AD-AU~~~~ 321 COLD REGIOWS RESEARCH AND ENGINEERING LAB HANOVER N H FF6 8/12 N

AXIAL DOIStE POINT—LOAD TESTS ON SNOW AND I C E, (U) N

MAR 78 A KOVACS NSF~~ DPP7le~~ 236MIRICLASSIFIED CRREL—78—1 Pttts

ENDD AT I

___________________________________________________________________________________________ FILM!

6— 78

a~~~~~~~~~~~~~~J

_ _

I~C R R E L 1

REPORT 781Ax ial double point-load tes tson snow and ice

I m D l

L A4

~- = l.l ± O.O5 LD

D_

BQ

-

01 L>O. 7D

• L:5OmmI. 0 to 1.4 i~~~~~~rnoN STATEMENYK

r i T ~~~~T _ _ _

Cover: Sample shapes and loading configurations usedin (A) axial, (B) diametral, and (C) Irregularlump double point-load tests. The length (L)/diameter (0) ratio for each rest, as recommendedby the Commission on Standardization of Labora-tory and Field Tests of the International Societyof Rock Mechanics, is also shown.

L _ _ _ _

_ _ _ _ _ _ _ _ _

CRREL Report 78-1

Ax ia l double point-load testson snow and ice

Austin Kovacs

March1978

ACCESSION fer -

-p

DOC Buff SectIon ~UNAN ’ JNC~

JUS ‘ICATION ____________

BY _ _ _ _ _ _ _

Prepared for _~~lI~i.IAwaIIy i(~

DIVISION OF POI~AR PROGRAMS ~~~ AVAIL am~/or $p~~~NATIONAL SCIENCE FOUNDATIONByCORPS OF ENGINEERS, U.S. ARMYCOLD REGIONS RESEARCH AND ENG INEER ING LABORATO RY

________________

HANOVER . NEW HAMPSHIRE

Approved for public refeave , distribution unlim ited

— d

— ~~~~~~~~~~~~~~ ---~ -

~~~~~ ---

~~~~~~~~~~

UnclassifiedSECU RI TY CLASSIFICATION OF ThIS PAGE (Wf~.~ D~~a E.,t.,.i ~

REPORT DOCUMENTATION PAGE BEFORE COMPLET!NG FORMI. REP T ~~~~~~~~ — 2. GOVT ACCESSION NO 3. RECIPIENT’S CATALOG NUMBERI _ _ _ _ _ _ _ _ _

4. TITLE (~~d S.aeJU.) 5. TYPE OF REPORT S PERIOD COVERED-t

AXIAL DOUBLE P(MNT-LOAD TESTS ON SNOW AND ICEá(1~. 6. PERFORNINO ORG. REPORT NUMBER

~~~ ç’ AUTNOR(.)

~~~~~~~ ~~~~~~ ~~~~~~~~~DPf~/4.236~~~~~~~~\

PERFORMING ORGANIZATION NAME AND ADDRESS / 10. PROGRAM ELEMENT. PROJECT , TA SKJ AREA & WOR K UNIT NUMBERS

U.S. Army Cold Regions Research and Engineering LaboratoryHanover, New Hampshire 03755

H. CONTROLUNG OFFICE NAME AND ADDRESS 12. REPORTDivision of Polar Programs ~~‘[fMar~~~~ 78 jNati onal Science Foundation 13. NUMBER OF PAG55Washington , D.C. 20550 14 (j~~ [7~ 10._J

14. MONITORING AGENCY NAME S ADDRESS(I( dlff.rw t from Controllind OWe.) 15. SECURITY CLAS~~~~~ 7~.t. r.po4

Unclassified

IS.. DECLASSIFICATION/OOWNGRAOINGSCHEDUL E

IS. DISTRIBUTION STATEMENT (of lbS. R.po rt)

Approved for public release ; distribution unlimited.

17. DISTRIBUTION STATEMENT (of lb. ab.te.cl .nt.,.d In Block 20, If dUf.r.Mt f rom R.po rt)

IS. SUPPLEMENTARY NOTES

IL KEY WORDS (CcnIlnu. on r.v.~.. .id. If onc.. . y aid Idsntlfy by block niaib.r)

Compression Point-load testsIce SnowIce strength Test methodsIndex properties Unconfined testsLoads (forces)

SO. ABSTRACT (CaNS... ai i.va~. .i ~~ If n.cr n y aid Id.n,Ify by block m b.r)

— ~ The results of axial double point-load tests on disk samples of snow and ice obtained from the area of McMurdoSound , Antarctica , are presented. They show the effects of temperature , sample length , load poin t diameter andspecific gravity on failure load. It was determined that 13 samples should be tested to obtain a representative meanstrength Index. The results show that the axial double point-load test has good possibilities as a rapid field test fordetermining the unconfined compressive strength of snow and ice but that further evaluation of the variables affect-ing test results must be made.

