Environmental Engineering Reference
In-Depth Information
Aggregated
clay grains
Silt grains
Sand
Sand
Sand
Sand
Meniscus
Sand
Water
Sand
Sand
Sand
(a)
(b)
(c)
Clay
buttress
Clay or
silt
Clay bridges
Sand
Sand
Sand
Sand
Clay
lump:
(d)
(e)
(f)
FIGURE 10.22
Typical collapsible soil structures: (a) capillary tension; (b) silt bond; (c) aggregated clay bond; (d) flocculated clay
bond; (e) mudflow type of separation; (f) clay bridge structure. (From Clemence, S.P. and Finbarr, A.O.,
Proceedings
of ASCE
, Preprint 80-116, 1980, 22 pp. Adapted from Barden, L. et al.,
Engineering Geology
, 1973, pp. 49-60.)
Valley Alluvium: Semiarid to Arid Climate
The origin and some characteristics of valley alluvium susceptible to collapse are
described in
Section 7.4.1.
In arid climates, occasional heavy rains carry fine soils and sol-
uble salts to the valley floor to form temporary lakes. The water evaporates rapidly leav-
ing a loosely structured, lightly cemented deposit susceptible to subsidence and erosion.
Often characteristic of these soils are numerous steep-sided gulleys as shown in the
oblique aerial photo (
Figure 10.23)
taken near Tucson, Arizona. Tucson suffers from the
collapsing soil problem, and damage to structures has been reported (Sultan, 1969).
Studies of subsidence from ground collapse
were undertaken by the U.S. Bureau of
Reclamation to evaluate problems and solutions for the construction of the California
aqueduct in the San Joaquin Valley of California (Curtin, 1973). Test sites were selected
after extensive ground and aerial surveys of the 2000 mi
2
study area. It is interesting to
note that the elevation of the entire valley had undergone
regional subsidence
and had been
lowered by as much as 30 ft by groundwater withdrawal since the early 1920s.
USBR test procedures to investigate subsidence potential involved either inundating the
ground surface by ponding or filling bottomless tanks with water as shown in
Figure 10.24.
One of the large ponds overlays 250 ft of collapsible soils. Water was applied to the pond
for 484 days, during which an average settlement of 11.5 ft occurred. Benchmarks had been
set at the surface and at 25 ft intervals to a depth of 150 ft. A plot of the subsidence and com-
paction between benchmarks as a function of time is given in
Figure 10.25.
It is seen that
the effects of the test influenced the soils to a depth of at least 150 ft. The subsidence appears
to be the summation of soil collapse plus compression from increasing overburden pressure
due to saturation. The test curves show an almost immediate subsidence of 1 ft upon satu-
ration within the upper 25 ft; thereafter, the shape of the curves follows the curve expected
from normal consolidation.
San Joaquin Valley soils
are described by Curtin (1973). They have a texture similar to that
of loess, characterized by voids between grains held in place by clay bonds, with bubble
cavities formed by entrapped air, interlaminar openings in thinly laminated sediments,
and unfilled polygonal cracks and voids left by the disintegration of entrapped vegetation.
The classification ranges from a poorly graded silty sand to a clay, in general with more
