Engineering Geology - Beavis

Prévia do material em texto

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, 
• 
SE RIE S EDII O R 
11. . HII. LLII.M 
Lapwonh Professor of Geology 
Un iversi ty of Birmingham 
Principles of Mineral Beha viour 
1\. P UT N I S and J. D. C. MC C ONNELL 
2 An Int roducti on to O re Geology 
A. M. EVAi\S 
3 Sed imentary Petrology: an Introduct ion 
MA URIC E E. TUCKER 
"1 
4 Geophysical Surveying 
P. K EARY and M . BROOKS 
5 Engineering Geology 
F. c. S EA V I S 
I 
(OI ~ ()S('I FN( 'E TF 'I S VO l I I M F S 
Engineering Geology 
F. . BEAV I 
MA . IlSc. Ph I), FGS 
1', ofc .... lH' nf Engineer ing .enloBY 
lJ ni vcr~i l Y of New South W:dc~ 
IlLII. KW ' L LS I ENT I F I r UB I.I II.T I ONS 
M I ' III OJ) I(NF( XFO I\J) I ON D ()N F IJ I N I)U I(I Ii Il OS I ON I' A I (lA I I i) 
To Joan and To Evan 
I \ ' H~ hy 
Il lut'\.,wcll Scientific Pu blica tions 
I d!lil1ill\ ofticc-;: 
1O/ II1I1 1Y Stree t, Carlton 
VleWI hL 1053, A\L stralia 
()~m'V M end , Oxford OX2 DEL. UK 
It 10 1111 Sh eet. London \VeiN 2ES. UK 
) \ AIII ' !!C I' la ce, Fdinburgh 
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~.) 11(."1 11..' (111 S ll cel , B o,,\o n 
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Cata log uing in publica tion data 
Beavis. F.e. (F ra ncis Clifford). 1924 - . 
Engi neer ing Gcology. 
I. Eng inee ring geology. I. Tit le. (Series: 
Geoscience text s: v.5) . 
Bibl iogra phy. 
Includes inde.x. 
IS BN 0 g6793 200 7. 
[SBN 0 86793 [280 (pbk.) . 
624. ['S[ 
Preface, ix 
Int roduction, 1 
2 Rock - the material, 4 
2. J General statemen t, 4 
2. 2 Rock-formi ng mineral s, 4 
Contents 
2.3 Geological classification of rocks , 9 
2.4 Rock fabric, 17 
2.5 The mec ha nical pro perties of rock mate ria l. 19 
2.6 Mechanical classificat io n of rock mater ial. 22 
3 The rock mass, 27 
1. I Genera l s tatement. 27 
1.2 Homogeneity and isotropy of the rock mass . 27 
1.3 Cont in uity, 28 
1.4 Physical and mechanical cha racteristics of discontinuities. 33 
1.5 Classification o f discontinuities. 39 
1.6 Analysis of fracture systems. 40 
3.7 Permeabi lit y of the rock mass. 43 
1.M Se ismic vc locit y in the rock mass. 45 
1.9 Rock quality designation (RQD), 45 
1. IU Mechanica l classificati on of the rock mass. 47 
4 Rock weathering, 52 
4. 1 Irllroduclion . 52 
11.2 WClI!hcri ng processes, 52 
,I 1 Patterns of weat hering, 61 
,I <I Produ cts of wea theri ng, 63 
.1 ~ C I:t .. sificalion of weathe red rock for engineering purposes. 66 
" () I he effects of weathering on geomeeha nical properties, 73 
,1 7 Weat herab ilit y a nd dura bility o r rock, 81 
I It Weathering of bioge nic rocks. 82 
I II Weatherin g in engineering practice. 83 
·1 10 Concluding remarks, g9 
~ I he lund surfnce, 92 
'\ I I lltl(ld ucl(l ry stat eme nt, 92 
.. ' Cllmut lc co ntrol of la ndform. 93 
'\ I ( i !:ohlHLcal c()ntrol of landform, 93 
'I I I il nd/ol 111 ' of temperat e climatic IOIlCS, 100 
'\ '\ I LIIHll il ilm of lHid l eg ion" 10 1 
, II ( :lu l' iu l lrIlUII Otlll 'i, I02 
'\ 1 ('011 >11 111 11\1I(11 \1IllI .. , 1011 
, H I l tI ~ ll1h l (· l lIlId l n llll ~. IO ~ 
'\ " (h\llIlIl llpl1ll ' tlljlp ~ 1111 (' lI " lll l'l' l llI ~ PIlIIJlI ~l'''' I O() 
(, Soil: material and mass, 110 
6. I (ic ncra l statemen t , 11 0 
6.2 M inera l composi tion of so ils, I I! 
(1 ,;\ I"cxtun: of so ils. 11 3 
(1,4 So il st ructure a nd fabri c. 1[5 
h.5 A nalysis o f so il struct ure a nd fabr ic , I [!:! 
6,6 Soi l consistency, 120 
6. 7 Cl a s~ i fi catio n of soils. 121 
6.X Mec ha nica l pro pert ies of soil s. 124 
(1 .9 C haracteristics of t he soi l mass, [24 
6. 10 Gco morp hology an d so ils. [27 
7 WlItcr in rock and soil masses, 131 
7. 1 Introduct ion, 131 
7.2 I he hydrologica l cycle. 131 
7.3 I he wa te r ta ble , 133 
7.4 Per mea bilit y, 134 
7.5 Flow nels, 139 
7. 6 Wat e r q uality, 140 
7.7 Grou ndwa te r reso urces eva lua t ion . 142 
7.K Grou nd water a nd c ivil e nginee ring, 143 
H Geological investigations, 149 
~ , I Il1Irod ueti on, 149 
K.2 T errai n evaluation , [49 
tU Gcological ma ps, 15S 
X,4 S ubsu rfa ce investigations. 156 
x.5 O th er methods o f invest iga t ion. 163 
x.6 l.ab o ra to ry invest igations. 163 
1(7 Report ing geologiea[ investigat ions, 164 
4) Geolo gical factors in engineering works, 166 
9.1 Introd uct ion, 166 
1) ,2 Da ms, 166 
9 ,) Buildi ngs, 171 
9.4 Road s and a irfi e ld runways. 173 
9.5 Brid ges, 175 
9.6 Un de rgro u nd open ings . [76 
9. 7 C oasts and harbou rs, 179 
9.K Engineering materi a ls, I!:II 
10 Environmental geology, 184 
10. 1 Int rod uctory remar ks, 184 
10.2 Geo logy and regional planning, 184 
10.3 Urban planning, 187 
[0.4 Was te d isposal , 190 
[0.5 Geological hazard s, 193 
10 .6 Enviro n mental impact sta tements. 195 
10,7 Earthqu a ke ha7.a rds and nuclea r power sta tio ns. 199 
Appendices, 201 
1 Ci colo gic1l 1 colu mn and time scal e, 201 
2 Propel'l it.: s and ide nti fi ca ti on o f mi nera ls, 202 
1 1- id el id entifica t io n of roc ks, 206 
4 S te reogra phic a nd eq ual area project io ns , 208 
A u1hor index, 2 13 
Index \l r rocks, mincntls nnd soils, 21 5 
(;c ll crlll index. 218 
Preface 
In troductory tex tboo ks o n geology for c ivil e ngineeri ng s t ud ents q u ile right ly 
p lace the emp ha sis o n geology. The a pplicat io ns o f geo logy to engineeri ng. 
ho we ver, a re o fte n di vo rced fr om the basic geology, a nd th e stu dent , in 
co nseq ue nce, so metimes fail s to comprehend as well a s he should the c lose 
H..: l;lli o nship between the two disci plines, In thi s boo k, the a im has bee n as close 
0 11 integrati on as possib le. Discussi on of geological principles has been link ed 
with Lh e a ppl icat ion of these principles to engineering practice. Ill ust rat ive 
exa mp les, draw n from a wide geograph ic ra nge, a re used freely. Th e ca se 
li i ... to ries were chosen from both fa il ures a nd successes, for it is from both, not 
I he fa ilures a lone, that we learn. So far as has been possi ble, the case histo ries 
,",e lected a re projects wi th which the writer has had some direct persona l co nta ct. 
I IIc two final chapters co llect, in a syste matic form, t he mai n geologica l fa ctors 
invo lved in t he safe and econom ic construct ion of engineering wo rks, and some 
npp licali ons of geo logy to en vironmental stud ies . 
A n emp ha sis is placed, on Ih egeological s ide, on those aspects o f geo logy of 
1I1I1'HH lanCe 10 th e e ngineer: min eralogy, pet ro logy , st ruct ura l geo logy, 
J.tl'() lI1 o rphology a nd rock wea th ering. No atte mp t has bee n made to includ e 
nhll C tha n th e basic princi ples of strat igra ph y. Detai led disc ussio n of loca l 
'1 ll l1l igra phic sequ cnces is o f littl c general interes t, or value, to the engineering 
'i lud cnt. a lth o ugh, of course, on a n actua l project kn owledge o f th e local 
Irj ll ll l ig ra phy is always essent ia l fo r the geologi st, an d at least helpful to th e" lt w ll ct.: r . 
I II prcpa ri II g I he boo k, the aut ho r h<ls kept in In i nd the needs o f In idd Ie-level 
1 1\ 11 c llg in l:c rin g stud ents, a nd a kn owledge o f seco nd ary sc hoo l geology has 
11 1'1' 11 il ss unl l! d . Reference lists have been la rgely restr ic ted 10 sugges ti ons fo r 
lUI I Il \: l readin g rro m more advanced sta nd a rcltexts. However, so me references 
III I II igi na lli tera tu re arc gi ve n to encoura ge the stud ent to o bt ai n Ill o re detail , 
IIl1d 10 gni n ex perience in crit ica l eva luatio n o r o riginal wo rk . 
11 11..' au th o r wishes to ac knowledge his grati tude 10 copyright hnlckrs fo r 
pn 111 1"; "'; 0 11 t o u:-;c publ ished Illill erial. Specific aeknowk dgl! l11 cnt i::; mad e inlh c,; 
II iii I M IIn y peop le hi.! ve co nt ri buted to I he preparati on of th l! book . and to a 11 or 
1111 I II , I (1 m ... in ce rely gra tefu l. My wire, Joan, Iw " a lwa ys give n her SUp pOl1 and 
I III tlt ll nge meill. Mrs Dia nc B lu ll k cardu l1 y un d di li ge ntl y l)1'cpn red th(,.' 
1\ PI"H r tpl , MIs M 1\I'i:t nna 11 01 v:tl h Ih e li ne (l n l wi ngs, li nd M I ( ;. S mu ll I he.: hull 
111 11 1' p l: !l l· ... , MI I'o Bl unk a nd Mrs I flr iSII S mith pn.: p:rred ti ll.' in([ cxt.:s, I lll il 
1111 1111 uilidy tt l/ ltt' flll lP my l'o ll l·OHtu.:, I)r M .. I, Kni ~h t. who pJO vid ('d 1I1U l' h II I 
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\\1 111 1111', hl l vl.' 1'11.'1.' 11 Ill voluohl e. l;innlly, I w i ~ h \0 II ck now lcdttc the 
l ' I Ii ' OI Il I I ~l' llI l' IH 11 f..'t'civcd mon y y c.:: lI s ago f !'om M I' E.L. Ri chH l'd. l o w holll ihis 
lHIO~ 1\ illllllly d cd iL'l ll c.:d , 
Fril nk Beavis 
Syd ncy 
1 
Introduction 
IllIgincc ring geo logy, as a science, is co ncerned with the applications of the 
Plillc ipies of geo logy Lo c ivi l (and to some extent, mining) engineering, so that the 
t' lI~dncc r can take in to account those geological aspects which control the 
l' l 'OIl(}I11Y. and sa fety, of the structure which he is required to design and 
i'OIl 'i lruc l. All c ivil engineering works are constructed on, or in, rock and soil 
11I1I'l~ es. It is essential. therefore, that the civil engir:.eer be aware of the history, 
11111 II re, and propcrties of the rocks and soils. Equally, it is essentia l for the 
p.I'olngist. who is to work with the engineer, to have some knowledge o f 
l' ll v,incc ring requi rements. 
