2010.Barton LowStressHighStressPhenomenaBasaltFlows IntWorkshopVolcanicRocks

8/9/2019 2010.Barton LowStressHighStressPhenomenaBasaltFlows IntWorkshopVolcanicRocks

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1 INTRODUCTION

The beauty of columnar basalt, and the huge arealextent of basalt flows across large tracts of manycountries, are erha s the features that characteri!e basalt most rofoundly" The Colombia Ri#er basaltsin U$%, and the &arana 'asin basalts of $"(" 'ra!il,are )ust two of these ma)or accumulations of 1*+s of thousands of m- of basalt" In this a er, someso histicated characteri!ation in the first locationmentioned, in the ho e of finding a nuclear wastedis osal candidate, and some ma)or roc engineering roblems due to extreme hori!ontal stress in thesecond location, will form the core of this a er"

.igure 1" 'asalt forms bloc s of many sha es and forms"- $TR($$/D(.OR0%TION C %R%CT(R

One of the U$%+s nuclear waste dis osal candidatesof the mid/eighties was the 2** m dee Cohassetflow of the extensi#e Colombia Ri#er basalts" Thiswas found some distance away at a more con#enient

shallow de th for reliminary but extensi#echaracteri!ation studies, at the so/called anford'3I& 4basalt waste isolation ro)ect5"

$ome interesting )oint deformation effects werecaused by the low hori!ontal stress le#els at this4too5 shallow location, as re#ealed in anin situ bloc test, and at larger scale in some cross/hole seismicmeasurements in a tunnel wall, showing strong (D6effects" %t each scale, beha#ior was affected ins ecial ways by the anisotro ic )oint ro erties and by anisotro ic stress le#els, articularly the lowhori!ontal stress" The latter could be controlled inthe bloc test, and thermal loading logically caused )oint closure7 the original state" %n unex ected linear stress/deformation beha#iour was measured in the bloc test, a arently due to the contribution of bothshear and normal com onents of )oint deformation"

$ome site characteri!ation was erformed by theauthor, along ex osures of the candidate Cohasset

8ow $tress and igh $tress &henomena in 'asalt .lows

N"R"'arton Nick Barton & Associates, Oslo, Norway

%'$TR%CT7 Contrasting geo hysical, roc mechanics and roc engineering ex erience in basalts, caused by either exceedingly low or extremely high stress are described, from ro)ects in the U$% and 'ra!il" Thefirst in#ol#es a nuclear waste characteri!ation ro)ect in anford basalts in the U$%, and the seconddescribes, in much more detail, stress/fracturing roblems in numerous large tunnels at the 19:* 03 Itahydro/electric ro)ect in $( 'ra!il+s basalts" &articular henomena that were noted, include linear stress/strainloading cur#es when columnar basalt is loaded hori!ontally, and a * #alue reaching about -*/-: at Ita (&"





.igure <" The #ertical and hori!ontal ray/ aths showed greatcontrast due to the low stress across columnar cooling )oints"

The two diagonal seismic/ray aths F1/9 and F-/9showed, in contrast, almost identical seismic#elocities, with a lateau at about :/:": mGs, andreduction to about ="< to 9"9 mGs in the outer - to =m"

.igure " $eismic and other hysical measurements in the face

of the tunnel in basalt" 6immermann and ing, 412?:5"

The large contrasts in H #alues for the #ertical ath 4F1 to F-5 and for the hori!ontal ath 4F= to F95close to the o ening 41": to -"* mGs difference5 arethe most clear indication of the easily disturbedcolumnar )ointing" There is also some indication of atangential stress concentration effect7 the bac ground 4far/field5 #elocity of about :"9 to :"?

mGs a ears to be ele#ated by about *": mGs fromabout 9 to ?m de th in the wall, with a lower bac ground #elocity" The authors registered no consistent trend in R D#alues with de th, but increased crac density wasseen close to the o ening" The #elocity reductionsseem to be a roduct of blast/damage, stress relief4and redistribution5 and ossible reduction inmoisture content" The authors noted water flow fromsome of the hori!ontal holes during the tests, andhad originally assumed more or less saturatedconditions" owe#er, .igure does show a changein saturation le#el" These results are resented in order to em hasisethe ossibility of drying out of some of the )oints,des ite water flow from some of the holes" Thetheoretical analysis of crac density did not a ear

to be su orted by the R D measurements ingeneral, but is erha s an ex ression of )oint #oidratio changes, with the )oints closest to the tunnelwall showing the largest #oids and thereforesuggesting an a arent 4but false5 increase in )ointdensity" % so histicated heated bloc test was one of themain com onents of thein situ testing at '3I&" .lat )ac s were used to load four sides of the large )ointed bloc and confinement on a fifth side wasa#ailable too" Unusually for )ointed roc masses,neither conca#e nor con#ex load/deformation cur#eswere roduced7 rather the load/deformation waslinear when erformed across the art/columnar artentablature )ointing" .igure ? suggests how this may be due to thecombination of )oint closure henomena 4conca#e5and )oint shearing tendencies 4con#ex5" UD(C/''models of these )oint configurations showed suchtrends"

