1975_ significance of in-situ tests on large rock specimens.pdf

7/28/2019 1975_ Significance of in-situ tests on large rock specimens.pdf

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Int. J. R ock M ech. Min. Sci. & Geomech. Abstr.Vol. 12, pp. 101-I 13. Pergamon Press 1975. Printed in Great Britain

The Signif icance of I n Si t u Testson Large Rock Spec imensZ . T . BIENIAWSKI*W. L. VAN HEERDEN Th is paper rev iews large sca le in situ t es t s on rock conducted throughou t the

wor ld . Based on the au thors" exper ience o f 8 years o f in situ testing involving66 la rge coa l spec imens (up to 2 m in wid th and he igh t and wi th wid th - to -he igh tra t ios f ro m 0.5 to 3.4), the value and meaning o f large scale in situ tes ts in com-pression are discussed. P ract ica l guidel ines are given for performing the tes ts,preferred test ing techniques are described and typical data to be expec ted fro mthe tes t s a re l i st ed . E mpir ica l fo rmu lae are der ived for the presen ta t ion o fs trength resul ts and applicat ion o f large scale tes t data to engineering designis demons tra ted . I t i s a l so show n how the resu l t s o f such tes t s can be d i rec t lyapplicable to predict ing the b ehaviour of fu l l s ize rock s tructures . Finally, thecosts o f large scale tes ts are considered, in term s of both t ime and mon ey, andactual f igu res are quoted. I t is concluded that large scale in situ tes ts can bean invaluable aid in engineering design and that their costs const i tu te a verysmal l percen tage o f the value o f p roduct ion f rom an average mine and tha t thecos t s o f t es t s a re more than o f f se t by the f inancia l re turns and techn ica l ga insobtaine d fro m large scale in situ tests.

INTRODUCTIONLarge scale tests on rock have been conducted in variousparts of the world; they are usually expensive, time con-suming and not always successful. The question, there-fore, arises as to what one really obtains from in situ testson large rock specimens and what value may be attachedto the results.

Very little information is available in published litera-ture to answer these questions.

The authors have conduc ted an extensive programmeof in situ tests on large coal specimens loaded in com-pression. The tests star ted in 1966 and, after eight seriesof tests, the programme was conc luded in 1973. These in-vestigations involved testing square coal specimens withsizes from 0"6 m to 2 m in side lengths and of width-to-height ratios from 0"5 to 3.4. The tests provided strengthand deformation data including post failure character-istics. In total 66 in situ tests were conducted over theperiod of 8 years.The results obtained and the experiences gainedenable a critical assessment of the meaning and the valueof large scale compression tests in rock mechanics.

The following questions will be dealt with:(1) What are the best techniques for large compression

tests?(2) What information do the tests yield?(3) How can one apply this information to practicaldesign'?(4) What costs and time-tables are involved for suchtests'?* Geomechanics Division, Council for Scientific and IndustrialResearch, P.O. Box 395, Pretoria 0001, South Africa.

NEED FOR I N S I T U LARGE SCALE TESTSDue to the fact that actual rock masses are discon-

tinua in most cases, tests conducted on small specimensin the laboratory generally do not yield strength anddeformation data of rock which would directly beapplicable to the rock mass from which the specimenswere taken. A specimen is usually a continuous structureor, at any rate, approaches such a state. The smaller thespecimen the fewer the discontinuities present and hencethe stronger the specimen. Thus, a smaller specimen maybe expected to have a higher strength than a large speci-men. A small specimen will seldom be representative ofthe rock mass characteristics and a large specimen may,therefore, provide a better estimate of these character-istics. Since there is no reliable method of predicting theoverall strength and deformation data of a rock massfrom the results of laboratory tests on small specimens,in situ tests on large specimens are necessary. Such testsalso have the advantage that the rock specimen is testedat the same environmental conditions as are prevailingat the rock mass.

