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668 DISCUSSION

apParentlY, suggests that the actual slress distribution in a pillar is ofno importance. Thisstatement' read in the proper context, clearly refers only to the determination of the stifness

REFERENCES1' SALAMoN M- D' G'' Rvo¡n J. A. a¡d ORTLEPP W, D. An analogue solution lor determining the elastic

response of strata surromding tabular mining excavarions. il-s. ,l¡r. Inst. M¡ti. l,tììüi.'ài, us-ttl(19Ø).2' cTr ry. G. w., H'EK E-, pnnon¡ous J. p. G., onrreee w. D. and SrLruoN M. D. G. Rock mechanics- applíed to the srudy ofrockbursts. il S. Afr. Iilst. Min. Menll.66,435_j2S (19;6j.3. coorc N- G- w. conrribution to Discussion. "// s. Afr. Insr. Min. Mctail. oa,

'r r:-írs lDo¿.

Jnt. f. Rock ,Vech. Mi¡. Sci. Vor. 9, pp. 669-497. Perganoa Prs 1972- P¡iolcd iÀ Great B¡itain

THE POINT-LOAD STRENGTH TEST

E. Bnocn

Tæhnical University of Nosay, Trondheim, Noruay

and

J. A. Fn.l¡xuN

Ræk Mechanics Ltd, Bracknell, Berks.

(Receiued 10 May 1972)

Abstact-An index test for s¡renglh classiñcation of rock materials is described. The restuses po¡tablc equipment. Spæimens re4uire no machining and can take the fom of eitherrock core or irregular lumps. A single'pointload strength index'can be obtained whateve¡the shape of specimen, provided that this shape is maintained within prescribed limits andthat a size coftection is made. Results were found to conelate closely w¡th rhose from uniaxial(unconfi¡ed) compressive strength testing, with the diametral poinGload test giving lesssetter. The development o[ a portable t$ting machine is dffiribed- Aspects of raring pro-cedure that were ínvstigated include size and shape effæts, the measurenent ol'anisorrooymd the influence of water content on strengfh results- A suggested standard t6ting præedureis given as an Appendix.

INTRODT.JCTION

IN s¡cINrsnrNc applications the conventional geological map should be regarded onlyas a uselul framework on which to build. Its classifications and terrninology are inten<iedto show the history ofrocks, rather than their potential as engineering ma¡erials. In makingthe conversion to an engineering geological map, lurther detail on lithology, joinring andother structural features should be added, and above all the mechanical character of soiland rock materials should be shown.

Visual observa¡ion ìs unreliable as a means of estimating mechanical properties. and onemust reso.rt to testing. Care is needed when selecting suitable tests. .A great many specimens

must be tested ifthe narural variability ofgeological materìals is to be adequately portrayed.'Index tests' are defrned âs ¡esrs that are sufficienrly quick and cheap to be used in classifica-

tion and mapping applications. Methods for index classification of soils have long been

established. but for ¡ocks the existing methods of index testing and classiûcation can bequesrioned.

Rock strength is an important property, and a suitabie strength-index test is requirtd.Simple 'hammer and penknife' tests can be used but this approach seldom gives objective,quanritative or reproducible results. The uniaxial (unconûned) compr*sion tesi has been

used widely for rock srrenglh classification but requires machined specimens end is therelorea slorv technique. essenrially confined to the laboratory. Ahernatives are available giving

results that can be correlated rvith uniaxial compressive strenglh. The Schmidt ReboundTest. ftrr erarnple is quick and cen be used in the field. but unfonunately the results are

insensirive to strètgth changes end are strongi] influenced bv va¡ietions in testing tech-nrque.

669

RcK 9/ó-^

670 E. BROCH AND J. A. FRANKLIN

In searching for a practical yet sensitive and reliable srrength index test, the authorsfound that one category in particular, namely the 'point-load strength tests' appearedsuitable. These tests had previously been used mainly for research, so it was trecessary toevolve testing procedures suitable for routine freld use, to develop portable testing equip-ment, and to re-examine various factors governing the accuracy and repeatability of testresults. The results of this work have been outlined elsewhere* and the technique has sub-sequently been applied with some success to pracrical problems. Suitable designs oftestingmachine a¡e now in commercial production.

Inùirect tensile tests RE\TEW

Early attempts to measure the tensile strength of rocks and concrere suffered fromexperirnental diñculties due to the stress concentrations set up by the steel jaws used togrip the specimens. Premature failu¡es were commonplace even when specially machined'dumbell'-shaped specirnens we¡e used. Various 'indirect tension' tests were investigatedas an alternative to direct methods for tensile-strength measurement.

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itporNT LoaD rr'ErHoos ç, #

Frc. l. Typical point-load and line-load tests.

Typical tests in this family are shown in Fig. l. The best known of these is probably theBrazilian 'cylinder-splirting'test [Fig. l(a)] frequently useC for quality control ol concreteand ceramics [1-13] and also for testing of rock [4, 5]. The fan:rilv may be subdivided intopointJoad and lineload categories. The venical concentrated applied load induces a hori-zontal tensile stress, and failure eventually occurs b-v splining along a plane or planes

t 135-39, ?5,441.

THE POINT-LOAD STRENGTH TEST 671

-f.6 -1.2 -0'8 -0¿Com prêss¡o n

04'fcnsion

Stresscs orè mutt¡plês of 2Plr

Flc.2. Stress distribution along the loaded diameter ola Bnzilian test sçrcimen (from FATRHURST [8]).

that run parallel to the direction of load application. Elastic stress analyses (e.g. Fig. 2)have been published for various tests in this category [6-12] and numerical techniques

have been used to consider departures from ideal elastic assumptions. From these analyses

one may draw the following general conclusions:

l. Stresses normal to the failure plane are in most places tensile, but are accompaniedby a compressive component in the direction ofloading.

2. The maximum tensile stress, occurring at the centre of the specimen, may be relatedto the applied load P, the distance betì'veen platens D, and the length of line loadingt, by an expression of the form:

t : kPiDt (line-load tests)

or

t : kPlD2 (point-load tests)

The constant k is found to assìrme values fron 0'5-l'0 (approximately) depending

on the geometr,v of the specimen.

These indirect tests on solid specimens gire results that are not strictly comparable uiththose from direct tensile testing, because of the existence of a compressive component

that may be t'ar greater in magnitude than the induced tensile stress. and whose influerrce

on failure cannot therefore be ignored. This problem led to trials ofvarious alternative forms

of indirecr rensile test using specimens in the form of disks with a machined central hole [13].In theory a state ofpure tension erists at the top and bottom ofthe central hole so that

the compressive stress problem is overcome, but other problems associated rvith high stress

gradients and the sensitivity of stresses to inelastic behaviour are introduced. Table Ishows that results f¡om various types of indirect tensile rest in l'act differ from each other,

¡nd from the direct tensile-strength values.


