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SUPPLE ETT TO

ENGINEERING GEOLOGY

OF THE

KE0 WEE T0XAWAY PROJECT

DUKE POWER COMPANY

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/4hrd_ /11 G J7 Gorin.,Atlanta, Georgia

Septe=ber 15, 1973- 1Qeelegist -

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SUPPLEMENT TO

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' ENGINEERING GEOLOGY*

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KE0 WEE - T0XAWAY PROJECT

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TABLE OF CONTENTS

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I. INTRODUCTION

| -A. Previous Reportsb. Final Foundation Report

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II. REGIONAL GEOLOGY

A. Structural GeologyB. Jointing-

III. RELATION OF SITEGEOLOGY TO CONSTRUCTION

A. GeneralB. Rock ClassificationC. Abutment WeatheringD. JointingE. Geology of the River Section

IV. SUMMARY,,

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SUPPLEMENT TO-

ENGINEERING GEOLOGY_

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KE0 WEE - T0XAWAY PROJECT'_

( I.INTRODUCTION .\ A.

Previous Recorts.

mation for feasibility and design st dDuring the site explorations cond.

ucted to provide infor-

prepared on the engineering geology u ies, a report dated December 16, 1965, wasof the project.

boring information; field studies; The report was based on core

described the foundation conditio and reference to the geologic litera ture. Itidentified the engineering and c ns which should be expected at the site, and itsubsurface materials. onstruction aspects which should be e

During construction, this inform xpected of the,

of the borings provided guidance on e ation and the geologic logs*

grouting procedures. This supplement xcavation grades; blasting criteria; and

will provide a description of the g will up-date the pre-constructionreport, and

Pre-construction reports on avail bileologic conditions actually encounter de.

materials were prepared by Law Engi ity and testing of constructiona

ineering Testing Company. |

The original engineering geology |

the Keovee-Toxaway Project. report described all of the three dam

relatively routine, and adequate fou dConstruction of Little River and Ks in

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eowee Dams was

Dam, because of its size and th n ation records are on file.However, Jocassee

a more detailed compilation of the vari de many foundation problems encounte_*

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red, requires $ 1

This supplement to the original construction techniques which were :|e.

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Jocassee, although many of the freport will therefor.e be primarily co fi,used. '

n ned totpply to the other two proj eatures on general and regional geol ,f

ects also. ogy would fi

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B. Final Foundation Reoort. It was originally proposed that on completion of

construction a final foundation report for the Jocassee Project would be prepared

covering all aspects of excavation, foundation preparation, and grouting. Such a*

record is an important part of any dam where other than routine foundation problems

are experienced, as it provides as - built information which is available in the

event later corrective measuras are required. Because of the large volume of

foundation records and the many people concerned with the several aspects of con-

struction, it was decided that the Board of Consultants Reports would represent the

best continuing narrative of the many foundation procedures which were put into

effect as the situation required during each phase of construction. The compilation

of all of these consultant's reports would be supplement $ by as - built construction

records, including drawings depicting grouting procedures, foundation geology and- .

topography, and areas of special treatment. Detailed photographic coverage was

made as the work progressed, and is' on file. All of this data, brought together,.

will comprise the final foundation report..

The purpose of this supplement to the pre-construction geology report is

to revise the original engineering geology studies and make the foundation geology

as disclosed during construction a matter of record. It will provide a backgroundi

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for interpretation of the final drawings on foundation geology and grouting, and will

be an integral part of the overall foundation record package. So far as possible

this supplement will follow the format and pertinent portions of the original

report.

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II. REGIONAL GEOLOGY

A. Structural Geologv. Concepts of the geology of western South Carolina

have been revised since the original report was compiled in 1965. Additional

dctailed geologic mapping and structural studies have changed previously accepted

formation nomenclature, and additional revisions may be expected as more worktws is done in this geimlogically complex area. However, the previously recognized

classification of the rock unit underlying the site as the Henderson gneiss re-

mains unchanged, and the present revisions and future progress in region:1 studies

will not materially change our understanding of the engineering characteristicsi

of the site.