.igure ?" Conce tual ex lanation of conca#e, linear andcon#ex load/deformation cur#es obtained from in situ testing,and UD(C/'' modelling result showing res ecti#e )ointshearing magnitudes" 'arton, 412?<5, and NJI modelling team"

In .igures 2 and 1*, a more sim le/mindedclassification of columnar basalt is suggested, usingthe first four / arameters" Kw and $R. might beextreme"



=" $TR($$ &RO'8(0$ %T IT% LDRO 'R%6I8

In the case of the 'ra!ilian 1,9:* 0w Itahydroelectric ro)ect, contrasting /#alues inad)acent columnar and entablature flows were thefocus of stress/fracturing redictions for #arioustunnels" It was found that the least )ointed flows with

high /#alues attracted extremely high stresses inthe to ogra hic ridge defining the ro)ect locationacross a meander in the ri#er" $tress fracturing andextensi#e, many meters dee , >dog/earing+ occurredin the fi#e large 1:* m- di#ersion tunnels" In thehigher/ele#ations of fi#e ressure tunnel linings,crac ing occurred when contact grouting,s ecifically in the = o+cloc and 2 o+cloc ositions,o#er total lengths of hundreds of meters" There wasalso extensi#e erosion loss of basalt in the firstflood/o eration of the s illway, which could beattributed to the stress/aligned fracturing" $tressratios * as high as -:71 could be inter reted in the:* to 1** m dee tunnels"

.igure 2" Columnar basalt dis laying large/scalesinuosity and small/scale undulation due tosuccessi#e cooling allowing )oint ro agation togreater de th" 8ocations7 Chile and Jreenland"

.igure 1*" 0ostly small/scale lanarity and small/scale roughness down each column" $uggested +classification would be 42*/1**5G2 x 41":/=5G1" Thesix sides of the hexagons are e;ui#alent to Kn M 9"



.igure 11a" The 19:* 03 Ita hydroelectric ro)ect was builtacross a roc ridge formed by a 1- m meander in the&araguay Ri#er in $outh (ast 'ra!il"

The first telltale signs of high hori!ontal stresses andstrong stress anisotro y de#elo ed gradually as the ro)ect itself rogressed, during the four years of construction time" $e arate henomena in differentlocations in the ro)ect e#entually built a con#incing icture of a highly stressed, narrow roc ridge inwhich the ri#er meander itself resumably had actedli e an >o#er/coring+ agent" 4$ee the satellite hotogra h re roduced in .igure 1-5" The assumedregional stress anisotro y was concentrated in thenarrow, illar/li e ridge, and with each newexca#ation,

stress concentrations ro#ed to be close to the limitof stress/induced fracturing and sometimesexceeded the limit, des ite the high strength of the basalts" &o ing noises, some thin slab e)ection, andlarger than ex ected deformations were recordedduring exca#ation of the five diversion tunnels , atde ths of only :* and 1**m beneath the ridge" $ince stress roblems and deformations weremore notable as the tunnels reached their full height,the re#iously ro#ided roc bolting in the arch of

each to heading ro#ed, in retros ect, to beinsufficient, as some areas of excessi#e scouring inthe arch and in#ert were later ex erienced followingri#er di#ersion" $e#eral metres thic ness of o#er/

stressed roc were lost in laces, in the arch and inthe in#ert"

.igure 11b" $tress/related roblems were noted first in the4Tuneis de Des#io5 di#ersion tunnels, later in the 4Tuneis.orcado5 ressure tunnels and finally in the s illway"

.igure 1-" $atellite image of ro)ect site" The Uruguai Ri#erflows from the bottom to the to of this satellite hotogra h"