DO ~~~ 1473 EDITION OF 1 NOV SI IS oBSOLETE Unclassified

SECURI’Y CLASSIFICATION or TIlls p*os (BV~.n D.e. Enl.e.d) -~~~

cr32 iØ~

PREFACE

This report was prepared by Austin Kovacs, Research Civil Engineer , of the Foundations andMate rials Research Branch, Experimental Engineering Division , U.S. Army Cold Regions Researchand Engineering Laboratory . Funding for this study was provided by Grant DPP 74-23654 fromthe Division of Polar Programs, Nati onal Science Foundation.

Dr. Anthony Gow, Edwin Chamberlain , and Dr. Donald Nevel technically reviewed the manu-script of this report. The author gratefully acknowledges the assistance of Dr. Gow durin g samplecollection and testing.

The contents of this report are not to be used for advertising or promotional purposes. Citationof bran d names does not constitute an official endorsement or approval of the use of such corn-mercial products.

‘I

IlL ~~ --~~-—~.-. ——---~— - - ---

CONTENTS

PageAbstrac t .

Preface iiIntroduction Test procedure 2Test program 2Test samples 2Number of test for determining strength index 3Effect of temperature 3Effect of sample length 5Effect of load point size 6Tests on snow 8Discussion 9Recommendations I ILiterature cited I I

ILLUSTRATIONS

Figure- Axial double point-load system 2

2. Representative thin sections of ice magnified x 1.2 from Koettlitz Glacier andRoss Island , Antarctica 3

3. Temperature vs mean failure load for axial double point-load tests on ice from theKoettl itz Glacier and Ross Island 4

4. Effect of sample length on mean failure load for the axial double point-load test 55. Configuration of the four ball points used in the axial double point-load tests 6

• 6. Effect of load point diameter on the axial double point-load test mean failure load 77. Axial double point-load test results on snow vs the unconfined compressive strength

of snow at —25 °C 98. Shape constant vs test sample diameter JO9. Axial compressive/tensile strength ratio for ice vs axial strain rate JO

TABLES

TableI. Axial double poin t -load test results using 76-mm-d iam , 78-mm-long samples of ice

from Ross Island 4II. Axial double point-load test results vs temperatu re 4

Ill. Ax ial doubl e point-load test results vs sample length 5IV. Axial double point-load test results vs point diameter for Koettl itz Glacier ice samples 7V. Axial double point-load test results vs density of snow from the McMurdo Ice Shelf .. 8

III

•--~~~~~~~~~~~~~ - -- - --~~~~~~~~~~~~~~~~~~---~~~

-i

AXIAL DOUBLE POINT-LOAD TESTSON SNOW AND ICE

Austin Kovacs

INTRODUCIJON P = failure loadD = sample diameter.

It is well recognized that sample preparation foraxial unconfined compression and tension testing is For the axial double-load test in which the load pointsdifficult at best and that test results are extremely are positioned at the center of each end of a rightvariable , depending upon the sample geometry and cylinder or disk (see cover), Broch and Franklin givetechnique used. This recognition has recently been the equationthe subject of considerable discussion among researchersengaged in ice engineering. To help resolve some of = (2)these difficulties , the Committee on Ice Problems of the C

International Association of Hydraulic Research (IAHR)decided in January 1974 to form a subcommittee for where L = sample length.the purpose of establishing standards specificall y fortesting the mechanical properties of ice (IAH R 1975). The axial tensile strength a~ may be determined (PengIn ice engineering, as in all structural engineering, 1976) from the latter test configuration bymaterial strength is an important fundamental propertywhich must be considered in design analysis. A suitable

= ~KL . (3)strength index test which can give consistent and re- IrDLliable values is therefore essential . Papers publishedsince 1972 have aroused renewed interest in the point- While this equation is in current use, Peng (1976)load test as a convenien t method for classifying rock points out that It is an unvalidated approximation ofstrength. In principle it is a simple test , which can the tensile strength. This is due in part to the equationeasily be performed in the field with lightweight , in- having been developed for an axially point-loaded diskexpensive equipment. And the resulting failure loads sample that is mathematically represented as an infmitehave been used to calculate the axial unconfined corn- elastic medium . As most materials undergo a certainpressive or axial tensile strength of the rock material , amount of nonelastic behavior durin g testing, the aboveBroch and Fran klin (1972) made a detailed study of equations can be used only u a guide. Therefore,the point-load test. They give the following empirical constants which allow the formulas to be used In pie-equation for determining the axial unconfined corn - dicting the axial compressive or tensile strength ofpressive strength of a cylindrical sample loaded dia- materials need to be developed from test results.metrically to failure between two points (see cover): Hoek (1977) suggests that Griffith’s fracture theory

indicates that the point oad test cannot be used toK13 (1) determine the tensile strength of soft material with anaxial compressive/tensile strength ratio unde r 8. Hewhere o~ = axial unconfined compressive strength further states that point-load testing of such materialK = shape constant (used to convert to a~) will normally give erratic results , even when the test is

= diametral point-load strength index used as an index in its own right. Since the axial corn-( P /D 2) pressive/tensile strength ratio for ice is a f’un~tion of

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

F strain rate and is generally less than 8, it appears that into two or three columnar pieces. After the peak forcethe axial double point-load test may not be a suitable was recorded on the digital display, the instrumen t wastest for ice, reset to zero for the next test by simply pressing a but-

