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D5731-08 Standard Test Method For Determination Of The Point Load Strength Index Of Rock And Application To Rock Strength Classifications

4.1 This index test is performed by subjecting a rock specimen to an increasingly concentrated load until failure occurs by splitting the specimen. The concentrated load is applied through coaxi...




Designation: D5731 − 08 Standard Test Method for Determination of the Point Load Strength Index of Rock and Application to Rock Strength Classifications 1 This standard is issued under the fixed designation D5731; the number immediately following the designation indicates the year of  original origin al adoption or, in the case of revis revision, ion, the year of last revision. revision. A number in paren parenthese thesess indicates the year of last reappr reapproval. oval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval. 1. Sco Scope* pe* 1.9   This standar standard d doe doess not purport purport to add addre ress ss all of the safety safe ty co conc ncern erns, s, if an anyy, as asso socia ciated ted wit with h its us use. e. It is th thee responsibility of the user of this standard to establish appro priate safety and health practices and determine the applicability of regulatory limitations prior to use. 1.1 This test method covers the guide guidelines, lines, requirements, requirements, and procedures for determining the point load strength index of  rock. This is an index test and is intended to be used to classify rock strength. 1.2 Specim Specimens ens in the form of rock cores, blocks, or irregular irregular lumps with a test diameter from 30 to 85 mm can be tested by this test method. 2. Referenc Referenced ed Documents 2.1   ASTM Standards: 2 D653 Termino erminology logy Relating to Soil, Rock, and Contain Contained ed Fluids D2216 Test D2216  Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass D3740 Practic Practicee for Minimu Minimum m Requir Requirements ements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction D5079 Practices D5079  Practices for Preserving and Transporting Rock Core Samples D6026 Practice D6026  Practice for Using Significant Significant Digits in Geotechnical Data D7012 Test D7012  Test Methods for Compressive Strength and Elastic Moduli Mod uli of Int Intact act Roc Rock k Cor Coree Spe Specim cimens ens und under er Vary arying ing States of Stress and Temperatures E18 Test E18  Test Methods for Rockwell Hardness of Metallic Materials E122 Practice E122  Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot or Process 1.3 This test method can be performed performed in either the field or laboratory. The test is typically used in the field because the testing machine is portable, little or minimal specimen preparation is required, and specimens can be tested within a short time frame of being collected. 1.4 This te 1.4 test st me meth thod od ap appl plie iess to me medi dium um st stre reng ngth th ro rock  ck  (compressive strength over 15 MPa (2200 psi)). 1.5 This test method method does not cover which which type of specimen specimen should be test should tested ed or whe whethe therr ani anisot sotrop ropic ic fac factor torss sho should uld be considered. The specifics of the point load test program need to be developed prior to testing and possibly even before sampling. Such specifics would be dependent on the intended use of the data, as wel welll as pos possib sible le bud budget getary ary constraint constraintss and possible other factors, which are outside the scope of this test method. 1.6 All observed observed and calculated values values shall conform conform to the guidelines guidel ines for signifi significant cant digits and rounding established established in Practice D6026 Practice  D6026.. 2.2   ISRM Standard: Suggested Methods for Determining Point Load Strength 3 1.7 The met method hod used to spe specif cify y how data are collected collected,, calculated, calculat ed, or record recorded ed in this standard is not directl directly y related to the accuracy to which the data can be applied in design or other uses, or both. How one applies the results obtained using this standard is beyond its scope. 3. Terminology 3.1 For definitions definitions of terms terms used in this test method refer refer to Terminology  D653  D653.. 3.2  Definitions of Terms Specific to This Standard: 1.8 The values values stated in the SI units are to be regarded regarded as standard. 2 For referenced ASTM standards, visit the ASTM website,, or contact ASTM Customer Service at [email protected] For  Annual Book of ASTM  Standards volume information, refer to the standard’s Document Summary page on the ASTM website. 3 “Suggested “Sugg ested Methods for Deter Determining mining Point Load Strength”, International International Society for Rock Mechanics Commission on Testing Methods,  Int. J. Rock. Mech.  Min. Sci. and Geomechanical Abstr Abstr.., Vol 22, No. 2, 1985, pp. 51–60. 