These lost meters of failed roc will be bac /calculated as indicators of stress magnitudes" 3ewill also return soon to the deformations measuredin the tunnels, when trying to bac /figure the li elyle#els of stress" The author+s in#ol#ement in the ro)ect started atthe s illway location, with a /system basedhistogram logging of the characteristics of the four basalt flows J, , I, K that were now well ex osed at



this downstream location" 8arge/scale crac ing had beennoted abo#e the ortal of the tunnels, ossibly alsodue to the high, hori!ontal, ridge/ arallel stress" 0ost of the di#ersion tunnels had been exca#atedin the central, and most massi#e and I flows wheremost of the > o ing+ was registered" The /loggingconfirmed the significant difference in the degree of )ointing between the basalt flow > airs+ J and K4abo#e and below5 and and I in the centre" In thiscase we were dealing with a >sandwich+ with a hardcentre, which was erha s res onsible for concentrating hori!ontal stresses to an e#en higher le#el in the N/$ oriented ridge 4see .igure =5" Therelati#e magnitudes of the / arameters in the two airs of flows were as follows7

.lows J and K7 general character7

1<<"

-1-:"1

2<2*D* ×

−

−×

−

−=

.lows and I7 general character 7

11

1D:"9:"1

<=1**2* ×

−

−×

−

−=

&rior to the assum tion of a significant stressdifferentiation between the two airs of flows, wecan gi#e the following reliminary /ranges of : to1=, and =* to 1** res ecti#ely" If we assume generalhigh stress for all these flows, and a reliminary$R. ranging from *": to -, the abo#e ranges areextended to -": to -<, and 1: to -** res ecti#ely"

Correlation of such /#alues with roc mass arameters such as deformation modulus andseismic #elocity are im ro#ed, following 'arton4122:, -**-5, by normali!ation with the uniaxialstrength @c"

The normali!ed #alue c is estimated as follows7

1**

II c

cσ×

= 415

%n estimate of &/wa#e #elocity 4for #erificationwith site characteri!ation5 is gi#en by the followingem irical relation for roc of low orosity, and isalso shown in .igure 97

( )sG,m:"=IlogH c += 4-5The basalt at U ( Ita was unusually hard, with a

range of uniaxial strengths of 19* to -?* 0&a" If weassume a mean of about -** 0&a for con#enience,the abo#e /#alue ranges for the two airs of flows become c estimates of : to :-, and =* to 9**res ecti#ely" Ranges of near/surface 4nominal -:mde th5 H&are therefore 9"- to :"-, and :"* to <"1 mGsres ecti#ely" These ranges ro#ed, ;uiteinde endently, to show reasonable agreement withthe 9"- to :"< mGs range for >sound roc + measuredabo#e the future di#ersion tunnels many years re#iously" The re#iously referred >sandwich+ of massi#eflows and I, in which the di#ersion tunnels were

dri#en, are li ely to ha#e attracted higher le#els ofhori!ontal stress than their neighbours, and this can

.igure 1=" Inter/correlations of or c and H and ( mass which

were used for differentiating the basalt flow ro erties" 'arton,4-**-5"

be indirectly assessed by the relati#e magnitudes of deformation moduli that can be estimated from thefollowing e;uation, again for near/surface 4nominal-:m de th5 and low orosity"

( )J&aI1*( =G1cmass = 4=5

The estimated contrasts in roc mass deformationmoduli were erha s in the range 1 to = J&a for flows J and K, and =1 to 9 J&a for flows and I,in fact roughly a doubling of moduli due to the moremassi#e nature of the central, and e#entually #erytroublesome basalt flows" 3ith greater hori!ontalstress in the and I flows, an anisotro icdistribution of moduli would robably ha#e been ino eration, but this ossibility has been ignored in thesim le treatment that follows"

9 '%C /C%8CU8%TION O. &O$$I'8($TR($$ 8(H(8$

3e can first address the magnitude of thedeformations actually recorded at u to twentymeasurement locations along each of the fi#edi#ersion tunnels" The con#ergences were lotted bysadly de arted colleague Nelson Infanti, in thea roximate log1* G4s an or height5 #ersus log1*

4con#ergence5 format of 'arton et al", 412295" (#enat the to heading stage, the deformations, whichranged from *": to 1=mm, were mostly higher thanex ected from the central em irical trend of numerous data7

(IJ Tor $&%N

5mm4 ≈∆

495In the case of TD/:, ten of the twenty instrument

locations that were monitored again after benchingdown to the full 1 m height, showed magnitudes of



con#ergence at the triangular monitoring stationsthat ranged from 1= to :*mm, with a median #alueof --mm, and a mean of -:mm"