Alt ho ugh engineering geo logy has been practised for centuries, systematic 
Ntlld y bcgan o nly in the 19th century. The present century saw the development of 
"j ill I mecha nics a nd rock mechanics ~ this has tended towards a so-ca lled 
'qunntificat io n' of enginee ring geology. Such a quantitative approach is 
dl" " ltb lc, but it must not be overlooked that much of the geology which is so 
/II jv,IIHica nt fo r the engineer is qualitative and descriptive. Rock mechanics and 
NIIII mec han ics supplement , or complement, the descriptive geology, they do not 
lI' plllec it. 
At the most fundamental level , geological principles must be used to explain 
11", var ia l io n in the mechanical properties of rocks and soils, and the behaviour 
II I lOcks tlnd so ils under stress. T he quantitative description of soil a nd rock 
Ili'hllv iour has to assume a n idealized material; the geologist is aware that actual 
Hlld li nd rock materials, in their texture, structure, and composition vary to a 
IIIIt<iderable degree fro m the ideal. 
I he scicncc of geo logy comprises a number of subdiscipl ines, all of which, to a 
Hll'lite r o r lesser ex tent, find some appl ication in engineering. An understanding 
III tlco loSY itse lf requ ires a sound basis in chemistry, physics and mathematics, 
", 111('1..' geo logy is co ncerned with processes which are chemical and physical , and 
wllh the produc ts of the various processes. 
Mi nenllogy is tha t branch of geology which is concerned with the origin, 
IHTII II'CIlCC. and Ihe propert ies of minerals. A ny mineral species is a unique 
IOllIhinHt io n of a definite chemical composition a nd crystalline structure. 
"\I' VC.' I III th ousunds of minera l species are known, but the civi l engineer, for the 
IlIilNI PIli t. is conce rned wi l h on ly a few: those which are known as rock-forming 
nlllll,' luls, So m ' n ... i ncrnl s fa I'C o f pa rticulH I' i n\e rcs t a nd impo rtance to, especially, 
I il lu,' ro l e CIIBin CCls. s ince they rca ct with ceme nt under certain conditio ns; the 
Iruclion CO lll CS ul1 in Ih c disrupti o n of co ncrete. 
II I{' 'l tlldy o f Ihc OCC Ul'I'cncc, dis tributi o n, origi n, co mpos itio n Hnl! lex lu re o f 
IIH ~ "I I N known us p ' l l' logy. All rocks a rc made up of o ne o r mo re mincnds. T he 
IIII1U' I,d "l which lIrc prese nt , the ir arrangement (fa bric) a nd the tex tu re, are 
tl t'h' l mined hy the origin o f the rock and the processes it has und ergo ne, a nd is 
IlIlth: ll-\oi ll ~, si nce it s fonna l io n. Rocks Cll n be c lassified ge netica lly into igneous, 
I'l l'" IIIH' UI II I Y II nd metam o rphic, depending o n the ir o rigin. Rocks in each of these 
1' llI NNl'N hu ve ec rt a in, we ll defined cha racteristics, which are reflected in their 
Iwhuviollr os enginee ring mate rials. 
AN 1\ I 'su it of the processes to which rocks a re subjected during and afte r their 
1lIIllInti () I1 , s tluc tura l features such a s bedd ing pla nes, joints, fault s, fo ld s and 
101111 110 11 ' mll y be prese nt. The o rig in, geometric patterns, and the fo rces wh ich 
1110tlllccd these struc tures co nstitute the subdiscipline of structural geology. 
Mosl slrt! ' tUI'CS represen t discontinu ities in the rock mass, whi le some represe nt 
(I h~l' () 11 I i nuit ics in t he rock ma teria I. As a res ult they have a major infl uence on the 
I' ligi liccri ng pro pe rties, and behavio ur, of both mass and material. Structural 
~ll() I ()HY is onc of the most im portant bas ic studies for engineering geology, 
Alth ough of less intcrcst to the cngineer, those branches of geology known as 
Pllhll'O ltt olO!;y a nd stra tigra phy a re of vital impo rta nce to the geologist , and a re 
lI"Inl, quite co mm o nly, to solve problems which confront the engi neer. 
PUl lll'on lo logy is the science of fo ss ils: the remains of past life on the ea rth. 
I+tI Ns iis /lrc used parti cula rly to determine the age of the rocks in which they a re 
11111 lid , O t he l' methods o f dating are available which give absolute ages, as distinct 
11 1)111 Ihe rc lative ages determ ined by means of fossils. By studying the age, the 
t. tlllcl III 1:, lind the fi eld relationships of rock masses, geologists are able to 
,Irl II mine the ir his to ry, a nd , in the fina l analysis, the histo ry of the who le earth. 
, Itt' hu sis of any geo logic;:d map is the stratigra phy of the region involved . T he 
hlNIIH y of t he regio n ca n ha ve consequences o f vita l importance to engineering. 
I lit' filtH I stage in the geo logical evolution of a ny region is the development of 
Ihl' pl csc nl land surfaces, T he processes respo nsible for this development are 
III I ~l'I y clinult ica lly co ntrolled , a lthough thc landforms themselves are fun ctions 
hllih 01 elilmlle (present and past) , and of und erlying geology. Geomorphology is 
I Ill' slud y o f the fo rm and origin of the p resent land surface. Engineering 
I-\l'O IlHII pho logy is emerging as a specialist discipl ine, particularly in the fi elds of 
"1 10111.' enginecring a nd enviro mental engineering. Nonetheless, a carefu l 
w'olllorp ho logica l ap praisa l o f a n engineering site has always been an essent ia l 
fU l'l' t of fin in ves ti gat io nprog ramme. 
ApII I'I fro m th e so mewhat p urely geo logica l subdisciplines ment.ioned , 
'"ItHlnccJ'inggeo logica l in ves ti ga tio ns req uire, in add ition , studies which ove rlap 
willi c Il HilH.:eri ng: so il mcchanics, rock mecha nics, a nd g ro und wate r hydro logy, 
lI yd l o~c() l ogy is conce rned with the o rigi n, qualit y, quantit y, distribut io n an d 
1l1OVclllcnt of wute r in, Hnd through, a rock mass, A lt ho ugh hydrogcologists arc 
I" tl' l l'S t ' (I pi i 1110 rily in gro undwa te r as a lI scn ble reso ll ree, engi neering geO logists 
IIll' l'o nc ' I ned wi l h I he hydro logy o f gro undwuter fll mosl engi nee ring s il es, bU I 
llN IH'l' ill ll y III dll ll' s it 's lind ill tunnels lind o lh ' r undc rg ro un(t excll vllti o ns, 
II hIlS h l'~ n nOled IllIIt Ilt l' hll s ic d lilil n.: llIlill tl to 1111; g 'o IOHY o f n l'cJ.\ io llllrc 
1I'I'lI llh'd iI n 11 ,.W \I 1 ()~dl'ld II III p, Wh il l' "I Il l' hU 1i1 p ,~ lil t" i nd i 'q 11: IIN1I hk 10 I Il l' t' II ~.d IIl'l' I', 
not infrequent ly they fail to show ma ny of the fa ctors o f importa nce for design 
a nd constructi o n, In recent yea rs, t he basic geo logica l map has come to be 
supplemented by specia lized maps record ing specifica lly e ngineering geological 
features. These are refe rred to as engineering geological maps, engineering 
geo morphic maps, a nd geotechnical maps. These are all prepared after fi eld , 
la boratory a nd office invest igat io n and research . 
In the following pages, the basic principles of geology are outlined. The 
applications of geo logy to civil engineering are presented , and examples of these 
Hpp lications a re discussed. T he reader who is concerned that the t reatment is for 
I he most part qua litative should note the words of Sir Harold Harding, a former 
Presid ent of the Inst itution of Engineers, Lo nd on: "Some engineers desp ise 
geo logy because it is not quantitative, there a re no formu lae a ttached to it. But to 
me, geology is the basis of civil engineering. 
2 
Rock - T he Material 
1. 1 (; Cllcrll l statement 
MOl'l l l ocks It I'C aggregates of mineral particles which are c rysta ls, more or less 
IWll l,'l' t ly for Il1c(l . or fragments of crys tals; some rocks may co ntain, or be ent irely 
\'1 1I1I pns(;<i of. no n-crys ta ll i nc ma tcr ia ls such as na t L1ral volcanic glass . T he size of 
tlH' Pllili c l ~s. th eir ar ra nge ment, and th e proporti on of eac h minera l present , 
't' l W II' Ih !.! ba sis for the geologica l class ifica tio n: the o rigin and history of the 
r Ol' ~ III C o f ITIHjor importance a lso since, to a large extent , these aspects wi ll have 
c! t' lt' tIl1im:d the si/e. arra ngement a nd co mpositio n of the minera l particles. 
HOI'ks UI"(,! nHlurai materials common ly lack ing the uniformity of man-made 
lIH1t1.' lIlIls slIch as stee l. While it is possible 1'01' man to contro l the properties or 
11111 rllllu ctlIl'cd engineering ma teria ls, he ca n exert only a ve ry limited co ntrol, if 
II II V. nvc l the properties of rocks a nd soils. 
In ' Il v,i ncc ring geo logy, it is essent ial to dist inguish between the rock or soil 
IIIofl '"nl lind thc rock or soi l mass. Because of textura l and composi ti ona l 
VII I 1111 lOllS , any sa mple of roc k or so il is a lmost certain ly not the same, in a ll 
1I'''I l lt'rh. II", th e mass from which it was ta ken. Apart from text ura l and 
I'tl llIllIl\llionH I va riati ons, the mass may be broken by discontinui t ies such as 
1111111\ (II' hcclcling planes wh ich influence the engineering properties, a nd the 
lu' h,IV101l l' und er stress. orthe rock mass. For these, and for other reaso ns which 
WI ll hcco me ap pa rent , it ca nnot be emphasized too much tha t the va lue of 
I\lOflcltics determined on a sma ll sa mp le of rock or soi l may bea r litt le 
Il' llit ionship lo t hc values fo r the mass. ln enginee ring geo logy it is essential that 
1Il'i 'lir dis tincti on be made between material and mass characterist ics. 