1 This test method is under the jurisdiction jurisdiction of ASTM Committee D18 Committee D18 on  on Soil and Rock and is the direct responsibility of Subcommittee  D18.12  D18.12 on  on Rock Mechanics. Current Curre nt editio edition n approv approved ed Jan. 1, 2008. Published Published Febru February ary 2008. Origin Originally ally approved in 1995. Last previous edition approved in 2007 as D5731 – 07. DOI: 10.1520/D5731-08. *A Summary of Changes section appears at the end of this standard Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States 1 D5731 − 08 3.2.1   diameter—D, for point load tests,  the dimension of the specime spe cimen n bet betwee ween n the opp opposi osing ng con conical ical,, test plat platens ens whe when n placed in the test machine be highly influenced by how the specimen is treated from the time it is obtained until the time it is tested. Therefore, it may be necessary to handle specimens in accordance with Practice D5079 and D5079  and to document moisture conditions in some manner in the data collection. 3.2.2   poi point nt loa load d str stren ength gth an aniso isotr trop opyy in index dex— — I a(D), the strength anisotropy index is defined as the ratio of mean I s(D) valu va lues es mea measu sure red d pe perp rpen endi dicu cula larr an and d pa para ralle llell to pl plan anes es of  weak we akne ness ss,, th that at is is,, th thee ra ratio tio of gr great eates estt to le least ast po poin intt lo load ad strength indices on different axes that result in the greatest and least ratio of point load strengths values. NOTE   1—The 1—The qu qual ality ity of th thee re resu sult lt pr prod oduc uced ed by th this is st stan anda dard rd is dependent upon the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 Practice D3740 are  are generally considered capable of competent and objective testing and sampling. Users of this standard are cautioned that compliance with Practi Practice ce   D3740   does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 Practice D3740 provides  provides a means of evaluating some of those factors. 3.2.3   size-corr size-corrected ected point load strength index— I s(D), the original point load strength index value multiplied by a factor to normalize the value that would have been obtained with diametral test of diameter ( D). 6. Appar Apparatus atus 3.2.4  uncorrected point load strength index—  ( I s), an indicatorr of str cato streng ength th (se (seee   10.1) 10.1) ob obtai taine ned d by su subj bjec ectin ting g a ro rock  ck  specimen to an increasingly concentrated point load, applied thro th roug ugh h a pa pair ir of tru trunc ncate ated, d, co coni nical cal pl plate atens ns,, un until til fa failu ilure re 3 occurs. 6.1   General— A basic point load tester (see Fig. (see  Fig. 1) 1) consists of a loading system typically comprised of a loading frame, platens, a measuring system for indicating load,  P , (required to break the specimen), and a means for measuring the distance,  D, between the two platen contact points at the start of testing and after failure. The equipment equipment shall be resistant to shock and vibrati vib ration on so tha thatt the accu accurac racy y of read reading ingss is not adversely adversely affected by repeated testing. Any special operational, maintenance or calibrations instructions provided by the manufacturer for the particular apparatus being used shall be followed. 4. Summ Summary ary of Test Test Method 4.1 This in 4.1 inde dex x tes testt is pe perf rfor orme med d by su subj bject ectin ing g a ro rock  ck  specime spe cimen n to an incr increasi easingl ngly y con concen centrat trated ed loa load d unt until il fai failur luree occurs occ urs by spl splitti itting ng the spe specim cimen. en. The con concen centra trated ted load is applied through coaxial, truncated conical platens platens.. The failur failuree load is used to calculate the point load strength index. 6.2   Loading System: 6.2.1 The loading loading system shall have a loading frame frame with a platen-to-platen clearance that allows testing of rock specimens in the required size range. Typically, this range is between 30 to 100 mm, or the maximum opening size of the load frame, so that an adjustable distance is available to accommodate both small and large specimens. 4.2 The point load strength strength index index can be used to classify the rocks. A common method used is by estimating the uniaxial compressive strength. 5. Sign Significan ificance ce and Use 5.1 The uniaxial uniaxial compression compression test (see Test Method D7012 Method D7012)) is used to determine compressive strength of rock specimens. Howe Ho weve verr, it is a tim timee-co cons nsum umin ing g an and d ex expe pens nsiv ivee tes testt th that at requires significant specimen preparation and the results may not be available for a long time after the samples are collected. When extensive testing and/or timely information is required for prelim preliminary inary and reconn reconnaissanc aissancee infor information mation,, alterna alternative tive tests such as the point load test can be used to reduce the time and cost of compressive strength tests, when used in the field. Such Su ch da data ta ca can n be us used ed to ma make ke tim timel ely y an and d mo more re in info form rmed ed decisions during the exploration phases and more efficient and cost co st ef effe fecti ctive ve se selec lectio tion n of sam sampl ples es fo forr mo more re pr preci ecise se an and d expensive expens ive labora laboratory tory tests. NOTE  