'ac /calculation according to e;uation 9suggested much lower >stressed+ /#alues, -*mmdeformation im lying B *"?, and :*mm im lying

B *"=" $o characterization rior to tunnelexca#ation, was suggesting /#alues for the massi#e

and I flows of the order of 1: to -**, whileclassification for tunnel design was, through bac /calculation from deformations, suggesting /#aluesin the a roximate range of *"= to 1":"

3e were clearly mostly within the >stress/slabbing+ $R. class 4Table 15 of :/:*, 'arton andJrimstad, 412295, which im lies a @cG@1 ratio of : to=, or an >elastic beha#iour+ tangential stress ratioassum tion 4 @AG@c 5 of *": to *"<<, i"e" a tangentialstress high enough to cause failure with roc strength scale effects considered"

Table 1" (xtract from /system $R., concerning stress/failureof massi#e roc " 'arton and Jrimstad, 41229"

In the case considered here, the ma)or rinci alstress is of course @ and the abo#e ratios aresuggesting that its #alue might be in the a roximaterange 9 to :< 0&a, when using the 19* and -?*0&a uniaxial strengths in the logical way in relationto the abo#e strengthGstress ratios of a roximately :to =" 0easurements erformed at the site with an older8N(C $TT 4stress tube tensor5 method wereinconsistent, but maximum stresses of -2, 9= and :90&a were recorded, and, significantly, the coreremo#ed from the 2m dee holes abo#e the o#er/coring sites, showed >dis ing+, which is a sure signof strong stress anisotro y and large magnitude"There was also some limited core/dis ing in dee er arts of two in#estigation boreholes" %n alternati#e way of bac /calculating the ossible hori!ontal stress le#el is to use the set ofem irical >de th/of/failure+ data assembled in .igure19" 3ith de ths of failure as seen in .igure 1: in therange - to =m for an a#erage tunnel >radius+ of about?m, we see in .igure 19 that ratios of @ maxG@c ofabout *"< to *" are im lied when Df Ga is in the rangeof 4? - or =m5G? M 1"-: to 1"=?" Ta ing @c as ana#erage -** 0&a, the abo#e im lies that themaximum tangential stress may ha#e been as high as

1-* to 19* 0&a" If we further assume rele#ant#erticalstress ranges from about 1"-: to -": 0&a from :*to1**m o#erburden de ths, and an elastic isotro ic

.igure 19" (m irical data for stress/induced de ths of failure inrelation to stressGstrength ratios" 0artin et al", 4-**-5"

.igure 1:" Consistent stress/induced fracturing of - to = mde th 4also in the in#ert in laces where scoured by flood flows" These large di#ersion tunnels measure 1: x 1 m"theoretical @P 4max5 M =@ @#, we obtain estimates of@ of about =2 to 9< 0&a" The im lication istherefore that the ratio of rinci al stresses 4@G@#5may be as high as a roximately -*/-:, which ofcourse is exce tional"

1"1 An estimation of negative minimum tangential stresses

%s the critical ressure tunnel exca#ations were at a#ery reliminary stage, some re/e#aluations of otential tangential stress anisotro y was



a ro riate, for the artly hori!ontal, artly stee lyinclined shafts" .or the case of exca#ation throughthe massi#e and I flows, a similar assum tion tothe abo#e, of @4max5 of =: to 9: 0&a was utili!ed,together with a #ertical stress range assum tion of 1to - 0&a" This is an unusually extreme stressanisotro y, but henomena from around the sitea ear to su ort it, as we shall see" 'ased on the abo#e, and a lication of sim le

irsch e;uations, the (3 oriented ressure tunnels,li e the (3 oriented di#ersion tunnels furtheru stream, might ha#emaximum tangential stressle#els in the range 1*= to 1=: 0&a, andminimum tangential stresses as low as 4/5 -2 to 4/5 9- 0&a,easilyenough to exceed the tensile strength of the basalts"

3e now that the former, whate#er their realmagnitude, had been sufficient to cause stressfracturing in the first tunnels exca#ated, and loss of 1**+s u to 1***+s of m= of stress/fractured roc during ri#er di#ersion 4.igure 195" The latter 4tensilestresses5 would clearly be large enough to causetensile fractures on N$ sides 4or = o+cloc and 2o+cloc ositions5 around the ressure tunnels,which were yet to be com leted if these ressureshafts assed through sufficiently massi#e flows for the abo#e irsch elastic isotro ic solutions to berele#ant 4max" tang" stress M=%/', min" tang" stressM='/%, where % and ' are the ma)or and minor rinci al stresses"5"