However, the apparent simplicity of the axial double ton.point-load test , the limited sample preparation required , The peak load recording system eliminated possibleand the resultant short time needed between sample inaccuracies in the Bourdon pressure gage generally usedcollection and testing are very attractive Lltributes. In in axial double point-load systems for measuring theaddition , the test may prove to give a reliable st rength peak hydraulic pump pressure developed during a test.index for ice and thus would be a practical field test This gage tends to have poor resolution because of itsfor ice engineers. An evaluation of the test for deter- large pressure range . Also eliminated by this peak loadmining the strength of snow and ice war therefore con- recording system were any effects of seal friction in thesidered timely and important. Tests were made in hydraulic ram .January 1977 on snow and ice collected in the areaof McMurdo Sound, Antarctica. The results of thesetests are discussed in this report . TEST PROGRAM

The purposes of the test program were to evaluateTEST PROCEDURE the effects of ice temperature , sample length and load

point diameter on failure load. Snow was also testedThe axial double point-load test method used in to determine if there was a correlation between the

this study is a modification of one suggested by the axial double point-load test results and the axial uncon-Commission on Standardization of Laboratory and fined compressive strength of snow of the same densityField Tests of the International Society of Rock (specific gravity). To achieve these goals over 250 sam -Mechanics (ISRM 1972). The test appara tus (Fig. 1) pIes were tested. Of these about 20 tests were rejectedconsisted of a small test frame , a hydraulic tam , a hand when the snow was crushed rather than split unde r thepump, a load cell, and an electric interface unit that loa d po ints or when the ice spalled radia lly away f romprovided a digital display of the peak force developed the load points .during a test. Fixed to the top of the ram and the bot-tom of the load cell were small steel balls, which servedas the load points. An upright cylindrical sample was TESI SAMPLEScentered between these balls. When the ram wasmoved upward , axial point loading of the sample Both snow and ice samples from Antarctica wereoccurred. When the rupture strength of the material tested. The snow specimens , obtained from thewas reached , samples failed by splitting apart , usually McMurdo Ice Shelf , varied from 0.6lto 0.73 in specific

2

3

•o ~~~

1

Figure 1. Axial double point-load system. From left to right: (1) hydraulic pump,(2) test frame with load cell (top), hydraulic ram (bottom) and test sample (center),(3) digital strain indicator.

2

---

~~~~~~~~~~

-----

~

- .

