ML20204D893
| ML20204D893 | |
| Person / Time | |
|---|---|
| Issue date: | 10/04/1988 |
| From: | Lohaus P NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Arthur W ENERGY, DEPT. OF |
| References | |
| REF-WM-86 NUDOCS 8810210245 | |
| Download: ML20204D893 (7) | |
Text
_ _ _ _
SW/SLICKROCK NT 4 4 W r
W. John Arthur, Project Manager Uranium Mill Tallings Project Office U.S. Department of Energy Albuquerque Operations Office P.O. Box 5400 Albuquerque, NM 87115 Dear Mr. Arthur; Enclosed are the NRC staff connents in the areas of surface water hydrology and geotechnical engineering on the preliminary design for the proposed remedial action at the Slick Rock, Colorado site. Staff connents in the areas of geology and groundwater hydrology will be provided to you as a part of the review of j
the preliminary final RAP and design.
If you have any questions regarding the enclosed comments, please contact Sandra L. Wastler of my staf t' at F15 492-0582.
Sincerely, WIGNALSIGNED BY l
Paul H. Lohaus, Chief Operations Branch t
Division of Low-Level Psste Management and Decomi, ~.,oning j
t t
Enclosure:
t As Stated i-L cc: D.Mann, DOE /AL M. Abrams, DOE /AL S. Mann, DOE /HQ H. Roitman, Colorado l
i r
,. A
]F; ;LLOB
- LLOSts/
- LLOB f
..... : - - ; - -. - - - : - -. -, y,'- - - - : - -..
NAME S(a MFliege
- PLoh s i
~*
t
.----:#///88.d..---.--.-:h-/88 h/68 DATE OFFICIAL RECORD COPY
[
lhd-bi 8810210245 881004 hvlf i
SLICK ROCK SURFACE WATER HYDROLOGY QUESTI0tlS AND COMMENTS 1.
Desigr of Rock Apron and Rock Side Slopes Based on a review of the erosion protection calculations, we note that a significant amount of backfill material will be placed directly over the riprap aprons and riprap on the side slopes of the embankment, particularly on the west side of the reclaimed pile.
We also note that the surface of this backfill, in some cases, will be shaped so as to provide, ir effect, diversion channels above the riprap layers.
We further note that the design of the riprap layers has considered the possible erosion of the backfill material where the riprap is exposed.
However, the assumptions used in the analyses may not be appropriate when other critical failure modes are considered, and the design has apparently not considered the possibility of gully intrusion at various locations around the pile.
First, the assumption of complete removal of the backfill material may not be conservative in designing the riprap aprons.
It appears that a much more critical failure mode would be one where only a portion of the backfill material is removed, so that the bottom width of the gully channel is significantly less than 29 feet.
If a triangular channel is assumed to carry the flow, for example, the rock size required to resist the increased velocities would be.nuch greater.
An adequate basis has not been presented for assuming complete removal of the backfill material.
In addition, a steeper channel slope than 2% could occur if more erosion occurs at the downstream end of the channel than at the upstream end.
Second, the calculations are not clear with regard to the assumption of a 10%
slope at the downstream end (120-feet wide) of the ditch, under Case I assumptions; this channel slope is not assumed to occur under Case !!
assumptiuns. The assumption of a different channel with different slopes under Case I conditions is not well documented.
It appears that if a smaller eroded section is assumed (as discussed above) and if a 10% slope is also assumed at the downstream end of the ditch, the rock size will be considerably larger.
Third, it appears that little or no protection has been provided where the oitch discharges ficws into the Corral Draw drainage.
Based on the information provided, the ditch flows will be forced into a relatively constricted section, resulting in the possibility of gully intrusion into the reclaimed pile.
Since protection for this area has apparently not been provided, it appears that significant erosion could occur, particularly if additional turbulence, energy dissipation, and non-uniform flow conditions occur.
