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Table 8.20: Riverward Deformation Results Comparison for the Earthquake Magnitude 8.5 Analyses.
METHOD OF
CALCULATION
RIVERWARD DISPLACEMENT: NORTH LEVEE CREST (cm)
Low river stage
Elev. NGVD 2.1 m
100 yr flood river stage
Elev. NGVD 8.8 m
Newmark 17 7 0
Makdisi and Seed 300 100 0
Bracketed Intensity 4 1 0
Results of the Parametric Study 40 30 1
Numerical Model 0 16 48
Levee crest river stage
Elev. NGVD 13.0 m
The direct comparison of the results of rigid-body, sliding block analyses with the results of the
numerical model is complicated by two important issues; (1) the former methods focus only on
the displacement of the slope along a single failure plane, and (2) benched slopes, like the
riverside slope of the levee, should be evaluated for three different modes of failure (shallow
crest, shallow bench, and deep-seated failure) when using limit equilibrium methods. In this
example, the displacement values calculated from the rigid body analyses are only for failure
wedges or circular slip planes associated with what was estimated to be a “critical” deep-seated
riverward failure (Figure 8.24). The location and shape of the potential deep slip surface was
established for the 100 year flood condition and this “critical circle” was used for the other two
river stages. In slopes of cohesionless soils the critical circles are usually associated with shallow
face and/or toe circles.
The shallow failure modes were excluded from consideration in this example, as they were
interpreted as indicating shallow sloughing. The displacements reported here did not take into
account movement by other potential failure masses within the levee. The results of the
Newmark and Bracketed Intensity methods show very little movement of the levee toward the
river. This is in contrast to the results of the Makdisi and Seed method, which is considered to be
very conservative for large magnitude earthquakes. Again, the critical failure surface indicated
by the slope stability analysis for deep-seated riverward failures was used as the basis for the
comparison.
20
Shallow Failure Surface
50
10
Elevation, NGVD (m)
0
-10
Deep Failure Surface
0
-50
Elevation, NGVD (ft)
-20
-30
-100
Figure 8.24: Schematic Illustration of Shallow and Deep-seated Failure Surfaces.
182