NATIONAL DAM SAFETY PROGRAM. UNIMIN TAILINGS DAM ... · 803 Front Street Norfolk, Virginia 23510 11. CONTROLLING OFFICE NAME AND ADDRESS Ii. REPORT DATE.Janomy 1981 13. NUMBER OF

AD-A103 52 BAKER (MICHAEL) JR INC BEAVER PA F/6 13/13NATIONAL DAM SAFETY PROGRAM. UNIMIN TAILINGS DAM (INVFNTORY NUM--ETC(U)JAN 81 J A WALSH OACW65-80-O-0032

UNCLASSIFIED NIEE'llllEEEmmlllIIEEEEIIEEIIIIEIIIIEIIIIEEEEIIIIIIIIIIIIIu'IIIEEE."

POTOMAC RIVER BASIN

S'Name of Dam: Unimin Tailings Dam~'-~Location: Frederick County, Commonwealth of Virginia LEVELS

Inventory Number: VA 06918

PHASE I INSPECTION REPORTIATIONAL DAM SAFETY PROGRAM

INC DDISTRIBUTION STATEMENT A

Approved for public releaseIDistribution Unlimited

PREPARED FOR

NORFOLK DISTRICT CORPS OF ENGINEERS803 FRONT STREET

NORFOLK, VIRGINIA 23510 DTIC

PREPARED BY SEP 1 1981MICHAEL BAKER, JR., INC. 0

BEAVER, PENNSYLVANIA 15009 D

January 11

SECURITY CLASSIFICATION OF THIS PAGE (When Date Entered)

REPORT DOCUMENTATION PAGE BEFORE COMPLETINGFI. REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECIPIENT'5 CATALOG NUMBER

"%36/04. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

Phase I Inspection Report Final

National Dam Safety Program

6. PERFORMING ORG. REPORT NUMBER -

7. AUTHOR(e) 8. COIAVIACT OR GRANT NUMBER(&)

Michael Baker, Jr., Inc.Beaver, Penn 15009 C' / Ac' 65-80-D-0032 .

9. PERFORMING ORGANIZATION NAME AND ADDRESS i0. PROGRAM ELEMENT, PROJECT, TASK

U. S. Army Engineering District AREA& WORK UNIT NUMBERS

803 Front StreetNorfolk, Virginia 23510

11. CONTROLLING OFFICE NAME AND ADDRESS Ii. REPORT DATE

.Janomy 198113. NUMBER OF PAGES

14, MONITOR National Dam Safety Program. Unimin 15. SECURITY CLASS. (of-f'/i-r)

Tailings Dam (Inventory Number VA 06918),Potomac River Basin, Frederick County, UnclassifiedCormmonwealth of Virginia. Phase I 1Sa. DECLASSIFICATION/DOWNGRADING

Inspection Report. SCHEDULE

16, OISTRIBUTtONI T rTEIIE T (of hfa Rf ort .. . ..

Approved for public release; distribution unlimited

17. DISTRIBUTION STATEMENT (of the absttact entered In Block 20, If different from Report)

It. SUPPLEMENTARY NOTES

Copies are obtainable from National Technical Information Service,Springfield, Virginia 22151

I. KEY WORDS (Continue on reveree aide If neceeery end Identify by block number)

Dams - VANational Dam Safety Program Phase IDam SafetyDam Inspection

20. AweT'RACT (Cawt*ew ,erew af Ifneeae and identltfy by block number)

DO t J 73 EDTION OF I NOV 66 IS OBSLETE

S ECURITY CLASSIFICATION OF THIS PAGE (len Data Entered)

,o ~

SECURITY CLASSIFICATION OF THIS PAGE(Whan Data Shteaed)

20. Abstract

Pursuant to Public Law 92-367, Phase I Inspection Reports are preparedunder guidance contained in the recommended guidelines for safetyinspection of dams, published by the Office of Chief of Engineers,

Washington, D. C. 20314. The purpose of a Phase I Inspection is toindentify expeditiously those dams which may pose hazards to human lifeor property. The assessment of the general conditions of the dam is

based upon available data and visual inspection. Detailed investigationand analyses involving topographic mapping, subsurface investigations,testing, and detailed computational evaluations are beyond the scope of aPhase r investigation; however, the investigation is intended to identifyany need for such studies.

Based upon the field conditions at the time of the field inspection andall available engineering data, the Phase I report addresses thehydraulic, hydrologic, geologic, geotechnic, and structural aspects of

the dam. The engineering techniques employed give a reasonably accurateassessment of the conditions of the dam. It should be realized thatcertain engineering aspects cannot be fully analyzed during a Phase Iinspection. Assessment and remedial measures in the report include therequirements of additional indepth study when necessary.

Phase I reports include project information of the dam appurtenenances,all existing engineering data, operational procedures,hydraulic/hydrologic data of the watershed, dam stability, visual

inspection report and an assessment including required remedial measures.

Accession For

NTIS GRAMIDTIC TABUninnouncedJust if icatlo .

Distribut icn/!

AvaflaiilGtv Cd03 SEP 1 1981.AvouA1 'rd/vr

Dict Special

SECURITY CLASSIFICATION OF THIS PAGE(Whn Dats Entord)

II . .. . .. . .. . . I . . . 1 'i - " rW: ,

PREFACE

This report is prepared under guidance contained in theRecommended Guidelines for Safety Inspection of Dams, forPhase I Investigations. Copies of these guidelines may beobtained from the Office of the Chief of Engineers,Washington, D.C. 20314. The purpose of a Phase I investi-gation is to identify expeditiously those dams which maypose hazards to human life or property. The assessment ofthe general condition of the dam is based upon availabledata and visual inspections. Detailed investigation andanalyses involving topographic mapping, subsurface investi-gations testing, and detailed computational evaluations arebeyond the scope of a Phase I investigation; however, theinvestigation is intended to identify any need for suchstudies.

In reviewing this report, it should be realized that thereported condition of the dam is based on observations offield conditions at the time of inspection along with dataavailable to the inspection team. In cases where the reser-voir was lowered or drained prior to inspection, such action,while improving the stability and safety of the dam, removesthe normal load on the structure and may obscure certainconditions which might otherwise be detectable if inspectedunder the normal operating environment of the structure.

