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Page Title: GEOTECHNICAL ENGINEERING ANALYSIS (cont.)
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ERDC TN-DOER-N5
July 2000
the particles reorient or collapse into a denser state. The clumps of dredged material in a matrix of
soft slurry from clamshell dredging could conceptually be in such a meta-stable condition, but too
little is known of the actual deposition conditions of a dredged material deposit for this to be any
more than a hypothesis. Even if such a collapsible structure were built into these man-made deposits,
knowledge of the collapse behavior or quantitative methods of analyzing the magnitude of the
potential collapse does not exist.
Both bearing capacity and slope failures are conventional geotechnical limit equilibrium problems
and are routinely analyzed in practice. In such problems, the factor of safety against failure is
calculated by dividing the available resistance due to the soil shear strength by the sum of the shear
stresses along a predefined slip or failure plane. These shear forces may develop from surface
loading, as is typically encountered in foundation problems, or they may be caused by gravity loads
reflecting the specific geometry of a slope as well as any superimposed surface loads. At a factor
of safety of 1.0, the stresses just equal the available soil strength, and the loading condition is
nominally stable along the analyzed plane for any factor of safety of 1.0 or greater. In practice,
bearing capacity factors of safety are typically 3 to 5, and slope stability factors of safety are often
1.3 or higher. Specific magnitudes of factors of safety used in design reflect the uncertainty
concerning soil or loading conditions or the seriousness of the failure occurring. Bearing capacity
failures in a soil mass develop when applied surface loads exceed the available soil strength. Failure
is a function of the magnitude of the load, size of the loaded area, and soil strength. Analysis of
slope stability usually requires analyzing a variety of trial surfaces and identifying the failure surface
with the lowest factor of safety. The results of slope stability calculations are heavily dependent on
the available soil strength, density of the soil, location of phreatic surface, and slope geometry (e.g.,
height, length, and angle of slope).
As stated earlier, the most critical time for shear displacements is relatively soon after deposition.
At this time, excess pore water pressures have not drained, so the analyses should use undrained
conditions. This is also called the = 0 condition. For slope stability problems, the most critical
geometries or worst-case conditions are steep slopes combined with thick dredged material deposits.
It is very obvious that the actual slopes attained in most dredged material deposits are quite small
when drawn to scale because of the low shear strength of the material; in fact, many mounds of
dredged material resemble pancakes on a griddle.
Other more unusual behaviors have been postulated concerning the behavior of soft dredged material
below a sand cap. These include:
A relatively rigid sand cap floating on fluid-like dredged material, then tipping to allow the
dredged material with excess pore pressure to escape to the surface of the cap.
A relatively rigid sand cap through which the pressurized dredged material erupts much as a
volcano erupts from the surface of the earth.
Multiple localized bearing capacity failures that allow columns of sand to penetrate the
dredged material until an equilibrium condition is reached in which the sand bridges the area
between the sand columns (this would be somewhat similar to the concept of stone columns
used to provide foundation support over very soft ground). The validity of such unusual
behavior cannot be assessed without further laboratory and field investigations. Whatever
9

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