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Appendix D-- Slope Stability Analysis for poFJrtt.-2001-1 l.
I Environmental Cap Rev. e cd1 n
Rodlinc/Strikcout
1 conservative approach, because most settlcmcnt of the engineered fill would have occurred
2 during the approximately 2-year construction period for the barrier, thus minimizing the potential
3 for additional settlement during the post-closure period.
4
5 Gcosynthetics were not used for the engineered surface barrier in the recommended cross
6 sections because the long-term (500-year) performance of geosynthetics cannot be extrapolated
7 from existing data. Layout requires that the engineered barrier must extend to at least a 1 H:1 V
8 projection upward and outward from the bottom comcrs of the 221-U Building.
9
10 The assumed geotechnical properties of the engineered barrier components are listed in
11 Table D-2. The basis for estimating each property is also included in the table.
12
13 D.2.4 Bottom Liner Function and Components
14
15 The exterior bottom liner would be placed only for Alternative 4 to function as bottom
16 containment for waste that is disposed outside of the concrete structure. A waterproof coating or
17 liner would be applied to the exterior wall of the 221-U Facility where it would be in contact
18 with the exterior waste fill. The liner system is intended only as a temporary filtrate collection
19 and barrier system while the exterior waste fill, engineered fil1, and engineered barrier are being
20 placed. After the engineered barrier is in place and excess pore water ( if any) within the exterior
(^N21 waste fill has dissipated, it is assumed that only very small amounts of additional infiltration
22 would reach the bottom liner. After facility closure, the bottom liner would no longer be relied
23 upon for long-term protection of human health and the environment or for leachate collection.
24
25 The general bottom liner cross section is shown in F'igure D-5. The height of seepage in the
26 drain gravel layers is assumed to be less than 15 cm (6 in.) for short-term conditions during
27 waste-filling operations (a conservative assumption because no specific analyses were performed
28 to determine the estimated seepage height for this pre-conceptual feasibility study) and zero for
29 long-term conditions. The cross section shown is Identical to the dual liner that has been used at
30 the Environmental Restoration Disposal Facility (ERDF) on the Hanford Site (Casbon 1995).
31 Throughout the geotechnical profession, it is generally agreed that geosynthetics can be
32 considered to have a useful life of at least 100 years. Because the liner must maintain drainage
33 and low-permeability functions for only a few years (until the engineered barrier is in place and
34 minimizes infiltration), geosynthefics are used in the external bottom liner cross section.
35
36 The clay admixture and drainagc material properties for the external bottom liner are assumed to
37 be similar to those listed in Tablc D-1 for the engineered surface barrier materials. The unit
38 weight of the 0.9-m (3-R)-thick native soil protective layer was conservatively estimated to
39 weigh 1,031 kglm3 ( 1101b/fO) and have an angle of internal friction of 26 degrees with no
40 cohesion.
41
42 For stability analyses, the textured high-density polyethylene (FIDPE)lnonwoven geocomposite
("43 interface is the weakest of all the materials and interfaces within the external bottom liner cross
44 section. Residual strengths are commonly used for assessing the stability of sloping liner or cap
Fiaal Feasibility Study jor dte Canyon Disposition laiuiat(ve(221-U Facility)
un '1 00Z D-3