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Appendix H - Detailed Description of Alternative 6: DoFnu.-2001-1 t
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1 1I3.1 Construct Environmental Cup
2
3 The environmental cap for Alternative 6 consists of three parts: engineered fill, engineered
4 barrier, and erosion protection (see Figure 11-1). This application of an engineered barrier is
5 unique in that the top of the barrier is nearly 13 m(40 ft) above the surrounding grade. This
6 affects the seismic event factors used for barrier design and consequently the barrier layout as
7 described below.
8
9 A preliminary two-dimensional stability analysis (Appendix D) was completed for the
10 environmental cap for Alternative 3. Results for Alternative 3 are considered to be applicable to
11 the environmental cap for Alternative 6. The controlling factor for this analysis was to select a
12 cap layout that could remain functional after enduring a design seismic event. This analysis was
13 key in determining the physical layout of the environmental cap geometry. Briefly, the analysis
14 finding was that the engineered barrier must be as flat as possible to prevent potential
15 deformations from reaching the drain gravels that are the barrier's capillary break. Therefore,
16 the barrier Is sloped at 2%. In addition, the batrier must extend out far enough from 221-U that a
17 potential crack (estimated to be 5 cm [2 in.] or Iess), due to movement in the 3H:1 V side slope,
18 would be outside the waste-area requiring infiltration protection from the barrier. With these
19 layout steps addressed, the environmental cap can provide the required containment during a
20 500-year life. During final design of the environmental cap, a finite element analysis method
21 should be used to define the final cap layout dimensions and confirm that the engineered barrier
22 is properly sized for the design seismic event. Additional discussion of the barrier is provided in
23 Section 4.2.
24
25 H.3.1.1 Place Engineered FJL The engineered fill would be clean compacted granular
26 material, which would be placed in lifts. Its source is assumed to be a Hanford Site borrow pit
27 within 24 km of 221-U. The actual source location has not been identified. The fill level along
28 the galleries walls would be maintained within a few meters of the grout elevation inside the
29 galleries. The volume of enginecred fill needed for Alternative 6 is 287,800 m3 (376,300 yd3
30 The extent of the engineered fill and environmental cap is shown in >:rgure 4.2.
31
32 The Gll would be compacted to a density in the range of 95% to 98% relative compaction where
33 relative compaction is determined by standard proctor (ASTM D698). Final design of the
34 engineered fill would determine the compaction requirements and the material specifications.
35
36 H3.1.2 Construct Engineered Barrier. The engineered barrier would be designed to prevent
37 unintentional human and biotic intrusion, minimize potential human and biotic exposures, and
38 control potential contaminant migration by preventing water Infiltration inte-the-waste-materiale
39 (221-U and demolition debris). The barrier thickness would be 5 m(16 ft) minimum, which
40 would meet the requirement for protection against inadvertent intruders.
41
42 The barrier would be a modified RCRA Subtitle C-compliant barrier design to provide protection
r`^ 43 against water infiltration and biotic intrusion for 500 years. For purposes of this final FS, it is
44 assumed that the engineered barrier would be replaced one time at the end of its 500-year design
45 life. The result is that 221-U would have containment for at least 1,000 years. The barrier would
Final Feasibiliry Study jor the Canyon Disposition lnitiative (221-U Facility)
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