Fourth, the basis for the design of the riprap apron at various portions of the pile has not been discussed or documented. While the use of 2.2 feet of Type C
2 riprap may be acceptable, it is not clear that this rock is adequate to prevent the headcutting and intrusion of gullies into the pile area.
In general, the need for backfilling the areas of the riprap aprons and toes is unclear.
Under such conditions, it is very difficult to predict the occurrence of the most critical design conditions, because so many are possible.
- Further, the use of hydraulically unstable backfill material may result in erosion damage and the need for subsequent maintenance (even though the maintenance may not be needed to maintain long term stability, because the rock is designed for a critical case).
It is unclear why such a design is provided, knowing that the use of backfill results in the use of rock sizes much larger than would be needed if the backfill were not provided.
In addition to the difficulties in justifying the design from a theoretical standpoint, it is unclear why a simpler riprap design requiring smaller rock has not been used.
In order to resolve our concerns, DOE should eliminate the use of backfill over the rock aprons and redesign the aprons accordingly.
Alternatively, additional justification is needed to document the effectiveness of the design under more critical design assumptions. Additionally, further justification of the adee.uacy of the design to prevent gully intrusion is necessary, regardless of the design selected.
9 6
4
f ~
GEOTECHNICAL ENGINEERING COMMENTS ON PRELIMINARY DESIGN FOR REMEDIAL ACTION AT SLICK ROCK SITE 5 LICK ROCK, COLORADO, UMTRCA PROJECT SITE References-Docenents Reviewed:
1.
Information for Reviewers, Preliminary Design for Review-UHTRAP, Slick Rock, Colorado, March 1988.
2.
Calculations, Preliminary Design for Review-UMTRAP, Slick Rock, Colorado, Volumes I,II,III, and IV, March 1988.
3.
Subcontract Documents, Preliminary Design for Review; Specifications and t
Subcontract Drawings - UMTRAP, Slick Rock, Colorado, March'1988.
1 4.
Effects of Freezing and Thawing on the Permeability of Compacted Clay -
Report by E. Chamberlain of Cold Regions Research and Engineering 2
Laboratory (CRREL), U.S. Anny Corps of Engineers; Prepared for DOE, July 1988.
GEOTECHNICAL ENGINEERING COMMENTS:
1.
Soil Properties, Table 4.1; Information for Rev1 ewers, Reference 1 The moist unit weight for the UC Site Contaminated Soils presented in Table 4.1 cf Reference 1 is 110 pcf whereas the value presented for this material in sheet 3 of Calculations i 11-250-04 in Volume I cf Reference 2 is 116 pcf.
This inconsistency should be clarified.
2.
Compaction Moisture Content for Tailings materials, page C-13; Information for Reviewers, Reference 1 The placement moisture content for tailings material, as stated in page C-13, is two percent above and two percent below optimum moisture contenc as determined in ASTM D698 test. The specifications for earthwork as presented in (Section 02200, Earthwork, Part 3-Execution; 3.8.D.1, page 02200-18) the Subcontract Documents (Reference 3) contains no specific guidance on moisture control.
In the past NRC has reconrended and DOE has implemented a requirement of compacting tailings at a moisture content of zero to two percent less than the optinum moisture content as determined by the ASTM D698 test.
The specifications should be re' vised to comply with the above recommendations and provide specific guidance on placement coisture content for tailings materials.
. 3.
Section 2.1 B.2.b. Radon Barrier Materials, page 02200-7; Subcontract Documents, Reference 3.
The gradation specifications for radon barrier material are very general.
For example a gravel material with a maximum particle size of 2-inches and a minimum of 20 percent passing No. 200 sieve would be in compliance with the specifications, but will not have the hydraulic conductivity and diffusion parameters used in the radon barrier design.
