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Appendix G -Detailed Description of Alternative 4: DoFJRI.-2oo1-1i
^ Entombment with Internal and Extertial Waste Disposal Rev. l Draft,Lt
Redline/Strikeout
I The interim barrier material would become incorporated into the bottom layer of the engineered
2 barrier. Therefore, no additional fill material is estimated for construction of the interim barrier.
3 The total volume of material for the engineered barrier for Alternative 4 is estimated as
4 1 135,000 m3 (176,567 ydt).
5
6 While not expected to be a significant concern, during final design of the engineered barrier, the
7 potential for differential settlement at the interface between the fill directly on top of the waste-
8 filled facility (221-U) and the fill adjacent to it should be evaluated fully. It is estimated that
9 filling the canyon with waste could take several years. This extended period of engineered fill
10 placement ( fill level must closely match the waste placement fill inside) around 221-U and the
I 1 fact that the engineered fill would be constructed of locally available coarse granular material
12 should allow for the majority of settlement of the fill prior to construction of the engineered
13 barrier. Therefore, differential settlement at this interface is expected to be minimal.
14
15 G.3.13 Placc Erosion Protection. The top of the engineered barrier would have a 2% slope;
16 the top layer would be vegetated and would contain pea gravel. Therefore, after vegetation is
17 established, concerns for erosion from precipitation and wind would be minimized. To reduce
18 the volume of the engineered fill while providing stability during a seismic event, a 3:H IN side
19 slope was selected for the engineered fill. This slope would require placement of a basalt riprap-
20 type layer for erosion protection. The erosion protection layer would also include gravel and
^ 21 sand filter layers to carry the runoff safely to the outer toe of the environmental cap. The erosion
22 protection slope would not be vegetated. The volume of the erosion protection is estimated as
23 140,700 0 (184,003 yd').
24
25 G.3.1A Stability Analysis of Environmental Cap. A two-dimensional stability analysis
26 (Appendix D) was completed for the environmental cap. The layout of the environmental cap at
27 221-U is a unique application because of the height of the engineered banier, which is nearly
28 24 m ( 80 ft) above the surrounding grade. The controlling factor for the stability analysis was
29 selection of a cap layout that would remain functional after enduring a design seismic event.
30 Results from this analysis were key in determining the physical layout of the components of the
31 cap for Alternative 4.
32
33 The analysis found that the engineered barrier slope must be as flat as possible to minimize the
34 potential for eatthquake-induced cap deformations from reaching the portion of the engineered
35 barrier that functions as a capillary break. Therefore, the engineered barrier is sloped at 2% and
36 does not extend down the sides of the environmental cap. In addition, the barrier must extend
37 out far enough from 221-U that a potential earthquake-induccd crack (estimated to be 5 cm
38 [2 in.] or less) resulting from movement in the 3:I1 to IN side slope of the environmental cap
39 would be outside the waste area requiring infiltration protection from the engineered barrier.
40 With these layout parameters addressed, the environmental cap can provide the required
41 containment during a 500-year life.
42
43 The environmental cap layout is affected most by the need for a minimal slope on the engineered
^44 barrier to remain functional and not by the waste type (such as soil remcdiation or containerized
45 waste) covered or the dual-liner system beneath the external waste unit. During final design of
Final Femsibility Srudyfnr the Cenyoar Diiposiifon lnitiative (221-U Facility)
Jura 1^ 0-22