TABLE 1. CoMpÀRIsoN oF 'TENstLE-srRENcrH' vALUEs oBTAINED FRoM .DfREcr, AND

vlnlous .n¡r¡nec¡' TENsroN TEsrs

(values in lbf/in,)(At'ter JAecen and Hosrrrs [13])

Gosfordsandstone

Canaramarble

Bomaltrachyte

Direct tensionBrzilian (15'contacr)PinchingDisk with hole (extemal loading)Disk with hole (internal loading)Bending-3-point loading

520540450

1200I 1001l.lO

10001265670

25002;001710

19901744t090350037003650

i

i

II

The main requirements are those of simplicity and reproducibility. The point-loadtests a¡e more tole¡ant ofirregularities than line-load rests and attentioû rvill be focused onpoint-load tests in later discussion. In these tests the c¡itically srressed region lies in theinterior of the specimen vr'here surface irregularities have least effect. Strength can beexpressed as the ratio of applied force to the square of the distance bet$,een platens, andprovided lhat ce¡tain restrictions are placed on the shape and size of specimens the actualspecimen geometry has little ínfluence on the strength r¿sults. This means that t1.pes of point-load test may bè selected to suit the shapes of samples commonly available-typicallyeithe¡ core samples or irregular lumps [Figs 1(d) 1(f) and l(h)]-and fo¡ rhese tÉsrs nomachining of specimens is required.

History of the irregular \wnp lest

The point-load test on irregular lumps, developed in Russia, is described by pnoro-DyAKoNov [5-18]. To obtain a strength index, protodyakanov divided the rupture loadb)', the 2i3 polver of rhe specimen l'olume, an approximarion ro the cross-secrional;rer of [helump. The volume was determined using a sand-displacement technique. The InternationalBureau lor Rock Mechanics [9] incorporated ¡he Prototj.vakanov test as a stand¿rrd tech-nique as follorvs:

Egg-shaped 'irregular' specimens r,vith ratio of longest-to-shortest-diameter of aboutl'5:l and lolume ofabout 100 cm3 are obtained bv breaking lumps olrock usingany suirable me¡hod and rounding off suirable specimens by light hammer blorvs.The¡e should be fiftç'en ro twenty flve specimens with mass difference of iess than 2 percent, and these are crushed parallel to theìr longest axis and perpendicular t.r theplane of any laminetions.

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In Britain, the method was examined by Horas [20] with a view to classiûcation ofsedimentary (Coal Measures) rocks. His main criticism of the Russian method v/as thatin laminated rocks the long axis of an irregular lump usually coincided with the plane oflaminations, whereas the tesr requires it to be perpendicular. Specimen preparation istherefo¡e difficult. He also expressed the view that strength measurements should not berestricted to a single orientation of the laninations, and that account should be taken ofstrength variations with the size of specimen. Hobbs used an alte¡native arrangement r¡r'here

irregular lumps, usually in the form of parallel-sided slabs that are easily obtained fromsedimentary rocks, were compressed between flat platens, with the direction of compres,sion perpendicular to the plane of laminations. Platen contact was therefore at severalpoints on the faces ofthe specimen. A strength index was obtained by dividing the ¡uptureload by an average area perpendicular to the loading direction; this area was calculatedas the rario of specimen mass ro the product of specimen height and density. Also used wasan index obtsined by dividing the rupture load by the platen contact area, measured byinterposing sheets of carbon paper and graph paper betrveen specimen and platens. Asimilar rechnique was described by EvnNs and Pournov [21] and was used successfully byPtceox [22] when investigating the performance of rockñll materials.

In France the irregular lump test has been investigated by Dtrnxlr and Durr.lur [23],Du¡rrur [2-l] and by Durrlur and MAURv U l]. Taking as a starting point the test desc¡ibedby Protod-u-akanov. they were parricularly concerned wi¡h the influence of the size, shapeand orientarion of the lump on lest results. They demonstrated that for granite lumps therecorded strength could be nearly doubled if the specimen size were halved. or if the longaxis of the lump were perpendicular rather than parallel to the applied load. Their experi-rrrenral studies were supplemented by photoelastic stress analyses using plane models ofvarious shapes. These showed that the strength olslender specimens is sensitive to slender-ness ratio, and led them to recommend testing with the applied load acting aiong theshorres¡ sper'imen axis.

The practicality ol strength-index foÍmulations proposed in the methods revieweci abovecan be questioned. Volume measurements using sand displacement are inaccurate anddifñcult, as are measurements ol density or of platen contact area. Htnevlrsu and Oxe [9]gave an alternative formularion that avoids these practical problems by expressing strengthin tenrrs of rhe retio of rupture load to the square ol the distance berween platen contactpoinrs. This distance can be easíly and accurately measured. They shoued that this type oftbrmulation applies exacrly lbr an elastic sphere loaded betrveen points along a diameter,and is a -lood approximrtion for irregular lumps and for other shapes of specimen.

The aurhors have used pointJoad tests on irregular lumps to study rock weathering [25].The tests proved most suitable for this pu¡pose because core \¡'as not available, and themore highly weathered rocks, which could be crumbled between the fingers, rvould have

presenred ins¡.rrmountable problems if preparation of regularly shaped specimens had been

attempted. Irregular lump tests $ere used in Bordeaux, France [26] to classify the strengthol rveak limestone pillars in rn area of building stone extraction beneath ¡he town. Alsoin Can:rd¿ []71 the irregular lump test has been used to classify and map strength varia-tions in granites, w'he¡e kaolinization had locally resulted in considerable rveakening. Thereâre mnn,v crscs such as these whe¡e to obtain samples in the lorm of core would not be

appropriate, and lvhere the inaccu¡acies associateC rvith irregular lump testing can be

tolerated. Results lrom irregular lump tests are unquestionably rnore sc¿ttered than those

for poinÞload tests on rock core. The scatrercan be to some extent compensared by iesting

673


a large number of lumps (the test is quick) and taking the median value as an index. Thisdoes not overcome problerns due to the influence of specimen size and shape, and somedegree of standardization or correction for these effects is needed.

