ML20206M726
| ML20206M726 | |
| Person / Time | |
|---|---|
| Issue date: | 06/04/1986 |
| From: | Nataraja M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Martin D NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-41 NUDOCS 8607010270 | |
| Download: ML20206M726 (5) | |
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JUN 4 26 uor v,, m, -1 FA^ TIN
-.1 MEMORANDUM FOR:
Dan E. Martin, Section Leader Low-Level Waste and Uranium Recovery Projects Branch Division of Waste Management FROM:
Mysore S. Nataraja, Section Leader Engineering Branch Division of Waste Management
SUBJECT:
WMEG REVIEW 0F THE PROPOSED RAP MODIFICATION FOR THE SALT LAKE CITY REMEDIAL ACTION PLAN (UMTRAP)
In accordance with Technical Assistance Request WM-86425, WMEG has reviewed the two modifications of the subject document. The two modifications involve moisture control specifications for contaminated materials, and rock cover /
filter zone gradations and filter criteria. We are able to conclude that the modification for rock cover / filter zone gradations and filter criteria is acceptable. However, we are not able to concur in DOE's modification for eliminating the moisture control requirement for placement of contaminated materials. Rather, wa are _ proposing an alternative modification which DOE should consider.
The results of our review are' enclosed as an attachment to this memorandum.
Should you require more information about this review, please direct your questions to Steve Smykowski of my staff on X74109.
l Mysore S. Nataraja, Section Leader j
Engineering Branch i
Division of Waste Management
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Enclosure:
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SS 06/04/86 MOISTURE ATTACHMENT 1 REVIEW 0F THE PROPOSED MODIFICATIONS TO THE SALT LAKE CITY REMEDIAL ACTION PLAN (UMTRAP) by Engineering Branch, Division of Waste Management By letter dated July 10, 1985 (Themelis (D0E) to Higginbotham (NRC)), DOE had proposed two modifications to the Salt Lake City Remedial Action Plan (RAP).
The first modification dealt with changes in the rock durability specifications for the erosion barrier material. A review of the modification was performed by WMGT and their conclusion was that D0E's proposed modification was acceptable.
However, it was recommended that the design of the filter layer be reviewed again since there was a change in the overlying riprap size. DOE has submitted to NRC another evaluation of the grading curves and RAP requirements for filter and erosion barrier material and has made recommendations based on their conclusions. We have reviewed DOE's recommendations and our conclusions are included in this enclosure.
The second proposed RAP modification dealt with the compaction specification for placement of contaminated material. The requirement identified in the RAP (December, 1984) calls for compacting the tailings to a minimum of 90 percent of the standard Proctor maximum dry density (ASTM D-698) and at a moisture content between minus 3 percent of optimum and optimum moisture. NRC concurred in this requirement at the time concurrence was granted in the RAP. The proposed modification by DOE is that the moisture requirement be eliminated and the 90 percent standard Proctor compaction be the only specification required.
The staff reviewed this proposed modification (memorandum from J. Greeves (WMEG) to L. Higginbotham (WMLU) dated August 23,1985) and concluded that the modification would not provide reasonable assurance that the design would meet the EPA standards without further analyses providing acceptable justification for this modification. As a result, DOE reviewed and analyzed field l
moisture / density data of contaminated fill already placed and also performed laboratory testing on a composite sample of tailings considered representative of the material that has been placed. The results of the laboratory testing, analysis of the field data, and DOE conclusions and recommended changes to the
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RAP were submitted to NRC for review (letter from Themelis (00E) to Knapp (NRC) dated April 28,1986).
The results of our review are included in this enclosure.
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9 SS 06/04/86 M0ISTURE Criteria for Filter & Erosion Barrier Materials Based on a review of the gradation limits for the filter material and erosion barrier rock in accordance with the staff Standard Review Plan, the proposed gradation modifications are acceptable. However, 00E identifies in their Technical Approoch Document (TAD) that the minimum size rock of the rock blanket should aqual approximately one-tenth of the blanket thickness.
It should be noted that the minimum size rock observed from their gradation curve does not meet this TAD criterion.
Moisture Control During Field Compaction When specifications for the placement moisture content and degree of compaction are normally established, their selections are dependent upon required engineering properties that would typically include strength, compressibility, and permeability in order to achieve a certain level of performance and stability (Ref. 1). The typical density requirement for placement of tailings.
during remedial action is to compact the soil in the field to a minimum of 90 percent of the standard Proctor maximum dry density.
In general, compacting soil to this degree of compaction has not been a difficult specification to meet.
It can be achieved over a broad range of moisture contents depending upon the amount of field compactive effort applied to the soil (Ref. 2).
