The plann ing of dam and reservoir sites must in

c lude careful consideration o f the geologic environment .

Knowledge o f the princ ipal elements o f this environment

topography , hydrology , and geology contributes to the

success of a proj ect . Various conditions in bedrock and

unconsolidated depos i ts,such as di scontinuities and the

presence o f shale or cavernous limestone , lead to problems

in dam and reservoir construct ion and maintenance . Avai l

abi l ity of construction materials , the effect of the project on the area 's mineral resources , and the balance be

tween si te quality and budget , are economic aspects o f dam

and reservoir p lanning for which geologic considerations

are important .

I NTRODUCT I ON

As man strives to improve his environment , the deve lopment o f a

dam and reservoir is frequent ly planned to serve one or more purposes , such

as flood control , water supply , and water-based recreat ion . Dams and reser

vo irs must be bui lt on the surface of the earth and , in many instances , ,

with

natural earth materials . Therefore , one o f the first steps in planning such

a proj ect is the evaluat ion of geologic conditions to determine the suita

b i l i ty o f the s ite for the proposed proj ect . Thi s note provides a brief

di scuss ion of the princ ipal factors that mus t be cons idered in evaluating

the geologic setting o f dam and reservoir proj ects in I l l inoi s .

GEOLOG I C FACTORS

The geo logic condit ions that exist at and in the vic inity of everyproposed dam and reservoir should be known and understood by those reS pon

s ible for its planning and design . The principal elements that must be con

sidered are topography , hydrology , hydrogeology , and geology .

Topography

I n the geological sense , topography i s the configuration of the

land surface , and it inc ludes the location , s ize , and shape o f such physical

features as hi l ls , ridges , val leys,streams , and lakes . Topographic maps

show these features .

As a short dam is most economical to construct , the s ite selected

should be at a point where the val ley i s narrow . The val ley upstream should

have s ides high enough to contain the planned reservoir without necess itating

construction of smal l dams or dikes in low divides . The reservoir s ides

should be relatively steep so that a minimal area would be exposed by the

lowering o f the water surface during normal Operat ion of the reservo ir .

However , i f swimming is one of the water-based act ivit ies being planned ,

there must be an adequate depth of water in the vic ini ty of gent ly l ping

shores . Topographic relief and steepness of the S I O pes in the entire water

shed influence the rate at which water runs o ff the surface of the land ,

which is an important factor in both dam and spi l lway design . They are im

portant also bec ause they influence the potenti al rate of eros ion , the prob

able amount of s i ltat ion to be expected , and the adaptabi l ity of the project

to certain uses .

I f the proposed proj ect involves the maintenance of wi ldl i fe , the

topography of the s i te must insure a suffic ient depth and supply of water

throughout the dry season . The reservoir should be planned to avoid large

areas of water less than 4 feet deep because aquatic plants become a nui

sance in such shal low water , and water less than 2 feet deep becomes choked

with marsh plants . The latter condition also creates a mosquito problem .

A biologi st should be consulted on matters influenc ing the bio logical envi

ronment .

I nspection o f a topographic map coupled with reconnaissance of

the land often is suffic ient to est abli sh the general tapographic sui tabi l ity

of a proposed dam and reservoir s ite . This i s the first and eas iest step in

determining the feas ibi l ity of a proposed proj ect .

Hydrology and Hydrogeology

Hydrology is the sc ience relating to the water of the earth , i ts

distribut ion and i ts phenomena . To be success ful , a dam and reservoir pro

ject must have an adequate and cont inuous supply of water suitable for the

- 3

intended uses of the reservoir . Hydrologic informat ion and investigation

wi l l be required in varying degree , depending upon the s ize of the proj ect .

The annual rainfall , the ratio of watershed area to reservoir area,and the

volume of stream flow at al l seasons of the year must be known . The study

of hydrogeology to determine whether ground water would contribute to the

reservoir or whether the reservoir would lose water to the ground-water

system is also essential . The reservoir capac i ty and maximum and minimum

reservoir yield also must be known so that commitments for water wi l l not

exceed the quantity of water avai lable .