History of point-load test@ on cylinders and disks of core

In the united states, RErcHMurH [28, 29] conducted an experimental study of size andshape effects. His starting Point was a simple formulation for strength simiiar to that ofHiramatsu and oka; in later work this formulation was progressively modifred in orderto frt enpirical curves to the experimental results. His modiñed formulae, one of whichincluded a compressive-strength term, were conplex and thei¡ practical value may bequestioned. Following Reichmuth's \¡/ork the united states Bureau of Mines used thediametral test on core as one of ten index tests, in an attempt at compressive-strengthprediction [30]. PointJoad strength gave the highest correlation with uniaxial compressivestrength (a correlation coeñcient of 0.947).

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Flc. 3' Conelation between the Resistance to Bl¿sting Field Index (as defined by Bencr-CurusrrNsrNand S¡rutn-O¡-s¡N [33]) and the dìametral pointload strcngthlsP5l ofcores drilled perpend.icular to the

rock foliarion.

Mcwn¿r¿,ns [31] used point-load tests on disks to study the relationship of weaknessplanes in rock core to microstructural defects in the rock material. Rock core was sawninto disks which were rested by point loading in the axial direction, and the broken corepieced together to shoç'rhe preferred failure direction. In Norway, the diame¡ral testhas been in use since 1965. It was frrst applied to blastabiliry research by Bencn-CnnrsrrssrNand Serr,an-orseN [32, 33]. Larer serrmn-orsrs and Buxosrrrr [34] included rhe resramong other laborator,v tests in their work on drillabilit¡r of ¡ocks. Further ç'ork alongthese lines has been carried out by the authors t35-391. Diagrams have been published

i, 1

THE POINT.LOAD STRENGTH TEST

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Frc. 4- Correlation between percussive drill'r¡g Ête (SELMER-OLsEN and Blrsosrw t34l) æd the diametralpointload stteãg¡h I" [25] of cores drilled perpendicular to the rock foliation'

showing correlations between point-load st¡ength and a'resistance to blasting freld index'

(Fig. 3), and also correlation between point-load strength and percussive drilling rates

Gig. a).

DESIGI{ OF A TFSTING }IACHL\E

A portable poiut-load testing machine was built (Fig. 5) comprising a loading system

(rarn and loading frame) with provision for measuring the load P and the distance D between

the two platen contacts.

The loading system

belween successive tests.

strong rocks are seldom cored at such large diameters'

676

tested wirh

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E. BROCH AND J. A. FRANKLIN

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curyature were identical, as sho;n in fig. Z¡'.

15Ir {MN/r:)

ilh'; ;:n::lilu'å-ii:i'""J.'."î¡fååil',üå.;r?i.;:',Ri:::lËí,¡'*_*l",,aram¿¡bre whencuryarure were ideñri.¡l "" "r^...-

,,1 :o.n" 3nd wedge angle and radius of

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¿l

:¡:'

1.

co¡e, not for axial oren alternating between

2. to be inconvenient.

occur along planes ofweakness. llowed failu¡e to freely3' wedge platens required p¡ecautions in manulactu¡e and assembry of the tester toensure paralrel alignment. These prec rutio.r, *.ra unnecessary with conical pratens.

Load measurement

Load was measured UV 1111o;n-e

rhe hydraulic pressure in t onram the conversion lrom pressure to forc" ou, Uo.ti, ,iapi. " onpressure gauges were ñtted lvith quick_release

"oÇtin-e, uo.t ro

Frc. 5. The portâble point-load testing machine (as manufactured by ELE Ltd)

Frc. 13. Cubes of Delabolc slate tailed by poinr lording pardlcl and perpendicular to the cleavage direstion.

RM t'.p. ó761


suit the strength and size of specimen, hence maintaining a satisfactory accuracy irrespectiveofthe rock to be tested. The gauges could also be uncoupled for calib¡ation, and could beused for other resting purposes. They were frtted wirh hydraulic 'snubbers' of oriûce typeto protecl the gauge against sudden decompression at specimen failure. A small oriûcewas required to prevent gauge damage. vet big enough to let the p¡essure reach equilibriumwithin a second or so of pumping. A maximum-pressure indicating needle \ryas necessarybecause it proved impossible to remembe¡ failure readings reliably. This had to be free tomove without rcstraining the needle of the pressure gauge, yet sufrciently restricted toprevent its being carried past the true maximum reading.

Distance nleasurement

A rnetal scale calib¡ated in millimet¡es was flred to the crosshead of the testing machine,and a pointer to the lowe¡ platen. in such a way as to allow measuremenr of platen separa-tion irrespective of crosshead posirion or ram t¡avel. The scale reading couid be adjustedto ze¡o when the two platens were brought into contact. This simple afiangement minimizedmistakes in reading. The scale could be read easily to I mm, corresponding to a possibleerror of 7 per cent in the strength-index value. In practice this error would be considerablyreduced by taking the median of several results.

The distance D used in calculating the strength index, defined as the distance bet'Àeenplaten'points'at the moment of failure. equals rhe distance at the start of the test onl;- ifthe specimen is hard and platens do not penetrate into the specimen. For hard rocks aninitial reading of D is suffciently accurate. For softer rocks where contact damage, slabbingor penetration occur¡ed before lailure of the specimen, an attempt was made to obtainthe coûect distance prior to failure, although some inaccuracy was then inevitable.

EVAIú-ATION OF TESTtr\G TECII]\IQLj-E

The dianzetral point-load test

In the diametral point-load test the failure load P is independent of the lengh of coreprovided that the disrance L (Fie- 7) is sufficiently lar-se. With this condition satisfied. thestrength index is not influenced b-v.. irregular geometry of the end faces, which therefbreneed not be machined flat.

677

Flc. 7. Critiel dimension-diometral pointload test.

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Flc. 8. Diamerral point-load rest; pNided.thar length r (Fig. 7) excæds 0.5 D, the point-load. strengrhis independent of the total rength of spæimen and of the conditions oî end'faces----

The shortest length of specimen that could be tested with consistent results had anoverall length 2L eqral to the core diameter; with shorter lengths, a lower srrength wasrecorded, and short specimens olten failed along an axial plane that intersected the endfaces.

These results arecore rength wlr pr Ïr:ff:ti:iìr'1:greater rhan the cor and as a generalrule tests should be anisotropy. Thetesting ol anisotrop

correction curves f¡om which could be obtained a'co¡rected'strength index independent


ol core diameter. Size effects must be taken into account in any strength classifrcationwhose function is to compare test results from a variety of sources. Five rock types were

selected, isotropic and homogeneous materials by engineering standards, yet covering awide range of strengtrh and geological origin. For each rock type, cores were drilled at frve

difrerent diameters. using a single block of rock and drilling in one direction only. A totalof 407 tests were perlormed taking the median value of approximately 16 tests at each

diamete¡. As an indication of ¡he simplicity of the testing technique it may be mentionedthat much rrore time was needed to d¡ill the cores than to actually do the testing, and forthis reason no tests were performed at diameters larger than 76 mm.

