GEOTECHNICAL ENGINEERING ANALYSIS (cont.)

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