~~~—

~~~~

-

I.

I

‘ SH~ ~•~~~~~

•• -

‘

.-

a. Thin section of ice from Koettlitz Glacier. b. Thin section of ice from Ross Island.

Figure 2. Representative thin sections of ice magnified xl.2 from Koettlitz Glacier andRoss Island, Antarctica.

gravity. The ice samples came from an “ice wall” on a statistically representative mean strength of rock. InRoss Island and from the floating tongue of the the testing of snow and ice the author has attempted toKoettlitz Glacier. A representative thin section of the use 15 or more samples (Kovacs et al. 1969, 1977).ice from each site is shown in Figure 2. The ice from To obtain some guidance on the number of axialRoss Island was found to have air bubbles that were double point-load tests which should be made on ice toup to four times as large and ice crystals on the order obtain a representative mean strength index , 22 iceof twice as large as the ice from the Koe t tlitz Glacier. samples were tested. These samples were 76 mm (3 in.)The Koettlitz ice was found to be laced with very thin in diameter and 78 mm (3.1 in.) long. They were testedbut healed fracture planes believed to be the result of at — 14°C between 12.7-mm- (0.5-in. .) diam load points.therm al strianing. The specific gravity of the ice from The test failure loads along with the mean failure loadRoss Island averaged 0.885 and that from the Koettlitz and standard deviation beg inning with the fifth test areGlacier 0.895. listed in Table I. The data show that after the 12th test

The samples, obtained with a CRRE L auger , had a the mean failure load and the standard deviation varied76-mm (°. 3-in.) diameter. They were cut on a band insignificantly. Based on this result 13 samples weresaw into right cylinders having lengths of 50, 78 or 100 used in a given test series.mm (= 2, 3.1 or 3.9 in.). Sample length variation waslimited to under 2 mm (= 0.1 in.) with the use of acutting guide. EFFECT OF TEMPERATURE

It has been inferred from the limited dat a availableNUMBER OF TESTS FOR that the tensile streng th of ice is not very sensitive toDETERMINING STRENGTh INDEX temperature variations (Hawkes and Mellor 1972) when

the temperature range considere d is on the order ofThe number of tests required to provide a representa- —5 ° to —35°C. As the axial double point-l oad test is an

tive mean failure strength or strength index for a ma- indirect tensile test , it would follow that axial doubletenal wilJ vary with the material being tested as well as point-load test results would likewise not be affectedwith the difficulty of performing a specific type of test , by temperature . To study the effect of temperature .From an analysis of unconfined compression and ax ial doubl e point-load tests were made on ice at —1 0°.Brazil tensile test results, Yamaguchi (1970) determined — 14° and —2 1°C. The results are listed in Table II andthat 10 or more test samples are required to determine graphically presented in Figure 3. The slope of the line3

Table I. Axial double point-load test results using -25

76-nun.diam , 78mm-long samples of ice from Ross /bland.Samples were test ed at —14°C with 12,70.mm.diam points. -20 - / / —

Koett l itz / /Mean Standard Glacier Ice/ ‘R lila dFailure load failure load devia don

~,, / / °?~.

°Sample (kg) (Sb) (kg) (Sb) (kg) (Sb) - 5 — / / —

I I SloPe:i.45k9/~~/75 220 485 191 420 30 66 ~ -10 - I ? —6 158 349 185 408 30 667 194 428 186 411 28 6!8 226 498 191 422 29 649 137 303 186 409 33 7210 242 534 191 421 35 78 5 -

11 160 353 188 41$ 35 7712 145 320 185 407 36 7913 195 429 186 409 34 7614 171 376 184 406 ~~15 179 395 184 406 32 70 0

300 350 400 450 500 lb16 181 399 184 405 32 7017 245 541 187 413 33 73 I I18 162 357 186 410 33 73 140 160 80 200 220 kg19 142 314 184 405 34 74 Mean Fail Load20 219 483 186 409 34 7421 15 1 332 184 405 34 7’ Figure 3. Temperature vs mean failure load for axial22 2 1 7 474 185 408 ~ double point-load tests on ice from the Koettlitz Glacier

and Ross island.

Table II. Axial double point-load test results vs temperature.Samp les were 76 mm in diameter , 78 mm long and tested using 12 .70-mm-diam load points.

Temperature (-10°C) Temperature (-14°C) Temperature (-21°C)Failure load * Fai lure load t Failure load° Failure loadf Failure Soad° Failure loadt

Sample (kg) (Sb) (kg) (Sb) (kg) (Sb) (kg) (Sb) (kg) (Sb) (kg) (Sb)

181 398 156 345 169 372 220 484 194 427 277 6102 191 422 15$ 341 203 447 192 423 170 374 196 4333 224 494 173 381 231 509 220 485 180 397 197 4344 184 407 196 432 170 314 194 428 215 474 204 4495 116 389 176 389 233 513 226 498 170 376 175 38$6 176 389 208 459 209 461 242 534 171 377 173 3827 171 377 191 421 152 335 19$ 429 193 426 182 4018 168 371 189 417 160 352 14$ 320 232 512 244 5319 154 339 142 314 192 423 160 353 227 500 199 43910 196 432 196 433 152 334 137 303 188 415 211 466It 147 324 156 343 208 458 158 349 168 370 173 38212 166 365 209 461 140 309 168 370 220 486 170 37513 161 355 205 453 151 333 153 338 16$ 363 170 374

Mean 116 389 18 1 399 182 401 185 409 192 423 198 436

Standard deviation 20 44 23 50 32 70 34 76 24 53 32 70

Koettlitz Glacier Ice , 0.89$ specific gravity.t Ross Island ice, 0.885 specIfic gravity.

4

-~~~~~~~~~ - -— -~~~~~~~~~—~~~~~~~~~~~~~~~~~~~~~~~~ —-~ --—~~~~~ ~~~~~~~~~~~~~~~~~~~~~

- ~~~—- .~~~~~ ~~~~ -~~~~~-

passing through the test data for each ice type is 1.45 Standard Devia t ion0 20 30 40 kgkgJ°C (3.1 lb/°C). This change in failure load vs tern- I

perature is equivalent to a tensile strength change ofmm in .0,20 kgf/cm 2 °C (2.85 psi/ °C) or an unconfined com- 150 -. ~

20 4~0 610 ~O 100 lb

pressive strength change of 0.65 kgf/crn2 °C (9.3 psi/°C). Both these changes are signific ant. 5

Kovacs et al. (1977) have found from an analysis ofpublished unconfined compressive strength vs tempera- ~,ioo - 4 - Standard

Cture dat a that the uncon fined compressive strength of Devi

n)ice changes at a rate of 0.75 kgf/cm 2 °C (° 1 1 rsi/°C). This shows that the failui~ strength of ice vstemperature noted in the axial double point-load test ~~~ 2 -

results is comparable with the change in the uncon finedI —compressive strength vs temperature found by others.

The difference in failure load shown in Figure 3 be

-

_________________________0- Ctween the ice from the Koettlitz Glacier and Ross 300 350 400 450 lb

isiand is interesting. The finer-grained , higher density I Iice from the Koettlitz Glacier might have been expected tAO 160 180 200 l’Q

Mean Fo i l Loadto be stronger than the lower density Ross Island icewhich had larger grains and larger , more irregular bubbles Figure 4. Effect of sample length on mean failure(see Fig. 2 and Coble and Parikh 1972). This was not load for the axial double point-load test.the case. The difference noted is well inside the boundsof the standard deviation for each data set , suggestingthat the difference is within experimental error. Never- Table Ill. Axial double point4oad test results vs mm-theless, the failure load difference may be due partl y pie length.to a structural weakness in the Koettlitz ice associated Ko et tlil z Glacier ice samples 76 mm In diameter were testedwith the very fine flaw seams previously discussed. at —21 ° C using 12.70-mm-diani load points.