Thus, although large scale tests are clearly called for,the crucial questions are: Is the chosen size sufficientlylarge'? Are the loading condit ions such that they simulatethe loads acting on the prototype'? Is extrapolation of theresults to still larger sizes permissible'?There are many types of in situ large scale tests suchas compression tests, shear tests, plate bearing tests,cable tests, flat jack tests and pressure chamber tests.They are used throughout the world for various pur-poses and in various applications. The present paperdeals solely with compression tests.

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1 02 Z . T . B i e n i a w sk i a n d W . L . V a n H e c r d e nL I T E R A T U R E R E V I E W O F L A R G E S C A L E

C O M P R E S S I O N T E S T SN u m e r o u s l a r g e s c a l e in s i tu c o m p r e s s i o n t e s t s ,

a l t h o u g h n o t a s c o m m o n a s in s i tu shea r t e s t s , have beenc o n d u c t e d s in c e 1 9 3 7 m a i n l y i n c o n n e c t i o n w i t h s u c hp r o j e c t s a s p i l l a r d e s i g n i n c o l l ie r ie s a n d i r o n o r e m i n e s .

T h e se t e s t s w e r e i n p a r t i c u l a r a i m e d a t o n e o r m o r eo f t h e f o l l o w i n g o b j e c t iv e s :( 1) T o d e t e r m i n e t h e s t r e n g t h b e h a v i o u r o f p i l l ar s ;

( 2) T o e s t a b l i sh t h e d e f o r m a t i o n c h a r a c t e r i s t i c s o fr o c k m a sse s ; a n d , m o r e r e c e n t l y ,

( 3 ) t o o b t a i n p o s t - f a i l u r e l o a d - d e f o r m a t i o n d a t a f o rcoa l p i l l a r s .

I n sp i t e o f 1 4 i n v e s t i g a t i o n s i n v o l v i n g i n s i t u t e s t s onl a r g e sp e c i m e n s b e i n g r e p o r t e d i n th e l i t e r a tu r e , o n l y f i v eo f t h e m c o m p r i z e d a su f f i c i e n t l y l a r g e n u m b e r o f t es t s t oe n a b l e a p p l i c a t i o n o f t h e i r r e su lt s t o t h e so l u t i o n o fa c t u a l e n g i n e e r i n g p r o b l e m s ; t h e m a j o r i t y o f t h e i n v e s ti -g a t o r s t e s t e d o n e t o f i v e sp e c i m e n s o n l y , a i m e d a to b t a i n i n g a n i n s ig h t i n t o t h e b e h a v i o u r o f r o c k m a s s e s.T h e t e st s c o n d u c t e d a r e r e v ie w e d b e l o w a n d f o r m u l a ed e r i v e d t h e r e f r o m a r e l i s t e d in T a b l e 1.

T h e f i r s t l a r g e c o m p r e s s i v e t e s t s i n s i t u w e r e c o n -d u c t e d i n 1 9 3 7 i n t h e U . S .A . b y G r e e n w a l d e t a l . [ 1 ] . T h et e s ts t o o k p l a c e i n a P i t t sb u r g h c o l l i e r y a n d w e r e a i m e da t d e t e r m i n i n g t h e s t r e n g t h a n d d e f o r m a t i o n c h a r a c t e r -i s ti c s o f la r g e c o a l sp e c i m e n s . T h e t e s t e d sp e c i m e n s w e r ea l l sq u a r e i n p l a n w i t h w i d t h s b e t w e e n 0 " 8 a n d 1 .6 m a n dw i t h w i d t h - t o - h e i g h t r a t i o s f r o m 0 ' 5 t o 1 -0 . T h e sp e ci -m e n s w e r e p r e p a r e d w i t h h a n d t o o l s w i t h o u t u s i n ge x p l o s i v e s . A t h i c k c o n c r e t e b l o c k w a s c a s t o n t o p o fe a c h s p e c i m e n a n d l o a d i n g w a s a c h i e v e d b y m e a n s o fo n e o r t w o l a r g e h y d r a u l i c j a c k s i n se r t e d b e t w e e n t h ec o n c r e t e a n d t h e r o o f o f t h e m i n e . L o a d i n g o f t h e s p ec i-m e n s w a s c a r r i e d o u t i n s t e p i n c r e m e n t s . T h e l o a d i n -c r e a se w a s h a l t e d a t e a c h i n c r e m e n t u n t i l n o f u r t h e r

d e l b r m a t i o n o f th e sp e c i m e n w a s n o t i c e d . S e v e n sp ec i-me ns were t e s t ed bu t a l l were of d i f fe ren t s i zes so tha in o c r o s s - c h e c k i n g o f r e su l ts w a s p o s s ib l e . S o m e , b u t n o ta l l, s p e c i m e n s f a i l e d i n a d o u b l e p y r a m i d f a sh i o n .

