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Appendix D - Slope Stability Analysis for poEIRL-2001-11
I Environmental Cap Rev. 0 1 Draft a
^ Redline/Slrikeout
I a return period of approximately 2.000 years. The design earthquake magnitude is estimated at
2 6.5, based on site-specific deaggregation information from the U.S. Geological Survey seismic
3 hazards web site (httn://@eohazards.cr.usgs. fov/eq) .
4
5 D.2.7 Acceptable Factors of Safety
6
7 For long-term static slope stability, the desired factor of safety is 1.5 or higher. For short-term
8 construction loading when no waste is in place, the desired minimum static factor of safety is
9 1.3. These minimum factors of safety are consistent with standard geotechnical engineering
10 practice. The performance goal under long-term seismic loading is to maintain the integrity of
12 the engineered barrier over any waste and to keep the post-earthquake static factor of safety at
12 1.5 or higher.
13
14 Because the amplification of the base acceleration is very high for the environmental cap under
15 Alternatives 3 and 4 ( and particularly for the engineered surface barrier), it is judged that there
16 would likely be some amount of movement ( i.e., the factor of safety under seismic loading is less
17 than 1.0) for any reasonable environmental cap configuration. Such movement is acceptable as
18 long as the goals listed above are met. It was judged that 2.5 or 5.0 cm (1 or 2 in.) of
19 deformation that is primarily horizontal within the silt barrier layers and does not penetrate the
20 bottom of the layer is acceptable. Deformations of several centimeters occurring outside of the
^21 boundaries of the engineered barsier, within engineered fill, would commonly be considered
22 acceptable in design of engineered barriers. For example, in a summary of current practice for
23 seismic design of liner and cover systems for waste fills, Seed and Bonaparte ( 1992) note that
24 seismic deformations less than 15 cm (6 in.) are viewed by designers as being acceptable levels
25 of seismic displacement. Studies conducted on seismic stability of surface barriers for waste
26 disposal sites in the 200 East Area of the Hanford Site have also concluded that seismically
27 induced deformations less than 15 cm (6 in.) are considered to be acceptable levels of
28 displacement ( Saleh and Daniel 1994). For the purposes of this pre-conceptual design for
29 Alternatives 3 and 4, the maximum allowable deformation for the environmental cap was
30 conservatively set at that 2.5 or 5.0 cm ( I to 2 in.). This judgement is consistent with current
31 engineering practice.
32
33
34 D3 METHODS OF ANALYSIS
35
36 D3.1 Stability Analyses
37
38 Slope stability was evaluated by one of two methods. The stabilitX of the cap layers under
39 drained conditions (no cohesion) was evaluated using a Microsoft Excel spreadsheet employing
40 the method of Druschel and Underwood ( 1993). This two-dimensional method employs a fixed
41 failure surface along a predetermined interface, which extends over the entire length of the slope.
^42 Buttressing effects from the cover soils are considered.
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Final Feasibility Study for the Canyon Disposition Initiative ( 221•U Facility)
it•+ D-5