The behavior of streams in the proj ec t watershed is important . I n

areas where vegetation is spar se , occasional short periods of high- intens i ty

rainfal l may produce flash floods with high peak discharges capable of co l

leeting a great quantity of soi l debri s and tran sporting it into the reser

voir . Run -off from large storms and flood flows must be estimated or meas

ured , as this informat ion is necessary for the proper des ign of the dam and

for the determination of S pi l lway capac ity .

Accumulation o f sediment may soon reduce the usefulness of a reser

voir and may ult imately destroy its capacity . In I l l inois , s i lt derived from

surfic ial deposits of loess becomes trapped in reservoi rs ' for this reason ,

a percentage o f reservoir capaci ty is al lotted to sediment s torage . Sediment

control is probably one of the most di fficult of soi l conservation problems

and requires the cooperation of al l land users in the drainage area tribu

tary to the reservoir . Where s i ltation is severe , it may be des irable or

even necessary to cons ider two reservoirs in series , the upstream one de

s igned to function as a sedimentat ion basin to prevent the rapid accumula

t ion of si lt in the down stream reservoir .

Turbidity of the proposed reservoir is another phase of hydrology

that should be considered . Too much turbidity , or suspended sediment , makes

a reservoir unsuitable for the propagat ion of des ired aquatic li fe or for

swimming . Pol lut ion of al l kinds must also be considered because it can be

detrimental to the recreational or industrial usefulness of the reservoir .

Sources and kinds of pol lut ion should be ascertained and control or regula

tory measures inc luded in the planning .

Whi le hydrology is properly an important factor in the evaluation

of a dam and reservoir proj ect ,the study of hydrologic factors is the work

of the spec ial ist in hydrology,espec ial ly in the planning of large proj ects .

G eology

I t has been said that construct ion of a dam and reservoir causes

more interference wi th natural condit ions than does any other civi l engi

neering Operation . Knowledge o f the geological s ituation is essent ial as a

bas is for sound engineering,espec ial ly in the investigation of dam and res

ervo ir s ites , for an error in geological interpretat ion or the fai lure to

discover some relatively minor geologic detai l may be cost ly and sometimes

hazardous .

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To j udge properly the feas i bi l i ty O f a proposed dam and reservoir

proj ect , it is necessary to know the kind , distribution , and success ion O f

the rocks and other geologic uni ts in the proj ect area , for the stabi l ity Of

the dam and the water-holding abi lity of the reservoir are direct ly related

to them . Facets Of geology that must be evaluated in determining the suita

b i l i ty of a proj ect s ite inc lude ( 1)the atti tudes of the units- that i s ,

whether they are flat lying or inc lined ' (2)the depth and extent of weath

ering ' (3)the presence and condition Of di scont inuit ies , such as Open or

c losed joints , faults , or solut ion channels ' (4)the presence O f layers of

sand or si lt and of old soi l zones . The engineering propert ies of the geo

logic units ' for example , their strength or abi l ity to bear the weight of

the dam , their reaction to alternate wett ing and drying , and thei r permea

b i l ity' are direct ly re lated to the kind Of rock or unconsolidated mat erial

involved and , therefore , to the geology .

Although the detai ls o f I l l inois geology are complex , most O f the

geologic units exposed at the surface be long to two general c lasses , sedi

mentary rocks and unconsolidated glac ial and al luvi al,or stream- laid , depos

i ts . Under confined condit ions nearly al l sedimentary rocks and many un

consolidated glac ial and al luvial depos its ,i f sound and unweathered , have

more than enough strength to support the weight of a dam .

Dams ite and reservoir problems general ly ari se from conditions

assoc iated with discont inuit ies in the bedrock and unconsol idated deposits

rather than from an inherent defic iency of the material . The characteris

t ics O f the sedimentary rocks and some condit ions that are commonly the

sources O f problems in dam and reservoir construct ion and maintenance are

described briefly below . I f the condi tions are known and understood , prob

lems can general ly be avoided by appropri ate des ign or by remedial treatment .