2—It is generally accepted that specimens smaller than 42 mm (BX cor cores) es) are not rec recomm ommend ended ed bec becaus ausee for sma smalle llerr dia diamet meters ers the loading points can not be considered as theoretical “points” in relation to specimen size.4 6.2.2 The loading capacity shall be suf suffficient to break the largest large st an and d st stro rong ngest est sp spec ecim imen enss to be tes teste ted. d. Po Poin intt lo load ad strength of rock is usually an order of magnitude lower than the compressive strength of rock. 6.2.3 The load frame shall be design designed ed and constructed constructed so that it does not permanently distort during repeated applications of the maximum test load, and so that the platens remain coaxiall within 60.2 mm throughout testing. No spherical seat coaxia or ot othe herr no nonr nrig igid id co comp mpon onen entt is pe perm rmitt itted ed in th thee lo load adin ing g system. Loading system rigidity is essential to avoid slippage when specimens of irregular geometry are tested. 6.2.4 Tru Truncated ncated,, conical platens, as shown on Fig. on  Fig. 2, 2,  are to be used. The 60° cone and 5-mm radius spherical platen tip shall meet tangentially. The platens shall be of hard material (Rockwell (Rock well 58 HRC, as explai explained ned in Test Method E18 Method  E18))  such as tungsten carbide or hardened steel so they remain undamaged during testing. 5.2 The point load strength strength test is used as an index test for strength classification of rock materials. The test results should not be used for design or analytical purposes. 5.3 Thi Thiss test method method is per perfor formed med to det determ ermine ine the poi point nt load strength strength index of roc rock k spe specim cimens ens and and,, if req requir uired, ed, the point load strength anisotropy index. 5.4 Rock specimens specimens in the form of either either core (the diametral diametral and axial tests), cut blocks (the block test), or irregular lumps (the irregular lump test) are tested by application of concentrated load through a pair of truncated, conical platens. Little or no specimen preparation is required and can therefore be tested shortly sho rtly after bei being ng obt obtaine ained d and any infl influen uence ce of moi moistu sture re condition on the test data minimized. However, the results can 6.3  Load Measuring System: 6.3.1 A load measuring measuring system, system, for example a load cell or a hydrau hyd raulic lic pre pressu ssure re gag gage, e, tha thatt wil willl ind indicat icatee fai failur luree loa load, d, P, 4 Bieniawski, Z.T., The Point Load Test in Geotechnical Practice, Engineering Geology (9), pages 1-11 Geology 1-11,, 1975. 2 D5731 − 08 NOTE  1—Load frame general information (figure 1) Load is applied to the specimens through two standard hardened points (1) Two column fixed crosshead frame (2) Scale (3) Scale pointer (4) Attached by a bolt (5) to the hydraulic pump body (6) Oil filler cap (7) The hydraulic piston assembly incorporated the oil reservoir, a single acting pump, pressure relief valve (9), and a handle (8) Pump handle (8) Pressure Press ure release valve (9) Case latched for top cover (10) Digital pressure readout (11) Point load tester top cover(12) FIG. 1 Exampl Example e of a Light-Weight Light-Weight Point Load Test Test Apparatus Apparatus required to break specimen. The system should conform to the requirements of  6.3.2 of  6.3.2 – 6.3.4. 6.3.4 . 6.3.2 Measur Measurements ements of failure load, P, shall be to a precision of  65 % or better of full-scale load-measuring system, irrespective of the size and strength of specimen that is tested. 6.3.3 6.3 .3 Fail Failure ure is oft often en sud sudden den,, the theref refore ore,, and a pea peak k loa load d indicator is required so the failure load can be recorded after each test. 6.3.4 6.3 .4 If req requir uired, ed, the sys system tem should should be cap capabl ablee of usi using ng interchangeable, mechanical or electronic gauge, load measuring devices in order to be consistent with the estimated strength of rock and have the desired reading accuracy. 6.4.3 The measuring measuring system shall shall allow a check of the “zero displacement” value when the two platens are in contact and should include a zero adjustment and a means to record or measure any penetration of the specimen by the point load platenss durin platen during g testing testing.. 6.4. 6. 4.4 4 An instru instrume ment nt su such ch as a ca calip liper er or a st steel eel rule is required to measure the width,  W , (with an accuracy of  65 %) of specimens for all but the diametral test. 6.5   Miscellan D epend ndin ing g on th thee ty type pe of  Miscellaneous eous Items Items—  — Depe samples (core or non core) and the type of specimens to be tested (diametral, Block, Axial, etc.), the following items may be needed: diamond saw, chisels, towels, marking pens, and plotting paper. 6.4   Distance Measuring System: 6.4.1 The distance measuring measuring system, an electronic or vernier direct reading scale, should connect to the loading frame for measuring the distance, D, between specimen-platen contact points at the start of testing and just prior to failure and conform to requirements  6.4.2 and and 6.4.3  6.4.3.. 