As expected, the strength of specimens was found to decrease with increasing core

diameter (Fig. 9). Strength changed more rapidly at smaller diameters (12'5-25 mm) when

the nature ofplaten contacts, which then could hardly be considered as theoretical'points'in relation to specimen size, may have had some influence on the strength ¡esults. Atlarger diameters the size effect was much less pronounced. Pennant sandstone (sample 219)

gave results that were anomalous. The size-correction curves (Fig. 25 in the Appendix)have been based on data for the remaining four samples. However, the probable existence

of rock materials that do not follow the general trend should not be forgotten.

ro 20 30 .oiå',o"#. o,',lo 7o 60

Frc. 9. Size effect in rhe diametÉl pointJoad testt point-load strength dære¿ses with increasing core

diometer so that a sìze correction is necessary (sæ Fig. 25).

Various methods ol'size corrèction' have been considered. The formulatioo, I,: P, D2

could have besn modified to take account of the size effect, as R¡tcnltUTn [29] had pre-

viously attempted, but the authors riecided to re[ain the simple formulation which has

sone theoretical justification and is also in stress units. Another approach would have

been to specify testing at a standafd core dianeter but this would have made the technique

impractical for use in field-core logging Instesd. the authors propose that the stlength

index .I, obrainerj at ony avaiiable core diameter D should be 'corrected' to a value ¿ (-i0)

at a 'reference diameter' of 50 mm. This retèrence diameter is selected to lie approximately

679

z=


midway between rhe maximun and minimum core sizes commonly obtained in the field,thus reducing extrapolation to a minimum.

Frc. 10. Size effæt in the diametml poinr_load test: results (Fig_.9) have been re_plorteC in terms of K. amultiplicarion factor to 'normarize' råurts ,o ; .,""ä;;Ë;;- å,"r",... K also represents rhe eror in notapplying a size :orrection.

Figure l0 shows the experimentar size-effect data re-protted in rerms of K, the factor bywhich the measured p/D2 musr be murtiplied to obtain /. corrected to D :50 mm. Thediagram illustrates the amount.ofcorrectìon ttut -uy-u. needed. end also the èrrors thatma-v be invorved in apptying trris co¡¡ecrion. ir no slze cor¡ection were appried, an erroro.f more than r00 per cenr could resurt *hen .ompuring resurts for 30 and 76 mm coresince the factor K inc¡eases f¡om 0.6 to 1.3 in r',ì.ì"ng". The size-correction cur'es ofFig. 25 take this into accounr and ifthis charr is usecl i!-," "..o, is unlikery ro exceed l5 percent and in most cases should be much smalle¡.

The axial pointJoad test

Specimen length and specimen diameter both influence the rc<,,rt. ^l.ro:--¡ :- +L;^.^-a.rh--- :- ^ ¡-L^-^r -6 , .there is a 'shape' effect in addition to the ,sizExperiments to finci ¡he extent to rvhich these , .tiuse of the tes of ,..

being¡rveak vD .:ionenrrc r^^L ^. - -!-,¿ligneous rock,"'*,o uurs¡¡Lsr

"o^p,"-217.¡. Lores were dr'led in a di¡ection perpendicular il3

THE POINT.LOAD STRENGTH TEST 681

to that for diametral tesrs so that the loading direction rvith respect to any slight anisotropywould be the same. Cores with diameters of 25, 38 and 54 mm were sawn into cylindrical

specimens with six different ratios of length to diameter; approximately 160 specirnens in

total. The results of axial point-load tests on these specirnens are presented in Fig. I I (a).

These results show a size effect comparable to that previously discussed in connexion

with diametral tesring; at a given lengthidiameter ratio. the larger specirnens were'lveaker.

However. this size effect was far smaller than the shape effect that resulted from testing at

different length/diameter ratios; long, slender specimens were much weaker than short,

tested and for all core diameters. The autho¡s therefo¡e propose a length/diameter ralio

of l'l as standard for the axial point-load test.

The tolerance of i0'02 represents a length accurac) olabout -l-l mm, which is unlikely

to be achieved without machining of specimens. Disks of approximately suitable shape

become available as a result ofprevious diametral testing. The er¡or in departing from the

prescribed lengrhidiameter ratio [Fig. 1 I (a)] can be seen to increase rapidly as the specimen

becomes eirher slender or plate-like. A toleranc e of Ll D : I' I - 0' 05 results in a maximum

discrepancy of l0 per cÈnt between axial and diametral test results. This requires a length

u..uro"y ol about * 2.5 mm, and is probably the besl that can be achieved when test-

ing in th; freld. The diamerral test is more reliable lor strength classitcation, because it suffe¡s

tesì from shape effects, and the axial test should onll'be used when stfength anisotropy

is to be measured.

Inel-e ?. Lr.*crufotrurren RÀTios ro crve 1' (rw rrsr) : 1, (DLdYETRAL rEsr)

Core diameter25mm 38m 54mm

Sample 215, Darley Dale sandstoneSample 217, Quarlz dolerite

108I 11

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Measurement of strength anisotropy

Most rocks are to some extent anisotropic in their mechanical propefiies, even if they

appe¿u to conrcin no visible planes trf seakness. The strength ol in¡act rock specimens

"un uuty by a factor of ten or more depending on the direction of loadirrg relative to that

of *eain.is planes. and it is rnisleading to classifll' the strength of these materials by a

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Frc. 12. Comparison of uial and diametral pointload stren$h results; uially loaded specimens wilhlength'diameter ratio 1.08 give strength lues equal to those obrained in diametral teting.

generated by the point loading is of inte¡est. With load applied in the cleavage directionfailure was. as expectèd, by splitting along the flat cleavage surfaces. In contrast, the fail-ure of speciúens with load applied perpendicular to the cleavage was highly irregular,

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FIc. 14. S¡rencrh rcsults for cubes of Delabole slare (Fíg. l3); a strength misotropy index ol l'3S rvæobtained. Only a small scatter sas evident for ræults of loading perpendicular to the cleavage, in spite of

the highly irregular appearance of fractured surfaces.