Length, 30 mm Length. 78 mm Length, 100 mmFailure load Failure load Failure loadEFFECT OF SAMPLE LENGTH Sample (kg) (Sb) ( ks) (Sb) (kg) (Ibi

As with the unconfined compression test , sample I 202 446 194 427 264 5552 162 357 170 374 240 528

shape and size affect the axial double point-load test 3 136 302 180 397 243 536results. In this study cylindrical samples were used 4 170 374 2 1 5 474 208 458

which had a constant diameter of 76 mm. This shape ~ 192 423 170 376 149 3286 196 432 171 371 167 369

was selected out of convenience in that this was the 7 14$ 326 193 426 162 357inner diameter of the CRREL auger barrel used to ob- 8 174 354 232 512 176 387

tam the samples. Inasmuch as this core barre l is widely 9 170 374 227 500 2 20 48610 196 433 18$ 41 5 172 379

used in glaciological and ice engineering studies , it is I I 2 1$ 475 168 370 176 388only reasonable that snow and ice samples fc,r the axial 12 152 402 220 486 166 365

double point-load test could become standardized as 13 !.~~ ~~~~~~ !±! !~~ !~3_ ~~!.!_

cylinders with this diameter. Mean 178 392 192 423 194 428

Broch and Franklin (1972) determined empirically Standarddeviation 22 49 24 53 37 51that a sam pIe length/diameter ratio of about 1.1 ± 0.05

to I will give an axial double point-load strength indexcomparable to that obtained from a diametral doublepoint-load test when the distance from the contact decreased) or a gradual decrease in the index (when thepoint and the nearest free end in the latter test is at LID ratio increased). The importance of maintaining aleast 0.7 D. As a result , this LID ratio has been sug- set sample length was clearly sh own in their study.gested as a standard by the International Society for Peng (1976) made a theoretical analysis of the axialRock Mechanics (ISRM 1972). Broch and Franklin double point-load test and found that the stress dis-also showed that departure from an LID ratio of 1.1 tribution in a test sample changes slightly when LID >

resulted m either a significant increase in the axial 1.33 and stabilizes when LID < 1.00. He thereforedouble point-load streng th index (when the LID ratio concluded that the tensile fracture strength should not

5

-—~~ - -~~~~~~~

- - - _ _ _ _ _

vary considerably for samples with LID < 1.00 and certain “soft” materials the point will penetrate , andthat the best sample geometry is one with LID < 1.00. wedging by the walls of the cone will occur.These findings do nc - agree with the empirical results To avoid wedging and to determine the effect ofof Broch and Franklin . point diameter on failure load , four spherical ball points

To further investigate the effect of sample length on with diameters of 4.70 , 12.70 , 15.88 and 25.40 mmaxial double point-load tests performed on ice , samples (0.185 ,0.500 , 0.625 and 1.000 in.) were used (Fig. 5).were cut to lengths of 50 , 78 and 100 mm (“ 2.0, 3.1 The test results made on ice from the Koettlitz Glacierand 4.0 in.) and tested at —2 1°C. The results are listed are listed in Table IV and shown in Figure 6. It wasin Table III and shown in Figure 4. The data show a found that the 4.70-mm- (0.185-in. -) diani pointsgradual increase in failure load and a large increase in would on occasion crush their way into the ice to athe standard deviation with increasing sample length. dep th that allowed contact with the conical pedestalThe most striking result is that the data do not agree (Fig. 5). The resulting wedging by the pedestal waswith the findings of either Broch and Fran klin (1972) considered undesirab le and these tests were discarded.or Peng (1976). The test results indicate that failure Failure load is shown to increase significantl y withload is little affected by changes in LID above 1 but point diameter , reachin g an apparent maximum at aquite sensitive to changes in LID below 1. However , point diameter of 25 .4 mm (1 in.). The standardfurther testing to verify these results is desirable. deviation was also found to increase appreciabl y with

increasing point diameter. This may be due to atendency of the ice samples to translate laterally a sligh t

EFFECT OF LOAD POINT SIZE amount during seating of the largest balls. This wouldresult in off-center nonaxial point loading, lower failure

The load point being recommended by the ISRM loads and thus a larger scatter in the test results. Thusas a standard in the point-load testing of rock is a the apparent peaking of the failure load and the acceler-spherically truncated conical shape (ISRM 1972), with ated increase in the standard deviation noted in the testa 60° cone and a 5-mm (0.20-in.) tip radius. The results with the use of the 25.40-mm-diam load pointsselection of this size point was presumabl y based upon may have been associated with nonaxial point loading.empirical test results ; however , it is apparent that in Further tests must be made to evaluate this.

FigureS. Configuration of the four ball points used in the axial double point-load tests.

6

Table IV. Axial double point-load test results vs point diameterfor Koetthtz Glacier ice samples (76 mm in diameter, 78 mmlong and tested at —21°C).