2, 2 nock-formin~ minerals 
1\ lllin l! l'ul is a natura lly occur ring crysta ll ine mater ia l with a defin ite chemi ca l 
l'O lliposil ion and a defi nite crystal structu re. Ma ny thousands of minera 1 species 
ho ve hec n I'ccogni /cd, but only a few of these commonly occur as rock-fo rming 
1I1 11l l! I'/l ls, Most o f the rock-fo rming minera ls a re silicates (Ta ble 2. 1). Each 
lllincrll i i ll ft rock has phys ica l, mec ha nica l a nd chemica l properti es whi ch differ 
11(1)) th osc of the o thel' min era ls present . a nel which wi ll have so me effect 0 11 the 
pl(lpt:l li l!~ 0 1 thc rock as H who le, However, it is ge ne nilly true tha t, since the 
I1 \( II vidunl minel'lll pHI'ticles in a rock arc sma ll , ustlallyeach particle ca n havc. of 
II 'w ll . lilt Ie d if l!CI in fI lIenc;e 011 I he mcc ha 11 ieol properl ies of t he rock as H whole, 
I'IPpt' lties MI ' l! us hll l'(inl:ss. 01 1\ wcll .. el cvc lo pl!d l11in l! 1'!I 1 clcl-l vtl gc, in II milleral 
whkl! I(ln il ,"! II signifil'fl llt pili I of Ill l! I'Ock CUll , und er' so me con diti ons, 
drll'lll ll llt' IIH' It'l ll'tl Oil 01 I Il l' lo(:k 10 l'Xll' ll lI 1I 100 l'l'''!, 1'0 1 l'XIIlllp lc. th l' 
1"l"it' l l l'd IIlll' !llnllOIl 01 111 1(.' 11 c l y~ tu l \. cHe h wi th II hlHllly dcve lopcd plllt y 
111'1lVII~l', l'UII IIIII' III I II Ntl ()IlB 1l\cl!hllni cu! unisotl'opy 10 u rock such as slatc, 
""Vl lli l' or 'eh"' !. 
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111 1\111\' Olivine 
Kaolinil e 
I hl\"11 Ill ite 
Monlmori ll onite 
I ,II hlllll. ll·\ Calci le 
Dolo mite 
r I I SII. ICATE M INERALS 
Siruetu ral 
ci a" 
Tc ktu sili cat e 
T ek tosilicate 
Phyll osilicatc 
[nosilicatc 
i nosilicale 
Ncsosi licat e 
P hy ll osilicat e 
Composition 
S i0 2 
KAI Si30 3 
NaAIS i30 s-CaAISi20 a 
KAI2(AISbOlO) - (O Hh 
K2( Mg Fe)6(SiA I)sOzo 
(O H). 
(NaCa lz(MgFeAl h 
(SiAI)s02'2(OHh 
Ca(Mg FeAI)(AlSihOs 
(MgFchSio.. 
AI4 Si10 lO(OH)8 
KA12 (AIS i3)01 0(OH) 2 
AbSi40 1O(OH h 1l H20 
CaC03 
CaMg(C0 3h 
I Il l' si li cate minerals co nsis t essentially of silicon- oxygen tetrahedra: linked 
111~l' t hl!r in a var iety of ways; silicate mineral classification is based on the type of 
II"k ltNe hctwcc lllhe tet ra hedra. Six structura l classes are recognized, as shown 
III l ithic 2.2. 
I ~ llh\ 1,1 Si lic:n e mll1eral ~ lru c l LJral da ~ses . 
'tlllt llllUI l'In." 
"'II , .. ~ III CI II C 
'"HI~ lll l'l ll c 
I \t Iml lll'n ie 
III,,,IIII"IIIC 
1'11\ Il!l~l hl': II C 
I I ~ 111~lllflllc 
Structure 
In<lcpenden l tet rahedra 
Doub le tetrahedra 
Ring ~tfu ctllfe 
C hai n .. Iructurc 
\inglc chain 
double chain 
Shec t .. lru Clure 
I hree-dimc nsional 
nel wo rk 
Example 
01 ivi nes 
Epidote 
Co rdierit e 
Pyrox enes 
Amp hi boles 
M icas. clays 
Quart z, feldspars 
I Ill' Ol ill'! l'O IllI"OIl l! 111 1'o o f the silica te structurcs such as ca tio ns, add ili onal 
II ~'Hl' tl , ll ydH\xy l t\I' O I1P~, ftnd wH ter, arc arranged wi th in the slructure in such a 
WilY !IN 10 PI'O(!II l'l' ph ys ico! stllhilil y lind l! lcClli cnl nCIlI I'H lil y, T he tI '('/(J,vili (,fllfS 
Hl lt l H H'W 'i /t C(I/ (',\' d o no t inc ludc any rea ll y impo rta nt rock-fo rmin g mine ra ls 
1I11i1 1) \1 ~ 1I e pid o le a nd co rd ie rite may be essc nt ia l co nsti tue n ts o f so mc 
1I1(' IIIIII IH pili ' I'oc ks. O n t. he o t he r ha nd , the lekfOsilicates inc ludc so mc o f the 
1111, , 1 Illl nO! lunl rock-forming m inera ls: qua rtz, fe ld spa rs , feldspa tho ids and 
II ' o l ! I(.' ~ , Of ull minera ls . q ua rt? a nd feld spa rs a rc themost abund a nt in rocks. A ll 
il l I lit' Il.' klOSiliclltc minera ls a rc co lo urless, white, o r grey, provided they are free 
j 1IlIlI IIlc ius io ns o r impuri lies. A s a t esu lt of the re la tive ly open fra mewo rk , their 
!I (' lHd tv le nds 10 be low, and t heir hard ness is un ifo rm between 5 and 7. O n the 
whoit- , th ' Ick tosilica tes fo rm a n ex t remely ho mogeneous g roup. Phyllosilicates 
,,, " "''' '(I lo!' Ihe h ighly deve loped platy basa l cleavage. T he ma in minerals are 
Ih,' "It('"" Ihe clays . py!'ophyll ite, ta lc, serpentine a nd chlo ri te. Al though some 
pl1 Vll oN i li cit IcS H rc sta ble to qu ite high tempe ra tures , ma ny of the m are formed a t 
low h: mp ' rBlures du r ing weather ing a nd sedimenta ry processes. The c hain 
~ II\I " 1111 's of the illosilicates arc respo nsible fo r t he well-defi ned prisma tic 
I'I l' IIVO/-tCS c hu rHcte r isti c o f t he am ph ibo les a nd pyroxenes. A ll o r t he mine ra ls in 
I h l ~ 1.' IIPi' ha ve striking ly s im ila r p ro perties. T he nesosilicates, which includ e 
f-\ 1 1 11l ~' I "; IIfH.l o li vine. have a ve ry d ense pack ing, wh ich is ren ected in t he re lat ive ly 
V. II ' ; I I ~' I h Ili d ncss and highe r d ensity tha n is fo und in correspond ing com pound s 
0 1 o t he r sl,'uctural c la sses, w hile the absence of chains a nd shee ts is ren ected in 
tll \' f.t ' IlCI'II l1 y C(lu idime nsio na l natu re o f the crys ta ls. 
" I, (' l AY M INERALS 
l il t' II SC o f the term clay to describe a materia l implies tha t it has a very fine 
1I' ;'; lIl1 t" with indi vid ual par tic les less than 0.002 m m in d ia meter, a nd that it 
iI (' w lo ps II p las t icity when mixed w it h wa ter. C lay, in this sense , is no t co mposed 
I ' ... r lll ", ivc ly o f c la y mi ne rals, but these d o consti t ute , a s a rule, a s ignifica nt 
PIII IHllii on of the mate rial , a nd it is certa inly the clay mine ra ls which are 
1l" I"," , ;b lc ro r th e plaslicity o r the clay. C lay minerals a re of maj or importa nce 
til t lli: c ng inee r beca use of the inn uence t ha t they exert on the mechanica l 
I'lt l lH' ll ics o f t he so ils a nd rocks in w hi ch they occu r. A ll cla y mine ra ls a re 
phyllos ilicHles (Figs 2. 1, 2.2). 
O= AI 0 = 0 @ =OH 
I" 
1' 111 . 1 I li lt' ~ IIIII I IIII II I l Ill Y 11 11 111' 1111 11 ( 11 ) 1\ 1101111 11 1' (h) 11 111 1' 
I, 
Ibl 
lal 
) 1M J. J I Ill' " I Il et tl n; nfct ay mincral s: (a) Mo ntm orillonite. (b) Chlor itc. (c) Hydrated halloysi te. 
111'(.' I! IISe o f their eco nomic importa nce, cla y minerals have been st udied in 
~1I11 1 <klni l; mu c h is kn own of t heir structu re and p hys ico-chemical proper ties. 
WIII II' til e c la y minera ls ca n be cla ss ified in a number of ways, a simple 
1 l'I "'~ !l l t' lJ li () 1l hased o n st ruct ure a nd phys ico-chemical p ro pe rties, w hich is o f 
,d ill ' 10 Ih c c.: ng inee r, is presented in T able 2.3. In this class ifica ti on , fi ve ma in 
I jd " ' " 111 1.' Iccogn ized . 
I ii" I : I Iypc c lay m inera l crys tal uni ts a re made up of one tet ra hedra l sil ica 
II! It .rll l' t nll tin g w ith o ne octa hedra l al um ina la yer. T he two sheets of each unit 
II! hrhl l Oti ' I he r by oxygen a tom s whic h arc s ha red by the silicon a nd a lum ini um 
·1111111 "1 III thei l' res pect ive layers. The u nits themselves are held togethe r by 
111 .11 1\I' lv ! il-t id hydrogen bo nding. As a co nseq uence, the lat ti ce is fi xed , a nd no 
I 1'"11'111 111 will no rma lly occur whe n wate r is ad ded . Neit her wa ter nor cations 
1·1111 II h.' 1 hl;l wccn the struc tura l unit s, so that kaol in, a nd o ther minera ls in this 
I jll 11I1vt' II low Cli t io n ad sorpti o n ca pac ity. The pla stic ity , cohes ion, shrinkage 
111.1 " w(' lI lng l.' il IJI/ IClc ri sti 'S II rc low. 
I hl' ' : I tvpe cX J1l1 l1d inf, c la ys ha ve c rys la l u nits cha racter ized by a n 
111111 11 1'11 11 11 l1 itllllill ll It IYl' I' co nfi neCI he tween two telra hedra l sil ica laye rs. T he 
I I .. 1111 11111 1'" lil t' loo,,' lv ho nd ,'d hy v ' ly w 'u k oxygen to o xyge n lin ks. Wa le I' 
IIlol~'nllt" li nd l'n ll \II I' IIle IIIt !uClcd 111 10 I Il l' illI CI-Ullil space, c~llI !<.i Il Hcx pn n s i o n 
III Ih,' ny'H!!\ la lt ice. 1 he nl ()vc rne r'll of wu ler and cH lio ns in betwee n un its 
l'XpONl"~ 1\ IIl1'ge int clnnl surface areH in co ntra st to the 1:1 type clays. 
I NO IlI O I pilolls su\)stituti on of Mg2+ for so me of the A 11+ in the octahedral layer 
ulld, 10 n lesse r degree of A 11-+ for SiH in th e tetrahedral layer leaves a mineral 
N 11l' 11 n..; mo nt mori llon it e wit h a high net negative charge, so that montmorillonite 
hn, n hi gh CU I ion abso rptio n capacity: up to 15 times that of kaolinite. The 
1I100111l1(lrillonil c minerals have a high plasticity and cohesion , and a high 
NIII inkn gc on drying. 
1,.h lt1 2,,1 ('Iu .... ificntion of clay minerals. 