Sources of Problems in Areas of Sedimentary Rocks

Discontinuit ies ' The princ ipal discont inuit ies in sedimentary

rocks are bedding planes,j oints ,

faults,and so lut ion openings .

'edding planes mark the upper and lower boundaries of each layer

of sediment and are present in al l sedimentary rocks . They may be c losely

or widely spaced , t ight ly c losed or Open , fresh or weathered . Thin layers

of c lay or soft shale may occur along a bedding plane , and i f the rocks and

bedding planes are inc l ined,these layers can become s liding surfaces that

create stabi lity problems .

Jo ints are fractures that cut across the bedding planes and tend

to cut the rock units into blocks . Joints may be vertical or inc lined , and

it i s common for more than one set o f subparal le l joints to be present .

Joints may be tight ly c losed or Open,and they offer avenues along which

surface weathering , inc luding solut ion , can penetrate the rock mass .

Faults are ruptures in the rock mass along which there has been

di fferent i al movement O f the rocks on Oppos ite s ides o f the rupture . Severe

fracturing of the rock commonly accompanies faulting , and a fault may be

marked by a zone O f fracturing rather than by a s ingle fracture . Faults

and the frac turing that accompanies them also afford avenues by which sur

face weathering and solut ion ac tivity can attack the rock . Fault zones

present a poss ible escape route for the reservoir water , and faults also indicata , in varying degree

,the hazard of cont inued or repeated movement .

F aults in the foundation and abutment rocks at a dams ite and i n a reser

voir area introduce problems of bearing strength , stabi lity , and water

tightness and , therefore , s ites that contain them should be avoided . S table

dams can be des igned for faulted foundat ions , but the cost O f the necessary

remedial treatment may be prohibit ive .

Solut ion cavit ies , as the name implies , are Openings developed in

the more so luble rocks by the solvent action of water . They are troublesome

discont inuit ies that fre'uent ly occur in l imes tone and related carbonate

rocks .

Openings made by underground mining are man-made discont inui ties .

Underground mining often is accompanied or fol lowed by subsidence of the

overlying rocks , produc ing frac tures which may reach the surface . I f a

proposed dam and reservo ir site i s located over a mined-out area , very care

ful considerat ion should be given to the possibi l i ty of leakage and of

whether the si te wi l l remain stable under the new conditions resulting from

construct ion of the proj ect . Trouble i s more likely in areas where mining

was shal low than where it was deep,and in connection with large projects

than with smal l . In some instances a dams ite can be stabi l i zed by plac ing

concrete in the port ion of the O ld mine Openings that lie direct ly beneath

it , but such corrective treatment i s expens ive . Smal l dri ft mines in hi l l

s ides , for which no records were kept and whose locations are not known ,

can be the cause of unexpected reservoir leakage in some areas O f I l linois .

Shale ' The term shale denotes a fine-grained sedimentary rock

composed largely O f c lay and s i lt and having a laminated or thinly layered

structure . The term i s also used to inc lude the nonlaminated c layey rocks ,

and many shale deposits are composed of both laminated and nonlaminated

uni ts . Shales vary widely in their propert ies ,the variat ions being c losely

related to such factors as c lay mineralogy,arrangement of mineral part ic les ,

environment O f depos it ion,and post-deposit ional hi story of the deposi t .

Some shales provide excellent foundations for dams,as they are rocks of low

permeabi l ity and may have strengths comparable to those of good concrete .

Sound shales present few dams ite problems and a minimum O f reservoir prob

lems .