It is important to note that the condition of a dam dependson numerous and constantly changing internal and externalconditions, and is evolutionary in nature. It would beincorrect to assume that the present condition of the damwill continue to represent the condition of the dam at somepoint in the future. Only through continued care and inspec-tion can there be any chance that unsafe conditions bedetected.

Phase I inspections are not intended to provide detailedhydrologic and hydraulic analyses. In accordance with theestablished guidelines, the spillway design flood is basedon the estimated "Probable Maximum Flood" for the region(flood discharges that may be expected from the most severecombination of critical meteorologic and hydrologic con-ditions that are reasonably possible), or fractions thereof.Because of the magnitude and rarity of such a storm event, afinding that a spillway will not pass the design floodshould not be interpreted as necessarily posing a highlyinadequate condition. The design flood provides a measureof relative spillway capacity and serves as an aid in deter-mining the need for more detailed hydrologic and hydraulicstudies, considering the size of the dam, its general con-dition, and the downstream damage potential.

Ii

- -I

PHASE I INSPECTION REPORTNATIONAL DAM SAFETY PROGRAM

CONTENTS

Page

Preface ......... ....................... i

Brief Assessment of Dam ....... ............... 1Overall View of Dam ........ ................. 5Section 1: Project Information ..... ........... 7Section 2: Engineering Data .... ............. . 13Section 3: Visual Inspection .... ............ 17Section 4: Operational Procedures ... .......... .21Section 5: Hydraulic/Hydrologic Data .. ........ 23Section 6: Dam Stability ..... .............. 25Section 7: Assessment/Remedial Measures ......... ... 29

Appendices

I. PlatesII. PhotographsIII. Visual Inspection Check ListIV. General References

NAME OF DAM: UNIMIN TAILINGS DAM

ii


Name of Dam: Unimin Tailings DamState: Commonwealth of VirginiaCounty: FrederickUSGS 7.5 Minute Quadrangle: Hayfield, VirginiaStream: Mine Spring RunDate of Inspection: 30 and 31 October 1980

BRIEF ASSESSMENT OF DAM

Unimin Tailings Dam is an earthfill embankment approximately84.9 feet high' and 574 feet long. The principal spillwayis a 30-inch bituminous coated corrugated metal pipe riserand the emergency spillway is a natural, topographic saddleon the right2 side of the reservoir. The dam is on MineSpring Run approximately 1.2 miles south of Gore, Virginiaand is used for waste tailing storage by the owner, UniminCorporation. Unimin Tailings Dam is an 'intermediate"size-"high" hazard structure as defined by the RecommendedGuidelines for Safety Inspection of Dams.

Adequate stability analyses were available for review duringthese investigations which evaluated the factors of safetyagainst complete and partial failures of both the upstreamand downstream embankments. All of the analyses performedindicated acceptable factors of safety against failure.

Using the Corps of Engineers' screening criteria for initialreview of spillway adequacy, the Probable Maximum Flood(PMF) was selected as the spillway design flood (SDF). Thespillways are capable of passing 100 percent of the PMFwithout overtopping the crest of the dam, therefore, theyare adjudged as adequate.-,-

The dam and appurtenant structures were found to be in goodoverall condition at the time of the inspection. Mainte-nance of the dam is not considered adequate. Some minorerosion has taken place below a bench on both the upstreamand downstream embankments. The junction of the downstreamembankment with both the left and right abutments below thebench showed moderate erosion. Some areas of the embankmentand the emergency spillway are sparsely vegetated.

'Measured from the downstream toe of the dam to the embank-ment crest.2 Facing downstream.


A formal warning system and emergency action plan should bedeveloped and put into operation as soon as possible. Theemergency action plan should list steps to be taken to helpprevent failure of the dam in an emergency.

Regular, formal inspections should be made of the dam andappurtenant structures. A check list should be compiled foruse by the owner's representative as a guide for the inspec-tions. Maintenance items should be completed annually.

The following repair items should be accomplished as part ofthe general maintenance of the dam:

1) Repair all areas of erosion on the embankment,particularly the erosion rills on the downstreamembankment below the bench.

2) Establish a grass cover over all the sparselyvegetated areas of the embankment.

3) Repair and riprap the eroded junctions of theleft and right abutments with the downstreamembankment.

4) Uncover the outlet pipes for the internal chimney

and lateral seepage control drains.

5) Establish a grass cover over the sparsely vegetatedareas of the emergency spillway.

6) Install a staff gage to monitor reservoir levelsabove normal pool. Original signed bi

JAME-S A. WL!MICHAEL BAKER, JR., INC. SUBMITTED:

James A. Walsh, P.E.Chief, Design Branch

Original signed by:

P.E. RECOMMENDED: D__________________:__Chairman of the Board and Jack G. Starr, P.E.Chief.Executive Officer Chief, Engineering

Original signed bl

APPROVED:JACK G. STARR

0 'Douglas L. HallerMICHAEL %Colonel, Corps of Engineers

BA-'Q:R III District Engineer

JAN . 0 1%1Date:


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NAME OF DAM: UNIMIN TAILINGS DAM ID# VA 06918

SECTION 1 - PROJECT INFORMATION

1.1 General

1.1.1 Authority: Public Law 92-367, 8 August 1972,authorized the Secretary of the Army, throughthe Corps of Engineers, to initiate a nationalprogram of safety inspections of dams through-out the United States. The Norfolk Districthas been assigned the responsbility of super-vising the inspection of dams in the Common-wealth of Virginia.

1.1.2 Purpose of Inspection: The purpose is toconduct a Phase I inspection according to theRecommended Guidelines for Safety Inspectionof Dams (Reference 12, Appendix IV). Themain responsibility is to expeditiouslyidentify those dams which may be a potentialhazard to human life or property.

1.2 Description of Project:

1.2.1 Description of Dam and Appurtenances: UniminTailings Dam is an earthfill embankmentapproximately 84.9 feet high and 574 feetlong.' The crest of the dam is about 21.6feet wide and the minimum elevation is at920.6 feet Temporary Bench Mark (T.B.M.).

