ML19256F965
| ML19256F965 | |
| Person / Time | |
|---|---|
| Issue date: | 12/10/1979 |
| From: | Rolonda Jackson Office of Nuclear Reactor Regulation |
| To: | Scarano R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| FOIA-80-529, FOIA-81-8, REF-WM-25 NUDOCS 7912270126 | |
| Download: ML19256F965 (6) | |
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DEC 10 1979 MEMORANDUM FOR:
Ross A. Scarano, Chief Uranium Recovery Licensing Branch, DWM FROM:
Robert E. Jackson, Chief Geosciences Branch, DSS
SUBJECT:
CHURCH ROCK REVIEW - OUTSTANDING GE0 TECHNICAL ENGINEERING REVIEW ISSUES (TAC 5294, R53)
PROJECi. Church Rock - United Nuclear Corporation RESPONSIBLE BRANCH: Uranium Recovery Licensing Branch and Office of. State Programs REQUESTED COMPLETION DATE: November 23, 1979 In the past, Uranium Recovery Licensing Branch has expressed the need for Geotechnical Engineering Section, Geosciences Branch, to be satisfied with the safety of the revised design of the Church Rock tailings dam which had failed in July 1979.
In more recent telephone conversations between you and J. Kane of my staff, you had requested Mr. Kane.to work closely with NMSS Consultant, J. Nelson, from Colorado State University in identifying the important geotechnical engineering safety issues which have not been satisfactorily resolved.
In response to your request we have enclosed a partial list of the outstanding issues.
It is our understanding that other issues are being separately forwarded to you by.your consultant and cover concerns on unacceptable levels of settlement, verification that drain materials meet required filter criteria, adequate stability of the internal cross dikes under dynamic loading and the adequacy of seepage collection measures and proposed borrow materials.
We have identified the issues by number in the attachment to simplify our review of the Applicant's responses, United Nuclear should be requested to directly respond to the numbered issue.
In the past, questions raised by the Corps of Engineers, J. Nelson and J. Kane of my staff have not been directly answered, but only partially responded to by the issuance of a new report which does not state or identify our original concerns. This response approach by the applicant has been the major reason that safety issues identified early in regulatory reviews are still continuing.
We would like to reiterate our past comment on the need for careful evaluation on the Church Rock project concerning geological, seismological and hydrological considerations. These considerations, which could.1 ave a significant impact on dam safety and performance, should be reviewed 1d followed up by NMSS and your consultants.
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Ross A. Scarano C rr 1 9 ;973 NRC should request a list.of license conditions to be imposed by the State of New Mexico to cover United Nuclear's plans for raising the Porth Starter Dam to Elevation 6980. Some of the important license conditions of interest to the Geosciences Branch would be related to embankment construction control, slope protection, dam safety monitoring and inspection requirements during operation.
/h QVD lcz.y c/
obert E. Jac
, Chief Geosciences (r nch Division of ystems Safety
Enclosure:
As stated cc: w/o enclosure R. Mattson J. Martin G. Kerr cc: w/ enclosure J. Knight R. Jackson L. Hulman T. Sullivan L. Heller L. Reiter J. Kendig H. Miller J. Linehan J. Nelson J. Kane PDR 1645 236
C!iURCH ROCK URA' IU:1 :tILL TAILING PROJECT TAC !:0. 5204, RS3 MRilAL LIST OF 00TSTA30ING GE0TECli:lICAL EMGINEERIllG ISSUES PREPARED BY: Joseph D. ::ane,
.'.RR, DSS, GB, GES SELECTED SMEAR STRENGTHS - STABILITY ANALYSES _
Cur concern for selecting reasonably conservative shear strengths representa-tive of the embankment and foundation materials is directly related to our concern that an adequate margin of safety against an embankment failure (as listed in R.G. 3.11) is available at the Church Rock project when ultimate tailings reservoir loading will be imposed. The summary plots of triaxial test results in Volumes 3 and 4 " Stability and Integrity Assessment" are aisleading and past selections of design strengths from these graphs have been unconservative. These plots cc:rbine shear test resulis on samples without regard to similar soil type, depth differences in foundation layering, and, most importantly, disregard the significant effect of raised soil moisture contents on shear strength. The end result of this past selection method is to adopt a shear strength which is heavily influenced by the preponderance of testing on s:mples at low insitu moisture contents. This method fails to fully anticipate the,;etting of soils as the ater round develops frun the r2ising tailings reservoir. The effect on shear strength from higher moisture contents can be observed in the following test results:
Boring Depth No. _
(ft.)