Some shales are subj ect to deformation under the applic at ion or

removal o f load , but thi s problem i s more critical for large dams than for

smal l . 'edding planes,vertical or horizontal joints , and faults , along

any O f which there may be thin veneers o f soft c lay , are examples of dis

cont inuities that reduce the res istance o f shale to s l iding and shear . I f

Open , these di scontinuities may also act as avenues O f leakage . Perhaps

the most common problem with shale as foundation rock i s its rapid datari

orat ion when subj ec ted to alternate wett ing and drying while exposed during

construction . Thi s can general ly be prevented by coat ing the surface to

Sandstone- Sandstones are rocks composed of sand grains held

together by some sort Of cement ing agent . Whi le sandstones generally havesufficient strength to support the weight of a dam , crumbly and poorly

cemented ones may permit suffic ient seepage to be troublesome ' thi s , how

ever , can general ly be overcome by remedi al treatment . Sandstones are

frequently interstrati fied with shale , and seepage through the sandstone

beds may lubricate the shale-sandstone contact and reduce i ts res istance

to s liding . The princ ipal problems with sandstones s tem from the fact that

they generally are cut by many joints . I t is not unusual for joint blocks

o f sandstone to creep down the s ide Of a val ley , in which case deep strip

ping O f the abutment material may be necessary . Open joints in the abut

ments below the level Of the reservoir may be grouted by inj ec ting a cement

s lurry , that is , a mixture Of cement and water . I f the jo ints are wide ,

they may be backfi l led with concrete to prevent excess ive leakage . Jointed

or permeable sands tone beneath a dam foundation must be grouted to reduce

undesirable upli ft effects . I n I l l inois , sandstone usual ly makes a sati s

factory foundation and abutment rock ,as undes irable condit ions commonly

can be corrected with a reasonable amount Of foundat ion treatment .

Limes tone ' Limestones and related carbonate rocks vary widely in

their suitabi l ity as foundat ions for dams and reservoirs . Thick-bedded and

undeformed limestones that are relatively free of solut ion cavities make

excel lent dam foundations and reservoir basins ' but thin-bedded , deformed ,

and cavernous limestones present serious problems that may be so extens ive

as to make corrective measures impracticable . The major problems with l ime

stones result from their solubi lity . Solubi li ty in the sense of changes

that may occur during or after construction is of l itt le or no importance ,

but the effects of solut ion act ivity that Operated through past geo logic

periods are O f tremendous importance and must be determined in evaluating

the suitabi l ity Of a dam and reservoir s ite .

Limestones and related carbonate rocks involved in a dam and res

ervo ir proj ect almost invariably contain solut ion cavities . They may be

thin cavit ies a long bedding planes and joints,or large caverns . They may

be open or water-fi l led conduits, or be partial ly or completely fi l led with

sediment . The s ize , form,abundance

,depth ,

and complexity Of the inter

connected system Of cavit ies depend upon the detai ls Of the local geologic

structure and the presence O f interstrati fied layers o f inso luble or imper

meable materials . S inkholes are depress ions in the land surface in a lime

stone region that are connected wi th underground passages formed by solution .

The presence of s inkholes in an area i s a warning that cavernous limestones

are present and that extreme caut ion must be exerc ised in selec ting dam and

reservoir s i tes .

Wi th proper remedial treatment a stable dam can be designed and

cons tructed on cavernous limestone,but the cost may be very high . Small

cavit ies in the foundat ion and abutments general ly can be sealed by first

washing them out with air and water under high pressure and then grout ing

them with a s lurry Of cement or other suitable material . Large cavit ies

may have to be mined out and backfi l led with concrete . Reservoirs should

be reasonably watertight , and cavernous limestone in the reservoir basin

may provide many avenues for the escape Of water . The problem of excessive

leakage from the reservoir may prove to be insurmountable because it is not

practicable to search out and seal O ff al l solution channels in a cavernous

limestone basin . Proj ect s ites that contain cavernous limes tone are gen

erally abandoned in favor of s i tes that offer better foundat ion condit ions

because of the cost and di fficulty of accompli shing effect ive remedial

treatment .

Large areas in extreme southern I l linois have been affected by

fault ing , and many of the faults involve limestones that are known to be

cavernous . The combina tion of faulting and cavernous limestones makes it

very di fficult to find satis factory sites for dams and reservoirs in thi s

area .