2

The upstream embankment has a 12 foot widebench about 17 feet below the crest of thedam at about elevation 904.0 feet. The slopeof the upstream embankment is approximately2.5H:IV (Horizontal to Vertical) above thebench and 2.8H:IV below the bench. Thedownstream embankment has a 12 foot widebench about 30 feet below the crest of thedam. The slope of the downstream embankmentis approximately 2.5H:IV above the bench and

'Measured from the downstream toe of dam to the lowest pointon the embankment crest.2All elevations are referenced to a Temporary Bench Marklocated on the top of a concrete marker on the top of dam,right abutment. The assumed elevation is 921.0 feet.


7

2.6H:lV below the bench. There is no slopeprotection on the upstream embankment.

According to the As-Built Report (Reference18, Appendix IV), a key trench was excavatedinto the foundation soils near the upstreamtoe of the embankment and backfilled withcompacted clayey soils. The embankment wasconstructed of on-site borrow materials fromupstream of the dam. The As-Built Reportindicates that chimney and lateral drainswere constructed for internal embankmentdrainage.

The principal spillway is a 30-inch diameterbituminous coated corrugated metal pipe(B.C.C.M.P.) serving as a fixed riser. Accord-ing to the owner's representative, the topportion of the 30-inch riser was broken and a42-inch diameter pipe was placed over the exist-ing pipe and anchored with cables. Concretewas used to fill the space between the 30-inchdiameter pipe and the 42-inch diameter pipe.

The spillway riser has a crest elevation of901.0 feet T.B.M. The riser- is located tothe right of center of the embankment.' Atrash rack has been fitted to the top of theriser. According to the As-Built Report, theriser extends down about 39 feet where itconnects to a 24-inch diameter B.C.C.M.P.outlet. The 24-inch B.C.C.M.P. is approximately414 feet long and passes beneath the embankmentat a slope of approximately 5.5 percent.This pipe exits into a riprap lined plungepool at the center of the downstream toe ofthe embankment. The outlet pipe extendsabout 5.5 feet beyond the embankment. Thisextension is unsupported. The invert elevationat the principal spillway outlet is at 838.7feet T.B.M. The tailwater elevation in theplunge pool is at 836.2 feet T.B.M.

A gate valve with accompanying valve stemextension and support brace is attached tothe spillway riser pipe approximately fourfeet below the crest of the riser pipe(Photo 1). This valve is used during periods

'Facing downstream.


8

of low flow to maintain the water supply intothe fresh water pond located downstream.

The emergency spillway makes use of the

natural depression in a topographic saddlelocated to the right and upstream of the 1embankment. Flood discharges would passthrough this saddle and return to Mine SpringRun 1.5 miles downstream of the dam. Inaddition to the natural depression, someexcavation and clearing has been done in theemergency spillway. A discharge channelapproximately 6 feet deep and 24.5 feet widewas excavated through the saddle. A temporaryaccess road was constructed across thisdischarge channel, filling in a portion ofthe channel and giving the emergency spillwayan effective minimum elevation of 903.6 feetT.B.M. (Photo 2).

The reservoir is fed by runoff and springsfrom a 2.03 square mile drainage area southand east of the dam. Mine Spring run drainsthe watershed from the south and a smallunnamed tributary drains from the east. Theupper reaches of the watershed are denselywooded, very steep slopes of Great NorthMountain. The lower reaches, while lesssteep, are also densely wooded.

There are no facilities for draining thereservoir.

1.2.2 Location: Unimin Tailings Dam is located inFrederick County, Virginia approximately 1.2miles south of Gore, Virginia on Mine SpringRun, a tributary to Back Creek. A LocationPlan is included in this report in AppendixI.

1.2.3 Size Classification: The Unimin Tailings Damis 84.9 feet high and the reservoir storageat the crest of the dam (elevation 920.6 feetT.B.M.) is 1,102 acre-feet. Therefore, thedam is in the "intermediate" size category asdefined by the Recommended Guidelines forSafety Inspection of Dams.

1.2.4 Hazard Classification: Three homes arelocated approximately 1.0 miles downstream ofthe dam. A business (C. E. Minerals) is


9

located approximately 1.2 miles downstreamand three more homes are situated about 1.4miles downstream of the dam. There is dangerof loss of human life from large flows down-stream of the dam. Therefore, Unimin TailingsDam is in the "high" hazard category asdefined by the Recommended Guidelines forSafety Inspection of Dams. The hazard classi-fication used to categorize dams is a functionof location only and is not related to itsstability or probability of failure.

1.2.5 Ownership: The dam is owned by Unimin Corpora-tion, P.O. Box 38, Gore, Virginia 22637.

1.2.6 Purpose: The dam is used to provide wastetailing storage for the mining and milling ofglass sand by Unimin Corporation.

1.2.7 Design and Construction History: The dam wasdesigned by D'Appolonia Consulting Engi-neers, Inc. in 1976. The dam was constructedby Perry Engineering Company in the fall of1976 and spring of 1977.

1.2.8 Normal Operating Procedures: The reservoiris normally operated at the crest of theprincipal spillway (elevation 901.0 feetT.B.M.). A gate valve (as described inSection 1.2.1) can be operated during periodsof low flow to lower the level in the reservoirapproximately 4 feet in order to maintain thewater supply into the fresh water pond locateddownstream. No formal operating proceduresare followed for this dam.

1.3 Pertinent Data

1.3.1 Drainage Area: The drainage area tributaryto the dam is 2.03 square miles.

1.3.2 Discharge at Dam Site: The maximum dischargefrom the reservoir is unknown.Principal and Emergency Spillways:Pool level at top of dam . . . . 46,011 c.f.s.

1.3.3 Dam and Reservoir Data: Pertinent data onthe dam and reservoir are provided in thefollowing table:


10

TABLE 1.1 DAM AND RESERVOIR DATA

ReservoirCapacity

Elevation Area Acre- Watershed LengthItem (feet T.B.M.) (acres) feet (inches) (feet)

Top of Dam 920.6 36.7 1102 10.2 1950(minimum)

Principal 901.0 23.0 518 4.8 1400SpillwayCrest

Streambed at 835.7 - - - -

downstreamtoe of dam


- , , "- ' -,' . .. , ' .... | ..... ....1

SECTION 2 - ENGINEERING DATA

2.1 Design: The site was selected and the impoundment wasdesigned by D'Appolonia Consulting Engineers, Inc.Subsurface conditions were investigated and the embank-ment was designed to provide maximum strength forstability. Stability analyses were conducted andfactors of safety were calculated and are explained indetail in Section 6 - Dam Stability (also see Plates 9and 10, Appendix I). Control of seepage was providedby an upstream impervious blanket, a cut-off beneaththe embankment, downstream lateral drains, and aninternal chimney drain within the embankment. Theembankment design and construction is presented onPlates 2 and 3 in Appendix I. Boring and Test Pitlocations are shown on Plate 6. Soils information andsubsurface conditions are shown on Plates 7 and 8.