Failure Ioad ° Failure loadt Failure load°- Failure loadttSample (kg) (Ib) (kg) (Ib) (kg) (ib) (kg) (ib)

1 113 249 194 427 227 500 316 6962 107 235 170 314 191 420 220 4863 95 209 180 397 214 471 342 7544 104 230 215 414 218 480 191 4215 89 197 170 376 236 521 200 4426 105 231 171 377 175 385 193 4267 116 25 6 193 426 248 547 199 439t 8 89 197 232 5 12 270 595 163 3599 82 182 227 500 230 506 197 435

10 100 220 188 415 186 409 219 48211 131 288 168 370 211 466 164 36112 101 222 220 486 185 407 259 57113 107 236 165 363 190 418 240 528

Mean 103 227 192 423 214 471 223 492Standard devIation 13 28 24 53 28 62 54 119

• Point dIameter 4.70 mm (0.185 In.)t Point dIameter 12.70 mm (0.500 In.)~~ Point diameter 15.88 mm (0.625 In.)tt Point diameter 25.40 mm (1.000 in.)

S t anda rd Deviat ion0 tO 20 30 40 50 60 70kg

I I I

mm In. 0 20 40 60 80 00 20 140 ft3 0- 1 .2 I I

25 - .0

20 — 0.8 - Standard -Dev iation

a,E03 5 - 0.6 - -

C0a-

I 0 — 0 . 4 Load

5 - 0.2 - -

0 — C I I I I200 250 300 350 400 450 500 lb

I I I ItOO 120 140 60 180 .200 2 2 0 k g

Mean Fail Load

Figure 6. Effect of load point diameter on the axial double point-load test meanfailure load. Results are from tests on ice samples.

7

— — —-—— . ..-- . .- ~— -— -~~~--~~ .--

TESTS ON SNOW corrected data are shown in Figure 7. The curve passingthrough the data represents the Greenland axial uncon-

Over 70 axial double point-load tests, all using the fined compression test results.12.7-mm- (0.5-in .-) diam load points, were made on It was expected that the axial double point-loadsnow at —2 1°C. It was found that below a specific test results would follow an exponential increase ingravity of 0.61 the points would crush into the snow failure load with increasing snow density similar to thatwithout splitting the sample in two. These tests were observed for the axial unconfined compression tests.discarded. Some penetration by crushing also occurred However , it was not expected that the double point-loadin samples with a specific gravity as high as 0.65, but test results would straddle the curve representative ofsplitting of these samples would always follow. These the unconfined compressive strength of the Greenlandtest results were retained and are included with the snow. The cause of this virtual one-to-one correlationtest data given in Table V. is a mystery — after all, one is a compressive test and the

No unconflned compressive strength tests were made other an indirect tensile test. Whether this correlationon the snow from the McMurdo ice shel f. However , Un- could be main tained over the full density range of snowconfined compressive strength tests were made on simi- simply by changing to a larger poin t diameter below alar density snow in Greenland (Kovacs et al. 1969), specific gravity of 0.61 is, of course, unkn own. Itpermitting a relatively close comparison. Since the would be desirable to explore this further. Unfortunately,uniaxial unconfined compressive strength of the Green- the ideal place to compare snow samples of varyingland snow samples was determined at —25°C, the axial density, the Inclined Drift at Camp Century, Greenland ,double point-load failure loads listed in Table V were is buried under a thick layer of new snow and is nocorrected to —25 °C using the temperature correction longer accessible. In this drift an unlimited supply offactor previously detemiined in this report. The snow or ice samples of any density could have been

Table V. Axial double point-load test results vs density of snow fromthe McMurdo Ice Shelf.Samples were 76 mm In diame ter. 78 mm Ions and tested at — 2 1 °C usIng 12.7-mm- (0.5-In.-) diam poi nta.

Specific Failure load Specific Failure loadSample gravity (kg) (ib) Sample gravI ty (kg) (I b)

I 0.613 142 312 22A 0.690 213 4692 0.610 12 1 267 23A 0.696 240 5293A 0.626 ISO 331 23B 0.696 222 4895 0.638 1 24 274 23C 0.696 186 4 117A 0.639 153 338 24A 0.697 208 4587C 0.652 198 436 24B 0.697 179 3958A 0.648 184 405 24C 0.697 225 497SB 0.648 196 433 26A 0 . 7 1 1 192 4239 0.658 128 282 26A 0.711 187 413b A 0.644 146 321 26A 0.711 181 413lOB 0 644 178 394 26B 0.696 222 490b C 0.644 126 217 26C 0.700 2 14 413h A 0.662 16 1 355 21A 0.694 206 454I I B 0.662 170 315 27B 0.694 234 5 1 $12 0.644 181 399 28A 0.704 222 490I 3A 0.662 163 359 28B 0.704 230 50713 B 0.662 166 367 28C 0.704 187 4 13I SA 0.667 160 352 29A 0.70 3 234 51 7I S B 0.667 167 369 29B 0.703 2 12 467I SC 0.667 16 1 355 29C 0.703 2 1 4 47 116 0.657 142 314 30A 0.7 14 238 524h A 0.666 I I I 378 308 0.714 18 1 40017A 0.666 226 498 30C 0.714 186 4 1 1b I B 0.663 182 402 3 1A 0.115 191 421118 0.663 184 406 31B 0.715 178 39 318 0.682 228 502 32A 0.727 205 453I Q A 0.671 2 17 478 328 0.127 t96 43319 B 0.67 1 271 479 33A 0.714 224 493198 0.671 194 429 33B 0.7 14 190 419