683

E. BROCH AND J. A. FRANKLIN

õ

I

Variation in uniaxial compressive strength of South African slate as a function of direction ofloading with respect to weakness planes (from ftoã ¡+t¡.¡

with the sp unequal pieces- It is perhaps surprising thar thisirregularity resurts, which sho*eieuenìess s"-atte¡i¡an trrosefor loading ction (Fig. l4).Tests on cubes were convenient in this case because a rarge brock sampre and a suitabresaw were availabre. More often strength anisotropy is miasured by ;ring;ìock inseveral directions (Fig. l5), again only possible if iarge samples are availabîe. An easiertechnique that may be used with explorarion core e¡e block samplingis impractical) involves the use of diametral and t_load testing. Thecore is first subjected to diamet¡al pointload te of weakness, thenro a*ial point-load tests with load applied perpendicular to the weakner. ptu*r. ft"optimum loading directions are shown in Fig. 16. Before diametr"r t.riing,-tt".o.. i.carefullv ma¡ked to ensure rhat ir wilr be broken into cylinders *itr r.nfn¡aî*.ì.. r"tioof I'l i 0'05, also to ensure that the platens of the testing machine make conractalong, rather than ac¡oss a prane ofweakness. Trre index ofpoinì-load *."ogìt, ìnirorropy

1" is obtained as the ¡atio of strengths in the srrongest and weakest directio-ns.

Ftc' 16' Anisotropy measurements-on ræk core: diametral tests precede axial tests on the same core. Indiametral testing the platens should make contacrãlong a single *."tn"., pi"n..-""

application of this technique is shorvn in Fig. l7 where a r'm lengtho re from south wares has been crassiûed usilng conventionul obseruu-ti the aid of a portable point_load tester [37]. The values shown for's diametrar tests. The core, drilled vertical cãnåined planes oi*""ko.r,that lay approximately horizontar, so that diametrar testing -.".o.Éd strengrh in theweakest direction. \l'eak horizons are clearly visible in the form of coal strata and some,but not all of the mudstones. Diametral testing was followed by axial t"*rog oilh" ,"r.core and an axial pointload strength log could, ofcourse, havebeen p....oi.d. However,

uÌs30¿560?590INCLINAIIOil fDEGREES) OF WEAKNESS PLANES

TO OIRECIION OF MAJOR PRINCIPAL gfRESS

TFIE POINT.LOAD STRENGTH TEST

Gaotogìcqt to9

Mudstone, dork gr€yCoqt (Rhyd)

Coot qnd pyrìtes

S.otcorih, mediuh groy

Sondstone, mcdium grcy-

Sittstonc, medium grey

S qndsto n., med iu m g rey--Strip.d b€ds, mcdium dãrk gr.y

lronstone, dork 9r€y

Mudstone, m.dium dork grcy

Coql {Gr.yìSeoteorth, dqrk greyS.otedrth. dqrk grey

Sqndstone, mgdiqm dqrk gr€y

Frocture Str¿ngth,^spocing, ft,mñ f:, YN/m¿

soo roo 50 rolor ¡s :{ Eot,,, t,,,,t,, r 1,,,r,,,.1,r,l

Ccr! rund.pth, m

Anrsotropy,IA

i 510I l l¡,rl

^,4:l.:::'.1::: :l: ,:l

Mudstonc, mediuh grey

-Striped beós, h.d¡uh grey

Mudston!. mld¡uh dqrk grey

Mqóstona, dork qr€yMudstoña. blockcoqt tfhin)S.ot¿orth. dork gr.y

-fClqy mytonit., m.di!ñ gtlt/Sèotco.th, dork grey

Siltlonr, medium gtey

Vuclstonc, dqrk grey

F¡c. 17. Core log of Coal Measures rocks showing logs of diametral point-load strengrh aod strengthanisotropy. (Reproduced lrom Ref- []tl by pcrmiss¡on of che Institntion oi lvlining and Metallurgy.)

information about the anisotropy of strength is often more valuable. Thus. as shown, thestrensih anisotropy'inde;'. 1. has been presentèd in rhe lorm oi- a coi:rinuous log. I'leasure-ment ofpoint-load strength anisotropy is essenrial when classiflrins even moderately aniso-tropic rocks using the pointJoad !est, si¡ce other$,ise the influence of loading direction isignored.

The irregular lump teslIn this test the size and shape effects are more se./ere than uhen testing specimens of

regular geornetry, so the irregular lurnp test is less :rccurate. and less suited to 'precise'a(rR I r;s


strength classification than the diametral test. Ho\value rvhen oo "o.. i, available. Iì i;;;;"hrwever'

it can be of considerable practicaloutcrop materials p.ià. ro exprorarory dri¡ine fcl Tt{ll *l*.:tassifying and mappingwearhered, arte¡ed ir brok.n ;";;;;ì,. ;;;,;å,;.1*" Tore

deraired ,ru¿y, un¿- io Ëiã¡,tilength for diametral test lmcult to recover in pieces of ,uñ"l"ni

With suitable precautithe discrepancy between thcan be ¡educed to a minifrom results of tests ocussed ea¡lier in theratio of I .l gave axial poinrng ratio for rectangularprisms was 1.4. A shape lactoas ¡he rario of dist¿nce between I,""å"a'Oå_rrî ,n.perpendicular direcrion, is ' --- "-1.0 should be used when teo.f a more prismaric shape.sible co 50 mm so as to minThe chart in the Appendixposals are quite similar toMechanics [19], who recommately 100 c¡n3 volume.

within acceptable limits, the results when corrected to ashould correspond to those that would have been obtained br. The laritude ailowed in rhe shape "i;;;;;;r".in results. Ïr. u".i.o.y'""n ould a:count.for a _¡15)( variation

specimens; the International y testing a larger number ofthis appears reasonable. One commends 15-25 lurnps, andsuch as adequate colerage o, this test that considerarionscan be more important thin t. mpling of outcrop materials

The anisotrofy of materiat ults.

nounced and visible anisorro no¡ed. When there is a pro-and weakest directions tabula :ally performed in suongestcalculation. Tests on rock tha )urposes ofanisotropy index

Lould be conducled as lar as

n to test streng¡h isndicular ion, .flaggy,

mbers of ecsier. This

¡nen selecríon anti tesiing. tlvo directiohs even though

The influence of water conrent on srrengthThe uniaxial compressive

te¡erari"¡ï;';il;il"jï.1,i*'.ï:ji.._ï;ï:ï;:,,å",îï:å11,::iäï jå:ï:::

U

'óz

z

ÉU


UNIAXIAL COMPRESSIVE STRÊNGTH tb/in2

Frc. 18. Inrìuence of specimen water conrent "r illï]"t conprssiYe srrengrh (from Cor¡ncr ¿nd WIID

rock types that were subjecred to the diametral point-load iest shorved a strength reduction

of up to 33 per cent on satu¡ation compared with their oven-dry strengrhs, and even the

granite sarrple with zero measurable porosity suffeled a 13 per cent strenglh reduction.