Sources of Problems in Areas of Unconsolidated Depos its

More than three- fourths o f the surface of I l linois has been

altered by glac iat ion . Some of the results of glac iat ion are buried bed

rock val leys , changed drainage systems,val leys deeply fi l led with glaci al

and recent alluvium , and extensive deposits of clayey ti l l , loess or wind

blown si lt , outwash sands and gravels,and water-depos ited s i lts . In gla

c ia l terranes, condi tions and earth materials vary within short distances ,

necess itat ing detai led Observation of surface features and careful subsur

face investigation of proposed dam and reservoir s ites . Knowledge of the

preglac ial , glac ial , and postglac ial hi story of a locality i s he lpful in

the preliminary evaluation of s i tes in that i t permits a general predict ion

O f the kinds o f condit ions that may be encountered .

C layey ti ll i s widespread in I l linois and makes excel lent founds

t ions and abutments for dams of moderate s ize . However , depos its of perme

able sand and gravel commonly occur within and between depos its O f ti l l ,

providing avenues Of poss ible leakage . Such permeable depos its in the abut

ments or beneath the foundat ion are general ly amenable to treatment , but

thi s increases construct ion costs .

A common damsite s ituation is one in which the abutments cons ist

of glac ial t i l l a heterogeneous mixture of sand,s i lt , c lay , and peb

bles and the val ley is fi l led with glac ial and recent al luvium . I t is

important to ascertain in cons iderable detai l the character , s ize , shape ,

and locat ion of the lenses and strata that make up the val ley fi l l in order

to make adequate provis ions for a foundat ion that wi l l be stable under the

proj ected load and for the negation Of the effects of percolat ing water and

hydrostat ic pressure .

Depos its Of s i lt frequent ly are the c ause of s tabi l ity problems

at dam and reservoir si tes in glac ial and al luvi al terranes . S i lt partic les

are smal ler than fine sand grains and larger than c lay partic les , and they

lack the property Of cohesiveness,or st icking together . When deposits of

s i lt become water- logged , they lose strength and are easi ly deformed under

load . S i lt depos its are common in the glac ial and recent al luvium that

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Avai labi li ty of Construction Materials

The di stance that construction materials must be hauled appre

ciab ly affects the cost of a project . Fer this reason suitable construe

t ion materials should be avai lable near the project . Durable stone to

protect exposed s lopes , crushed stone for concrete aggregate , granular ma

terials for the pervious fi lters , and impervious material for the embank

ment are re'uired in some degree for al l earth dams . C layey ti l l makes an

excel lent embankment materia l and is wide ly avai lable . Weathered loess al so

may be used i f better materi al i s not avai lable .IOrganic soi l , watery si lt ,

and peat must be discarded . The locat ions O f potential depos its of suitable

construction materi als should be noted in the preliminary phase of damsite

investigation . I f there is a 'uestion of the suitabi lity or avai labi lityO f construct ion materi als

,potentia l deposits should be exp lored and tested

during the detai led invest igat ion of the damsite . In I l l inoi s , materi als

suitable for construction are usual ly avai lable near the damsite .

E ffect O f Proj ec t on. Mineral Industri es

Another economic-geologic e lemen t that is important in the consid

aration of a proposed dam and reservoir proj ect is the effect the project

might have on exist ing or potent ial insta l lations of the extractive mineral

industries , such as oi l wel ls , mines,gravel pi ts

,or 'uarries . The proj ect

may f lood a quarry , a gravel pit,or a mine

,making cont inued operation of

the installat ion impossible . It may cover al l or part O f an Oi l field ,

forc ing abandonment of produc ing wel ls . The impact of such an occurrence

on the economy of the area and of the state must be weighed careful ly against

the benefits to be derived from the dam and reservoir proj ect . I f the net

effect is adverse , it may be desirable to de lay the project unti l such t ime

as the natural resources have been depleted and the installations shut down .