The principal and emergency spillways were designed todecant clarified water and discharge flood peaks. Theprincipal spillway was designed as two 30-inch diametervertical inlet pipes connected to a 24-inch diameterpipe passing beneath the embankment. However, duringthe field investigation, only one 42-inch diametervertical inlet pipe connected to a 24-inch diameteroutlet pipe was observed. A plunge pool was designedto still water exiting from the outlet pipe. An emer-gency spillway was designed to use the natural topo-graphic saddle on the right side of the reservoir todischarge flood flows. The principal and emergencyspillway designs and pertinent information are shown onPlates 4, 5, and 11 in Appendix I.

2.2 Construction: The dam was constructed by Perry Engineer-ing Company in the fall and early winter of 1976 andspring of 1977. A majority of the construction wasinspected by D'Appolonia Consulting Engineers' repre-sentatives. The upstream embankment materials(typically clayey) were placed and compacted at moisturecontents 5 to 6 percent above optimum according to theStandard Proctor Method (ASTM D698-70) to allow forexpected differential settlement over variable founda-tion and abutment materials without detrimental affectsto the water retention capabilities. The downstreamembankment materials (typically sandy) were placed atmoisture contents just slightly (2 to 3 percent) aboveoptimum to also allow for differential movement betweenthe valley floor and abutments. However, they were notplaced as wet as the upstream embankment materials toprovide the necessary strength and stability for thestructure.


13

Fill was placed in lifts of 6 to 9 inches. Compactionwas achieved with 6 to 8 passes of a Caterpillar 815compactor and Bros. SP-255 DA compactor. Much of thefill placement was observed by a D'Appolonia engineer.Periodic fill density testing was completed byD'Appolonia when a representative was present. Thedensities were compared to the results of StandardProctor Tests conducted on various samples of borrow toprovide comparison of fill densities to maximum densi-ties and optimum water contents. Recompaction wasconducted in areas where tests indicated unsatisfactorycompaction. In areas where fill placement was notinspected by the engineer, borings were taken to obtainundisturbed samples of the fill that were tested fordensity. The available summary of all density testresults indicates final densities were in excess of 93percent of maximum dry density.

Four piezometers and four settlement plates were in-stalled beneath the upstream embankment area prior toconstruction to monitor foundation behavior and ground-water levels.

Eight additional piezometers were installed beneath andwithin the downstream embankment after construction tomonitor actual water levels for comparison with assumeddesign water levels. The locations and depths of theupstream piezometers and settlement plates are shown onPlates 2 and 7 in Appendix I and are designated as 1-1through 1-4 (both a piezometer and an adjacent settlementplate were assigned the same identification number).The locations and depths of the eight downstream piezo-meters, designated B-1 through B-6, are also shown onPlates 2 and 7. Typical piezometer and settlementplate construction details are shown on Plate 5.

During the construction of approximately the lowerone-third of the embankment, no extensive differentialsettlement reportedly occurred. However, settlementmeasurements were not taken thereafter. Measurementsin the piezometers during and after construction do notindicate detrimental water levels within the embankment.

A key trench, varying between 5 feet to 13 feet deepand 17 to 23 feet wide, was excavated into the founda-tion soils near the upstream toe of the dam usingnormal excavating techniques. The trench did notextend to bedrock. Depth of the trench was governed bythe original groundwater level and was shallow in thecreek area. Initial clayey backfill could not becompacted in thin lifts due to water levels in the


14

trench. The initial lift was as thick as four feet andwas compacted as well as possible. After the firstlift, the trench was backfilled in 9 to 12 inch thicklayers compacted to specified density. In addition tothe key trench, an impervious blanket of clayey soilwas placed over the toe of the upstream embankment, theimmediate reservoir bottom, and the upstream abutments.The extent of the key trench and impervious blanket areillustrated on Plates 2 and 3.

2.3 Evaluation: The as-built drawings, design informationand construction information were adequate to assessall aspects of design and construction. The hydrologicand hydraulic data provided were adequate for review.The assessments made in this report are based on thisengineering data along with field observations.


15

- ,"'* * * * *

SECTION 3 - VISUAL INSPECTION

3.1 Findings

3.1.1 General: The field inspection was conductedon 30 and 31 October 1980. At the time ofinspection, the pool elevation was 897.4 feetT.B.M. and the elevation of the tailwater inthe stilling basin for the principal spillwaywas 836.2 feet T.B.M. The weather was clearwith temperatures in the 50's (degreesFahrenheit). The ground surface of theembankment and abutments was generally dry.The dam and appurtenant structures were foundto be in fair to good overall condition atthe time of the inspection. Deficienciesfound during the inspection will requireremedial treatment. The following are briefsummaries of deficiencies found during theinspection. A field sketch of conditionsnoted during the inspection is presented asPlate 1 in Appendix I. The complete visualinspection check list is provided in Appen-dix III. Control surveys of the dam areperformed regularly but were unavailable forthis report. In addition, state MiningInspectors visually review the dam as part oftheir regular inspections.

3.1.2 Dam: The embankment was found to be ingenerally good condition, with no surfacecracks or sloughs. The bench on the upstreamembankment and the embankment above the benchare well vegetated with grass and are noteroded. The upstream embankment bench appearsto be a reverse terrace and water appears tohave lain on the back side of the benchperiodically. Minor erosion has occurredjust below the bench down to the edge ofwater. This segment is below normal poollevel. The downstream embankment is benched.Above this bench there are a few erosionrills. Generally, these rills do not extendto the crest of the dam. With the exceptionof the rills, the downstream embankment abovethe bench is well vegetated. The downstreamembankment below the bench is moderatelyeroded. Erosion rills up to one foot deepextend from the bench to the toe of the dam.The downstream embankment below the bench hasmany areas of sparse vegetation.