%s it ha ens it was also disco#ered duringthis first in#ol#ement with the ro)ect that the mostmassi#e and I flows had >mysteriously+ gi#en thehighest ermeabilities" This mystery is easilyex lained if theminimum hori!ontal stresses werealso of the same order of magnitude as the abo#e#ertical stress assum tion" Hertical tension crac salong the N and $ sides of these arts of thein#estigation boreholes could readily ex lain the>inex licable+ high ermeabilities in the mostmassi#e roc mass" This is another illustration of theneed for se aratecharacterization and classificationfor before and after exca#ation, at whate#er scale"$uch differentiation when using the /system isem hasised in 'arton, -**-"

- CR%C INJ O. T ( &R($$UR( TUNN(8$

The foregoing >situation re ort+, which can besummari!ed effecti#ely by .igure 1<, was deli#eredin 122 , two years before the author+s second #isit tothe site in 1222, following com leted exca#ation andlining of the fi#e ressure shaftsGtunnels 4mostly a:: degrees inclined section of 19*m length and alower hori!ontal section containing the final steel enstoc s5" Raised boring of the >central+ core of each inclined shaft had been followed by drill/and/

blast exca#ation of the com lex 4sometimes double/cur#ed5 2m diameter ressure conduits, which had been tem orarily su orted with fibre/reinforcedshotcrete and roc bolts, followed by about *":m

.igure 1<" $ummary of the robable origin of an ele#atedhori!ontal stress and stress anisotro y at U ( Ita, and thereasons for two otential ty es of roc failure around the tunnelexca#ations"

of reinforced concrete but with reinforcement onlyin the lower half of each shaft , where the water

ressure head would #ary from :: to 11*m" Thisomission of reinforcement ro#ed in the end to be afalse economy" $ome > o ing+ had been recorded duringexca#ation of the shafts, but only the lower art of each shaft was exca#ated in the massi#e and Iflows" The number of locations 4=, :, <, 2 or 115 inthe fi#e shafts where o ing noises had beenrecorded during exca#ation, were gi#en ele#ated$R. #alues 4-":, : or -:5 in the follow/u /logging" % concentrated and rather continuous !oneof roc noises and > o ing+ a ears to ha#eoccurred in the central section of ressure shaft T./-, with ? to 1* close occurrences" ere an $R. of -:was assumed, gi#ingclassification /#alues as lowas 1"<"



The henomenon of concern on the occasionof the author+s second site #isit was intermittent butrather regular crac s along the N$ sides 4or =o+cloc and 2 o+cloc ositions5 of the ressure shaftconcrete linings" The rather linear, sometimess oradic, sometimes semi/continuous crac ingmostly stretched for some <* to 2*m down eachshaft, and was a arently caused when the contact

grouting had been erformed, behind the re#iouslysound, cast concrete sli /formed linings" Thisgrouting had been limited to *"- 0&a excess ressure" The a erture of these mostly lea ing crac sin the concrete was from *"- to -mm, with many inthe range *"9 to *"?mm" The crac s were rough on asmall scale 4KRC* about -* to -:5 but remar ablylinear on a scale of meters" >.ortunately+ they hadoccurred before filling the ressure tunnels"

.igure 1 summari!es in gra hic form, what isassumed to ha#e ha ened as a result of the contactgrouting in already tension/bearing, and erha s re/crac ed, but otherwise massi#e roc " The existingnegati#e total stress 4σ h minimum5 at = o+cloc and 2o+cloc was already robably an e#en more negati#eeffecti#e stress because of the near/by reser#oir filling 4/u5, and this was made e#en more negati#e by the grouting ressure 4/Qu5" 3e thus ha#e otentially four stages of crac de#elo ment, if thefinal ob)ecti#e of ressure tunnel filling is included,as suggested in .igure 1?"

It is clear that this situation was com letelyunacce table for ressure tunnel o eration, and an

extensi#e re air o eration was already underway,using >e oxy ta ing+ following the method suggested by %ndrioli et al", 122?" This re air o eration wasextended considerably when the unstable nature of the henomenon was fully a reciated" Of articular concern were of course the conse;uences of uncontrolled lea age from the ressure shafts intothe roc mass abo#e the dee owerhouseexca#ation" %n extension of the drainage fans froman extension of the existing drainage gallery wasrecommended, in order to be able to intersect theabo#e crac s 4between each ressure shaft5 at moreacute angles, to im ro#e drainage efficiency" 0orefans of drain holes were added" &ie!ometers werealready installed and more were added"