A I t I Yilt: Oue .. ilic;! lelra hcdral layer alte rn ating with o nc a lu min a octa hedra l layer 
Kltnlinile 
I r lI!loy';,c 
Anlt uxi l C 
Dickite 
II 1 I ypc l' xpal1ding An alumina oeta hed ra l laycr cO nl ained bet ween two silica tet rahedral 
la yer, 
II( I) Mlwtmo rill onitc Group 
Mon t mo rill onite 
Beide llit e 
NOlH ronitc 
Supo nilc 
H(2} Vcrm ie ulit e 
( ' l I I ype no n-e xpand ing 
Il lite 
I) 2 1 I ypc I wo .., ilica tctrahedra l s heet s a nd IWO magnes ium octa hedral sheets 
( 'hIOllle' 
~hxc\1 Inye! c la y' 
I JIll OIl .. e ln y' 
Pa lygo rskit c 
Sep iolite 
" he ve rmiculites ha ve sltictural characteristics similar to those of the 
tllol1tmori ll oniles, but in so me the octahedral layer is dominated by magnesium 
1'11111\.;1' than al uminium. Water molecu les and magnesium ions are strongly 
i\dsOI'bed between the crys tal units. However, these tend to hold the units 
lo~ethc r , so that the degree of swell ing is less t han in the montmorillonites. The 
cn t ion nd sorpt ion ca pacity of vermiculite exceeds that of all other clay minerals , 
dtle to Ihe high nega tive charge in the tetrahedral layer. 
' rhe 2: I non-ex panding clay minera ls arc sometimes referred to as hydrous 
llIi t:lts. Ill ile is the 1110S1 in1portHnt member of the type. In this type, some 15% of 
tile letnlhedra l silico n sites are occupied by a luminium a toms, wh ich result s in a 
Ili ~h Il c~lI tive cha rge in Ihe tetrahedra l laye r. To satisfy th is charge, potassium 
ion, II I t.' , I I'ongly nl I meled hct wee n I he Ullii S. The ionic radius of I he potass ium is 
I il l' !<.ltIl H: 11/'1 ,~ plt ces in I h ~ rid jn cc n I 1 cll'lI hcd 1'11110 yc rs, so t hftl I he pot ass i \1m Hcts as 
II hondilll-\ :tgl'IlI , P1 l'vl,: ll li ll» ( . .'x pnllsiull of lI u,: clys lnl lUll icc, As U l'cs ull, 
IIIIIIII'IIU" 'lIl' lI " .. hYl ltHlioll , l'Ul ioll Hd~o ljlll o n , ,,;wt,.' lltn l-\, 'i lltlnk llgc, lind 
1'1.1 1I!'11V !I ll' I '!<., III lillI e Ihlln in InO nl lll() ltll oll lte. No nctheless, ill ilc has thcse 
I" IJpl'l t1l'''i IlIl"lH':CSS of Ihc 1:1 clay rn inera ls. 
lil t' 2:2 type mincl'lds fi re represe nted by chlorite whi ch is co mposed o f 
,till! lUI I~' 1!l Ie II nd hrllci 1 e l M g (0 1\ ),1] laycrs. Thus the crystal u nit co nsists of two 
III! II lI' tllth cUll1i sheets and Iwo magnesi um octahedral sheets. The cation 
• I hllllHt' l'lIPIICil y is abolll Ihe same as il lite . Relati ve ly little water absorption 
111111111 1)(.'1 Ween I he lu yers, so that this group of minerals is relatively noo-
111111"1 Vl', 
" Vt' ! Y important gro up or clay minerals is the co-called 'mixed layer' or 
1IIIII III IIH lified' clays. Within a given crystal , the crystal units may be of more 
,IHlu 1IIIl' Iype, e.g. illi te montmorill on ite. In some soils and weathered rocks, 
1111 II II I!lyer cl ~l ys may be morc common than single structuretypes. 
I htl type unci amo unt o f clay mineral present in a rock, but particularly in a 
.111 t 1111 hllve H profound effe ct on engineering properties. The nature of the 
I I hnll~l'lIhle ion, and the ion-exchange capacity of the clay mineral are also 
11111'1111 11111 10 the engi nee r. es pecia lly in soil stabilization processes. 
tA I( IIONA t t' MINERALS 
~ IIII \, 1'11 1 homll e mincra ls ex ist, some of which are important as metall ic ores. 
I Illh: I WO, however, are of real importance as rock-forming minerals; calcite and 
.tHinOlite. ulth ough others, such as siderite, FeC03, magnesite, MgC03, and 
III d. nlllt~·, ' n '03, may be important as rock constituent s. The fun damental 
IIIH HilI' 111111 of I he CH rbo na ICS, cot is a planar structure, having a carbon atom 
0111111 ' !'t'lI l te "ru ll clj llilateral trian gle, with an oxygen atom at each of the ap ices . 
1111 ~ IIIIC lurc is such that so lution can occur readily, although CaCOa is 
III ,dllh1,' und er a lkaline co nditions, and in sea water. Many of the carbonates, 
III! lIHllll1-t cu lcitc and dolomite, are relatively sol uble in slightly acidic water. 
I 111111111 I h .' is marked ly less readily soluble than calcite. 
I '1 111'11 (.' is I hc principal, sometimes the sole, constituent of the rocks limestone 
wd 111111 hit: , while dolom ite is an important constituent of the dolomitic marbles 
uld diliom il ic limcs tone, and is the principal mineral in the rock dolomite. Both 
II! Itl ,,,,,1 do lomi te have a st rong rh ombohedral cleavage and both develop 
1 lin 1I1111l' II 11C us strain phenomena when subjected to stress. Both of these 
! 11111 II \'It ' I 11<11 IcS IIrc impo rtant in determining the mechanical properties of the 
11111111111 ... 
(:Colonicul classification of rocks 
14 III ~ ... HIII V hl' classified in a number of ways, depend ing on the purpose for 
111111 tilt' ~' I fissificll li o ll is requircd. In a geological classification, the main 
• 1111 htl ' lIttin ns lI rc Ihe mineral co mpos iti on of the rock , the physical and 
• hllllhlllH ltl'llll' lio ll belween Ihe minera l gra in s, and the processes wh ich have 
1111 III d I Itt.' IlIl' k d lll ill l-\ :tl1d aflcl' its formai io n. In an engineering classifica tion 
Itl Ill! ~ 1IU1 ll' l wi. ~' lIlpIiIl Si:-; j:-, J') luccd o n those as pects which innuenceengineering 
1111111\1111 11 (;I,'IIIt1gll'lI l r lu s,..; il i 'I tt ions ha vc only a ve ry limited app lica tion in 
I 1t.'IIIII · IIII ~ /·wo lotW: (li l'Y \,'11111101 hI.' iHnol'cd sincc often an engineerin g meaning 
'I 
I ~ 11 0 , \\'1 11'1111) II H H' ~ IHlIlI I,', IlI ld \,( 11111' 1111'\' d \l IIl /-'. IIIr ~ 11I SO II II,' 11.'111\11('''1 ti l /I lock 
wlnl'i l CI III hI.' 101P()I11I 111 to I lie ' tl t\ ltl f.'l' l 11 1I·"'t' III I.' llId l.' th t.: Iwoccssl's II lVolved in 
ItIl'k 101111111\011 . I ll ' 111incI'ni '(1 111P(h IIH)II , 11.·\ IU Il'/ Hld Inblie, Funcl:tmt,; nta lly, 
til ' 111 11 in gco logica l cla sses a rc ge netic. ,"'0 0101 locks Hrc class ifi ed acco rding to 
1I1 1 ~i llll '" igneo us, sedimenlary o r ll1et II I1H H pil i " Finer subdi vision is based on 
IIl IIlCIItI co rn pos iti on: tex ture, which refers to the si7e a nd shape of gra ins; and 
lohl ie, which refe rs to the mutual a rrangement of gra ins. Geologists recognize 
thOlI'tll llcb of diffe rent rock types based on diffe rences. often slight, in 
(,lI lllp ()~ iti o n , tex ture a nd fabric. In engineering geology many of these rocks can 
he.: ~ I o llped together without any serious conseque nces. 
) , I IGN FOUS ROC KS 
H 0 'ks included in th is ge netic class are those which have coo led and crystallized 
11'0 111 a mo lten mat erial ca lled magma. T he type of rock produced from the 
Illngll'lfl depend s o n the cool ing history, during which a number of physical and 
chernica I processes. kn own as differentiation, occur; and on the composition and 
ell vi ronment of the magma. So metimes the magma coo ls and crys tallizes before 
H~ , 2, '\ I ~neo u .. rod .. : ( I ) 
( i l IIlIlIC, YulwlI l (' reck, Au,lralia 
( ~ 1(,) (2) 1\11 ,:111, Ml lIogol1 1,;. 
A\l~ l lfd lU (-16) , 
it ICll ches the eart h's sur fa ce, Igneous roc ks form ed in th is way a rc said to be 
I"II/o l/h ': th l:Y nrc ex posed only nft er the overlying Hnd enclosing count ry rock. 
illi o whi l' !! tll t'V IlII ve h 'en intfudcd. hu s hec ll croded HWII Y, BecHllsc of the 
III 
111 llIlilillP l'Ik l l III IIl ~' (' II Vt' IOPIII K r Olllll lV 1 1Il' ~ , 011 1.' 11 IIU1I1 ;' ~ J101l1 I: tH:S tllU,.' k, 
lillIll1l1l1 lur k\,( 1'001 !11I(11.' lys tnlli /l' UWI II lo ll g pl' ll od tl f I lill I:, pCIllnJ1 1'1 mi lli ons of 
II '\It II q"w lt . tl u,· ... C loc ks hu vc 11 COlli S!: tcx tll1 C, with indi vidual grain s 
I, IlIHnp lI ' III IIWly IU lgc ... i/'S , Ir thc 1l1f1 g nlll hrcak s through to the surface, a 
"Ii It II I I", (k vl.' lopccl . lind Ihe locks <l I'C sa id to be vo/callic, Cool ing and 
dldl\11 1\1 111 11 11"'(' \11 ve ry lapidly: often ove r a period to be measured in hours, 
~II 111 WI,t'k ... , Iktll US' of I h e rapidity of cooling. volcanic rocks have a very fin e 
11111 I1l1 d 11 11.' M)ln ,t imes even glassy. the cooling having been too rapid for 
I 1.l lIl/ntlll ll 10 lU ke pla ce, Under some circumstances, such as explosive 
IHln IIIIIIt , t lw vo lcanic roc ks may be fragmenta l tuffs and agglomerates. In any 
i I lin I I !low, POS1'ICSS a hroken base and a broken upper surface due to the 
I qHd \111 11 111110 11 of H cr'lIst on the now. Since igneous rocks, both plutonic and 
"It diU! . ho w h 'en in a liq uid state and have flowed, a now fabric may be 
III' I 11 1. 1'\ 11I 1.·~SC(1 tiS n preferred dimensiona l orie ntation of platy and elongated 
!l1I1II!!d ~ I I IIII S, find in vo lcanic rock s. of gas vesicles , This flow fabric has the 
III 11 .11 plodllein g H plana r or linea r mechanical anisotropy in the rock. 
\ 1111 ' 1 !oI lIhdi vision o f igneous rocks is based on mineral composition which is a 
I II! 1111111 01 rock chemi stry. Four groups a re generally recognized: acid, 
11111 I IIII'd IlI ll', bll ..; ic a nd lilt rabasic. Acid rocks, with a silica content of 66% or 
1111111 I tlllt ll lll free quartz in relative abu ndance; in the intermediate rocks, with 
II I hl' twl'l' n 54% and 66%, quartz is a minor constituent, with the feldspars 
.1'111111111 01 Bu sic rocks have Si0 2 contents between 45% and 54%. Quartz is 
, 111 111 , IllId , while feld spars are present, they may be dominated by dark 
1111111 Hdfrl!, hut over 50% feldspar is common. Ultrabasic rocks, wi th Si0 2 
,41111111 kss thlln 45%, are composed almost excl usively of the dark minerals. 