Report Source 27 29.5' SHB Report, Vol. 4, App. B, Plates S45 thru B47 28 34.S' SHB Report, Vol. 4, Apn. B, Plates B51 thru B53 64.5' SHB Report, Vol. 4, App. 8, Plates B54 thru 856 42 29.5' DUt Report, Section 3 44 39.5' 001 Report, Section 3 47 9.5' DUI Report, Section 2 1645 237 J
t./..cr shear strcngths more representative of the strength available following
' ting have been indicated in some of the results on samples recovered and Usted since the July 1979 failure. The reduced shear strengths that were used in the stability study in " Final Design Analysis Report" (October 15,1979) appear more reasonable, with the exception of the adopted cohesion values, but no basis for the strengths selected has been given.
The correctness of the selected design strengths takes on greater importance because the results of the stability study presented in the Final Report (page 7) indicate that flRC minimum required factors of safety for staedy scepage condition are just being met w'.cn the dam would be raised to crest Elevation 7004. The following item 362.1 attempts to give guidance for pro-viding the information that should permit resolution of the shear strength issue.
Item 362.2 calls attention to the work which may need to be performed when the shear strength issue is fully resolved.
362.1 flew sur. mary plots of flohr circles should be presented from which the appro-priate design shear strengths should be selected. Considerations which should
'e addressed in the de';elopment of the new plots..ould include:
The grouping of results on materials that exhibit similar soil a.
classification and characteristics (e.g., plasticity, etc.) at comparable foundation stratification and depth and within similar embankment zones.
b.
The grouping of materials whi h attempts to clearly-distinguish the effect of higher moistura contents on collapsing foundation soils.
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The presentation of separate graphs for each type of anticipated drainage condition (U-U, C-U, and C-D) with the data labeled with the appropriate boring number, depth of sample and moisture content of the tested samples.
362.2 The need for additional stability studies and the determination of the margin of safety against failure should be established following an evaluation of the new graphs and selected design shear strengths for each embankment and foundation material.
CONSTRUCTION The comments of items 362.3 through 362.6 attempt to correct apparent omissions or errors in the previously submitted construction plans.
362.3 The specifications for controlling fill placement moisture content should be revised (Sheet 4 of 7, Construction Plans) to require an upper limit.
.ae Construction Notes (Sheet 4 of 7) and the specifications for Zone IV material (Sheet 4 of 7) should limit moisture content between optimum and two percent above optimum.
362.4 Zone III and IV materials are engineered fill materials therefore, Construction Note 3 (Sheet 4 of 7) would not apply to chimney and finger drain materials and these materials should be excepted in Note 3.
362.5 Note 1 under Tailings Deposition Sequence (Sheet 2 cf 7, Construction Plans) should be revised to exclude the use of tailings for Chimney Drain material.
362.6 The frequency of field testing and laboratory testing to meet proposed construction specifications should be submitted. This testing includes gradation, field density and moisture content, Atterberg Limits, compaction and relative density tests.
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. EMBANKMENT SAFE q INSTRUMENTATION Items 362.7 and 362.8 address the adequacy of needed embankment instrumentation which will permit observation and evaluation of the retention dam stability during construction and operation.
362.7 United Nuclear's plans and installation details should be presented for approval for installing a reasonable number of piezometers beneath the new embankment at varying depths within the compressible foundation materials. The piezometers are need to monitor possible development of excess pore pressures and to locate the developing top seepage line within the foundation.
362.8 The survey monuments discussed in Vol. 4 " Stability and Integrity Assessment" would eventually be lost beneath tailings sand beaches and are not suitably located for the new construction to Elevation 6980.
Plans for continued monitoring of settlement should be provided and include type of monument; approximate locations, elevations and time to be installed; typical installation details; and frequency of readings to be surveyed. Settlement plates installed only at the downstream toe are not considered adequate for measuring settlement beneath the new embankment construction.
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