%lthough some additional drainage ca acitywas installed, there was reference for the moreextensi#e crac 4and otential crac 5 re air using>e oxy ta ing+" 8ea age under first filling was wellcontrolled, though was higher than desirable" %scientifically interesting henomenon was alsodisco#ered" There was a #ery minor rotation of the owerhouse inclinometers, when a ressure tunnelwas ta en out of o eration for ins ection, suggestinga cou led 4effecti#e stress controlled5 deformation of the roc mass surrounding the ressure shafts, in thehillside abo#e the ower house" Needless to say,extensi#e im ro#ement of the grouting in this area

was recommended, in order to reduce sensiti#ity to otential effecti#e stress changes, and extra drainholes were drilled from the drainage gallery"

.igure 1 " Tensile stress enhancement during contact groutingis assumed to be the reason for the extensi#e crac ing"



.igure 1?" 8ongitudinal im ression of the grouting inducedcrac ing along considerable lengths of each of the inclined ressure shafts, which unfortunately did not ha#e steelreinforcement in their u er :* to <* m"

= %U I8I%RL $&I883%L O&(R%TION

The fourth, ma)or stress related henomenon atU ( Ita was ex erienced in the flat bottomeds illway exca#ations, in other words right to thesurface in exca#ations intersecting the two massi#eflows and I" &artly unlined, or mostly unlinedauxiliarys illways ha#e o erated with success in 'ra!iliandam ro)ects built in basalts in the ast, andex erienced consultants acce ted a similar designfor U ( Ita" One of the s ecific reasons that the basaltic roc s strongly resist erosion during theinfre;uent, but sometimes extreme water flows, isthat they ha#e an interloc ed, >)igsaw/li e+ attern of #ertical and sub/#ertical )ointing, fre;uently withcur#ed interloc ing roughness 4e"g .igures 2 and1*5" %lternati#ely the )oints may ha#e a smaller/scale >o en saw/tooth+ roughness, caused by the brittle/ductile cooling front that allows coolingcrac s to de#elo downwards from the surface, butonly as an intermittent rocess, a few centimetres ata time" There are often minor changes of directionwith each cooling/)oint ro agation, which hel s to

add to the deformation resistance of bloc s that are )ointed in this way"The sus icion of a fourth stress/related

henomenon was occasioned by an un recedentedloss of 1?,***m= of roc from the floor of the-*,***m=Gs auxiliary s illway, but during only -Shours 4 5 of s illway o eration with flows of only?**m=Gs for 1 hour, and 1,<**m=Gs for 1S hours inthe -***G-**1 rainy season" The author+s third #isitto the site in the dry season in mid -**1 coincidedwith the ossibility to ins ect, in dry conditions, if there was any e#idence to suggest stress/enhancederosion"

'esides a re#iously hidden >)unta falha+ or )oint/fault beneath art of the s illway, the mostim ortant henomenon ro#ed to be the existence of numerous, well/oriented tension fractures, whichcrossed or ran sub/ arallel with some existing N(trending )oints" The stress/induced fractures had ine#itably the familiar N$ 4ridge/ arallel5orientation" There was also an e;ually er#asi#ede#elo ment of sub/hori!ontal and e;ually fresh4unweathered5 tension fractures, which also satisfy a N$ maximum rinci al stress orientation" Thus the basalt had lost its rime ro erty of non/systematic )ointing 4if we ignore the familiar columnar )ointing,which resists erosion with reasonable efficiency5"

The now systematic fracturing of the basalteffecti#ely di#ided existing, irregular/sha ed bloc sinto smaller, more easily eroded units, and as theu er or front arts were remo#ed by traction andGor ore ressure during s illway o eration, the nextsub/bloc s were ex osed for a similar treatment" %nexisting basalt bloc , if di#ided by )ust one #erticaland one hori!ontal fracture, becomes in the rocess

9 bloc s" % finer di#ision of each existing bloc withtwo #ertical and two hori!ontal fractures becomes inthe rocess 2 bloc s" 3ith res ect to erosionresistance this is a catastro hic increase and wasreadily obser#ed in the floor of the s illway,sometimes with greater fre;uency than this, in oneof the directions of fracturing"$e#eral 1m long cored and instrumented slots had been drilled in the floor of the s illway, followingthe scouring e#ent, and these showed slow closurewhen oriented roughly (3 4u to *":mm surface/

measured closure5, while an almost N$ aligned slotshowed o ening, by u to *"1:mm" $uch is broadlyconsistent with a strong stress anisotro y, but wouldneed to be modelled in three dimensions forinter retation to be meaningful" The measurementscould also be influenced by sub/hori!ontalfracturing"