17 I.AUIN PAO& SANOT F10

The junctions of the abutments and the up-stream embankment are not eroded. The rightupstream abutment contains the access roadsto the upstream bench and crest of the dam.The slopes for the access roads are verysteep and appear to be subject to erosion andminor slides. The access roads are primarilycut into bedrock and are generally situatedon the abutment rather than the dam. Thejunctions of the abutments and the downstreamembankment are slightly to moderately eroded.Moderate erosion occurs from the toe of thedam about three quarters of the way up to thebench on both the left and right abutmentjunctions.

A seep is located immediately to the left ofthe principal spillway outlet. The flow rateof this seep was estimated to be three tofive gallons per minute (g.p.m.). Thisseepage is believed to be from a sedimentcovered outlet pipe for the internal chimneyand lateral seepage relief drainage system.In the past, runoff from the junction of theleft abutment with the downstream embankmentwas directed through a separate diversionchannel away from the approximate area ofthis outlet. However, runoff now dischargesover the outlet area and sediment has probablycovered the pipe. A very minor seep islocated beneath the decant outlet pipe. Noother seepage or drainage was observed at thetime of inspection.

3.1.3 Appurtenant Structures: The principal spill-way riser and accompanying gate valve (asdescribed in Section 1.2.1) appear to be ingood condition and operable. The outlet pipefor the principal spillway is in good condi-tion, although the pipe does extend, un-supported, out from the embankment approxi-mately 5.5 feet. The plunge pool dischargearea for the principal spillway is also ingood condition. The principal spillwayinlet, outlet, discharge area and downstreamchannel are all free from debris and otherobstructions.

The emergency spillway (as described inSection 1.2.1) is primarily a natural topo-graphic saddle and is in relatively good


18

condition. The saddle has been cleared andsome areas of the spillway have been exca-vated for a discharge channel while otherareas have been filled. No significant signs 4

of erosion were observed, although vegetationis sparse, particularly on the excavation andfill areas. The emergency spillway, with theexception of the temporary access road acrossthe discharge channel, is free from debrisand is unobstructed.

3.1.4 Reservoir Area: The slopes immediatelysurrounding the reservoir are steep andheavily wooded. The banks of the reservoirare well vegetated with grasses and sometrees down to the edge of the banks. Noindications of significant erosion wereobserved. Soundings taken at the principalspillway riser show the water to be approxi-mately 5 feet deep at the time of inspection.

3.1.5 Downstream Channel: Discharge from theprincipal spillway of Unimin Tailings Damflows into the reservoir for the Unimin FreshWater Pond Dam almost immediately after leav-ing the discharge area. The short length ofdischarge channel is free from debris or otherobstructions. Beyond the Fresh Water Pond Dam,the downstream channel is a natural streamchannel, known as Mine Spring Run, which drainsinto Back Creek at the town of Gore, Virginia.There is some minor debris and miscellaneousjunk present in the channel immediately down-stream of the Fresh Water Pond Dam.

3.1.6 Instrumentation: Eight piezometers wereobserved on the dam at the time of inspection.Five piezometers were on the crest of thedam; one at either end and a cluster of threenear the center. The piezometer on the leftend of the crest measured 60 feet to thebottom with no water. The piezometer on theright end of the crest and the three clusterednear the center were plugged near the surfaceand could not be measured.

The remaining three piezometers were clusterednear the center of the bench on the downstreamembankment. The piezometer with the highestelevation could not be measured because of


19

equipment malfunction. The next lower piezo-meter measured 15.17 feet to the bottom withno water. The lowest piezometer measured32.87 feet to the bottom with no water.

There was no other instrumentation observedat the dam during the inspection.

3.2 Evaluation: The embankment was found to be in generallyfair to good condition, with no surface cracks orsloughs. All areas of erosion on the embankment shouldbe repaired, particularly the erosion rills below thebench on the downstream embankment. A grass covershould be established on all the sparsely vegetatedareas of the embankment. The eroded junctions of theleft and right abutments with the downstream embankmentshould be repaired and riprap placed to minimize erosion.The drains for seepage control that are reportedlylocated to the left and right of the principal spillwayoutlet should be uncovered.

The appurtenant structures were found to be in generallygood condition. A grass cover should be establishedthroughout the natural saddle that comprises the emer-gency spillway. A staff gage should be installed tomonitor reservoir levels above normal pool.


20

HA -.

SECTION 4 - OPERATION PROCEDURES

4.1 Procedures: The operation of the dam is primarily anautomatic function controlled by the principal andemergency spillways. Water entering the reservoirflows into the principal spillway at elevation 901.0feet T.B.M. When inflow is sufficient, the reservoirlevel rises above elevation 903.6 feet T.B.M. anddischarges through the emergency spillway. When inflowis insufficient to meet the water needs downstream, thegate valve attached to the principal spillway riser canbe operated to lower the reservoir approximately fourfeet below the crest of the principal spillway.

4.2 Maintenance of Dam: Maintenance of the dam is theresponsibility of the owner. Brief daily inspectionsare made by the owner. Also, the alignment of thecrest and elevations are surveyed regularly. However,a formal inspection and maintenance schedule has notbeen instituted.

4.3 Maintenance of Operating Facilities: The only oper-ating facility at the dam is the gate valve attached tothe spillway riser. The maintenance of this facilityis the responsibility of the owner. A formal inspec-tion and maintenance schedule has not been instituted.

4.4 Warning System: At the present time, there is no warn-ing system or emergency action plan in operation.

4.5 Evaluation: Past maintenance of the dam has been in-adequate. Regular, formal inspections should be made ofthe dam and appurtenant structures. A check listshould be compiled for use by the owner's representa-tive as a guide for the inspections. Maintenance itemsshould be corrected annually. A warning system andemergency action plan should be developed and put intooperation as soon as possible.


21

SECTION 5 - HYDRAULIC/HYDROLOGIC DATA

5.1 Design: Hydraulic and hydrologic design data were pro-vided by D'Appolonia Consulting Engineers, Inc. in theform of an As-Built Report (Reference 18, Appendix IV).The hydraulic and hydrologic data provided were adequatefor review and are presented on Plates 4, 5 and 11 ofthis report.

5.2 Hydrologic Information: No rainfall, stream gage or

reservoir stage records are maintained for this dam.