I Ol~ ~l'() l ot\ i cu l class ifications of igneous rocks have been developed. In 
11 1111 n 111 ~ f-tco logy. a sim ple classification such as that given in Table 2.4 is 
1"/11 1I 1I 1'qll ll le, T hi s classifi cat ion is due to the Geological Society Engineering 
IIIHUP WOIkin8 Party on the Description of Rock Masses ' for Engineering 
PllljllUWH, li nd should be ado pted as a standard , The classification is based on 
11111'\1 I" It crln wh ich can be determined read ily in the field : an essential 
I' IjIl/U lIl l' IIL 
~ I I) I M I ' N rA I\Y ROC KS 
II ! 1I1 11 11' IIt ill Y roc ks owe their origin to a long series of processes, the last of 
hll Ii ,II \' Ik po' ... it io n and the con version, by pressure and chemical reactions, of 
I III IHI! I' IV cI ' pos it ed sedim ents into a co mpact rock. Deposition of the materials 
I ~ 11111 \\ hh'h scdil'l'lcntary rocks are formed usually occurs in layers or lenses which 
II 11I1I\\: \1 , \ " heds, or stra ta . Fo r this reason, a ll sedimentary rocks possess, to a 
I 1\11 1)( 1 ~· ... o.;l.' l degree, a pla nar mechan ica l an istropy. Bedding is one of the 
III'j I I hlll llrl l' l iSli features of th is group of rocks.Many kinds of sedimentary 
I 111. 1'1 1, hut they Cll n bc grouped into th ree main classes: detrital or clastic; 
II. 11111 IlI lsli l' lind r yt'oc lastic: and chemi ca l-o rganic. 
I III Nli r Nt' (!11l1 ' nllll Y roc ks ha ve been derived from other, pre-existing rocks 
II 1111 11111 Inn,: I.' IOI-l ion; IIlld trul1sportnti on of the products of erosion, by wind , 
III I 11 1 li 'I': II lid lin ll ll y dill 1-\e ncs is, tile physica l ilnd chemical processes wh ich 
II 
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"II I ,\ II lOON\" IIlll' OIl ~ oild l l ll' d <.;\.'{IlIlll' lI l IlItO" l' OIl"ohtlill l'd IlIck. Ikpo .. IIIOIi 
til.' III' ~ lIh I H'l illl 0 1 Nuhll\l m'ou" . lind Ill ... 10(.'101. l 0l-W lllc i w , tl, t h !,; modI.' 01 
II lit III 11111111111, will i nllllcncc I hI.: III I'l l i ', II lid I (l son) • ex tent tile co mposi! ion . o f 
tltt 1!l1~ , IIl1d hell " i ts m 'chnn icn l propcl'lies. I)ill gcnclic processes ma y be 
I'" Ii 1.1 III l'IH" llll'lI l . 0 1 ho th . " he 111a in phys ica l process is cOinpactio n which 
I" dill 1''11 II H' dul' t10 1l i ll pOl'Osity and ll1 0 isture conte nt. The chemical processes 
II ' 1IIIII'h'\;, hUl1 lu.! res ldl is thecc ll1cntin g, o r welding, togc lhcr of grains. and 
11111111 IIla'll t' li o ns wi th in ilnd betwee n grain s whi ch produce new minerals. The 
II 111111 ti l d illl.tc ll l' , i, is or vi tal im portance in enginee ring geology. If diagenesis 
I I'lorl y ph vs l 'lI l cOIl"tpa clio n, the rock s arc much less stable physically and 
II, lui, nllv 1111111 th ose in which chemi cal cc mcnta tion and reaction have taken 
Id .!, 
II~ " 1111111I'nl11l\ HlcJ.,, ; (I) 
ti l l' lill 1 11\\1t ' 1 ~ (illp , i\\ I, l ral i:1 
I 1'1 "llh l ' ltl r, 1 ,)\\ll'I\ ('lIp. 
I 11 ,1 1 Ifd NUII' lh,' 
'jII' 11111\ Ili ul dIWillltIlHIIIIC'. 
I 111111'111 111111 .'iI.'dill1 c I1I S, and cspecially the fin er textured types, tend to 
II 11111.' 11111 ' wlwil suhj clecl 10 wctt ing, o r 10 alternat e wetting a nd drying; they 
1111 011 II ~ lIl1w "'xl'cs"livc swe llin g a nd shrinking, and abnormally high creep 
III II 1111" I Oi.' k is 101ld <.:d o r unloa ded . Unless th e cement is highly so luble, 
1I111i1111 11111 "1('( 1 i 1l ll,.'Il IS do not show th e ex treme reacti ons to weuinga nd drying, 
11'1 !I" IIII' '' 1 '~ h"'''1 Ill , "llnnc degree or lime-depend ent strain when loaded or 
1111"101"11 1 1 k llli ul "I1.'di m ' III II IY roc ks for which diage nes is was esse ntially 
1'1111 11 JlI IIII ' II NlIlIll y I(' ltl l iw l s illb l\,; holl1 ph ys ica ll y 1Ind chcmically. 
i1 
Wildt' tl t' ll it :d 'Wdllll l' llI llI \' Il Ii ~ . 11111 \ III' ~ 11I "'''' lll l'd III pllli: ly pli Y'IH'1I1 tel t1h 
"111' 11 II" PI;lltl "Ill' , Ih \.! ~i.:O IO~ l l'ul rhl",,, IIH !I I HIli IIIO Y tak c il1tO II tcount Hillin sile, 
11 10111 "I hl1p r.:. Inh l Ie lind lIlillclll l ("OlllPO "l IlI Otl , As studi f.!s in scdi n'll.: ntary 
1a'1 1 oW II pit y nd Vil li 'C, t 1H; clll ss il iCli t iO Il 01 dl' t I ilttl sedimentary rocks is bccOIn ing 
tlll' ll"I\lll }.l ly Inlticll te. The classi fi catioll 0 1 tll e!o.c roc ks. included in Tab le 2.5 . 
\ I'I~'" thos \! IIlclors which can he dC lermined simply. and read ily, in the lield , and 
Whl l'll IIIIVC so me effect on the mcchanical properti es of the rock. In the 
II I l' IHi l'C() tIS rock s, t he rock na me may be qua I i fi ed by prefix i ng a term descriptivc 
olthl' nutul'e of the ccmen!. orof the domi nanl mine ra l present. Unfortuna tely. 
Ihl .. l ' I!I ..... ;j lieatio n, due to the Geo logical Society Wo rking Party, takes no 
IH'l'Olltlt of the dia ge netic processes. 
PYI (lclu "; l ie sediment s .. of which the most common examples are agglomera te 
li nd I uff , a I'C t hc result of ex plosive volcanic activi ty a nd the associated lahars and 
mudflows . Mi xtures of co untry rock, volcanic rock, and lava are thrown into the 
lIil lind arc th f.! Jl deposited .. usua lly wi th a high moisture co ntent due to rain and 
l' (lIl(kn ~ ing steam .. in layers. Each layer tends to represent a single exp losive 
vo ltllllic cve nt. 
Wit hin the class of chemica l-organic sediments is to be found a wide variety 
01 lock types. Th e orga ni c sedi ments have their origi n in living organisms, bot h 
II 11 i 111:1 I and plant. Plant remains ultimately become coa l. while the calca reous 
.. h ' letons of marine organi sms accum ulate to fo rm limestones. Here, the 
dill ge neti c processes progress ive ly ceme nt the loose accumulation of skeletons 
lInd skf.! lf.! tal fra gments to a co mpact crysta lline roc k, so that the term limestone 
"OWlS it wide range of rock condi tions, all ex hibi ting different mechanica l 
plopertics. Since the original skeletons are calca reous, limestone is composed 
dOlll illllnt ly of ca lcite or aragonite. Because many limesto nes have a high 
Plopoltio n of detrital im puriti es, some workers place these rocks in a special 
do .... of detrita l calcareou s rocks. Although coal is norma lly regarded as a fuel. 
therc life parts of the world where it has form ed foun dations for engineering 
Ntlilctures. Lit tle is known of its foundation characte rist ics. but it is a materia l 
I'w jll ~ more close ly studied in geo mechanics as the stabi li ty of ope n-cut coa l 
Illines beco mes a major prob lem req ui ring solution. 
( 'hemica l sed iment s arc not very com mon. but large masses of these rocks do 
Ot'C II I'. They ma y result from the precipita tion of sa lts from so luti on .. by chemical 
lellc ti on, or as 1.1 result of evaporation , or from biochem ica l activity, The most 
'0111 111011 chemica l sediments are trave rtine, chert and Oin t. and ca p rocks such as 
'Hlclete , silcrete a nd ferr icrete. 
) 11 MI- I A MORPIl IC ROCKS 
Both the mineral asscmbla gc which constitut es an y rock, and the fabri c ofl hat 
IOt'k, al'e stahle within ve ry na lTOW ran ges of press ure and temperature, In the 
l'W llt of eit her, or both .. of th e pressure and tempera ture stabi li ty ranges being 
"<l,'l'e<i cd. ch:ll1ges OCCllr in both rock compos iti on and fabri c in res ponse to th f.! 
I1 l'W l'o ndilioll": i,e. 11 lIew minerul tlssc mh lnge, li nd fabr ic, arc estuh li shed which 
li lt· slnhl ' lInd el' lit · new ly imposed press lil es nnd tcmpcrutu l'cs. The changes 
wll k'h Ol'Clt t I,.'o l\stitut , lll ' tIlI11 OI'pllislll : lite I'ocks T)I'odu ced as ft res ult ofl hesc 
~1 r:!l 91 I ~ 
., 
§, 
.. " v~r ~a " 3 
~u C '5 _ a 
3 ,. •. 0 iii 
3 ~ " , n 
8. 
0. 
., ,. 
'" " 0"<::1>0 
'" 
;:; 0 ~ 01:1 :5 
8. g [s- r;. 
~ ::I .. 3 c.. 
ri' <"') 
OJ 
;; 
1"\ 
~ 
1 0-
" 
~~ :e 
" . ;. s· 0 
" ~ 0.. 
< - ~ ~; ~ 
. " 
" " 0 ;:;' ...., 
a 
0 
~ 
15 
~ 
~ 
:-
0 
" 0 
" ." 
o 
s. 
§. 
" '< a 
0 g 
n· 
'" ;,. 
changes are called metamorphic rocks. Rocks resulting primarily from elevated 
temperatures. as for e,'\ample. around a plutonic intrusion. are knov"n as thermal 
metamorphic rock~. When pressure is the main agent of metamorphism. 
dynamic metamorphic rocks are formed. This latter type of metamorphism is 
usually effecti\c o\'er a large volume of the crust. in major fold belts. while 
thermal metamorphism tend!:. to be very localized. Usually. the effects ofthe-rmal 
metamorphism are compositional.with less ob\'ious. but important changes in 
texture and fabric. On the other hand. dynamic metamorphism results not onl~ 
in major changes in mineral assemblage. but also in quite dramatic changes in 
texture and fabriC. It is the metamorphic fabric which is so important in 
determining the engineering properties of these rocks. Dynamic metamorphism 
results almost always in the mineral constituents de\'eloping a preferred 
dimensional orientation. and this imparts a planar anisotropy to the rocks. 
kno\\n as foliation. clea\'age or schistosity. In mechanical terms. foliation is one 
of the most important characteristics of metamorphic rocks. 
I-ig. 2.5 \ktarnllTphic rod.,: (II 
Slate. Bendigo. \u'tralia 1 ".'(1). 
