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Appendix F- Detailed Description of Alternative 3: DoFJRt,-2001-11
Entombment with Internal Waste Disposal Rev. p^fp
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1 retards vertical moisture movement. At the bottom of the compacted silt, sand and gravel layers
2 provide a capillary break in the barrier cross section. The capillary break causes moisture to be
3 retained in the overlying compacted silt layer. The sand and gravel layers are sloped to the
4 outside edge of the barrier to carry any water that migrates vertically through the silt horizontally
5 to the outside edge of the barrier. The drain gravels/sand are placed on a 1-m (3.3-ft)-thick clay
6 admixture layer. This layer provides a second bartier of low hydraulic conductivity. Below the '
7 admixture layer is compacted clean fill of adequate thickness to provide a total barrier depth of
8 5 m(16 ft) over the waste, as required for an intruder barrier.
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10 The total volume of material for the engineered barrier for Alternative 3 is estimated as
11 135,000 mi (176,567 yd').
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14 While not expected to be a significant concern, during final design of the engineered barrier, the
15 potential for differential settlement at the interface between the fill directly on top of the waste-
16 filled facility (221-U) and the fill adjacent to it should be evaluated fully. It is estimated that
17 filling the canyon with waste could take several years. This extended period of engineered fill
18 placement ( fill level must match the waste placement fill inside) around the 221-U Facility and
19 the fact that the engineered fill would be constructed of locally available coarse granular material
20 should allow for the majority of settlement of the fill prior to construction of the engineered
(0'%1 barrier. Therefore, differential settlement at this interface is expected to be minimal.
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23 F.3.1.2 Place Erosion Protection. The top of the engineered barrier would have a 2% slope.
24 the top layer would be vegetated, and it would contain pea gravel. Therefore, after vegetation is
25 established, concerns for erosion from precipitation and wind would be minimized. To reduce
26 the volume of the engineered fill while prnviding stability during a seismic event, a 3:horizontal
27 to l:vertical (H:V) side slope was selected for the engineered fill. This slope would also require
28 placement of a basalt riprap-type layer for erosion protection. The erosion protection layer
29 would include gravel and sand filter layers to carry the runoff safely to the outer toe of the
30 environmental cap. The erosion protection slope would not be vegetated. The volume of the
31 erosion protection is estimated as 140,700 m^ (184,003 yd^).
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33 F3.13 Stability Analysis of Environmental Cap. A two-dimensional stability analysis
34 (Appendix D) was completed for the environmental cap. The layout of the environmental cap at
35 221-U is a unique application because of the height of the engineered barrier, which is nearly
36 24 m(80 ft) above the surrounding grade. The controlling factor for the stability analysis was
37 selection of a cap layout that would remain functional after enduring a design seismic event.
38 Results from this analysis were key in determining the physical layout of the components of the
39 cap for Alternative 3.
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41 The analysis found that the engineered barrier slope must be as flat as possible to minimize the
42 potential for earthquake-induced cap deformations from reaching the portion of the engineered
(0-143 barrier that functions as a capillary break. Therefore, the engineered barrier is sloped at 2% and
44 does not extend down the sides of the environmental cap. In addition, the barrier must extend
45 out far enough from 221-U that a potential earthquake-induced crack (estimated to be 5 cm
Final Feasibility Study jorthc Canyon Aispasition Initiative (221-U Facility)
' 007 F-21