Attempts to standardize this aspect of testing procedure 3re not without problems. Itmay be considered both practical and realistic to tesr at 'natural' water content, bur then

the same rock, depending on whether it is sampled in e humici or an arid environment, rvill

be allocated diffe¡ent strength values. The alternarive, which permits a universal and

unambiguous classification. is to specify testing urder'standa¡d'water-content conditions.

This, however. c¡n lead to results ¡hac are unrealistic, particularly if ¡he 'standard' water

content bea¡s little ¡elation to that found in nature-

The authors suggest that srandard strength-classificütion tests should ernploy water-

saturared specimens. StricCy. the specimens to be classified should be wa¡¿r saturated inva.cuum. then sto¡ed for several dsys under water prior !o ',esling. However. the strength

difference between fully and paltiall) saiu¡¿teci speciinens is likely to be snall (Fig. l8). Inprac¡ice therefore, ifco¡e is tesred soon after driliing. or having been stored in such a way

Trsr¡ 3- INFLUENCE oF WATE¡. cLrtvrE\T oN srRE\crFr REsuLTs

ti¡1r.

l;I1

Granitesam¡le 106

Drrlc'; Dale saniistonesamDle ll5

Pennanr sands¡onesamole ll9

C¡re diamerr'r inm)Porositv ( 9l)I ùven dry (lvl\ n:).f" sarurared ilvl\ nr)Strergth reduction ( i")

Jù

0.0r0.69.2

1i '3

38lt.53.51.4

30.3

5ll.54.2ló

i3.0

23 .51.6

!0.98.5

22.-o

-ori.d der Co Cl 2

50? R.totiYÊ humió¡tyos zero dotum

I'

lril:It-¡ì't:

¡it,,

IJ

iit,

I

r,#'t

r,i

t¡,i


as to preserve natu¡al water content, the strength results should be comparable with'standard' results on satu¡ated.rocks. Rewetting'oimateriars that have been ailowed todry out should be avoided, particularly with argilaceous materials, since this often leadsto mechanical dete¡ioration. The wate¡ condition and srorage hirt;.t oirp;.;r,r.-ens stouiaalways be stated rvhen reporting test results.

strengths of partially saturated and .fullyfor example when an engineering sire isrelevant, but tests should still be at natural

rials will invariably give results that are un_

furrher tesrs in lhe sarurated condition *,u o"oirlåtild to a universal classificarion then

Number of tests, and computation of test resultsThe strength index ma¡i be obrained as either the mean or the median of test ¡esurts. Themedian is ro be prererred when the sample contains only a few specimens b.;";r; it is lesssensirive to extreme results, and ¿lso bicause it invoh,es l.ss

"åmpotation urrãii.r.for.e median of a set of ¡esults may be obtàined by systematicallyvalues until onlv tlvo values remain, the average oi these beingian value. The sequence of compntation is iriportant. tn theiameter D is consrant, one.can economize on Àmpuration by

hen computing the equivalent indei p/D: andlump testing hotvever, since D varies, each in_

ted and corrected for size; cornputation of thecan be used to assist in calculating the strength

It is dilñcult, perhaps even- incolrect, to specify that teststnust be performed. Often only limited supplies of rock andabundant supplies from others. In such cãses indexes testsare better than none at a[, provided the rimited accuracy of results is not forgotten.The authors carried out a long series of tests to obtain some guidance * ,-nr-ìn¡u"n..of the number of tests on the accuracy ofl results. Both mean and median strength values

I

I

I.fM N/m 2)

SANûSTONE 215

o 25mm corcr d.i'td ln di.dióã I¡ 25oD cru ddl.d rormot to d¡..dionI

10 3o¿o-50.rEST

NCI¿5ER

Frc. 19' variarion in meao and median diâmerral poinÞroad strengrh of Darrey Dale sandstone afunction of the num ler of lests complãted.


v/ere plotted against the number ol tests completed (Fig. l9). It can be seen that the meanand median values differed iittle from each other, and that aiter 10-12 rests only slightimprovement to the accuracy of the index was obtained by further testing. As a generalguide (see also [43]), the authors recommend that a minimum of l0 specimens per sampleshould be used whenever possible in diametral and axial testing- In irregular lump testingthe number oi specimens should be approximatelv doubled if the median value is to lallrvithin the same confidence interval.

COMPARISON OF POTÑ'T LOAD À\D UNAXIAL COMPRESSION TESTS

The point-load test is proposed as standa¡<i lor strength classifica¡ion of rock materials,and therefore as a replacemenl for rhe uniaxial or unconfined compression test usuallyernployed for this purpose. To support this proposal the advantages of both types of testare summarized below. Resuhs are also gìven of an experimental study to ccmpare thereproducibility of the two rvpes of tesr and of another to examine correlation bet*een thetwo.Advantages olthe point-load test:

I . Smailer lorces are needed so that a small and portable testing machine may be used.2. Specimens in the form of core or irregular lumps are used and require no machinins.3. ){urre tests may be made tbr the same cost and this aliows lor adequate sampling

even rvhen rock conditions are variable.4. Fragile or broken mate¡ials can be tested so there is less chance ol results being

biased in favour of more competent s¡rata.5. Results show less scalter than ihose for uniaxial testing (Fig. 20).6. lvf easurement of strene¡h anisotropy is simplified.

Advantaees of the uniaxial compiession test:

7. Tire testing procedure is berter knoun and evaluated.8. Results are available for a wide variety of rock types, together with experience in

linking these ¡esults to frelcì performance.

