Environmental Engineering Reference
In-Depth Information
FIGURE 9.47
Scarred surface remaining after a rock and debris avalanche in a limestone quarry. The rock is heavily jointed
with sets oriented more or less parallel to the slope and across the bedding plane. Failure was induced by
wedging from water and ice pressures, and occurred in the early spring.
Climatic conditions
: Hundreds of avalanches and slides occurred in Rio de Janeiro and
the nearby mountains during the unusually heavy rains of 1966 and 1967 when intensities
as high as 200 mm/h (8 in./h) were recorded (Jones, 1973).
During a 3-day storm beginning on January 10, 1966, a gaging station at Alto da Boa
Vista, in the mountains a few kilometers from the city, recorded 675 mm (26.2 in.) of rain-
fall. It was an unprecedented amount. Although heavy rains occur each year during the
summer months of January and February, with rainfall averaging 171 mm (6.7 in.) during
January, most of the rain falls during intense storms. The potential for slope failures is very
much dependent upon the accumulated rainfall and associated water-table conditions for
a given rainy season (see
Section 9.3.4)
.
Local geology
: Typical profiles in the residual soils (see
Figure 6.82
and
Figure 9.69)
along
the coastal mountains of Brazil show that these soils are most impervious near the surface
and that permeability increases downward through the soil profile into the underlying
decomposed and fractured crystalline rocks. Fissures in the outer portions of the residual
and colluvial soils close during rainfall; they thereby block drainage and cause a rapid
increase in pore pressures, resulting in sudden failure, which combined with high runoff
develops into an avalanche or even a flow. To minimize the slope-failure hazard, the city
of Rio has zoned some areas of high steep slopes to prohibit construction, and has under-
taken the construction of numerous stabilization works throughout the city.