6.4.2 Measur Measurements ements of  D  shall be to an accuracy of  62 % or better of distan distance ce between contact points, irrespective irrespective of the size and strength of specimen that is tested. 7. Test Samples Samples 7.1 Rock samples samples are grouped grouped on the basis of rock type, type, test direction if rock is aniasotropic, and estimated strength. 7.2   Sample Size 7.2. 7. 2.1 1 When When tes testin ting g co core re or bl bloc ock k sa samp mple less at lea least st te ten n specimens are selected for the samples. 3 D5731 − 08 FIG. 2 Tru Truncated ncated,, Conical Platen Dimensions Dimensions for Point Load Apparatus 7.2.2 When testing irregular-shap irregular-shaped ed specimens obtained by other means at least 20 specimens are selected for the samples. 7.2. 7. 2.3 3 Sa Samp mple le siz sizes es ma may y ne need ed to be lar large gerr if th thee ro rock ck is anisotropic or heterogeneous. 7.2.4 If needed, needed, Practice Practice E122  E122 can  can be used to more precisely determine the sample size. load te load test ster er re reco comm mmen ends ds th that at a mi mini nimu mum m ra rati tio o of co core re di diam amete eterr to maximum aggregate size of 4 be used.5 This ratio may be used until guidelines are developed for rock. 8.3   Water ater er co cont nten entt of th thee sp speci ecime men n ca can n Water Conten Content—  t— Wat affect the value of the point load strength. Therefore, the testing plan shall include how water content will be included in the point load testing program. This may include the recording, controlling, and measurement of water content. 7.3 Sam Sample pless in the for form m of core are pre prefer ferred red for a mor moree precise classification. 8.4   Marking specimens ens Marking and Measu Measuring ring Specim Specimens—  ens—   The specim should be properly marked and measured as shown in  Fig. 4. 4. 8.4.1   Marking— The The desired test orientation of the specimen shall be indicated by marking lines on the specimen. These lines lin es ar aree us used ed fo forr cen center terin ing g th thee sp spec ecime imen n in th thee tes testin ting g machin mac hine, e, and to ens ensure ure pro proper per ori orienta entatio tion n dur during ing test testing ing,, including any issues involving anisotrophic rocks (see  Fig. 3). 3). These lines may also be used as reference lines for measuring width, length, and diameter. 8.4.2   Measuring— Measure Measure each dimension of a specimen at three different places, and calculate the averages. 7.4 For anisotropic anisotropic rocks rocks the best results for core core samples is when the core axis is perpendicular to the plane of weakness. 8. Test Specimens 8.1   Test Diameter— The The specimen’s external test diameter shall not be less than 30 mm and not more than 85 mm with the preferred test diameter of about 50 mm. 8.2   Size and Shape— The The size and shape requirements for diametral, diametr al, axial, block block,, or irregular lump testing shall conform with the recommendations shown on  Fig. 3. 3. The sides of the specime spe cimens ns sha shall ll be fre freee fro from m abr abrupt upt irr irregu egular laritie itiess tha thatt can genera gen erate te str stress ess con concen centra tration tions. s. No spe specime cimen n pre prepar paratio ation n is required, however a rock saw or chisels may be required for block blo ck or irr irregu egular lar spe specime cimens. ns. Pro Proper per pla planni nning ng of dia diametr metral al tests on rock cores can produce suitable lengths of core for subsequent axial testing provided they are not weakened by the diametral test. Otherwise, suitable specimens can be obtained from the cores by saw-cutting, or core splitting. 9. Pro Procedu cedure re 9.1 Develo Develop p a testing plan and, if needed, needed, sampling plan to providee spe provid specime cimens ns for poi point nt loa load d test testing ing acco accordi rding ng to the follo fo llowi wing ng pr proc oced edur ures es fo forr th thee sp speci ecific fic sp speci ecime men n sh shape ape (diametral, axial, block or irregular). 9.2   Diametral Test  NOTE  3—While there are no established established speci specimen men guidelines guidelines for grain size ver size versus sus specimen specimen size thi thiss sub subjec jectt is sti still ll imp import ortant ant and mus mustt be included in the testing and use of the data. Concrete testing using a point 5 Robins, P.J., The Point Load Strength Test for Concrete Cores, Magazine of  Concrete Research, Vol. 32, No. 111, June 1980. 4 D5731 − 08 NOTE  1—Legend: L = distance between contact points and nearest free face, and De = equivalent core diame diameter ter (see (see 10.1  10.1)). FIG. 3 Load Configurations and Specimen Shape Requirement for (a) the Diametral Test, (b) the Axial Test, (c) the Block Test, and (d) the Irregular Lump Test 3 FIG. 4 Aniso Anisotropy tropy measurements measurements and testin testing g for maximum and minim minimum um indice indices s 9.2.1 9.2 .1 Cor Coree spe specime cimens ns with leng length/ th/diam diameter eter rat ratio io gre greater ater than one are suitable for diametral testing. 9.2. 9. 