Environmental Engineering Reference
In-Depth Information
Midwest Tills
Care is required in the Midwest so that heavily loaded foundations are not placed over the
relatively compressible peat and gumbotil layers, although in general the till itself is a
strong material. An investigation for a high rise in Toledo, Ohio, encountered “hardpan”
at a depth of 70 ft described as a “hard, silty clay, mixed with varying percentages of sand
and gravel.” SPT values were erratic, partially because of the gravel particles, ranging
from 12 to 100. On the basis of these data an allowable bearing value of 6 tsf for caissons
bearing in the till was selected. Subsequent tests with the Menard pressuremeter (see
Section 3.5.4) yielded compression modules values ranging from 570 to 880 tsf, which per-
mitted the assignment of an allowable bearing value of 12 tsf at a penetration depth into
the till of 8 ft (Ward, 1972).
7.6.3
Glacial-Fluvial Stratified Drift
Origin
During warm periods numerous streams flow from the glacier, which are literally choked
with sediments. The streams are usually braided and shallow, and because of the excep-
tionally heavy loads being carried, large thicknesses of soils can be deposited in a rela-
tively short time. Some of the streams terminate in moraine-formed lakes where they
deposit lacustrine soils; other streams flow to the sea. A general section illustrating the
relationship between a recessional moraine and fluvial and lacustrine deposits is given in
Figure 7.84.
Classes of Stratified Drift
Proglacial deposits form beyond the limits of the glacier and include stream, lake, and
marine deposits.
Ice-contact stratified drift consists of deposits built in immediate contact with the glacier
and includes only fluvial formations.
Modes of Glacial-Fluvial Deposition (Figure 7 .77)
Outwash , the streambed-load materials, consisting of sands and silts, is highly stratified
(Figure 7.85).
Outwash plain is formed from the bed load of several coalescing streams and can blan-
ket large areas. A portion of the terminal moraine and the outwash plain along the south
shore of Long Island, New York, is shown in the topographic map ( Figure 7.86).
Kettles occur where the outwash is deposited over blocks of ice that subsequently melt
and the surface subsides (Figure 7.75). Formerly lakes, these depressions are now com-
Glacial advance
Lacustrine
plain
Recessional moraine
Recessional or
terminal moraine
Kame
Outwash
Varved clays
and silts
Rock
FIGURE 7.84
Geologic section showing relationship
between recessional moraine and fluvial
and lacustrine deposits.
Ground moraine
 
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