'\ote the ~lnf!le planar ani,otrop!.::-
'lat~ clea\age. 0.) Chlorite ,chi,\. 
ra"onga. Au~tralia ("JIl). '\ote 
t" 0 planar ani.,otrop1c~. ~chi~o.,1t\ 
and strain slip clea\age, (3) \1arb"le, 
Wombeyan. Australia 1'(36). 
In those rocks which have undergone only a low grade of dynamic 
metamorphism, foliation is expressed as a fine slaty cleavage such as is seen in 
slate and phyllite. When the metamorphism has been more intense. the foliation 
is expressed as a coarse schistosity. often combined with a lithological layering 
resulting from the processes of metamorphic differentiation. In some areas. 
16 
metamorphism and deformation ma~ have occurrcd more than once. so that 
more than one foliation may have becn imposcd. E\'cn in the simplest case of a 
.... mgle metamorphic e\,ent. tv-o physical anisotropies may be present: bedding of 
the original sedimentary rock. and the imposed metamorphic foliation. For two 
metamorphic events. three planar anisotropic.., may be present. although with 
increasing complexity of metamorphism and deformation. some earlier 
foliations may be destroyed. 
Table 2.6 is a classification of metamorphic rocks which is essentially 
geological. but which also include.., criteria of importance to the engineer. The 
composition of the parent rock, the nature of the metamorphic proces!:.es and the 
intensity of metamorphism \\ill determine the compo..,illon and fabric of the 
metamorphic rock. Metamorphic rock names are commonly prefixed by the 
name.., of the more prominent metamorphic mineral.., present. e.g. cordierite 
hornfels: a'mandine sillimanite schist. 
Table 2.6 A cla~~irication or mctamorphic rock, 
Fabric 
Granular 
Foliated 
Foliated and 
layered 
Rock 
Hornfel, 
!\Ieta4uafl/Jle 
\larble 
Granulite 
Slate 
Phyllite 
Schi,t 
layered ,chi,t 
Gnei~~ 
2.4 Rock fabric 
Gram \lIe 
Fine 
Fine to coar~c 
\1edium 10 coar\e 
Coar~c 
\"er~ finl' 
\"er~ fine 
\1edium 
\1edium 
Coar,e 
CompmJ\!on 
Quartl >70"( 
Calcitc >""0'1 
Fine mica, and cla\ mlncrah 
Hne mica, 
Quart/. ield'par. mICa, 
Quartt, feld'par. m1ca, 
Quartt. feld'par. m1ca, 
As it is applied to rocks and soils. the term fahric is defined as cO\'ering the 
complete spatial and geometrical configuration of all oft hose components which 
make up the rock and soil. Within this definition is included texture and 
structure: i.e. the si7e and the preferred orientation of components. such as 
grains. crystal elements and structures, pores and \oids. and the interrelationship 
between mineral grains. Put more simply. the fabric describes the shapes. sizes. 
orientations and any other relevant characteristics of the individual components 
of the rock or soil material (and mass). For purposes of statistical analysis of 
fabric. it is usual to regard as fabric elements only those features which are 
penetrative. i.e. which occur everywhere \\ ithin the rock. Here. however. the 
scale of observation must be taken into account since an element which may be 
penetrative in terms of the rock mass. say joints and bedding planes. may not be 
penetrative in terms of the rock material. Some elements are penetrative 
irrespective of scale. for example. foliation in a metamorphic rock. It is necessary 
When discussing fabric always to define both the fabric element under discussion. 
and to specify the scale of observation. 
The statistical analysis of fabric at all scales is important in the study of the 
response of rock to stress. Elements usually considered at a small scale. in terms 
17 
III Illl' HI\ ~ 1I1II'!t' llItI , l lI t't lll' l lIt'll'Iln l IItll l llllllllllltlIIU' q ll l1 l1ll.' l yS l ld .. 01 Hi llin .. , 
IIlId ti ll' pl t' h ' ll l ' tI Ilt l l' lll tllitlll 01 Inil Hllil'll'lt .. /'j ill 1111 '1 Ill i ' lul l tll' IIII I.", lilll ltlld 
t ' k I I V II Ht'~ , IIt1d .. 11 111 11 tWill 11I 11 \l' Il II\ ' I l lldl ' l "111 111 1.' ..." p..:c inl cil' llI llSt tlIlC":S. 
\ tVS lltlIO I-\ II,phil' l.' h,' 1I1I:1I1 ... ..." UI: I1 II"': 1l]l I H' ,\ \l' " ti l 1111 111.' 11 11 .. (e.g . qU:!11! Hnd clilcit e) 
IIIIIY hI.' !t l ll ll y~t..: d to dt..: t '1'lIlin t..! Ihe l'X I,' ll' ll l'l' til II prd ": I'red orielli l-l iio n whi ch 
"(\l lld IIIIIIICIIC' the Il1ccha llicu l h ' hllVilH l1 0 1 11 lock, and which may ot herwise 
tllIl hI.' ohsc l'ved . O n H sorncw !1 al lurgel' sCllle of iI hancl spec imen Of' single 
(I UIl' I OP , ~ uch clcrne lii s as fo li <ll ion an d bedding ma y be exami ned , while o n Ihc 
1III I-\l· ... 1 "' l ' U lc, t hat of the rock m::t ss, foliat ion, bedding, fo ld a xes a nd jo in ts will be 
~'l(lIlI l it \(.' d , FII I)!'ic a na lys is at th is sca le a nd of these clement s is esse llti a l in a 
HI.' o l() ~iclI l engi nee ring stud y of a rock mass. 
I hl' purpose of fabr ic ana lys is is to determine. in a qua nti tati ve manner. th e 
I." "tcnee IIlld patt ern of clements, often mechanical defects, which influencc the 
'No t t opy and co ntin uit y of both the mate rial a nd the mass. It wi ll be show n later 
Ihlll a lli ,oll'OPY. li nd disco nt inui ty. eve n at the sma llest sca le, can innucnce stich 
11111101 ttlll t propert ies as strength a nd elasticity of a rock. Fabric analyses arc 
u"Ic,:lul , too. in the ini lial sta ges of the stress ana lys is of a rock ma ss . Some 
II ludyses cun he performed on ly by specia lists. c.g. clay minera l fa brics. Others, 
sitch li S I he ana lys is of fo liatio ns, bedding a nd joints is a simple task for any 
~l' oIOHist 0 1' geotechnica l engineer, es pecia lly if use is made of modern co mputer 
tech n iq lies. 
h lhric dllw a rc usuall y analysed, and presented, in the form of co ntoured 
\'quul II l'en d iagrams (Fig. 2.6). T he two most importa nt characterist ics of the 
I\lh l i " thc geo metry of th e clements and the symmetry, can be determined in thi s 
WilY , I' igu rc 2.6 is actually an a nalys is of a rock mass fa bric, but it serves well to 
t1i1nHlul e geo metry and symmetry. Po les to 1513 jo ints in a granitic gneiss (fro m 
110 1 th eustern Victoria , Austra lia) we re plotted, and concentrat ions contoured . 
N 
l ' III , l , l' l 'I. IUlII III CIi jllojcclion of 
1' 1 \ 111111 1\, KICWII , ALl~lm IHl . 
( oUlln" , (t ~ II 1 2 1(';. 
11 t"11 n I'll' 'itT Il Illn 1 I 1I l.' 1'\.! is [\ wide I'H n~ , ill I hI.! o l'i 'n lol iOI1 of t he jo in l"l , hilt I h t"ce 
r! IIH.' I.' lI t , :tl llI ll"l' X"1 I.' tllt t'" polldi ng to :o. llik c Il mlh ·en .. t II lId vC ll ienl dip ; ~ll'i k e 
1M 
Itlllll! \\n t IlUd \l' t t ll.'!d d ip : lind Il011l1l1l1 1tl , 1\ dl' llllII l' I'Wl tl.! rt1 lind 1111 
"1111111111111 111 11.' ", !lIII Il' tI Y 101 l it " juhl lc IIll' Cll:llll , l' , lnh1i ... h l:ll. 
IlIh tl ~' '1 V!l llI ll.' tI Y is illlPOl'tllllt 10 1' h0111 I lieo l'et iCl-I I and practical stud ies in 
. I 11111"1 11 11 1I 1i.' " " IHllt e 2. 7 s h ow~ 1\ SC I ics of fabri cs of rock mat eria ls (orientations 
It 1 1 I lIhlle , ) mmctIY: (a) 
I,d ~\ IIlIll l' t I y. (h) ()n thH h() l11hic 
11 11111 11 \ (r) Motlocliuic 
III I!!! 11\ (tl) Il lclllll C ,ymrnclry. 
11 1 1 tllll1 Cllllid \Vci,,(1963)1 
HI i I t' lIvl!~es in mica ) each with a different symmetry, the symmetry being 
1I11I tlilincd by the existence of a centre, axes, and planes of symmetry. T he basic 
jllilH·tple , a nd the importance. of fa bric symmetry lies in the fact that it is a 
II I h'i lion of I he symmetry of both the defo rming stress, or stresses. a nd of the 
ItIIl\I' I1I i,: llIS which prod uced the fabric. It is possible, then, by determining the 
IIIIIH' !l y of the fabr ic, to learn much abo ut the st resses which deformed the 
1\1\ ~ Because th e types of symmetry so fa r observed in rocks and soils are 
[l llllt Vc ly few, a nd co nta in only 00 - and 2-fold axes of symmetry, the only 
\ 1I1111ct I'y clemcnt o bserved in projection is the plane of symmetry (m). Details 
Itl lulHi l.! Ny mmetry are shown in Table 2.7. 
11 11111' 11 V 
1, 1111 11 Il lll i 
1 111 111111 
Planes 
J 
I 
o 
Example 
Figure 2.4a 
Figure 2.4 b 
Figure 2Ac 
Figure 2Ad 
11"like the symmetry of c rys tallogra phy, the determinat ion of fabri c 
IIIltH' t, y is stllti slicHI. For thi s reason it is not a lways possible to state the 
IIIItH·tI Y rH ccisely. since it ma y have aspects of mo re tha n one class. 
T ill' IIICdllllllcu l Ilropcrt ics of rock material 
II III ~ IIl11ll' l illl 1)o'ISl'S,"':C., I I Hum her of phys ica l and mecha nica l prope rties, most 
I II \\ hl\ II I II \' (Il' tl' llil ill l'd hy I 's l i n~ i ll I he Inhorntory, a nel to a lcsser ex tent , in the 
1'1 
ficld. It is most important to be aware that values for any of these properties of 
Ihc maler ial may bear only a slight relationship, if a ny, to the value for that 
propcrty of the rock mass. The main mechanical and phys ical propert ies of rock 
matcrial are: 
(a) St rcngth: 
(i) uniax ia l compressive strength; 
(ii) shea r strength ; 
(i ii) tensi le strength; 
(b) Deformation properties: 
(i) modulus of elasticity; 
(i i) Poisson's ratio; 
(iii) 'creep constants'; 
(e) Hydro logical properties 
porosity; 
wa ter sorption; 
I"n oist ure content; 
primary permeability; 
(d) Ol hc r propert ies: 
(i) un il weight; 
dry unit weight; 
saturated unit weight; 
(i i) durabi li ty. 
When testing a rock sam ple, it is essent ia l that the material is intact, i.e. that it is 
Iree from dcfccts, and also that the materia l is homogeneous. As it is desirable 
IIIIIIIhc mate rial tested should be typical ofthe mass, it may be necessary to test a 
lrll'ge number of samples. If a linear or planar anisotropy is present in the rock 
11111101·i,,!. thcn a ll of thc properties listed (with the exception of porosity, water 
sO I'plion , moi sture co ntent, unit weight and durability) will vary with direction. 