In expe¡iments to compare rhe reproducibility ofresults fo¡ the two types of test, 25 nmco¡es were drilled fronr a block ofPennant sandstone. sample 219. From this core tv/en¡yc¡lindrical specimens rxith length,diameter ratio of 2'0 were prepared lor uniaxial compres-sion testing, end faces being carefully sau'n and ground flat. Specimens were carelullycentralized in the testing machine, and rvere loaded ihrough a single spherical seat. Theremainder ol the core was used for diamet¡al pointJoad testing. The results (Fig. 20) have

219

AXT $NOSfOIE¡

l##^''zFrc.20. Comparativeaccuraclrofuni¿xialanddiametralpoint-loadtesting;point-loadtestsgaveastandard

deviation of 3'7 per cent compared with 18 8 per cent for uniaxi¡l ¡es¡s.


ed by expressing each_as a percentage of the average point load or uniaxialrengrh' The pointJoad strength resulìs show far ress-scätter, *ith altanoa.d'7 per cent for diametral point-load tests, cornpared with 1g.g per cent forIn experiments t ofcorrelation between results ofuniaxial compres_síve and diametral rther ten rock types were tested in addition to thefive that had been studìes. Tl

results being cor¡ected to a relerence dline correlation whose slope of 23.strength to point-load strength, ave

Fra' 2r' correration between.lttl;iöaóiili+.i[:f;,j"*î"**.,."t or0.88 is sufficientryhigh to allow prediction of one from the othtr.

coefficient of 0.88 compares with a value oeither value is'sufrciently high to warrant uscompressive strength when this is required. Dratio; his results ¡elate to pointload tests onto a 50 mm ¡eference diameter is made, the rin the present study. The value of 23.7 shobased on results for a limited number ofrock types, and also because no account has beentaken of size effects in uniaxial compress

One may conclude that the point-load tclassification, but as yer there is only lithe results in terms of Âeld pertbrmance.making use of the close cor¡elarion (Fig. 2l)tests. Clsssiflcations esrablished on the basiscan, as 5horvn in the lollcwing paragraphs, btions based on rhe sjmpler point-loaã teìting method.

STRENGTH CLASSINCÂTION

. classiflcation ¡equires three steps; choice ofa measurement on which to base the classifica_tion; sr.rbdivision of the scare of measurenent into pàr.i,ion, each of which constitutes a'class'; assi,enment of a name or'designation'to eaåhcrass [44]. subdivision and nomen_clature a¡e largely arbirrary,and indeid may be diÇnsea r.r.ith entirely if the measuredvalue is appiied directly to the problem. However, ii is often mo¡e convenient to talk ofa'strong ¡ock'than to quote strength values.

E

E

r

t

¿.l¡I!r,i'It,'1

i,

i

fi

ì..

.

;+.1

rÍi:úil LtET

rLltïil¿1,:t

!::rÌ.

i¡\lt''11

i,.'!ii1;-

fi:

rs(so),MN/m

Str.ngth d.3ignq!lon - Fronkl,lh d ot

Approximoto strcngth ruñgcs lor æmmon

r0 100

ur'raxraL coMpREsstve sreetreix,vñf-2


Frc. 22. Strength classification: two alternative systems of subdivision and nomenclature are shown,together with the correlation betwæn uniuial

#Íoïî::-l::a strengttls and tvpical rangs of values fo¡

The proposed point-load strength classification is shown ín Fig.22. Because of the wide

range of strength values to be accommodated, a logarithmic scale has been used. Uniadalcompressive strength has been shown on the same diagram by juxtaposing the two scales

using the r¿tio of uniaxial to point-load strengths given in Ftg.2l.The choice of a system for subdividing the point-load scale into 'classes' may now call

directly on previous subdivision schemes for uniaxial strength; the report olthe Geological

Society of London Working Party on Rock Core Logging [45] was particularly helpfulsince it had reviewed the najority of prevíous work. The subdivision and nomenclaturescheme proposed by this Working Pafy is also shown in Fig. 22 and may be compared

with the authors'proposed system which incorporates several modifrcations [37]'Six subdivisions (seven classes)'rvere chosen, progressing in multiples of I and 3 MNim'z

on the logarithmic scale so that approximately equal spacing would be achieved. The autho¡s

could find no valid reason for choosing unequal subdivisions; equal ciivisions are easier toremember. The range of values was chosen to covet soil as well as rock materials. As aresult fresh, unaltered rocks commonly occupy only the toP four or ôve classes, and one

could argue that a fi¡er system of subdivision might be needed.

In choosing a system of srrength designation, tems such as /ol'. nt edium. high strengtltwere

selected rather than 'strong' or 'weak'. This to some extent avoids problems of ambiguitythat can arise, particularly in a classiûcation encompassing both soil and rock materials.

For example a weak ¡ock may be equivalent to a strong soil; a rveak granite rnay yet be a

strong rock. Terminology should as far as possible avoid conflict with everyday usage. We

suggest that terms might be used as follo*'s:'Weak granite: â granite weaker rhan usual for granites, for example a weathered or

altered granite.Weak ¡ock: a rock weake¡ than usual for rocks, e.g. chalk.

Low srrength material: a rock or soil with d (50) betrveen 0'1-O'3 MN/m'?.

692E. BROCH AND J. A. FRANKLIN


ENCES

sile st¡ength by diametral compression of discs andks- J. appt. Mech., Trans. Am. Soc. mech. Engrc 55,

the modulus of materials of a low tensile resistance.

for brittle materials. Int. J. Rock Meclt. Min- Sci. l,

iïì';,"Ï:ft or rock bv conpression test or an

in rings of rock loaded in diameftal tension or com-compressive stress. /Rock Mech. Engng Geol. 4,

U-S.S.R., pt oceetlings).. Translated by Israel

i::;:i';!,¿:Åi,,;å{":::,i:Ãi!;:Å"ial compressive stren$h of rock. Iu¡. t. Rock Mech.

(te66).

e First

24. Durrru¡ P. Size effect on crushing blocks of inegular shape. Reue Ind. møer. (Special No.) pp' 62-67

(1968).ZS. Èoo<ü p. G., De.lnvrN W. and FnlNrux J. A. Some engineering aspects of rock weathering with

fieldexamplesfromDartmoorandelsewhere. Q.JIEngngGeol'4,(3) 139-185(1971)'

26. LoLrs C. Private communication (1971).

27.28. acement Data perties of Rock for

(C. Fairhurst, 63)'

29. Materials to D trength and Relativen Rock Mechan olorado, PP. 134-159

30. R. L. and FocelsoN D. Ê. Prediction of Compressice Strength of Rock from.S. Bueau of Mines, Report of Investigations 6702 (1965)'

31. ole of mic¡ostructure in the physical properties of rock' Proc' Am' Soc' Test

Mater.S-fP 402' 175-189 (1966)'

32. Bsncs-C¡rnrsr¡xseN J. On the Blastabilíty of Rocks, Lic. Techn. Thsis, Geological Institute of the

Technical University of Norway, Trondheim 11968)'

:¡. Èi*ån-C¡¡*rsrr:rs¡N J. and Ser.vgn-Orsev R. On üe Resistance to Blasting in Tunnelling, Proceedings

ãj ri" Siiond Congr"tt of the Internøtional Sociery for Rock Mechanics, Belgrade, Vol. 3, Paper 5-7

(1970).34. ÙrrvÉn-Ors¿s R. and Brmon¡¡v O. T. On the Drillabiliry of Rocks by Percussive Dàlling, Prcceedinçs-

áì tn" Si*i¿ Congress of the International Society for Rock Mechanics, Belgrade, Vol. 3. Pâper 5-S

(1970).¡S. Ètíàr¡ J. il. Classifcation of Rock According to its Mechqnica[ Properries'Ph.D. Thesis, University

36. of London (1970)'

31. cal charâcter oflrock' Trans' [nsrn 'l[itt'

38. punktlast metoden' Tek' Ukebl' ll8'(18) 2l-24 (1e71).