8

_ _ _ _ _ _ _ _ _ _ _ _ - -

kg l b PSi kgf/Cm 2

6OC~ I J ~0O

250 - / 40

500 - .~. - 500 - 35 ~

200 - ~~~

, .-30 0

400 - - .1- ’ - 400 ~

~~I5O .;t -2 5

300 - / ,.. - 300 2 0 °

00 / - S C200 - - 200 ~

5 0 - 100/

- 100

( )- C I I 3 - O0.4 0.5 0.6 0.7 0.8

Sp ecif i c Gro v ~t y

Figure 7. Axial double point-load test results on snow (dots) vsthe unconfined compressive strength of snow (solid line) at —25°C.

obtained , and because the axial unconfined compressive Kovacs et al. (1969) on ice of a similar density. Thestrength vs density of the snow and ice in the drift has Koett litz ice tests were not used because of the internalalready been determined (Kovacs et al. 1969) it would flaw seams noted, which may have affecte d the test re-have been very convenient to compare these strengths suits. Results from axial double point-load tests madewith axial double point-load test results. on 78-mm-long ice samples were selected because they

gave an LID ratio of 1.04 to 1, which is close to the 1.1to I ratio suggested as a standard for axial double point-

DISCUSSION load test samples by the International Society of RockMechanics (ISRM 1972). Using the axial double point-

The shape constant K in eq 1-3 is used to convert load test failure load (extrapolated to —25°C) of 201 kgthe axial double point-load strength index J ~ to either (444 Ib) from Figure 3, an axial unconfmed compressivethe axial unconfined compressive or tensile strength. strength of 89.2 kgf/cm 2 (1268 psi) was obtained from -

The appropriate value of K is dependent upon the eq 2. For ice of the same specific gravity (0.885) anddiamete r of the core tested. Bieniawski (1975) com- temperature (—25°C), Kovacs et al. (1969) obtained ,pared the axial uncon fined compressive strength of from tests on 210-mm-long and 76-mm-diam core, anrock with the point-load strength index obtained from axial unconfined compressive strength of 91.1 kgf!cm2the testin g of cores 21.5 , 42 and 54 mm in diameter. (1296 psi). There is virtually no difference (about 2%)From his data a plot of the shape constant vs core between the two strengths.diameter has been constructed in Figure 8. When the Using eq 3, the calculated axial tensile strength forline passing through the test data is extended , a the Ross Island ice at —25 °C is 28.8 kgf/cm 2 (410 psi).representative K value for the 76-mm-diam core tested The result ing calculated axial unconfined compressive!in this stud y would appear to be 27. tensile strength ratio is 3.1 to I. This ratio is an artifact

To determine if this shape constant is an appropr iate of eq 2 and 3. It will change only if sample length orone for ice and if the axial double point-load test can diameteT change.be used to predict its axial unconfined compressive Hawkes and Mellor (1972) and Haynes* (unpub-strength , a comparison was made between the strength lished data) have shown that the tensile strength of icecalculated from eq 2 for the Ross Island ice and the ________________________

axial unconfined compressive strength determined by • F.D. Haynes, Ma terials Research Engineer , CRREL .

