Fundamentals of Engineering Design
Table 5.10 Soil Properties and the Averaging Method
The natural bank height is 18.3 feet
Cohesion c )
Soil
Depth
Friction Angle
Unit Weight
M)
Description
(ft)
(lbs/sf)
(lbs/cf)
(degrees)
Silty clay
0 - 4.5
14
300
115
Clayey sand
4.5 - 7.5
17
700
140
Clay
7.5 - 15.0
15
550
130
Stiff clay
15.0 - 26.0
17
1,100
145
Clay
---
---
---
---
Cohesion c )
% of
Friction Angle
Unit Weight
M)
Bank Height
(lbs/sf)
(lbs/cf)
(degrees)
19.3
2.70
57.94
22.21
12.9
2.18
90.13
18.03
32.2
4.82
177.04
41.85
35.6
6.05
391.85
51.65
Total
100
15.78
716.95
133.73
5.3.9.4 Stability of Mild Slopes
The majority of the streams within the Yazoo River Drainage basin have mild stream bank slopes.
Mild slopes are less then 60 degrees to the horizontal. To determine the stability of these mild slopes the
DECBank computer program was developed.
The DECBank computer program reads and interprets HEC-2 input data files to determine the
required bank heights and angles for stability calculations. Users of DECBank have the ability to visually
inspect each cross-section prior to stability calculations to ensure that they agree with the determined bank
angles. If the user does not agree with the computer determined bank angle, they have the ability to alter
the bank angles and visually inspect the new bank angle of the natural cross section.
Mild slope stability (bank angles between 30 and 60 degrees) is determined by use of numerical
representations of Osman's (1985) stability curves. Osman's (1985) stability curves were developed
using numerous stability computations based upon the simplified Bishop method of slices. Singular or
average values of internal friction angle, cohesion, and unit weight are used in conjunction with the equations
representing Osman's (1985) stability curves to determine the stability of both banks of a cross section.
Osman's (1985) stability curves are dimensionless, so the DECBank computer program is applicable for
both English and Metric units. After determining the stability of all appropriate cross sections DECBank
automatically determines the average factor of safety for the entire river reach.
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