39. È;ó; È. Èuntsjonett klassifuering av fjell, generell oversikt-punktlætmetoden, Proceedíngs of the

Symposium on Rock Mechanics' Oslo (1971)'

40. DTTRELLT A. J. and prnxs J. Innu.n"L of size and shape on the tensile strength of brittle materials.

41.42. ive Strength of

43. ch' Min' Sci' 7'

209-277 (1970).¡+. f""i*tÑ ¡. e. Obsewarions and Tests for Engìneering Description and Mappìng of Rocks.-Proceeclings'

ài iniii"à"¿ Congres o¡ tni Interrutional Slociety ¡or Rock Mechanics, Belgrade, Vol. l, Paper I -3

(1 _45- G g Party-Report on the ìogging of ræk cores for engineer-

in ); Discussion 4' 131-132 (1971)'

46. D and Delornnbility Delerminqîion' lntcrnarional Society

for RocklT.SuggesredtheSlaking,svelling,Porosity,DensilyandRelutedRocklndex

Proþenies Rock ìllechanics, Lisbon, January (1971)'

APPEI'¡DIX

SuEgested 'Vetiod for Derer:niníng the Poittt-Loat! Strength Indext

menls oû rock core and outcrop specimens.

' Incorporating ræommendations from Rei- [46]'


ttJli.'.4: ¡.i"1Í;'i.iì ,

l':1 tl,i- rl.F¡Ét; ii ti:i- ).

i+lirl:ì;:: i'

'r: li*li¡,Èj i::if i;¿¡ 1,

:1,.'

lïi !:,rìì¡ij-¡ ì

ii:ir'' '

::: I

:':iui ,,

Apparatut

Procedure

Frc. ?3. Suggested standard platen dimension for poìnt_load resting.

I


(d) The írregular lamp lest- Rock lumps with rypical diameter approximately 50 m and with a ratioof longmt to shortest diameter of between 1 0 and l'4 a¡e selæted and trimed using any convenienttechnique. At leæt twenty lumps shou.ld be tested per sample- Each lump is inserted in the tsring machineand the platms closed to make contact along the longest diameter of the lmp, away from edges and cornerswhere possible. The distance D is ræorded and the load increæed to failue. The load P is ræorded andthe procedwe repeated for the remaining tesr in tbe sample-

(e) Anisotropic rock. With rock that is beCded, schistose or otheruise shows obseflable anisotropy,tests should be made in both weakest and strongesr directions. when testing horizontally bedded core, forexample, dimetral tests will nomally give a set of lower boud-strength values, i.e. the strength perpen-dicular to planes of weakness. Care should be laken to ensure that loading is strictly along and perpendicular

P

failur? tood

fsparnt load strength inde¡

0plqten 9cporotion

S;RENGTHCESIGNA'ION

M N/m2

kN tbr

- 10000

'=-¡oI

+-J:ooo

ro-!f-1+-)

-Fl0

Extrê1ê yhrgh

h¡gh

hm ¡n

i++--t_1l

I

-1

300

!00

lhÍ l t^zr0000

1000

100

10

=

r

T--F

I

=---

ftt --+

=--a

_F

_rtr-{0.1 ---l-

L=-:-

70 --f

t0{I

3C-:roo-_L¿

+-o _:xometc :- 0 - ¿: 5 ñfr ; P. ¿ t0 :N!¡.2 3 MN/m2; l¡(50) '2 r MN/m2

Strèngth dêsignati¡n.'high'

NOMOGRAM FoR COMPUTING PcINT LoaD STRENGTH rruCeX r.=$

Frc' 24. Nomogram for computing point-load strength in dex I, : pr pz ' For testing machines that employgruges calibreteã in terms of h¡-drsuìic .:ressure, the pr.'ssure reading shouid bc multiplied by the eÍfective

iu- urer to ob¿ain load; the nomosxf,m crn be adjusted lor diræ¡ readings !y a ,erticâl displacement oflbe rhilrre load scele.

0

Erlr.icly


Cqlculøtions

," 1,!1'rÏ" Poinl-Load strength Index 1', defined as the rario p/ D2, may be obrained from rhe nomo*,am

nect-

t and

constant, tbe median failure load pand the size conætion applied. In

t lìrs¡ be obhined and coriécted for

Is (5O)

THE POINT.LOAD STRENGTH TEST 697

(e) The Strength Anisotropy Index 1. (50) may be compuled as the ratio of corræted median strengthindexes tbr lests perpendicular and parallel to planes of weakness- /. (50) æsumes values close to 1.0 fbrisotropic rocks, and higher values when the rock is anisotropic.

Reporthtg of results

5. Results for diametral tests, axial tests and inegular lump tests perpendicular and parallel to planes ofweakness sbould be tabulated separately. The repon should contain the following infomation for eachsample tested:

(a) Sample number and location, also its water content condition, and storage history. If possiblenumerical valuæ for both water content and saturation should be given. The orientation and nature of anyweakness planes present in the rock should be described.

(b) .{ tabularion of failure load P and platen separation D for each test-(c) Computed values of.l" and I, (50) for each rest- These computed values can be omitted in the case

of diametral test ¡esults, but the median value of failure load P should be reported.(d) Median values for /. (50) parallel and perpendicular to planes of weakness, together with the

computed strength anisotropy index /. (50).

6. This test is intended as a simple procedure for field classification of rock materials, and when necessarythe recommended procedures can be modified to overcome practical limitations. Such modifications toprocedure should however be clearly srated in the repon.

7. When required, the strength index values can be used to assign a strength designation to the rock('high, medim, low strength'etc. as for example in Fí9. ]2), but the numerical strength values should alsobe retained.

8. Point-load strength is closely conclated with rhe resuits of uniaxial comoression and other strengthtests. An approximate conversion Uniaxial Compressive Strength : 21 x I, (50) can be used.

E

llr,rrllAW BW NW HW PW

NOMINAL CORE SIZES

Ftc. Ls. Size correction chart for point_load strength testing (from Fig. 9).

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