On the other hand, O i l wel l s or coal mines may be located in the

reservoir area in such positions that drainage from them may adverse ly

affect the quality of the reservoir water . Thi s problem must also be con

sidered and regulatory measures inc luded in the planning .

The presence in the reservoir area O f minera l resource reserves

that may be recoverable in the future presents another problem . Such re

serves should not be rendered unrecoverable by the project and thereby lost

to the economy . Conversely,development of a proj ect in an area coul d be

O f material ass istance in hastening the development O f nearby natural re

sources .

'alance 'etween S ite 'ua li ty and 'udget

I f a dam and reservoir proj ect i s to be bui lt , the antic ipated

benefits , of course , should exceed the est imated cost . Among the many consid

erations in determining the benefit-to-cost ratio are the estimated cost of

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foundation preparation , of remedial treatment Of the foundation and abut

ments , and the contingency that cost ly problems Of stabi l ity or leakage may

develop during or after construct ion and require addit ional treatment . At

nearly a l l s ites unexpected foundation or reservoir condit ions are encoun

terad that require special treatment , and i t i s customary to provide for

thi s in the budget . I t i s always prudent and general ly necessary to select

a project s ite whose foundation and reservoir problems can be handled wi thin

the framework of the expected construc tion and maintenance budget . That i s

to say , s ite qual ity and budget should be in balance . In many instances ,

s ites with cost ly foundat ion and/or reservoir problems can be eliminated by

eva luation of the geologic condit ions early in the consideration of the

proposed project .

CONCLUD I NG STATEMENT

Evaluation of the geologic environment i s an important early step

in the consideration of a proposed dam and reservoir proj ect . Ass istance in

the select ion O f a suitable proj ect s i te i s one of the most useful services

provided by the geologist . I f poss ible , more than one s ite should be eval

uated so that the best s ite can be selected for the project . I n many instances ,

the I l l inoi s State Geological Survey can provide from information already

avai lable a general idea of the geologic conditions at proposed s ites in 1 1

l inois . I n such a preliminary evaluation,facts and problems can be pointed

out that may eliminate from cons iderat ion the Obvious ly unsuitable s i tes and

focus attent ion on the sites worthy of detai led investigat ion .

ENV IRONMENTAL GEOLOGY NOTES SER I E S

Con tro l led Dri l l ing Program in Northeastern I l linois' J . E . Hackett

and G . M . Hughes . Apri l 19 65 .

Data from Con tro lled Dri l ling P rogram in DuPage Coun ty,I l linois

Jean I . Larsen and C . R . Lund . May 1965 .

Activit ies in E nvironmen ta l Geo logy in Northeastern I l l inois

Jean I . Larsen and J . E . Hackett . June 196 5 .

Geo logica l and Geophys ica l Investigations for a Ground-Water Supply

at Macomb ,I l linois' Keros Cartwright and D . A . Stephenson . Ju ly 19 65 ,

Prob lems in Providing Minera l s for an Expanding Popu lation

H . E . Risser . July 19 65 .

Da ta from Con trol led Dri l l ing Program in Kane,Kenda l l

,and DeKalb

Coun ties ,I l l inois' C . R . Lund . October 19 65 .

Data from Con tro l led Dri l l ing Program in McHenry Coun ty ,I l l ino is

C . R . Lund . November 196 5 .

An Appl ication O f Geo logic I n format ion to Land Use in the Chicago

Metropoli tan Region ' J . E . Hackett . January 19 66 .

Data from Control led Dri l ling Program in Lake Coun ty and the Northern

Part o f Cook Coun ty ,I l l in ois' C . R . Lund . March 19 66 .

Data from Controlled Dri l ling Program in Wi l l and Southern Cook

Coun ties,I l l inois' C . R . Lund . May 19 66 .

Ground-Water Supplies A long the In terstate Highway System in I l linois

Keros Cartwright . June 196 6 .

E ffects O f a Soap ,a Detergent , and a Water So ftener on the P last ici ty

o f Earth Ma teria ls' w. Arthur White and Shir ley M . 'remser .

August 19 66 .