2.2 2 In Inse sert rt a sp speci ecime men n in th thee te test st de devi vice ce an and d cl clos osee th thee platens to make contact along a core diameter. Ensure that the distance,  L , between the contact points and the neare nearest st free end is at least 0.5 times the core diameter (see Fig. (see Fig. 3 and 3  and Fig.  Fig. 4( 4(a)). 9.2.3 Determi Determine ne and record the distances distances  D  and  L  (see  (see Fig.  Fig. 3). 9.2.4 9.2 .4 Stea Steadily dily increase increase the loa load d suc such h tha thatt fai failur luree occ occurs urs within 10 to 60 s, and record failure load,  P . The test should be rejected if the fracture surface passes through only one platen loading point (see  Fig. 5( 5(d )). )). 9.2.5 The procedures procedures in 9.2.2 in  9.2.2 – 9.2.4  are repeated for each specimen of the rock type. 9.3   Axial Test  9.3.1 Core specimens specimens with length length/diamet /diameter er ratio of  1 ⁄ 3 to 1 are suitable for axial testing (see Fig. (see Fig. 3( 3(b)). Suitable specimens can be obt obtain ained ed by saw saw-cu -cuttin tting g or chi chiselsel-spl splitti itting ng the cor coree sample sam ple,, or by usi using ng sui suitab table le pie pieces ces pro produc duced ed by car carefu efully lly planned diametral tests (see 9.2 9.2)). 9.3.2 Ins 9.3.2 Insert ert a spe specim cimen en in the test machine machine and close close the platens to make contact along a line perpendicular to the core end faces (in the case of isotropic rock, the core axis, but see Fig. 5 and 9.5 9.5   for anisotropic rock). 9.3.3 Record the the distance, distance,  D , between platen contact points (see Fig. (see  Fig. 3) 3). Record the specimen width, W , perpendicular to the loading direction, with an accuracy of  65 %. 5 D5731 − 08 FIG. 5 Typical Modes of Failure for Valid and Invalid Tests—(a) Valid diametral tests; (b) valid axial tests; (c) valid block tests; (d) invalid core test; and (e) invalid axial test (point load strength index test). 3 9.3.4 9.3 .4 Stea Steadily dily increase increase the loa load d suc such h tha thatt fai failur luree occ occurs urs within 10 to 60 s, and record the failure load,  P. The test should be rej rejecte ected d if the fracture fracture sur surfac facee pas passes ses thr throug ough h onl only y one loading point (see  Fig. 6( 6(e)). 9.3.5 Proced Procedures ures   9.3.2 9.3.2 – 9.3 9.3.4 .4   are repeated repeated for each test specimen of the rock type. 9.4  Block and Irregular Lump Tests : 9.4.1 Rock blocks blocks or lumps, 30 to 85 mm, and of the shape shown in Fig. in  Fig. 3( 3(c) and ( d ) are suitable for the block and the irregular lump tests. The ratio,  D/W , should be between 1 ⁄ 3  and 1, preferably close to 1. The distance  L  should be at least 0.5 FIG. 6 Procedure Procedure for Graphical Determinat Determination ion of I  (50) (50)  from a Set of Results at D   Values Other Than 50 mm 3 s  e  6 D5731 − 08 Suitab table le spe specime cimens ns can be obt obtain ained ed by saw saw-cu -cuttin tting g or W . Sui chisel-splitting larger samples or specimens if needed. 9.4.2 Insert a specimen specimen in the testing machine machine and close the platens to make contact with the smallest dimension of the lump or block, away from edges and corners (see Fig. (see  Fig. 3( 3(c) and (d ). ). 9.4.3 Record the distance distance  D  between platen contact points. Record the smallest specimen width, W , perpendicular to the loading loadin g direction. If the sides are not parallel, then calculate W  as (W 1  +  W 2)/2 as shown on  Fig. 3. 3.  This width, W , is used in calculating point load strength index irrespective of the actual mode of failure (see  Fig. 5 (c). 9.4.4 9.4 .4 Stea Steadily dily increase increase the loa load d suc such h tha thatt fai failur luree occ occurs urs within 10 to 60 s, and record the failure load,  P. The test should be rej rejecte ected d if the fracture fracture sur surfac facee pas passes ses thr throug ough h onl only y one loading point (see examples for other shapes in  Fig. 5( 5(d ) or ( e). 9.4.5 Proced Procedures ures   9.4.2 9.4.2 – 9.4 9.4.4 .4   are repeated repeated for each test specimen in the sample. 10. Calc Calculat ulation ion 10.1   Uncorrected The uncor uncor-Uncorrected Point Load Str Strength ength Index— The rected point load strength, I s  , is calculated as:  I  s 5 P /  D 2 , MPa e (1) where: = failur failuree loa load, d, N, P = equ equiva ivalent lent core core diam diameter eter (see (see   Fig. 3), 3), mm, and is  De given by: 2 = D2 for diametral core tests without penetration, penetration, mm2,  De or 2 = 4A/ π  for axial, block, and lump tests, mm 2;  De where: minimum m cross-section cross-sectional al area of a plane plane through through  A = WD = minimu the platen contact points (see  Fig. 3). 3). NOTE   4—If significant platen penetration occurs in the test, such as  D  should be the final value of  when testing weak sandstones, the value of  D the separation of the loading points, D '. Measurements of core diameter,  D, or spe specim cimen en wid width, th, W , mad madee per perpen pendic dicula ularr to the line joi joinin ning g the loading points are not affected by this platen penetration and should be  De can be calculated retained at the original values. The modified values of  D from: 9.5   Anisotropic Rock: 9.5.1 9.5 .1 Whe When n a roc rock k samp sample le is sha shaly ly,, bed bedded ded,, sch schisto istose, se, or otherwise observably anisotropic, it should be tested in directions that will give the greatest and least strength values, in general, parallel and normal to the planes of anisotropy. 