Environmental Engineering Reference
In-Depth Information
formation of hard consistency 300 ft or more above river level, yielding SPT values of 50
blows per foot or more.
The varved clays along the lower side slopes of the Hudson Valley (
Figure 7.94)
suffer
many slope failures. At the campus of Rensselaer Polytechnic Institute in Troy, New York,
240 ft above sea level, soils exposed on slopes have progressed to form stiff fissured clays
yielding SPT values from 20 to 50 blows per foot. They are subject to instability and slid-
ing, particularly during spring thaws and rains. Formations in depressions above river
level, such as existing in Albany and Troy, both high above the Hudson River, or along the
lower valley slopes where the materials tend to remain saturated, have much less pre-
stress, as shown in
Figure 7.96b.
Engineering Characteristics
General:
Because varved clays underlie many large urban areas, their unusual engineering
characteristics are very important. Two factors are most significant: the amount of pre-
stress that has occurred in a given formation and the effect that the sand lenses and silt
varves have an internal drainage. The sand and silt permit consolidation of the clay varves
to occur much more rapidly than in a normal clay deposit. Both factors may vary sub-
stantially from location to location.
Where desiccation has occurred, a stiff crust is formed and the deposit varies from being
heavily preconsolidated near the surface, decreasing to moderately preconsolidated with
depth, and eventually becoming normally consolidated.
FIGURE 7.94
Excavated face in varved clays exposed in slope (Roseton, New York). Location is about 10 m above level of
the Hudson River.