An up-dated discussion of the regional geology, together with a pertinent

bibliography, is contained in report to Duke Power Company on " Engineering Geology

of Bad Creek Pumped Storage Project", revised August 1,1973.,

B. Jointing. Study of topographic maps covering the upper tributaries of the

Keowee River suggests that river alignments are largely joint controlled. This

was confirmed during the excavation of the Jocassee abutments and river sections.

The regional joint pattern was reflected in the dam foundation, and was one of

the primary sources of construction difficulty. The impact of the jointing on

the dam foundation is referred to in many of the Board of Consultants reports,i

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|-and will be further discussed elsewhere in this supplement.

III. RELATION OF SITEGEOLOGY TO CONSTRUCTION

A. General. The original exploratory borings made at the Jocassee site pro- -

vided a reasonably complete picture of the geologic problems which could be expected

during construction. Additional borings night have .provided a slightly more

!detailed picture of the anticipated foundation def ects, but there were actually

no " surprises" when the foundation was finally uncovered. The great depths of

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weathering on the abutments; the alternate hard and weathered rock layers; the

general joint pattern; and the deeply weathered river bottom trench; all were anti .

cipated from the pre-construction boring data and geologic report. Each of theseii features will be examined in more detail in the following sections.

B. Rock Classification. The original petrographic examinations identified''

9the rock as a quartz-microcline augen gneiss, and this classification was found

to apply to most of th'e foundation rock. Exceptions are the layers of hornblende-

biotite gneiss (in many regional studies classified as "amphibolite") which were

encountered in the upstream part of the foundation at the base of the lef t abutment.

Local bands of pegmatite also occurred at intervals along the foliation. Thesei

had been identified in the exploratory borings, but it was not expected that they

would be as continuous throughout the abutments as they proved to be.

The early report stressed the characteristic of the site rocks to weather

deeply into silty sands without chemically altering into the red su rface clays so

characteristic of the balance of the Southeastern Piedmont. This was born out

during construction, when it was found that borrow areas contained only thin surface

layers of red lean clays. Since the clay - like materials were more desirable

for compacted core material than the gray silty sands, they were used to plate

the abutment rock prior to placing the predominantly silty sand core. However,

the exact reason for the poor clay - soil forming characteristic of the ninerals

comprising the Jocassee rock remains unclear, and the discussions on this feature,

contained in th'e criginal. report still apply.

C. Abutment Weathering. The weathering characteristics of the site rocks

also directly effected the depth of general rock disentigration in the abutments,

and it was this feature which proved to be the one of the most time consuming and

expensive efforts during construction. The preliminary borings had served notice

that unathering uas deep, and also that af ter sound rock was initially reached,

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local weathered zones could again be expected at icwer elevations. This proved

to be the case, and abutment stripping became the major operation during the early

phases of the work.

At the start of abutment excavation it was obvious that the same qualityI of foundation would not be required for the areas to be covered by the rock shell

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zones as would the foundations under the core and filter zones. It was agreed thati

removal of.only the most highly weathered material was all that would be required

to support the rock shell. In instances whera there was concern for point contact

of the rock of the embankment fill penetrating the weathered abutment and contributing

to settle =ent, a' layer of transition material was placed between the embankment rock

and the abutment.

Excavation of the abutments in the areas to be occupied by the impervious

core presented a greater problem. As in most instances where dams are constructed

on weathered foundations, the decision had to be made as to where abutment stripping ,

should stop, and where foundation grouting should begin. It became apparent that

a conventional core trench with precise side slopes and alignment would be difficult

to secure. Further, it was desirable to have as wide a contact as possible between

the entire impervious core width and the prepared abutment. It was therefore decided

to excavate to partially weathered but groutable rock in the entire abutment

underlying the core section rather than providing a narrow core trench to sounder,

rock.

Other features which influenced the extent and configuration of abutment

stripping were the pegmatite zones which were found along the foliation at several~

elevations in each abutment. These zones averaged one to two feet thick, and the~ -

feldspars weathered more readily than the surrounding rock. The result was a series

| of weathered seams extending along the foliation back into the abutments. During

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stripping opurations the contractor's haul roads were often along the base of

these seams, resulting in a stepped configuration to both abutments, rather than

the a re regular slopes which would be produced in uniform rock. Special treatment"

of these seams included flaring the impervious cut-off material upstream to provide.

a longer path of seepage; concrete plugs along the outcrops; additional filter

sections downstream of the core; and additional grouting along the cut-off.j;

In summary, the final grades and configuration of the abutments reflect'

both the depth of weathering of the mass of the abutment rock and the isolated

weatherec planes produced by the pegmatite zones. Joint orientation was also

important in defining abutment stripping and is discussed below.