5.3 Flood Experience: No records were available.

5.4 Flood Potential: The Probable Maximum Flood (PMF) and1/2 Probable Maximum Flood (1/2 PMF) were routed through

the reservoir by the use of the HEC-l DB computer pro-gram (Reference 9, Appendix IV) and appropriate unithydrograph, precipitation, and storage-outflow data.Clark's Tc and R coefficients for the local drainageareas were estimated from basin characteristics. Therainfall applied to the unit hydrograph was taken frompublications by the U.S. Weather Bureau and the NationalOceanic and Atmospheric Administration (References 16and 17, Appendix IV). *Rainfall losses for the PMF and1/2 PMF were estimated at an initial loss of 1.0 inchesand a constant loss rate of 0.05 inches per hour there-after.

5.5 Reservoir Regulation: Pertinent dam and reservoir dataare provided in Table 1.1, paragraph 1.3.3.

Regulation of flow from the reservoir is primarily auto-matic. Normal flows are maintained by the crest of theprincipal spillway at elevation 901.0 feet T.B.M. Somemanual regulation of flow is possible by the operationof the gate valve attached to the principal spillwayriser which is approximately four feet below the spill-way crest.

The outlet discharge capacity was computed by hand; res-ervoir area was planimetered from the Hayfield, Virginia,7.5 minute USGS quadrangle; and storage capacity wascomputed by the HEC-1 DB program. Outlet dischargecapacity and storage capacity curves were computed toelevations above the crest of the dam. All flood rout-ings were begun with the dam at normal pool. Flowthrough both spillways was included in the routings.

NAME OF DAM: UNIMIN T--PACEDUMG PAZ B .AJ-NT 11iM

23

2,

5.6 Overtopping Potential: The probable rise of the res-ervoir and other pertinent information on reservoirperformance are shown in the following table:

TABLE 5.1 RESERVOIR PERFORMANCE

Hydrographs1/2

Item Normal' PMF PMF2

Peak flow, c.f.s.Inflow 1 7,177 14,353Outflow 1 7,001 14,105

Peak elev., ft. T.B.M. 897.4 909.3 911.7Emergency spillway'(elev. 903.6 ft. T.B.M.)Depth of flow, ft. - 5.7 8.2

Average velocity, f.p.s. - 11.1 13.3Duration of flow, hrs. - 25.0 30.7

Non-overflow section'(elev. 920.6 ft. T.B.M.)Depth of flow, ft. - -

Average velocity, f.p.s. - -

Total duration of over-topping, hrs. - -

Tailwater elev.,ft. T.B.M. 836.2 -

* Conditions at time of inspection.

2 The PMF is an estimate of flood discharges that may be

expected from the most severe combination of criticalmeteorologic and hydrologic conditions that arereasonably possible in a region.

' Velocity estimates were based on critical depth atcontrol section.

5.7 Reservoir Emptying Potential: There are no facilitiesfor draining the reservoir.

5.8 Evaluation: Unimin Tailings Dam is an "intermediate"size - "high" hazard dam requiring evaluation for aspillway design flood (SDF) equal to the PMF. Thespillways are capable of passing 100 percent of the PMFwithout overtopping the crest of the dam. The spillwayis adjudged as adequate.


24

SECTION 6 - DAM STABILITY

6.1 Foundation and Abutments: The dam is located in theValley and Ridge physiographic province of Virginiawhich is characterized by folded and faulted sedimentaryrocks. Bedrock in the vicinity of the dam consists ofsteeply dipping late Silurian to early Devonian agelimestone, sandstone (Oriskany), and shale. The Oriskanysandstone is clearly exposed on the northern abutment.The actual foundation for the dam consists of thickcombination residual, alluvial, and colluvial soilsaccording to the As-Built Report (Reference 18, AppendixIV). The principal soils underlying the dam are clayeysilts. Some sandy soils are present.

The dam foundation was not keyed into bedrock. Seepagecontrol consists of a 17 to 23 foot wide, by 5 to 13foot deep key trench excavated into the foundationsoils near the upstream toe of the dam and backfilledwith compacted clayey soils. In addition, an imperviousblanket of clayey soils was placed over the toe of theupstream embankment, the immediate reservoir bottom,and the upstream abutments.

6.2 Embankment

6.2.1 Materials: According to the available as-builtreport, the embankment materials range fromclayey to sandy soils. The clayey soils wereplaced on the upstream side of the structure.The sandy soils were placed on the downstreamside of the structure. Brown silt with alittle fine to medium sand and numerous sand-stone fragments (ML group soil-Unified Classi-fication System) was noted as the typicalsoil on the downstream embankment during thefield inspection. Most of the soil samplesclassified by D'Appolonia from test pits inthe borrow area were ML group soils.

6.2.2 Stability: Stability analyses were conductedby D Appolonia Consulting Engineers, Inc.using a computerized version of Bishop'sMethod of Slices. Analyses were conducted toaddress short term conditions occurringduring construction (not applicable now) andlong term conditions. Shear strength valuesof representative embankment and foundationmaterials used in the analyses were derived


25

"- - -- " . .. ", *.- : , ' '

from laboratory consolidated undrainedtriaxial shear tests, direct shear strengthtests, and field investigations. The valuesfor the long term analyses are as follows:

Generalized Angle ofSoil Internal Cohesion

Description Material Friction (0) (C(p.s.f.))

Clayey Silt Borrow Embankment 280 02% to 3% AboveOptimum 1

Clayey Silt Borrow Embankment 260 4005% to 6% AboveOptimum

Boulders, Rock Foundation 340 0Fragments, SomeSilty Sand

Clayey Silt, Some Foundation 250 200Rock FragmentsSoft Clayey Silt, Foundation 240 500 Some Rock Frag-mentsClayey Sand, Some Foundation 300 0Rock Fragments

Individual analyses were made for failureswithin the upstream and downstream embankmentsand failures through the foundation soils onthe upstream and downstream sides. Theconfiguration of the dam used in these analysesis the same as the as-built configuration.The phreatic surface was assumed to be con-trolled by the internal chimney drain anddownstream lateral drains. Measurementstaken in the piezometers that were not cloggedduring the field inspection generally showthat the phreatic surface is lower than thatassumed during the stability analyses andindicate that the drainage system is perform-ing satisfactorily. The factor of safetycalculated against the four failure casesdescribed above are as follows:


26

Type of Factor of SafetyFailure Against Failure

Downstream EmbankmentFailure From Crest toBench 1.46Failure Through FoundationAffecting Entire DownstreamEmbankment and Crest of Dam 1.72

Upstream Embankment FailureAffecting Crest and Upper3/4 of Embankment 2.16

Failure Through FoundationAffecting Entire UpstreamEmbankment and Crest of Dam 2.71

The visual inspection of the structure revealedno signs of instability such as slumping,tension cracks, or unusual alignment alongthe crest. However, moderate erosion of thedownstream embankment (primarily below thebench) has occurred and should be correctedto prevent additional damage that couldaffect the stability of the existing slopes.The as-built plans indicate that there shouldbe two 8-inch*C.M.P. outlets, one on eachside of the principal spillway outlet, forthe internal chimney drain and lateraldrains within the downstream embankment.Neither of these pipes was observed duringthe visual inspection. A discharge (3-5 g.p.m.)was occurring from the approximate area of theleft drain. No discharge was occurring fromthe approximate area of the right drain. Theoutlets may have been covered over by sedimenteroded from the embankment above the drains.It is not clear from available informationdescribing construction history whether thedrains were constructed as designed. If thetwo outlets were constructed as designed, theyshould be uncovered and cleaned out to ensureproper operations.

6.2.3 Seismic Stability: The dam is located inSeismic Zone 2 which presents no hazard fromearthquakes according to the RecommendedGuidelines for Safety Inspection of Dams bythe Department of the Army, Office of the


27

- .-... A

Chief of Engineers. This determination iscontingent on the requirements that staticstability conditions are satisfactory andconventional safety margins exist.

6.3 Evaluation: The design of the dam is such that accept-able factors of safety against failure have been pro-vided. The existing configuration of the embankment isthe same as that assumed during stability analysesconducted during design. The visual inspection did notindicate any stability problems. Only minor mainte-nance is required to ensure structural integrity.Measurements taken in the existing piezometers thatwere not clogged revealed that the phreatic surface isadequately controlled by the internal drainage system.Adequate stability analyses, considering the presentconditions of the dam, have been conducted.


28

SECTION 7 - ASSESSMENT/REMEDIAL MEASURES

7.1 Dam Assessment: The engineering data provided wereadequate to assess the design and construction ofUnimin Tailings Dam. Hydraulic and hydrologic dataprovided were adequate for review. Adequate stabilityanalyses were available for review during these investi-gations which indicated acceptable factors of safetyagainst failure. Deficiencies discovered during thefield inspection and office analyses will requireremedial treatment. The dam and appurtenant structuresare generally in good overall condition. Maintenanceof the dam is not considered adequate.

Using the Corps of Engineers' screening criteria forinitial review of spillway adequacy, the PMF was se-lected as the SDF for the "intermediate" size - "high"hazard classification of Unimin Tailings Dam. Thespillways are capable of passing 100 percent of the PMFwithout overtopping the crest of the dam and are ad-judged as adequate.

Some minor erosion has taken place below the benches onboth the upstream and downstream embankments. Thejunction of the downstream embankment with both theleft and right abutments below the bench showed moderateerosion. Some areas of the embankment and the emer-gency spillway are sparsely vegetated. The seepagecontrol drain outlets have been covered by sediment.The principal spillway outlet extends unsupported overthe plunge pool.

There is no warning system or emergency action plancurrently in operation.

7.2 Recommended Remedial Measures: A formal warning systemand emergency action plan should be developed and putinto operation as soon as possible. The emergencyaction plan should list steps to be taken to helpprevent failure of the dam in an emergency.

Regular, formal inspections should be made of the damand appurtenant structures. A check list should becompiled for use by the owner's representative as aguide for the inspections. Maintenance items should becompleted annually.

The following repair items should be accomplished aspart of the general maintenance of the dam:


29

1) Repair all areas of erosion on the embankment,particularly the erosion rills on the downstreamembankment below the bench.

2) Establish a grass cover over all the sparselyvegetated areas of the embankment.

3) Repair and riprap the eroded junctions of the leftand right abutments with the downstream embankment.

4) Uncover the outlet pipes for the internal chimneyand lateral seepage control drains.

5) Establish a grass cover over the sparsely vegetatedareas of the emergency spillway.

6) Install a staff gage to monitor reservoir levelsabove normal pool.


30 ,a.. '4*.

2, $ .

APPENDIX I

PLATES

I. -f'-

CONTENTS

Location Plan

Plate 1: Field Sketch i

Plate 2: General Plan

Plate 3: Typical Section Through Dam

Plate 4: Plan and Profile Decant System

Plate 5: Details

Plate 6: Plan and Location of Borings TestPits and Auger Borings

Plate 7: Subsurface Investigation Section C-C and D-D

Plate 8: Test Pit Logs T-12 Through T-16

Plate 9: Slope Stability Analysis

Plate 10: Laboratory Data Triaxial and Direct ShearStrength Tests

Plate 11: Hydrology and Hydraulic Data

Plate 12: Top of Dam and Spillway Profile

Plate 13: Typical Dam Cross Section


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KEY TRENCH DIMENSIONS

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_______ (FT) 'II 'F1)

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DAM CLT L 92, (PQOJECTLEII ______ _______ ______ _________ ______ ____

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GNAT CLC!T C;~. NEATOI R 6-~UWA, CBTAINEO SOLELY FOR USEC.S'N'.CO

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I

SOIL PARtAMETERS UISED ~ STAILITY ANALYSIS .-

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-940

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C-ESTti. 2, -920

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840

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SOIL 6

780

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720

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6. ' 2500

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0

S- TL OAP 50% PROBABLE MAX!MUM PREC!"ITATI

STORM INFLOW- OUTFLOV HY,,OGRAP

0 ' 8 I20 24 26

TIME. (HOURS)

A RE A. ACRES

PROBABLE MAXIMUM PRECIPITATION STORM 100 o 0o 70 60 50 40 E _-20

000INFLOW- OUTFLOW HYDROGRAPHS

4 , / / '" OAREA CURVE :,

9,-

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STAGE D SCHA GE RELATIONSHIP! Io-

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ELEVATION Vs tR.AA RAOD VOLU

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TiME (HRS) DISCHIARGE Q0,CF S

MAX!NIUM PREC!PIT-ATI ON STAGE DISCHARGE RELATIONSHIP FOR-OUTFLOW HYDROGRAPHS DECANT STRUCTURE

AREA. ACRES60 50 40 30 20 cl 0 20

1900

I6 10 PRECCPITATION.50,6~ ..4 .- c~qvt NO, 75

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V*~~w HOURS;VOUM

Ve9* A2, AN VOLUME

MICHAEL BAKER, JR., INC.