In conjunction with specifying a density, a range of placement moistures are usually specified and are specified for good reasons. The strength and compressibility of a cohesive soil are known to depend on the placement water content and density, and both must be considered (Ref. 3, Ref. 4).
If enough field compactive effort is applied to a soil, 90 percent of standard Proctor density can be achieved at moistures well below optimum moisture (Ref. 1). However, soils compacted too dry of optimum moisture content have been known to settle considerably upon subsequent wetting (Ref. 5). Without a moisture specification, it is possible that the tailings at the Salt Lake City site could be compacted to the required minimum of 90 percent of standard Proctor density at moisture contents well below optimum moisture (and less than 70 percent saturatior.). Significant settlement of the material could occur if the tailings were later saturated.
If cracking of the cover should occur, water could infiltrate the tailings and possibly cause localized areas of saturation. This could lead to pronounced differential settlements which would contribute to additional cracking of the radon barrier and/or the creation of preferential drainage pathways.
For these reasons, the staff recommends that i
the minus 3 percent specification limit remain as the lower bound for moisture
- control, l
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l SS 06/04/86 MOISTURE Shear strengths of soils compacted wet of optimum are generally less than those i
obtained from soils compacted dry of optimum, particularly immediately following construction (Ref. 2, Ref. 3).
In addition to reduced strength, soils compacted wet of optimum become more ductile which allows for significantly larger strains (Ref. 2, Ref. 3). Settlements increase as the placement moisture increases on the wet side of optimum (Ref. 4). This is shown by DOE's test results which indicate that relatively larger strains occur with those samples compacted at moistures greater than 90 percent saturation.
Reference 5 (p.254) also shows that there is a significant increase in compressibility of soil placed wet of optimum over that of soil placed dry of optimum when both are placed at the same dry densities. We do agree with DOE's statement that the test results indicate that the tailings show increased sensitivity to settlement when placed at moisture contents resulting.in saturations greater than 88-90%. Therefore, in recognition of this soil behavior and DOE's own test results, we recomend that an upper. bound for moisture control be specified so that 90 percent of standard Proctor density will be achieved and the required strength and settlement characteristics will be assured. The placement moisture content of the tailings recomended by the staff should not exceed plus 5 percent of optimum moisture.
The DOE should have little difficulty achieving the upper moisture bound recomended by the staff for'two reasons.
First, the average field moisture content exceeds the average optimum moisture content by 2.1 percent which is less than the NRC recommended upper bound. Secondly, DOE reports that the maximum percent saturation for which compaction could be achieved in the field has been 86 percent. This corresponds to approximately plus 6 percent above optimum moisture for 90% standard Proctor density.
Furthar, it is our understanding that most of the tailings at the time of excavation are at noisture contents greater than optimum. Therefore, DOE should have little difficulty meeting the lower bounds of the staff's'recomended limit.
In sumary, based on our review of the information presented, we recommend that the material be placed with a moisture control specification. However, we agree that the present RAP requirement is too restrictive and can be modified.
Increasing the range of placement moisture from minus 3 percent of optimum to optimum moisture as originally required by the RAP to new limits of minus 3 percent to plus 5 percent of optimum moisture are acceptable to the staff and will provide the material placement conditions required to minimize differential settlement and maintain stability. We are not able to support DOE's technical
-argument that the stability of the pile will be assured and differential settlement will be minimized if the tailing are placed at moisture contents i
outside this specification.
In order to demonstrate that the modified moisture control limits are adequate and acceptable, the staff would agree with DOE's plan for monitoring of embankment settlement and recommend that this be l
performed as stated in the RAP.
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SS 06/04/86 MOISTURE REFERENCES 1.
Terzaghi, K. and Peck, R.
B., Soil Mechanics in Engineering Practice, John Wiley & Sons, Inc., New York, 1967, pp. 440-451, 2.
National Engineering Handbook, U.S. Department of Agriculture - Soil Conservation Service, prepared by Geotechnical Engineers Inc... Project 80762, October 1982 Section 7, Chapters 9 & 12.
3.
Banks, D.C., " Embankment - Design Concepts: Water Content - Density Relations and Effects on Design Parameters," from Notes for Construction of Earth and Rock-Fill Dams Course, U.S. Army Engineer Waterways Experiment Station, Corp of Engineers, Vicksburg, Mississippi, 1980.
4.
Sowers, G.F., Introductory Soil Mechanics and Foundations, MacMillan Publishing Co., Inc., New York, 1979, pp. 237-263.
5.
Winterkorn, H.F., and Fang, H.Y., Foundation Engineering Handbook, Van Nostrand Reinhold Company, New York, 1975, pp. 253-256.
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