9.5.2 If the sample consists of core drilled through weakness weakness planes, a set of diametral tests may be completed first, spaced at intervals that will yield pieces that can then be tested axially. 9.5.3 Stron Strongest gest test results are obtained when the core axis is perpendicular to the planes of weakness; therefore, when possible, the core should be drilled in this direction. The angle between the core axis and the normal to the direction of least strength should preferably not exceed 30°. 9.5.4 For measurement measurement of the point load strength index ( I s) valuee in the direction valu direction of lea least st str streng ength, th, ensure ensure tha thatt loa load d is applied along a single weakness weakness plane. Similarly, Similarly, when testing for the  I s  value in the direction of greatest strength, ensure that thee lo th load ad is ap appl plied ied pe perp rpen endi dicu cular lar to th thee di dire recti ction on of lea least st strength streng th (see (see Fig.  Fig. 4). 4). 9.5.5 If the sample consists of blocks or irregular lumps, lumps, it should sho uld be test tested ed as two subsampl subsamples, es, with loa load d firs firstt app applied lied perpendicular to, then along the observable planes of weakness. Again, the required minimum strength value is obtained when the platens make contact and are loaded to failure along a single plane of weakness.  D 2 5  D 3 D ' for cores 5 4/ π  W  3 D ' fo forr ot othe herr sh shap apes es e (2) 10.2  Size Corrected Point Load Index : 10.2.1 10.2. 1 The point load index, index, Is, varies as a function of  D  in the diametral test, and as a function of  D e  in axial, block, and irregular lump tests, so that a size correction must be applied, if the  D  values for all the specimens are not the same, to obtain an unique point load strength value for the rock specimen and one that can be used for purposes of rock strength classification. See Fig. See  Fig. 7. 7. 10.2.2 10.2. 2 The size corrected point point load strength index, index,  I s(D), of  a rock specimen is defined in this procedure as the value of  I s that would have been measured by a diametral test with D = 50 mm and given the symbol I s(50). The diameter of 50 mm has been the preferred diameter since that diameter is associated with rock quality designations (RQD) and predominance of Nx core samples samples.. 10.2.3 10. 2.3 Whe When n a pre precise cise rock cla classifi ssificati cation on is ess essent ential, ial, the most reliable method of obtaining  I s(50)  is to conduct diametral test te stss at or cl clos osee to D = 50 mm mm.. Siz Sizee co corr rrect ectio ion n is th then en unnecessary. For example, in case of diametral tests on NX, core diameter = 54 mm and size correction to D  = 50 mm is nott ne no nece cess ssar ary y. Mo Most st po poin intt lo load ad st stre reng ngth th tes tests ts ar aree in fa fact ct performed using other specimen sizes or shapes. In such cases, the size cor correc rectio tion n des descri cribed bed in   10.2.4 or   10.2.5   must must be applied. 10.2.4 10.2. 4 The most reliable reliable method of size correction correction is to test thee sp th speci ecime men n ov over er a ra rang ngee of  D or De   value valuess an and d to pl plot ot graphically graph ically the relatio relation n between P and  D e. If a log-log plot is used, the relation is a straight line (see   Fig. 6) 6). Poi Points nts that deviate substantially from the straight line may be disregarded (alth (a lthou ough gh th they ey sh shou ould ld no nott be de dele leted ted). ). Th Thee va valu luee of  I s(50) 2 2 correspond corre sponding ing to D e = 2500 2500 mm ( De   = 50 mm) can be obtained by interpolation and use of the size-corrected point load strength index calculated as shown in  10.2.5  10.2.5.. 10.2.5 10. 2.5 Whe When n nei neither ther   10.2.3 nor   10.2.4   is pra practic ctical al (fo (forr example when testing single-sized core at a diameter other than 9.6 If significant significant platen penetration occurs, occurs, the dimension D to be used in calculating point load strength should be the value  D' measured at the instant of failure, that will be smaller than the initial value suggested in 9.2.3 in  9.2.3,,  9.3.3  9.3.3,, and 9.4.3 and  9.4.3.. The error in assuming D   to be its initial value is negligible when the specime spe cimen n is lar large ge or str strong ong.. The dimensio dimension n at fail failure ure may always be used as an alternative to the initial value and is preferred. 9.7   Water Content  9.7.1 For precise measurements, follow Test Method Method D2216  D2216 to det determ ermine ine the wate waterr con content tent of each rock specimen specimen and report the moisture condition (see Section 11 11)). 9.7.2 9.7 .2 At the minimum, minimum, water content content sha shall ll be recorded recorded as air-dried, air-d ried, satura saturated, ted, as-re as-received, ceived, etc. 7 D5731 − 08 FIG. 7 Exampl Example e of Descrip Descriptive tive Strength Strength Classification Classification and Using a Nomo Nomograph graph to Compute the Point Load Index. Other Strength Strength Classifications May be Used. 50 mm or if on only ly a fe few w sm small all pieces pieces ar aree av avail ailab able) le),, si size ze correction may be accomplished using the formula:  I  ~ 50! 