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Appendix D- Slope Stability Analysis for DOFJR1-2001-1 i
Environmental Cap Rev.A l Dreft lt
I
Redlinc/Strikeout
I final design may also consider additional configurations (e.g., during construction).
2 The recommendations are pre-cronceptual in nature and should be reevaluated if the design is
3 finalized and specific slope geometry, material availability, material properties, and construction
4 requirements are known. The engineered barrier and exterior bottom liner sections (for
5 Alternative 4 only) analyzed and described in this appendix were developed by others; their
6 suitability for limiting infiltration and supporting vegetation were not assessed as part of this
7 final feasibility study. Such evaluations should be performed during a final design effort.
8
9
10 D.2 DESIGN CRITERIA
11
12 Design criteria and assumptions that were used in the slope stability analyses are discussed in the
13 following subsections.
14
15 D.2.1 Design Life
16
17 The performance period for the 221-U Facility under Alternatives 3 and 4 is 1,000 years. The
18 RCRA Subtitle C barrier design life, however, is 500 years (DOE-Ri. 1996). It is assumed that
19 the engineered barrier would be replaced in kind after 500 years so that the 1,000-year
20 pcrformance period would be achieved. It is understood that construction and initial
^21 maintenance would proceed for several years following closure of the facility, but after a period
22 of roughly 50 years, the facility is to be self-maintaining.
23
24 D.2.2 Engineered Fill, Waste, and Subgrade Properties
25
26 The assumed properties for the engineered fill, exterior waste ftll (Alternative 4 only), and
27 subgrade are listed in Table D-1. These subgrade design strengths were taken from the
28 . calculations prepared by Baxter (2000) for structural evaluation of the building and are judged to
29 be reasonable based on the logs of existing subgradc conditions and the primarily granular nature
30 of waste and fill materials in the area.
31
32 D.2.3 Engineered Barrier Function and Components
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34 The engineered barrier is patterned after the Modified RCRA Subtitle C design (DOE-RL 1996),
35 which has a minimum surface slope of 2% for drainage, uses evapotranspiration in an upper zone
36 combined with a capillary break as a primary hydraulic barrier to infiltration, and has an
37 additional layer of low-permeability material to act as a secondary hydraulic barrier. The general
38 engineered barrier cross section is shown in Fgure D-4. The height of seepage above any of the
39 layers is assumed to be zero. This cross section is similar to that used for capping similar waete
40 inatetialswaste on the Hanford Site, except that a 0.9-m (3-ft)-thick clay admixture was used
41 instead of an asphaltic layer for the secondary barrier. Because of the potential, although judged
42 to be very low, for differential settlement of the environmental cap around the 221-U Facility, it
io^'43 is believed that secondary settlement of the clay-admixture secondary would accommodate
44 settlement, if any, more satisfactorily than a comparatively thin asphalt layer. This is a
Final FtasibifiryStady jor dit Canyon Disposition Giltiativt (221-U Facility)
lune 300:1 D-2