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Appendix D - Slope Stability Analysis for poFJRt,-2001-1 t
Environmental Cap Rev. o Draft ts
I To remain within acceptable displacements, a slope of 2% was selected for the engineered
2 barrier. The weakest portion of the engineered barrier would be the two upper silt layers. At a
3 2% slope, the yield acceleration for the engineered barrier is approximately 0.8 g. The average
4 design acceleration for the critical displacement plane is approximately 1.3 g. There is a higher
5 acceleration for the barrier than for the lower portions of the environmental cap because the
6 barrier is at the crest, which has a higher seismic amplification factor. The resulting deformation
7 for the 2% barrier slope is approximately 2.5 cm ( 1 in.). The deformation during a seismic event
8 is most likely to occur along a shallow sliding surface within the silt that is parallel to the
9 finished 2% slope of the engineered barrier. The sliding surface is unlikely to extend completely
10 through the uppermost silt layers. The static factor of safety is well above 1.5.
11
12 A slope of 3H:IV was selected for the portion of the environmental cap below the engineered
13 barrier. The average yield acceleration for the rock-armored embankment sloped at 3H:1 V, as
14 shown in Figure D-2, is 0.4 S. The average design acceleration along the critical displacement
15 plane is approximately 0.8 g. Therefore, the predicted deformation of the 3H:1 V-sfopc is about
16 2.5 to 5.0 em (t to 2 in.). The static factor of safety for the 311:1 V embankment is well above
17 1.5. A 2H:1 V slope would have an average yield acceleration of 0.3 g and only experience a few
18 more inches of deformation. The more conservative 3H:1 V slope was selected to limit
19 movement, decrease long-term erosion concerns, and ensure that construction could be
20 performed adequately and safely.
21
22 The head scarp for the critical failure surface for the entire embankment is approximately 6 in
23 (20 ft) back from the top of the 31-1:1V slope, as shown in Figure D-2. The deformation at the
24 head scarp is predicted to be nearly vertical. Although the deformation at the head scarp may not
25 result in an open crack, it was conservatively determined that the engineered barrier should not
26 extend across the potential head scarp. Therefore, the embankment should extend at least 6 in
27 (20 ft) horizontally past the required limits of the engineered barrier before being sloped at
28 3H:1 V.
29
30 A summary of the stability analysis results for the recommended configurations for Alternative 3
31 is provided in Table D-3. Results for Alternative 3 are considered to be applicable to the
32 environmental cap for Alternative 6: Close in Place - Collapsed Structure, which is also
33 addressed in this final feasibility study. Alternative 6 involves disposal of waste in the lower
34 part of the concrete structure, demolition of the upper part of the structure, and containment with
35 an environmental cap that is lower in height and has an overall footprint that i s smaller than the
36 environmental cap for Alternative 3. Therefore, slope stability analyses for Alternative 3 are
37 considered to be useful in developing the Alternative 6 environmental cap design and layout.
38
39 D.4.2 Results for Alternative 4
40
41 The layout and engineering basis for the environmental cap for Alternative 4 is identical to
42 Alternative 3. However, unlike Alternative 3, Alternative 4 includes a bottom liner and waste fill
(^ 43 around the exterior perimeter of the 221-U Facility. The external bottom liner and small amount
44 of exterior waste fill would not adversely impact the slope or stability of the finished
Final Featibiliy Study jor the Canyon Disporition Initiative (221-U Faeiliry)
une 2001 D-7