5 F  3 I  s FIG. 8 Size Correction Correction Factor Factor Chart 8 3 s (3 ) D5731 − 08 The “Size Correction Factor F ” can be obtained from the chart in Fig. in  Fig. 8, 8,  or from the expression: F  5 ~ D  /50 ! 0.45 e s 5 size correction factor For tests near the standard 50-mm size, only slight error is introduced by using the approximate expression: F  5 =~ D  /50 ! e 5 K * I  (6 ) s where: uniaxial ial compressi compressive ve strength, strength, MPa sc = uniax index ex to str streng ength th con conver versio sion n fac factor tor that dep depend endss on K  = ind site-specific site-s pecific correlation between sc  and Is  for a specific specimen with a test diameter (D), MPa and uncorrected rected point point load strength strength index index from from a specimen  I s = uncor with a specific test diameter ( D). (4 ) where: F  c 10.5.1 If site-specific 10.5.1 site-specific correlation correlation factor factor “ K ” is not available, the generalized values may be used in  Table 1. 1. (5 ) instead of using the procedure outlined in  10.2.4 on on Fig.  Fig. 6. 6. 10.3   Mean Value Calculation: 10.3.1 10.3. 1 Mean values values of  I  in  10.3.2,, are to be  I s(50), as defined in 10.3.2 used when classifying samples with regard to their point load strength and point load strength anisotropy indices. 10.3 10 .3.2 .2 The mea mean n va valu luee of  I s(50)   is to be ca calc lcul ulat ated ed by deleting deletin g the two highest and two lowest values from the ten, or more, valid tests, and calculating the mean of the remaining values. valu es. If sig signifi nifican cantly tly few fewer er spe specime cimens ns are tes tested, ted, onl only y the high hi ghest est an and d lo lowe west st va valu lues es ar aree to be de delet leted ed an and d th thee mea mean n calculated from those remaining. TABLE 1 General Generalized ized Index to Strength Conversion Conversion Facto Factorr (K) forA Core Size, mm 21.5 (Ex Core) 30 42 (Bx Core) 50 54 (Nx Core) 60 Value of “K” (Generalized) 18 19 21 23 24 2 4. 5 A Bieniawski, Bienia wski, Z.T. The PointPoint-Load Load Test in Geotec Geotechnical hnical Practice, Engineering Geology (9) 1-11. 10.4   Point Load Strength Anisotropy The strength Anisotropy Index— The anisotropy anisotr opy index I a(50)   is defined as the ratio of mean I s(50) valu va lues es mea measu sure red d pe perp rpen endi dicu cula larr an and d pa para ralle llell to pl plan anes es of  weak we akne ness ss,, th that at is is,, th thee ra ratio tio of gr great eates estt to le least ast po poin intt lo load ad 6 strength indices. See Fig. See  Fig. 9 . 10.5.2 10. 5.2 If any specimen specimen in a rock type gives a val value ue 20 % under the average, it should be examined for defects and a decision made on the validity of the results. 10.5   Estimat The Estimation ion of Uni Uniaxi axial al Com Compr press essive ive Str Streng ength—  th— The estimated uniax estimated uniaxial ial compr compressive essive strength can be obtained by using Fig. using  Fig. 9, 9, for Nx core, or using the following formula: 11. Repo Report rt 11.1 11 .1 A typ typical ical rep report ort (ex (examp ample le sho shown wn in   Fig. Fig. 10 10)) ma may y include the follow include following: ing: 11.1.1 11 .1.1 Sour Source ce of sample including project name, location, how collected (drill hole, block sample, etc.) and, if known, storage (curatorial history) environment. The location may be 6 D’Andrea, D.V., Fisher, R.L., and Fogelson, D.E., Prediction of Compressive Strength of Rock from Other Rock Properties, Strength Properties,  U.S. Bureau of Mines Rep. Invest., , 6702, 1965. FIG. 9 Relati Relationship onship Between Point Load Strength Index and Uniaxial Compressive Compressive Streng Strength th from 125 Tests On Sandst Sandstone, one, Quartzite, Quartzite, Marikana Marikan a Norit Norite, e, and Belfa Belfast st Norit Norite e6 9 D5731 − 08 FIG. 10 Te Test st Record Exa Example mple3 specified in terms of borehole number and depth of specimen from the collar of the hole, 11.1.2 11 .1.2 Physi Physical cal descr description iption of sample including including rock type and location and orien orientation tation of discon discontinuitie tinuities, s, such as, appar appar-ent wea weakne kness ss pla planes nes,, bed beddin ding g pla planes nes,, sch schisto istosit sity y, or lar large ge inclusions, if any, 11.1.3 11 .1.3 Date and perso personnel nnel involved with sampling, specimen preparation, and testing, 11.1.4 Test apparatus apparatus used, model number, number, and calibrations, 11.1.5 11 .1.5 As a minimu minimum, m, a gener general al indication of the moisture moisture condit con dition ion of tes testt spe specime cimens ns at the time of testing, testing, suc such h as, saturated, as received, laboratory air dry, or oven dry. In some cases,, es cases espe peci ciall ally y wh wher eree th thee re resu sults lts ar aree sen sensit sitiv ivee to wa wate terr content, it may be necessary to report the actual water content as determined in accordance with Test Method  D2216  D2216,, 11.1.6 11 .1.6 Averag veragee thickness and average diameter of the test specimen, 11.1.7 11 .1.7 The maximum maximum applied load “  P ”, 11.1.8 11 .1.8 The distance distance “ D” or D', or both, if required, 11.1.9 Direction of loading (parallel to or normal to plane of  weakness or anisotrophy directions), 11.1.10 11 .1.10 The number of specimens tested and how prepared, prepared, 11.1. 