THE BAKER ENGINEERS

Box 280Beaver, Pa. 15009

IT

00

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MICHAEL BAKER. JR., INC.

THE BAKER ENGINEERS

Box 280Beaver. Pa. 1500O9

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C.C

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APPENDIX II

PHOTOGRAPHS

• • ' .

CONTENTS

Photo I - Emergency Spillway (Natural Saddle), Decant Inlet

Photo 2 - View Across Emergency Spillway Control Section, MenStanding on Small Berm in Eroded Discharge Channel

Photo 3 - Downstream Embankment, Decant Outlet, SeepageArea (Covered Drain?) in Channel to Right ofDecant Outlet

Photo 4 - Decant Outlet, Stilling Basin

Photo 5 - Seep (Covered Drain?) Emanating from ChannelImmediately Left of Decant Outlet

Photo 6 - Erosion on Downstream Embankment

Photo 7 - Erosion at Junction of Downstream Embankmentwith Left Abutment

Photo 8 - Downstream View from Crest

Note: Photographs taken on 30 October 1980.

N! NAME OF DAM: UNIMIN TAILINGS DAM

UNIMIN TAILINGS DAM

PHOTO 1. Emergency Spillway (Natural Saddle) and Decant Inlet

PHOTO 2. View Across Emergency Spillway Control SectionNt:Men Standing on Small Baerm In Eroded Discharge Channel

UNIMIN TAILINGS DAM

","

PHOTO 3. Downstream Embankment, Decant Outlet and Seepage Area InChannel to Right of Decant Outlet

PHOTO 4. Decant Outlet and Stilling Basin

UNIMIN TAILINGS DAM

II

44

PHOTO 5. Seep Emanating from Channel Immediately Left of Decant Outlet

t 3IIPHOTO 6. Erosion on Downstream Embankment

UNIMIN TAILINGS DAM

PHOTO 7. Erosion at Junction of Downstream Embankment withLeft Abutment

PHOTO 8. Downstream View from Crest

APPENDIX III

VISUAL INSPECTION CHECK LIST

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APPENDIX IV

GENERAL REFERENCES

GENERAL REFERENCES

1. Bureau of Reclamation, U.S. Department of the Interior,Design of Small Dams, A Water Resources TechnicalPublication, Revised Reprint, 1977.

2. Chow, Ven Te, Handbook of Applied Hydrology, McGraw -

Hill Book Company, New York, 1964.

3. Chow, Ven Te, Open Channel Hydraulics, McGraw - HillBook Company, New York, First Edition, 1959.

4. Commonwealth of Virginia, "Geologic Map of Virginia,"Department of Conservation and Economic Development,and Division of Mineral Resources, 1963.

5. HR 33, "Seasonal Variations of Probable Maximum Precipita-tion, East of the 105th Meridian for Areas 10 to 1000Square Miles and Durations of 6 to 48 Hours," (1956).

6. King, Horace Williams and Brater, Ernest F., Handbookof Hydraulics, Fifth Edition, McGraw - Hill Book Company,New York, 1963.

7. Soil Conservation Service, "National Engineering Handbook -Section 4, Hydrology," U.S. Department of Agriculture,1964.

8. Soil Conservation Service, "National Engineering Handbook -Section 5, Hydraulics," U.S. Department of Agriculture.

9. U.S. Army, Hydrologic Engineering Center, "Flood HydrographPackage (HEC-l), Dam Safety Investigations, UsersManual," Corps of Engineers, Davis, California, September1978.

10. U.S. Army, Hydrologic Engineering Center, "HEC-2 WaterSurface Profiles, Users Manual," Corps of Engineers,Davis, California, October 1973.

11. U.S. Army, "Inventory of United States Dams," Corps ofEngineers, 9 September 1978.

12. U.S. Army, Office of the Chief of Engineers, "Appendix D,Recommended Guidelines for Safety Inspection of Dams,"National Program of Inspection of Dams, Volume 1, Corpsof Engineers, Washington, D.C., May 1975.


IV-1

13. U.S. Army, Office of the Chief of Engineers, EngineeringCircular EC-1l10-2-163 (Draft Engineering Manual),"Spillway and Freeboard Requirements for Dams, Appendix C,Hydrometeorological Criteria and Hyetograph Estimates,"(August 1975).

14. U.S. Army, Office of the Chief of Engineers, EngineeringCircular EC-ll10-2-188, "Engineering and Design, NationalProgram of Inspection of Non-Federal Dams," Corps ofEngineers, Washington, D.C., 30 December 1977.

15. U.S. Army, Office of the Chief of Engineers, EngineerTechnical Letter No. ETL 1110-2-234, "Engineering andDesign, National Program of Inspection of Non-FederalDams, Review of Spillway Adequacy," Corps of Engineers,Washington, D.C., 10 May 1978.

16. U.S. Department of Commerce, "Technical Paper No. 40,Rainfall Frequency Atlas of the United States for Dura-tions from 30 Minutes to 24 Hours and Return Periodsfrom 1 to 100 Years," Weather Bureau, Washington, D.C.,May 1961.

17. U.S. Department of Commerce, National Oceanic andAtmospheric Administration, "Hydrometeorological ReportNo. 51, Probable Maximum Precipitation Estimates,United States East of the 105th Meridian," Washington,D.C., June 1978.

18. D'Appolonia Consulting Engineers, Inc., "As-BuiltReport, Stage 1 Embankment, Tailings Disposal Facili-ties, Gore, Virginia," Unisil Corporation, Greenwich,Connecticut, May 1978.


IV-2

A1

Documents

NATIONAL DAM SAFETY PROGRAM. UNIMIN TAILINGS DAM ... · 803 Front Street Norfolk, Virginia 23510 11. CONTROLLING OFFICE NAME AND ADDRESS Ii. REPORT DATE.Janomy 1981 13. NUMBER OF