In a mechanical sense, the strength of rack material is defined as the ability to 
1 esist strcss without large scale fa ilure. Small scale failure with the development 
of I'll iCl'ofrac t u res occurs under st resses well below the strength of the rock. Since, 
in rock. large scale failure on ly occurs beyond the e lastic stress limit , this is the 
most COinm on ly used parameter, especially for britt le rocks. 
Un iaxia l, o r unconfined , compressive st rength defines the fai lu re of the rock 
,~ pcc il1"1cl1 subjected to a compressive st ress in one direction . The shear st rengt h is 
thc strcss at which the rock fai ls in shear. 
Te nsile strength is the s tress at which the rock fails in tension . T his tensile 
.'\ t !'engt h is u SUl:! lI y detcrm incd ind irectly. Flex ura l strength is the stress at fa ilure 
of it hcum . supported at bot h ends. and loaded at the cent re. As well as 
deter'mining the s trengths o f rocks unde r uniaxia l conditions, st rengths are 
(\ ' te rmined often unde r a co nfining stress: these arc ca lled tr iaxia l tcsts, and 
rll! CllIpt to simu lut c In ,\';(11 co nditi o ns as well as being used todctcrminc thcshcar 
Si"'IlKII1 of Ihe rock . 
Hi s IW l11 ct irncs nss um cd thut rocks UI'C pel'fect ly c ill s tic substullces, i. e. they 
ohey Ilook c'M ""W: 
'II 
.JJ 
( 
(2, I ) 
where a is t he stress, f is the strain ; and E is a constant known as Young's 
mod ulus or the modulus of elastic ity. In fact , ve ry few r ocks are perfect ly elastic, 
\0 that E varies with stress, and the st ress -strain relat ionsh ip is a curve. Because 
o f Ihis, three values of E may be cited (Fig. 2.8): 
I III . 2.8 Gcneral i7cd curve 
. 1111\\ ing stress - slrain relationships 
1I111lc.:k. 
E, Tangent modulus 
_-_ucs 
"+--- E, Secant modulus 
, 
~ E Init ial tangent mOdulus 
Strain 
(i) th e tangent modulus, Et, which is the va lue o fE at a specificst ress; Et is t he 
~llItl i e nt of the tangent to the stress-s tra in curve at the specific stress, usually, 
hilt not necessarily taken at 50% ultimate st rength ; 
(i i) the secant modulus, Et , which is the modulus over a given st ress range, 
li nd is the gradient of the secant joining the two points on the stress- stra in curve; 
II lid 
(iii) the initia l tangent modulus, a special case of Et for a = O. 
\Iwtller important parameter iIi elas tic theory is Po isson's ratio, v, defined as: 
" v= --
" 
(2.2) 
wlll.·re Ell , Ez are the s tra ins in the lateral and axial directions, respectively. As 
with E, the value of u can vary with stress. 
Most rocks exh ibit an e lement of time-dependent deformation in their 
1I '~ I Hl nsc to stress. Time-dependent strain, or creep. is frequently an important 
IllItlJ"IOnent o f the to tal defo rmation . A rock subjected to stress suffers an 
tll~lnnlnneo li s c lastic strain which is completely recoverable on removal of 
1II''iS , T he instantaneous e last ic stra in is fo llowed by creep, which will continue 
I" ',,'ell" i I1d cfi ni tc ly, but a t a decreasing ra te, during stress applica tion (Fig. 2.9). 
WIH,1Il Ihe s tress is removed, o nly a part of the creep stra in is recoverable . .In 
I Ill-I I til , I he less britt le n rock, th e greater is the c reep co mponent of the total 
11,llII ltllttl On, 
I'o lll 'l iiv• 1l1lli Nlllt r t'O llt l' IH . sO I'plio!! nile! pl'1l11l1ry permcabi lity are, 10 a 
II IHlIlll'.\ 1i' tlt , I l' l rll l-'iI 1' ll 1o'l iI Y (k l ;n~'''' tl H' 1"101(.' spun.·. 0 1 vo id s, in H rock I I ~ 11 
Fill . 2.9 Ti mc~d c pc nd c nt 
dc inrmalion in rock. 
t" S It css appllud 
t Sl t CSS tIltno vcd 
Elastic strain 
Time 
--1 
Creep strain 
__ 1 
1 a t io of t he total vo lume of rock and voids. Sorpt io n exp resses the a bility of a 
lock to a bsorb wa te r. and primary permeabilit y is th e capac it y of the intacl rock 
IIwferial to tra ns mi t water. Permeability and po rosity can be rela ted as follows: 
p = cry' (2.3) 
where p is permeability; ry is the apparent porosity and c is constant. A high 
porosity does not necessarily mean that a rock will have high pe rmeabil ity and 
sorp tion since these are very much d epend ent on the pore fabric ; size, 
ar ra ngement and degree of interconnection of the pores. A shale, for example. 
may have porosity several times that of a sandstone. but primary permeability of 
the sha le may be orders of magnitude less than that of sandstone. Voids in shale 
tend to be small and discrete, whereas those in sandstones are relatively large and 
a re interconnected. 
Moisture content of a rock is expressed as a percentage of the dry weight of 
the rock material. Field moisture content or saturation moisture content may 
each be imp o rtant , depending on the circum stan ccs . U nit weight of a rock is the 
weight of the ma teria l per unit volume and is usuall y ex pressed as Mg m ~ 3 . Again 
the unit weight may refer to the material at field moisture content, or saturation 
mo isture content. It is frequently expressed as dry unit weight, i.e. at zero 
mo isture content. The durability of a rock material is a measureof its resistance 
to degradatio n over a relatively short term . Durability is now expressed as the 
slake durability index I., which is a measure of the d egradation of the rock 
ma te rial under conditions of abrasion and alternate wetting and drying. 
Altho ugh this has tended to replace the standard sodium sulphate test of the 
co ncrete engineer, its significance and value are dubious. 
2.6 M echanical classification of rock material 
ilcc'-l usc the geo log ica l class ifi cation s o f rock have such limited applicat ions in 
cnginec ri ng geo logy, ma ny attempts havc been mad e to develo p a mechanica l. o r 
II 
t' ngillcc ri ng. i.:Ia ...... iIH.:Ht lo n. No s llch c la ... ~ ifi c nti o n . which wo ul d Illn· t a ll 
Icquircmcilt s. has ye t becn rorm ula tcd . In pract ica l terms. the mo~ t c ~sc nt i!l l 
Icquircmcn t is a c1ass irica tion o f th e roc k ma ss, b ut it is esscnt ia l, as a ba s i ~ 101 
thi .... t ha t t he rock mate rial be c lass ified . 
It is necessary initially, in class ifying rock mate rial , to assess its degree o f 
ho mogene ity. S uch an assess ment is large ly qualitative a nd subjective. In 
ho mogeneo us ma teria l, the minera l constituents are so distributed that a small 
", ulnp lc cut fro m a ny pa rt o f the materia l will have the same co nst ituents in the 
... " me pro po rt io ns, a nd will have th e sa me pro perties as the ma teria l as a who le. It 
1 ... a]o., o nccessa ry to d ete rm ine any mecha nica l a nisotro py which may be present , 
and to spec ify it s nature and o rientation . Ani so tro py is a meas ure o f th e 
dll ct; tiona l pro pe rt ies of the roc k. Statistically, the rock will be isot ro pic if a ll 
rl li nc ral grains have random o rientat io n of both dimen sional and c rys tall o-
gl il phi t; para mete rs. Isotropy also requ ires that a plane of equal dimensio ns 
Ill tersect ing the rock in any d irection ex poses an equal number of grflins. 
( 'o lltinui ty, whi ch is probably o f more imp ortance in a rock mass, refers to th e 
illl lO unt and o rientati o n o ffi ss ure space in the mass. In an ideal. so-called inta ct 
loc k mater ial. fi ssu res are a bse nt and the ma teria l is therefore continu o us. In 
lut t. m icro frac tures due to weat hering, as well as grain boundaries and mineral 
l'h:a vages. a re discontinuities . The ve ry exis tence of t hese microdisco nt inu itic .. 
\\ II I influ cnce the cohes io n of the rock a nd a lso the distribut ion of slress in th l: 
l oc k ma teria l. 
C lassifica ti on ofrock mate rial for engi neering purposes req uires a stat C\TlI.· 1l t 
oil thl! fo ll owing crite ria : 
( I) Petrogra phy 
(a) co mposition; 
(b) degree of weathering; 
(c) texture; 
(d) fa bric, including a statement on microdiscontinuity. 
(2) lI o moge neity 
(a) ho mogeno us; 
(b) inho mogeneous. 
(1 ) Isot ropy 
(a) isot ro pic; 
( h) a nisotro pic 
(i) n um ber of anistropies; 
(ii) na tu re and re la tive development of anistropies; 
(i ii ) o ri enta ti o n of a nistropies. 
(4) S tre ngth 
, ItHlu lly. un iuxia l co mpress ive s t re ngth o n Iy is spcci fi ed, but some c i reu Il1sta nccs 
Illll y Iliso req uire H statemen t o n tensil e s trc ll ~th fi nd o n shea r st.rength porn-
2 1 
1I \l' t l' I '. A stull (lIlld sltltCll1cnt on unia xial co mprcss ivc strength tI \l'lrj til l' telms: 
(a) vel y hi gh strength , > 50 M Pa; 
(b) high , Irenglh, 16 50 M 1',,; 
(c) medium !'!trcngth , 5 16 MPa: 
(d) low st rength , 1.6- 5 M Pa; 
(e) ve ry low slrenglh < 1.6 MPa 
(5) Fiasl icil y 
1 hi li can be exp ressed by the modulus of elasticit y. but it is becoming increasingly 
co mmo n to usc th e m odulus raJio. Etlouh . where Et is th e langent modulus at 
50( 1'(1 ultimate strength. and O ult is the ultimate strength. The standard descriptive 
te rm!'! a re: 
(a) high modulu s ratio , > 500; 
(b) medium modulus ratio, 200- 500; 
(e) low modu lus ratio,< 200. 
The mechanical classification of rock developed may depend on the precise 
purpose of the classification. The bases outlined here are intended for the 
laboratory stud y of the rock material. Deere (1968) has proposed a classification 
of rock material based on uniaxial compressive strength and modulus ratio (Fig. 
2. 10) . It has been pointed out by Deere that rocks possessing an interlocking 
I'i /.: 2. 10 Geo logical classificat io n 
o f ruck materi,tI based on uniaxial 
c()mpre\~ i \'c st rengt h and modu lus 
ra t io . [After Dee re ( 1968)] 
Low Medium 
Hro 
High modulus r3 t io 
High 
.. ,, " 
, 
Vmy High Stlength 
Low modulus ra t io 
l L-~~ __ L-~ ________ ~~ ________ ~ 
1 10 100 1000 
Uniaxial compreSSive strength (MPa! 
• Hawkesbury Sandstone 
o Calcareous Phyll ite, Fowler's Gap 
o GraniCil, Copeton 
.. Bald Hill Claystone 
• Dolomite, Fowle"s Gap 
• Basalt. Kiewa 
• Doleflte, Prospect 
<> Marble. FOwler 's Gap 
• Schist. Broken Hill 
fabric, a nd with little or no anisotropy, fall within the field of medium modulus 
rati o: thi s includes most igneous rocks and sandstones. Rocks with a strong 
plana r a nisotropy, fo r example shales, phyllites and schists, may lie in either the 
24 
ft l' ltl o f low modulu" 111 11 11 , II tile uni sotl opy lies ul un a ngle lI ppl ()uc hlll ~ 9(1 1 to 
the directi on of I ()lt dln ~, 01 lhey lie in the fi e ld or high mod ulus rat io, if the 
nnisotropy is at a low angle to the loading ax is. 