11 .1.11 11 The calc calcula ulated ted unc uncorr orrecte ected d ( I s) an and d co corr rrec ected ted (D=50 mm), I s(50)  point load strength index values, 10 D5731 − 08 11. 1.1. 1.12 12 The est estima imated ted va value lue of un uniax iaxial ial co comp mpres ressiv sivee strength (σc) and the strength classification, 11.1.13 11 .1.13 The calculated value of streng strength th anisotropy index ( I a(50)), and 11.1.14 11 .1.14 Type and locatio location n of failure, including any photographs of the tested specimens before and after the test. tested, Subcommittee D18.12 cannot determine the variation between tests since any variation observed is just as likely to be due to specimen variation as to operator or testing variation. Subcom Sub committ mittee ee D18 D18.12 .12 wel welcom comes es pro propos posals als to res resolv olvee thi thiss problem prob lem and would allow for development development of a valid precision precision statement. 12. Pre Precisi cision on and Bias 12.2   Bias— There There is no accepted reference value for this test method; therefore, bias cannot be determined. 12.1   Precision— Due Due to the nature of rock materials tested by this test meth method, od, mul multip tiple le spe specim cimens ens tha thatt hav havee uni unifor form m physical properties have not been produced for testing. Since specimens that would yield the same test results have not been 13. Keyw Keywords ords 13.1 compr compressive essive strength; strength; index test; point load; rock; rock  classification SUMMARY OF CHANGES Comm Co mmitt ittee ee D1 D18 8 ha hass id iden enti tified fied th thee lo loca catio tion n of sel select ected ed ch chan ange gess to th this is st stan anda dard rd sin since ce th thee las lastt is issu suee (D5731 – 07) that may impact the use of this standard. (Approved January 1, 2008.) 8.4 revised.  revised. (1)  Section 8.4 (2)  Eq 6 was corrected to have symbols to be universal with the liter lit erat atur uree an and d wi with th ot othe herr AS ASTM TM st stan anda dard rds. s. Th Thee in inde dex x to stre st reng ngth th fa fact ctor or sy symb mbol ol wa wass ch chan ange ged d fr from om C to K to be consistent with the literature. 10  expanded. (3)  Fig. 10 expanded. Committ Comm ittee ee D1 D18 8 ha hass id iden enti tified fied th thee lo loca catio tion n of sel select ected ed ch chan ange gess to th this is st stan anda dard rd sin since ce th thee las lastt is issu suee (D5731 – 05) that may impact the use of this standard. (Approved February 1, 2007.) (1)  Change in title to include Rock Strength Classifications. 1.1.. (2)  Revised Section 1.1 1.3.. (3)  Added Sections 1.2 and 1.3 1.4.. (4)  Revised Section 1.4 1.5.. (5)  Added Section 1.5  Added E122  E122 to 2. (6)  Added 3.2.. (7)  Terms added to Section 3.2  Revised 5.2  5.2 – 5.4. 5.4. (8)  Revised 1  replaced with newer version of the apparatus. (9)  Fig. 1 replaced  Revised 6.2.1  6.2.1,,  6.2.2  6.2.2,,  a  and nd 6.5  6.5.. (10)  Revised 2  was added. (11)  Note 2 was  6.3.4,,  6.4.1  6.4.1,, and 6.4.3 and  6.4.3.. (12)  Updated Sections  6.3.4 (13)  Added reference to Bieniawski. 4 Chan ange ged d pr prev evio ious us Se Sect ctio ion n 7 on Sp Speci ecime mens ns in into to tw two o (14) Ch sections, 7 and 8. Sectionss  7.2.3  7.2.3,,  7.2.4  7.2.4,, and 7.4 and  7.4.. (15)  Added Section 8.2 expanded.  expanded. (16)  Section 8.2 3. (17)  Added  Note 3. 8.3 revised.  revised. (18)  Section 8.3 8.4,,  8.4.1  8.4.1,, and 9.1 9.1 were  were added. (19)  Sections 8.4 and 9.4.5  9.4.5   were revised. (20)  9.3.1 and  9.7.2 added.  added. (21)  9.7.2 (22)  Eq 1  revised.  10.2   heading revised. (23)  10.2  10.2.1 revised.  revised. (24)  10.2.1  10.2.2 expanded.  expanded. (25)  10.2.2  Section 10.5  10.5  Compressive Strength was changed to Uni(26)  Section axial Compressive Strength and the figure number was corrected to the correct figure number.  10.5.2 clarified.  clarified. (27)  10.5.2  11 revised.  revised. (28)  11 2  title revised. (29)  Fig. 2 title 3   notation revised. (30)  Fig. 3  4  added. (31)  Fig. 4 added. 5  corrected. (32)  Fig. 5 corrected. 7  added. (33)  Fig. 7 added. (34)  13  revised. 1  title changed and in-text reference added. (35)  Table 1 title 11 D5731 − 08 ASTM International International takes no positi position on respecting the validi validity ty of any patent rights assert asserted ed in connec connection tion with any item mentio mentioned  ned  in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk  of infringement of such rights, are entirely their own responsibility. This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and  if not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standards  and should be addressed to ASTM International Headquarters. 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