Interestingly, occasional >radial+ blast/gas inducedfracturing seen at the base of some remnant, #ertical blast holes, was actually not radial but elli tical with the long axis ine#itably oriented N$, with some

articularly extended 4gas/and/stress/induced5fractures in this direction" %t another location in thes illway floor, a newly stress/fractured slab hadlifted 4buc led5 ma ing a ga of se#eral centimetres, beneath its bridge/li e structure" igh stressanisotro y was e#ident in many forms at U ( Ita,and was a #aluable learning ex erience for all arties in#ol#ed, including the author who wasengaged by the consortium of Contractors"

" CONC8U$ION$

%n unex ected linear stress/deformation beha#iour was measured in the bloc test that was erformed at the '3I& ro)ect in anford, U$%"This a arently was due to the contribution of bothshear and normal com onents of )oint deformation"Koints that are closing normally exhibit conca#eload/deformation cur#es, while )oints that are loadedin shear exhibit con#ex load/deformation cur#es"These contrasting trends a ear to >cancel+ and alinear cur#e results"

'asalt flows encountered near the surface mayexhibit low hori!ontal stress, if there are noto ogra hic or tectonic reasons for stressconcentration" This would seem to be related to the



tensile nature of )oint formation, and is clearest inthe case columnar basalt" %t the '3I& site, thethermal and flat/)ac a lied stress in the heated bloc test was able to close the #ertical columnar, art entablature )oints, gi#ing much stiffer beha#iour"

Cross/hole seismic erformed between four

boreholes drilled into the wall of an ex erimentaltunnel, also at shallow de th, exhibited strongcontrasts in #elocity between hori!ontal 4lowest#elocity5 and #ertical 4 highest #elocity 5 ray aths"Diagonal ray

.igure 12" Hiews of the site during construction and after reser#oir im oundment" The narrowness of the ridge of roc made for a #ery com act site 4com ared to 1<"< m of mountain tunnels for a much larger ri#er meander at Kin ingII5"

aths showed intermediate #elocities" In all casesthere was a strong reduction of #elocity of about -mGs in the outer - to = m of the holes, due to an

(D6 caused by blast damage, stress reduction, andsomewhat reduced moisture content"

%t the candidate waste site, the Cohasset flowwas encountered at about 2** m de th, and due tostrong stress anisotro y and high stresses 4<*, 9*, =*0&a5 core discing was ex erienced in the moremassi#e columnar/)ointed roc " $tress/inducedfracturing was redicted for the tunnels in thismassi#eflow, but )oint deformation maybe would

ha#e rotected tunnels from roc bursting in themore )ointed entablature"

%t the Ita 1,9:* 03 (& in 'ra!il, an anisotro ichori!ontal stress distribution of >normal+ magnitudefor $( 'ra!il a ears to ha#e been seriouslyconcentrated by maximum/stress/aligned ri#er erosion" This occurred in the narrow 1:*m highridge se arating a 1- m ri#er meander, which waschosen for the site of this hydroelectric lant"

.urther concentation of hori!ontal stress in thisridge was caused by general stri ing andexca#ation of large surface structures such as theauxiliary s illway" 3ithin the ridge of basalt are two articularly massi#e, high /#alue, high modulusflows, which robably concentrated the hori!ontalstress e#en more" Tunnelling in these flows roduced many sur rises, e#en when tunnel de thswere only :* to 1**m"

(xca#ation of the fi#e di#ersion tunnels in an (/3 direction beneath this highly stressed ridge

a arently increased the already high stresses withinthe massi#e flows, to tangential stress le#els as highas 1-* to 19* 0&a, e#en at :*m de th" .or the large1: by 1 m tem orary di#ersion tunnels withminimum roc bolting and shotcreting, the resulting- to = m dee stress/fracturing allowed ma)or erosion, amounting in laces to loss of se#eralmeters of hard basaltic roc in the in#ert as a resultof ri#er/flood di#ersion through the tunnels, and of course a similar loss of - to = m of stress/fracturedroc in the arch"