These design parameters were determined by laboratory tests performed on soil samples obtained from the designated borrow areas. To ensure that the material pla:ed as radon barrier is similar to that obtained from the designated borrow areas (which were tested in the laboratory to establish the design parameters), the specification should he revised to provide an allowable range for the intermediate particle sizes so that any material meeting the minimum requirements of the specification will have properties similar to those used in the radon barrier design.
4 Section 3.6.B. page 02200- 16, Field Quality Control; Subcontract r
Documents, Reference 3.
The specified frequencies of testing to be performed during construction as part of field quality control are not in agreement with the NRC Branch Technical Position on Inspection and Testing.
The NRC will be using this Cranch Technical Position in reviewing the Remedial Action Inspection Plan I
(RAIP) for this preject. Therefore the spec.ified testing frequencia* should be revised to comply with those in the NRC Branch Technical Position on Testing and Inspection.
5.
Radon Barrier Design, Calculations 11-224-01-01; Calculations, Volume I, f
Reference 2.
The parameters used in the design of radon barrier were not finalized at the preliminary design stage and therefore, the design is likely to be revised during the preparation of the final RAP.
However, the NRC would like to emphasize that the radon barrier should also be designed for the freezing and thawing conditions at the site, and the results of the CRREL study on this topic should be used as a reference in this review (Reference 4). An evaluation of the value of the hydraulic conductivity and integrity of the l
radon barrier layer af ter 15 cycles of freeze and thaw, and assessment of the predicted cover cracking due to differential settlement and/or shrinkage should i
be carried out by the DOE in the final design / final RAP.
i 6.
Settlement and Cover Cracking Analyses, Calculation 11-225-01-00; Calculaticns, Volume II, Reference 2.
t It is stated in the document that the calculations performed to evaluate the potential for cover cracking are preliminary and that the final evaluation
. will be presented with the final design / final RAP.
Howevcr, the NRC would like to point out that the bedrock surface under the northern side of the tailings ernbankment ( see sheet 12 of calculations 11-225-01-00 ) raises at approximately a 45 degree slope.
It will be difficult (in the absence of any specific provisions in the compaction specifications to ensure adequate compaction at this location) to compact the tailings placed adjacent to the sloping rock surface.
This will result in a nonuniformly compacted tailings layer.
This localized under-compaction will result in a differential settlement higher than that calculated assuming a uniformly compacted tailings layer. As it is difficult to quantify this anomaly, it is recommended that as a conservative approach the DOE should consider cutting benches in the rock slope. This will enable uniform compaction of tailings in t.he vicinity of rock slopes and mitigate the potential for excessive differential settlement / cover cracking in the northern sih of the tailings embankment.
7.
Embankment Toe Detail, Section B in Drawing SRK-PS-10-0216; Subcontract Documents, Reference 3 The toe of the tailings embankment is difficult to construct as per the detail presented in the drawing.
If there is any overriding reason for this recommendation it should be indicated on the drawing so that the subcontractor would understand the reason for 'It.
A general concern is that the bedrock surface slopes towards the center of the embankment.
Thus the depression in the rock surface, which is like a bowl, would be a collecting water that seeps along the bedrock - insitu gravel interface (point for any see bedrock contours on sheet 3 of calculations 11-221-02 Volume I). Although there is a gravel / cobble backfill ditch around the entankment,,the water will have a tendency to drain down by gravity into the lower gravel / cobble deposit and subsequently seep along the bedrock - gravel deposit interface.
This water will come in contact with the tailings and is likely to seep down through the bedrock. As a defense against this scenario the bedrock under the tailings embankment should be shown to be an aquiclude with out any fractures or local zones of high permeability. Since the information presented is not detailed enough to allow an assessment to be made of the path of least resistance for the flow of water, the DOE should address this concern before the RAP is finalized.
SW/SLICKROCK 1-Of3TRIBUT10H:
vCentral file, WM-86 NMSS r/f LLOB r/f i
SWastler MFliegel DGillen MRKnapp JGreeves MBell r
PLohaus JJSurneier RDSmith, URF0 i
l t
h
- - -