9

~~~~--- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I I I28 - -

—:24 - ..-‘ “ -

- Figure & Shape constant vs test sam-pie diameter.

• Core Diameter , mm

Figure 9. Axial compressive/ tensile strength

2 —

ratio /br ice vs axial strain rate.

p0-6 10~ ,o-~ io~ ~ -2

A x ial Strain Rat• , i~

is virtually unaffected by strain rates between 10.6 and some unique way to the agreement found in the strength102 5~b , but that the axial unconfined compressive values. In any event the comparison is very encouragingstrength is highly affected by strain rate. A plot of the and indicates that further evaluation of the axial doubleaxial unconfmed compressive/tensil e strength ratio vs point-load test on ice samples is warranted.axial strain rate was constructed using their data. As The usefulness of the axial double point-load test onshown in Figure 9 there is an exponential increase in snow samples remains to be determined. The limitedthe axial compressive/tensile strength ratio with in- snow test result s presented are encouraging but alsocreasing strain rates, confusing. They are encouraging in that there is an

Figure 9 indicates that the axial compressive/tensile exceedingly good correlation between the failure loadstrength ratio of 3.1 to I found above should be obtained from the axial double point-load test and therepresentative of ice tested in direct compression and related axial uncontined compressive strength for simi-tension at a strain rate of 7.Ox io~ s~ - This is in- lar density snow. However , the reason for this correla-deed interesting, as the ice samples tested by Kovacs tion is not known , since the axial double point-load testet al. (1969) were loaded at a strain 1ate of 3.O x i0~ is an indirect tensile test , not a compressive test. The5 I — a difference of only 30x 10~’ ~~ This was failure load for these tests was nearl y equal to the tensilefortuitous and is perhaps a reason for the exceptional strength calculated from eq 3. Therefore , the correlationagreement between the axial compressive streng th and was essentially between tensil e failure loads and com-double point-load test results. Shape and size effects pressive strengths and not between two compressivebetween the axial unconfined compression and axial strengths.double point-load test samples also contributed in

10

RECOMMENDATIONS International Association of Hydraulic Research (1975) Reportof task-committee on standardIzing testing methods forice. Internstionah Association of Hydraulic Research ,Many more tests must be made to determine the Committee on Ice Problems , 3rd m t . Symposium on

value of the axial double point-load test in ice engineer- Ice Problems, Hanover. New Hampshire .ing. The effect of temperature on test results , especially International Society for Rock Mechanics (1972) Su~~ested

above —10 °C, needs to be determined. It is believed methods for determining the uniaxial compressivestrength of rock materials and the point-load strength

that the effect of strain rate on axial double point-load In dex Internationa l Society for Rock Mechanics , Corn-iest results is negligible because the test is an indirect mitt es on Laboratory Tests , Document No. I.

Kovacs , A., WF. Weeks and F. Mich ltt i (1969) VarIation of Itensile test . This belief is based on the work of Hawkes some mechanical properties of polar snow, Camp Century,and Mellor (1972), who show that the tensile strength Greenland. CRREL Research Report 276 , AD 100995.

• of ice is essentially unaffected by a change in strain Kovacs, A., F. Mich itti and J. Kalafu t (1977) Unconfined corn-

rate between 10’ and l0~ s~ - Nevertheless, this pre ssion tests on snow: A comparative study. CRRE LSpecial Rep ort 77-20.

should be verified for axial double point-load tests on Peng, S.S. (1976) Stress ana lysis of cylindrical rock discs sub-ice and snow. The effect of nonaxial point loading J ect to axial double point Iosd. Internationa l Journal

needs to be determined, and if foun d to be a serious of Rock Mechanics and Mining Sciences and Geo-mechanics Abstracts, vol. 13.

problem , sample alignment guides will have to be Yarnsguchi , U. (1970) Test pieces required to determine theincorporated into the test apparatus. strength of rock. International Journal of Rock Mechanics

From the results of this study it is recommended and Mining Sciences. vol. 7.

that test samples have an LID ratio of 1.1 ± 005 to 1.This ratio conforms with that suggested as a standardby the International Society of Rock Mechanics(ISRM 1972). Test samples should be standardized at76 ± 1 mm (3.0 ± 0.1 in.) in diameter. This is thediameter of core take n by the CRREL auger , which is

• The best ball point diameter for axial double point-load testing of ice has not been conclusively determined.

widely used by ice engineers and glaciologists.

• However, it is recommended at this time that 15-mm-(0.6-in.-) Warn ball points be used. Smaller points mayresult in undesirable crushing and penetration into

• warm or low density ice and into snow. For snowwith a specific gravity below 0.6 1 , ball points on theorder of 40 mm in diameter may be necessary to avoidball crushing penetration into the material.

In these tests it was determined that 13 or moresamples should be tested to obtain a statisticallyrepresentative mean failure strength . In no case shouldless than 10 samples be tested.

LITERATURE CITED

Blenlawaki, Z.T. (1915) The point-load teat in geotechnicslprac tice. Engineering Geology, vol. 9.

Droch . E. and l.A. Franklin (1972) The point-load strengthtest . Internat ional Journal of Rock Mechanic, andMining Science& vol. 9.

• Coble , R .L. and N.M. Parl kh (1972) Fracture of polycrysta llineceram ics. In Fmctu, ’e, vol. VI I (H. Liebowft z , Ed.),New York : Academic Press .

Hawkse, I. and M. MeUor (1912) DeformatIon and fracture ofice under unlaxia l stress. Journa l of Glaciology , vol. II ,no. 61.

Hock , F. (1917) Rock mechanic s labo r atory testing in thecontext of a consult ing engineering organization.International Journal of Rock Meclienj cs and Mining

• Sciences and Geomechanics Abstracts, vol. 14.

I I

In accordance with letter from DAEN—RDC , DAEN-ASI dated22 July 1977, Subject: Facsimile Catalog Cards forLaboratory Technical Publications, a facsimile catalogcard in Library of Congress MARC format is reproducedbelow.

Kovacs, AustinAxial double point—load tests on snow

and ice / by Austin Kovacs. Hanover, N.H.:U.S. Cold Regions Research and EngineeringLaboratory , 1978.iii, 11 p. ; 27 Cm. C CRREL report ; 78—2 .

Prepared for Office, Chief of Engineers,under Grant no. DPP 74-23654, U.S. ArmyCold Regions i~esearch and Engineering

• Laboratory.Bibliography: p.11.1. Ice. 2. Ice strength. 3. Loads(forces)

4. Snow . I.United States . Army Cold RegionsResearch and Engineering Laboratory, Hanover,LH. II. Title III.. Series

~U.S. GOVERNMENT PRINTING OPPICE~ 1977—102-4 11/342

_ _ _ _ _ _ _ _ --~~~~~~~~~~~~ ~~~~~~~~ .-~~~~~~~~~~~•• • - -