.or the fi#e ressure shafts exca#ated in the same(/3 direction through the ridge, the most seriousconse;uence of the extreme stress anisotro y wasthe highly negati#e minimum tangential stress,which caused tensile crac ing of the roc and later of the concrete lining 4 in the = o+cloc and 2o+cloc ositions5, when e#en low ressure contactgrouting was erformed" The location of thecrac ing followed sim le roc mechanics theory,and a ears to ha#e been re eated earlier whendrilling #ertical in#estigation boreholes, whichshowed greatest permeability in the most massive

flows , robably due to N/ and $/side tension crac sdown the massi#e/roc arts of the boreholes"



8essons to be learned include the need for stressmeasurements in general, when lightly reinforcedlined 4or unlined5 ressure tunnels are contem lated,and to ogra hic reasons suggest insufficientminimum roc stress" The extreme stress anisotro yat Ita (& would ha#e far exceeded the limits for hydraulic/fracturing based stress measurement dueto drilling induced tensile crac s that would not ha#e

gi#en a brea /down ressure nor a shut/in ressure,exce t at a larger, un nown radius" The maximum rinci al stress could not then ha#e been estimatedin the normal manner"

(xtreme, roc stress/induced, systematic tensilefracturing around ros ecti#e ressure tunnels, rior to their o eration, is rather unusual, and resents adilemma" igh ressure grouting of the roc could robably ha#e hel ed to eliminate the two regions of negati#e tangential stress along each ressure shaft, rior to reinforced concrete lining"

&artial >homogeni!ation+ of the tangential stressesand general roc mass im ro#ements throughsystematic grouting would also ha#e reduce the needfor hea#y reinforcement of the concrete liner, butwould need to be ro#ed by ost/treatment ermeability and stress measurement, and localcross/hole seismic and more general tunnel wallrefraction seismic"

?" R(.(R(NC($

%ndrioli, ."R", 0affei, C"("0" Rui!, 0"D" 122?"Im ro#ing the lining of the headrace tunnel inCharcani H &ower &lant, Rio Chili, &eru" &roc" :th

$" %merican Conf" on Roc 0ech" and -nd

'ra!ilian Conf on Roc 0ech", $%ROC $ 2?,$antos, 'ra!il"

'arton, N" 12?<" Deformation henomena in )ointedroc " ?th 8aurits ')errum 0emorial 8ecture,Oslo" &ubl" in Jeotechni;ue, Hol" =<7 -7 19 /1< "

'arton, N", 'y, T"8", Chryssantha is, &", Tunbridge,8", ristiansen, K", 8Vset, .", 'hasin, R" ",3esterdahl, " Hi , J" 1229" &redicted and

0easured &erformance of the <-m s an Norwegian Olym ic Ice oc ey Ca#ern atJ)V#i " Int" K" Roc 0ech, 0in" $ci" Jeomech"%bstr" =17<7 <1 /<91" &ergamon"

'arton, N" Jrimstad, (" 1229" The /systemfollowing twenty years of a lication in N0Tsu ort selection" 9=rd Jeomechanic Collo;uy,$al!burg" .elsbau, <G29" " 9-?/9=<"

'arton, N" 122:" The Influence of Koint &ro erties in0odelling Kointed Roc 0asses" eynote8ecture, ?th I$R0 Congress, To yo, =W 1*-=/1*=-, 'al ema, Rotterdam"

'arton, N" -**-" $ome new /#alue correlations toassist in site characteri!ation and tunnel design"Int" K" Roc 0ech" 0in" $ci" =2G-7 1?:/-1<"Infanti Kr, N", Tassi, &"%", 0a!!utti, R", &iller, 0" 0afra, K"0" " 1222" TensXes residuais nas obrassubterrYneas da U ( ItZ" III $eminZrio

Nacional de Jrandes 'arragens, Comit['rasileiro de 'arragens" 'elo ori!onte, 'ra!il"

ing, 0"$", 0yer, 8"R" Re!owalli, K"K" 12?<"(x erimental studies of elastic/wa#e ro agationin a columnar/)ointed roc mass" eophys!

"rospect! , #$7 11?: 1122"0artin, D"C", ChristianssonW R" $oderhall, K" -**-"

Roc stability considerations for siting and

constructing a '$/= re ository, based onex erience from \s ] R8, %(C8+s R8,tunnelling and mining" $ ' 4$wedish Nuclear .uel Co"5 $toc holm, TR/*1/=?"

6immerman, R"3" ing, 0"$" 12?:" &ro agationof acoustic wa#es through crac ed roc "% th '( (ymp! On )ock *echanics, )apid +ity ( !%shworth 4ed"5" =2 9:" Rotterdam7 'al ema"

2010.Barton LowStressHighStressPhenomenaBasaltFlows IntWorkshopVolcanicRocks

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