the crater of a much larger 500-year-old failure (deter-
mined by carbon-14 dating). Soil stratigraphy consisted of about 100 ft of disturbed clays
with sand pockets from the ancient slide debris, overlying a deep layer of undisturbed
glaciomarine clay. Occurring just after the first heavy rains following the spring thaw, the
flow apparently began as a series of slumps from the bank of a small creek, which formed
a temporary dam. Pressure built up behind the dam, causing it to fail, and 9 million yd
3
of
completely liquefied soils flowed downstream with a wavefront 60 ft in height and a
velocity estimated at 16 m/h. The flow finally discharged into the valley of the Saguenay
River, 2 mi from its source.
Causes:
Slope failures in the marine clays of Quebec are concentrated in areas that seem
to be associated with a groundwater flow regime resulting from the existence of valleys in
the underlying rock surface (Tavenas et al., 1971). The valleys cause an upward flow gra-
dient and an artesian pressure at the slope toes. The upper part of the soil profile is sub-
jected to a downward percolation of surface water because of the existence of sand strata.
The downward percolation and upward flow produce an intense leaching of the clay,
resulting in a decrease of the undrained shear strength and an increase in sensitivity (see
Section 7.6.5).
The evidence tends to indicate that the leaching is a function of the gradi-
ent. Field studies have shown a close relationship between the configuration of the
bedrock and the properties of the underlying clay deposit.
Norwegian “Quick” Clays
Regional geology:
Approximately 40,000 km
2
of Norway has deposits of glaciomarine clays
which overlie an irregular surface of granite gneiss, similar to conditions in Quebec.
During postglacial times, the area has been uplifted to place the present surface about
180 m above sea level. Typical stratigraphy includes 5 to 7 m of a stiff, fissured clay over-
lying normally consolidated soft marine clay which extends to depths greater than 70 m in