D. Jointing. A cursory study of topographic maps shcws the stream patterns.

!of western South Carolina to be greatly influenced by joint control. The Jocassee

Dam site is no exception. Measurement of joint orientations on outcrops at the

site during initial exploration. studies showed that the joints were primarily parallel

to the strike of the foliation (strike joints) and perpendicular to the strike (dip

joints). The joint pattern was identified also in the exploratory borings, although

the steeply dipping to near - vertical joints were not well delineated by the

vertical borings.

While preliminary studies recognized that the jointing system would be ani! important feature in controlling excavation, the difficulty in treating abutment

jointing for cut-off purposes was perhaps underestimated. The intersection of

the dip and strike joints at nearly right angles produced block-like rock masses*

in the abutments which were sepa' rated by the joints. Where these joints were

sufficiently far below the abutment surface they were of little importance. Butf

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near the surface the joints were highly weathered, and the blocks formed discrete

Slumping of the blocks as the river eroded the stream valley widened themasses.

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joint planes, producing sand-filled joints both parallel and perpendicular to the

das axis. Much of this slu=p block pattern was removed by excavation for the

impervious core contact. However, it was not considered practical to excavate

deeply enough into the abutments to reach a point where the jointing became tight, and

supplementary grout holes were required to insure the integrity of the cut-off.

E. Geolorv of the River Section. The portion of the dam embankment founded

on the river bottom or flood plain portion of the~ site presented different problems

than the deeply weathered abutments described above', and requir'ed special treatment.

It was recognized early in the preliminary investigations that the gorge

through which the Keowee River passes at the Jocassee site was probably joint

controlled. The remarkably continuous alignment 6f the Keowee River at the site

with the.Whitewater River upstream of Camp Jocassee (Sales quad sheet) suggests,

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that they were possibly continuous at an earlier period of the stream's history.|'

It was therefore not surprising when a preliminary boring disclosed a deeply weathered|

; zone of closely spaced joints in the river bed on t.he lef t side of the valley. At3

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that time it was not feasible to explore this area in more detail, but all evidence

indicated that it represented a deeply weathered, joint controlled zone, which

I would require some type of treatment to depths of about 80 feet..

Excavation of the river valley section confirmed the pre-construction as--

sumptions. The river at one time apparently occupied a channel along this con-| .' ' centration of joints, producing a zone of deep weathering parallel to the base of| the left abutment. The rock disentigration along these joints na aggravated by

_the proximity of the zone of biotite-hornblende gneiss, which decays more rapidly __

than the gneiss underlying most of the site. The only feasible and positive

. cut-off treatment for this zone was to completely remove the weathered material

down to groutable rock, and backfill with selected fill. The trench thus formed

in the foundation extended to approximately , elevation 690, the deepest excavation

on the project.

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The jointed zone was closely inspected during foundation clean-up to

determine if the closely spaced dip joints actually represented a fault. No

indication of movement was found. However, in preparing the detailed geologic

map of the foundation, it was noted that the orientation of joints on the right

abutment was slightly different from those on the lef t abutment (about N 57*W'

compared with N 46 W). This would indicate that the deeply weathered zone does0

in fact represent a fault. However, movement was apparently slight, and, as -in,

all faulting in the Piedmont and Blue Ridge Provinces, it is geologically very

ancient.

j IV SUmfARY

The preliminary foundation investigations for Jocassee Dam indicated deep'

abutment weathering; adverse joint orientation; and a deeply weathered trench under

the river section. During constuction it was found that the geologic conditions

were essentially as anticipated. Additional exploration effort during the pre-

construction studies would not have changed the basic design or the methods of*

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1 treating the foundation defects.;

During the construction of the project, techniques were developed to solve

foundation problems as they were encountered. The most detailed continuing,

narrative of the procedures used for each situaticn is a compilation of the

many Board of Consultants Reports. These reports, together with foundation maps,

grouting records, and photographic coverage, will provide a complete foundation-

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record for future reference as required.

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