T his classifica ti on, a lth ough ba sed prima rily on strength a nd elas tic it y, is 
vc ry useful beca use it is clearly sensitive to such important aspects of the rock as 
mine ra logy. fabric and anisotropy. Complete class ification of a rock ma teria l 
'Ii ll . however, requires a brief description of litho logy and fabric. For exa mple: 
ca lcareous phyllite, high strength (HS), low modulus ra tio (LM ), fine gra ined, 
fi nely fo liated. 
I{cfcrcnces and further reading 
Anon, (1977) The desc ription of rock masses fo r engineering pu rposes . Report by the Geo logica l 
Society Engi nee ring Group Wo rking Pa rty. Quart. JI. £n~. Geol. 10, 335- 88. 
Illu!I . II .. Middleto n G. & Murray K. ( 1972) 'ow Urigill (~fSedilllen /{/rr Uocks. Pre ntice I-I a ll. New 
11·"CV. 
I )ccre D. U. (1968) Geological considerations. In Rock Mechanics in Engineering Practice (cd . K.G, 
Slllgg and O.c. Zienkiewicz). John Wiley, New Yo rk. 
I IIImer I. W. (1968) Engineering Proper/ies of R()('ks. Spl1 n. Londo n. 
0 11111 R.E. ( 1953) Clay Mineralo~y. McGraw~H i ll. New York 
lI obbs B .. Mean s W.D. & Williams P.F. ( 1976) A n OUTline of STructural Geolo~y. Joh n Wiley. New 
Yurko 
II vnd man D.W. (1972) Perro logy of J~neo/Js and Metamorphic Rocks. McGraw~ Hill, New York. 
Ma \oll B. & Berry L.G. (1959 ) Elem efll s 0/ M inl'ralogy. W.H . Freeman. San Francisco. 
Spry A . ( 1969) Metamorphic Tex/ures. Perga mo n P ress. O xfo rd . 
r w'ner F.J. & Weiss L.E . (1963) Sfrll(,lImd A I/ (lI,I'~i.\ of Metamorphic Tec/Dlliles . McG ra\\ ~ lI ill. 
\I~'\\ York. 
25 
3 
The Rock Mass 
3.1 Gen eral sta tement 
The term rock mass means a large volume of rock on which, or in which, some 
engineering work is to be carried out. If the mass is made up of a single rock 
material, it will be, for engineering purposes, relatively homogeneous; if it is 
made up of more than one materia l, it may be grossly inhomogeneous. Masses of 
igneous rock, such as granite, are often homogeneous, alt hough volcanic rock 
masses may not be. On the other hand, masses of sedimentary and metamorphic 
rocks a re usually inhomogeneous in the extreme. No rock mass is tru ly 
con tinuous: it will be broken by cooling and shrinkage joints and fractures , 
bedding separation surfaces, tectonic joints and fau lts, and minor igneous 
intrusions such as hydrothermal veins, dykes and sills. In addit ion, the near-
surface segmentmay be weathered. The intensity and nature of the 
discontinuities depe nd s on the origin , nature, homogeneity and geologica l 
his tory of the rock mass. Di scontinuities in an initially homogeneous rock mass 
not only contribute to a destruction of continuity, but in a red uction in 
homogeneity since it is along fractures that dykes and sills are intruded, and that 
weathering is most active. Faulting produces a cataclastic rock which differs 
from the parent rock, in that it is completely crushed , and may have different 
mineral assemblage; whi le faults and unconformities may bring into con tact 
rocks which are completely different, in a ll respects, from eac h other. 
3.2 Homogeneity and isotropy of the rock mass 
From a strictly geological point of view, even the sma llest sam ple of rock lacks 
true homogeneity and true isotropy. If this is the case, then neither homogenei ty 
nor isotropy could be expected in a large mass of rock. Whi le th is is true, t he 
engineering geologist and engineer are concerned with inhomogeneity and 
anisotropy sufficient ly developed to exert some influence on the engineering 
properties of the rock mass. In these terms, homogeneity and isotropy a re more 
likely to occur in igneous rock masses and inhomogeneity and anisotropy in 
sedimentary and metamorphic masses. 
3.2. I IGNEOUS ROCKS 
If chilled border phases, such as are developed about the margins of illllll"ive 
masses, a re exclud ed , most plutonic rock masses can be 110111°1-\("111.011 '- Wllh 
respect 10 bo th co mpos ition and texture on a scale of N I~l1dII IIlH' 111 1 lIu' 
1'I1~'. 1 1Il·l' 1 II I'" !I Il l' Ihnl If Ih t' Il ll t SS is c losc ly cxa mined iIlI1 Oll1ogcnciln.:s 111111 (.' 
111 1111 of .'<eno lith ... a lid 1ll111cl a l segregati o ns will be fo und , bUI Ihesc will he 0 1 
li lt h'. li nd usuall y o f no, s ign ifi cance in the mechanical beha vio ur of the loc k 
I JlII ...... . II is rca sonable to ass ume that , in the fresh conditi o n, plut o nic ma sses of 
PllIllItc, sye nite . dio rite, gabbro a nd re la ted roc ks are ho m ogeneo us, altho ug h 
111m' p lutonic masses may be la yered , e .g. gabbro. It is also a rcaso nable 
U" lI lll pt io n tha t, unless flow folia tion is present , these rock s are isotro pic a t the 
11 111"" sca le . So me plutonic masses have undergone such an intrusive histo ry that 
Ilmv s tructures are sufficiently strong to impose a quite definite plannr 
11111 "101 ro py o n the rock mass. 
Most masses of volcanic rock are made up of a se ries of indi vidual fl ows, 
lin d , while the differe nces between individual flows may not be grea t, it is 
\ IlI nll\ o n 10 find between each successive fl o w such features as ash beds, fo.,!- il 
li d IlO rilo ns, and the bro ke n fragmental rock characteristic of la va flow 
"I: lll lnccs. At least until detailed studies are completed , it is sound practice to 
' 1 ..... lIl1l e lh a t vo lcanic masses are inhomogeneous. Thi s is no t to say tha t, within 
IIIl W ve ry thick indi vidual flows, homogeneity will not be fo und . 
S lll ce fl ow is an essential process in the formation of volcanic rock s, fl ow 
, I I Ift' lures are often strongly deve loped throughout the ma ss. The mass, thCll , 
1 ,111 be ex pected to possess some degree of linear or planar anisotropy. In some 
'I I 'I l · ... I his is sufric iently strong to have mechanical significance; in oth ers il is 
'\rll~. a nd o f lillie or no importance. 
, SFDIMENTARY AND METAMORPHIC ROCKS 
\,'d Il11cnlary rocks, and metamorphic rocks derived fr o m them , arc laye red 
I, njlll: nces. ea ch layer in the seque nce di fferin g in composition, tex ture. and 
1. ,111 it fro m th ose above and below it. Some sedimentary layers are graded bed s. 
wti h coa rse material at the base grading up to fine material at the top, while bed s 
II I .. Iu lle contain internal laminations of differing texture. Sedimentary layers 
within them se lves may, then, be inhomogeneous; the who le mass is strongly 
Ilt ho ltl oge neo us. Similarly, me tamorphic rock s derived from sedimentary rocks 
",II he inh o mogeneous. The possible exception is to be found in some ma sses of 
1111 11 h ie, which are relatively homogeneous, these having been derived fro m a 
hli lil ogc neous limestone ma ss. Metamorphic rocks derived from a homogeneous 
1. !l lt' IH IS (l"la SS te nd to inhe rit the homogeneity of the parent rock, but they may 
IIt ' vI' lo p a p lanar s tructure due to deformation, e.g. s lates. 
II . d uring metamorphism, metamorphic differentiation of a type which leads 
III I,th o logicallayering takes place, then the degree of inhomogeneity ma y be 
1111 I(,,, ... cd . T his a lso te nd s to in crease both the intensilY and complex it y o r th e 
plU ll il l a ni so l ro py whi ch exis ts in most sedime nt s. The d evelo pment o f clea vages. 
, 1"' I()s il y and lith o logical layering may increase the complexity s ince these 
IJlr lll 1l10rphic struc tural c leme nts may be imposed with orie nta tio ns diffe rcnt to 
Ihu l 01 Ih e o ri ginal bedding, a nd sometimes different to each o the r. Somc hig h 
~1I ,u k gJ ul1ulil CS and amphibo lit es may be bo th ho mogeneo us a nd isotro pic . 11 11<1 
\\I lllh' <';O I11 C l1laJ'hlcs may hc ho moge neo lls, lhe ir isolro py is appHrcllt rali1c rthl1n 
27 
rea l. C,tlcful stud y usual ly reveals quite stlOng tTH.!chani cll l lIlIl ~OIIlIPV . due to 
defo rma tion of these rocks during their fonnalion. 
3.3 Continuity 
It has already been stated tha t perfect continuity is not to be found in any rock 
mass. In sedimentary rock ma sses the disco ntinuities are bedding separat ion 
surfaces, joints a nd fa ults; in metamorphic rocks, inherited bedding surfaces, 
joints and fau lts; in igneo us rocks, jo ints and faults, with flow sepa ration surfaces 
in volcanic masses. All rock masses, irrespective of origin, may contain 
unconformities, and dykes and sills as small intrusions. The boun dary of a 
riutonic mass const itutes a major discontinuity. Disco ntinu ities such as bedding, 
joints and fau lts have a defin ite pattern , although the pattern may vary 
throughout the mass. Joint patterns, especially, a re more in tense a nd more 
comple x near the surface, where weathering is active , than they are at depth. The 
effect of di scontinuities is to break a mon ol ithic mass into a series of layers and 
blocks. The mec hanical importance of a ny system of fractures wi ll depend on 
their nature, spacing. o rientat ion and co ndition. Some wri te rs refe r to 
d iscontinuities in a rock mass as defects; th is is an apt term , si nce their presence 
effects a mec han ical deterioration of th e ma ss in relat ion to the material. 
Moreover, the occurrence of discontinui ties leads to variations in the stress 
dist ributi on in the mass, and may lead to stress concentrations of cri tical 
magnitude in the rock about excavatio ns. 
3.3. 1 BEDDI NG SEPARATION SURFACES 
When the sediments are first deposi ted , bedding separat ion surfaces are usually 
planar and horizontal. T hey form the boundaries between layers, or bed s, of 
sediment which differ from each other in texture a nd composition. Each 
Fi~. 3.1 Sedimenta ry bed s folded into an a nt icline, Cape Liptrap. Austra li a. 
28 
llitil'v ldUll1 h ... ·d plohnh ly I ~· fl l l· ... cn ts a si ngle depositiona l eve nt. With tectonic 
tlrlOl Il1Uli o n of the scdilllcn lHIY mass, th e surfaces beco me tilted and fo lded 
II I~' 1 1, 3.2) and so metim es show a high degrec of co mplex ity. At de plh , 
" . '1 SedLmentary beds lolded into a sy ncl ine. Ca pe Liptra p. Australia . 
I" 01111118 planes a re usually tightly closed, and become open only with 
\'I' lIlh cring, and with stress release due to eros ional unloading. Spacing varies 
Itlllh' ly from a few millimetres to many metres. Under some circumstances,