Environmental Engineering Reference
In-Depth Information
The flocs settle out quickly, leaving the liquid clear, to form a very weak and compressible
structure.
Postdeposition
When the formation is lifted above sea level by isostasy, freshwater may leach salt from
the deposit, producing a clay of high sensitivity (see
Table 3.18,
quick clays). The sensitiv-
ity increases with time as groundwater continues to leach the salt, slowly weakening the
deposit until it can no longer retain its natural slope. A failure results, often in the form of
a flow. Leaching occurs both from the downward percolation of rainfall and the lateral
movement of groundwater. The upward percolation is caused by artesian pressures in
fractured rock underlying the marine clays, a common condition. Bjerrum et al. (1969)
found that the sensitivity of Norwegian marine clays, directly related to the amount of
leaching and the salt content, is greater where rock is relatively shallow, about 50 to 100 ft
(15-35 m) than where it is deeper.
Engineering Characteristics
As described above, marine clays often become extremely sensitive (see
Section 5.3.3)
and
are known as “quick clays.” Land areas can be highly susceptible to vibrations and other
disturbances, and even on shallow slopes they readily become fluid and flow (see
Section
9.2.11).
Norwegian Marine Glacial Clays
Logs of boring and laboratory test results from two locations are given in
Figure 7.101
and
Figure 7.102.
At Manglerud, a crust has formed and sensitivities (St) are about 500; the nat-
ural water content is high above the liquid limit. At Drammen, where there is no crust,
sensitivities are much lower (St
7) and the natural water contents are in the range of the
liquid limit. The difference is caused by the uplifting and leaching of the Manglerud clays
(Bjerrum, 1954).
Canadian Leda Clays
The Leda clays of the St. Lawrence and Champlain lowlands are essentially nonswelling,
but below a typical crust can have void ratios as high as 2.0 (Crawford and Eden, 1969).
Some index and strength properties are given in
Figure 7.103.
As with the Norwegian
clays, the natural water content is typically above the liquid limit although the values for
LL, PL, and
w
are higher for the Canadian clays. Sensitivity is high, ranging from St = 34
to 150 below the crust, but lower than the clays at Manglerud. It is seen from Figure 7.103
that the OCR is about 2.0, which is less than for most varved clay formations.
Boston Blue Clays
The Boston blue clays, so named because of their characteristic color, are a glacio-marine
deposit that has undergone uplift, submergence, and reuplift (Lambe and Horn, 1965). As
shown in
Figure 7.104,
from the shear strength data, their sensitivity is relatively
low compared with other glacio-marine clays. They are substantially overconsolidated
and the zone of prestress extends far below the surface of the clay. Moisture contents are
generally below the liquid limit. Many large structures are supported on mat foundations
bearing almost directly on the clay (DiSimone and Gould, 1972). In one of the cases
cited, total settlements were about 0.14 ft (43 mm) and were essentially complete within
7 years.
