Fundamentals of Engineering Design
where: W
=
weight of a unit width of bank;
=
failure plane angle in degrees;
c)
=
effective cohesion;
L
=
length of the failure plane;
N
=
normal force; and
M)
=
friction angle of the soil.
Often, just prior to failure, a tension crack will develop parallel to the stream bank and can be
observed from the top bank. A tension crack is a vertical separation of the soil resulting in a cavity or
crevice. Vertical tension cracks at the surface of a slope, possibly occurring along natural cleavage planes,
reduce the overall stability of a slope. The presence of tension cracks reduces the critical bank height. At
failure, tension cracks may quickly develop to depths greater than half the bank height. As a conservative
measure, Thorne and Abt (1989) recommend using a tension crack depth of half the bank height if no site-
specific data are available. Generally, varying a tension crack depth from 30 to 50 percent of the bank
height is a realistic range and does not change the factor of safety by more then 10%.
5.3.9.1 Required Geotechnical Data
Bank stability determination relies heavily on soil properties. Review of the principal equation
governing limited equilibrium analysis indicate that the shear strength of a soil, and subsequently the resisting
force, is based on the cohesion and the angle of internal friction of a soil. The driving force is based on the
weight of the soil, and is a function of the failure block geometry and the unit weight of the soil. The three
soil properties required for bank stability calculations in the DEC watersheds are cohesion, unit weight, and
internal friction angle. These three soil properties, bank height, and bank angle are the minimum
requirements for slope stability calculations. The methods currently employed to determine the stability of
stream banks within the DEC watersheds require a composite or average values of cohesion, unit weight,
and internal friction angle.
An important consideration in stream bank stability analysis is whether to employ a total or and
effective stress analysis. A total stress analysis using undrained shear strength parameters (c, ) is limited
to slopes where pore pressures are governed by external stress changes. These conditions are
characteristic of post-construction problems. A total stress analysis does not require a determination of
pore pressure in the bank and is an important advantage for a total stress analysis. An effective stress
analysis is warranted when pore pressures are governed by steady state seepage conditions, or if long-term
stability is a consideration. Steady state seepage is the usual condition for natural stream banks. Effective
stress parameters (c ,N )can be determined from either drained or undrained triaxial tests with pore
) )
pressure measurements. However, if the pore pressures within a stream bank are unknown or cannot be
determined, there is little point to an effective stress analysis, and a total stress analysis should be employed.
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