T h e a u t h o r s d e r i v e d a r e l a t i o n s h i p b e t w e e n t h es t r e n g t h a n d w i d t h - t o - h e i g h t r a t i o s , a s g i v e n i n T a b l e Ia n d d e a l t w i t h la t e r. H o w e v e r . w i t h t h e a i d o f t h e r e su l t so f a d d i t i o n a l t iv e t e st s c a r r i e d o u t i n 1 9 4 1 b y t h e s a m ea u t h o r s [ 2 ] a d i f f e r e n t r e l a t i o n sh i p w a s d e r i v e d a s g i v e ni n t h e s e c o n d l i n e o f T a b l e I .

I n d e r i v i n g t h is l a s t r e l a t i o n sh i p t w o t e s ts ( o u t o fseven) o f t he f i rs t se ri e s were re j ec t ed an d th ree t e s t s (ou to f fi ve ) o f t h e s e c o n d s e r ie s w e r e c o n s i d e r e d u n r e l i a b l eb e c a u se o f t h e sm a l l sp e c i m e n w i d t h s 0 ' 3 0 - 5 m . T h u se v e n t u a l l y s e v e n te s t r e su l ts w e r e u se d t o d e r i v e t h es e c o n d s t r e n g t h f o r m u l a .

I n 1 9 6 4 , o v e r 2 5 y e a r s a f t e r th e t e s t s b y G r e e n w a l d e ta l. [ I , 2 ] , N o se [ 3 ] r e p o r t e d o n 1 2 c o m p r e s s i o n t e s tsc o n d u c t e d a t th e s it e o f K u r o b e g a w a N o . 4 D a m i nJa p a n . G r a n i t e sp e c i m e n s w e r e t e s t e d m e a su r i n g 1 . 4 1 .8 m i n c r o s s - s e c t io n a n d 2 -8 m i n h e i g h t . T h e a i m o ft h e t e s t s w a s t o d e t e r m i n e t h e sh e a r s t r e n g t h a n dm o d u l u s o f d e f o r m a t i o n o f th e f o u n d a t i o n r o c k m a s s .T h r e e sp e c i m e n s w e r e l o a d e d i n u n i a x i a l c o m p r e s s i o na n d n i n e in tr i a x ia l c o m p r e s s i o n . H o w e v e r , o u t o f t h e 1 2sp e c i m e n s t e s te d , e i g h t d i d n o t f a il ( i n c l u d i n g a l l u n i a x i a lsp e c i m e n s ) b e c a u se t h e l o a d i n g c a p a c i t y o f t h e e q u i p -m e n t w a s n o t s u f f ic i en t ly h i gh . A l t h o u g h t h e a u t h o r d i dn o t g i v e d e ta i l s o f t h e t e s t i n g t e c h n i q u e s u se d , h e p r e -s e n t e d a l l t h e r e su l t s o f t h e in s i tu t e s t s a s we l l a s t hoseo f th e c o r r e s p o n d i n g 1 2 l a b o r a t o r y t e st s o n 5 2 m m d i asp e c i m e n s (8 2 1 3 4 m m i n h e i g h t) o f t h e s a m e r o c km a t e r i a l . T h e u n i a x i a l c o m p r e s s i v e s t r e n g t h o f in s i reg r a n i t e w a s e s t i m a t e d a t b e t w e e n 1 2 a n d 15 M P a , b yc o m p a r i s o n w i t h t h e la b o r a t o r y s t r e n g th o f b e t w e e n 2 3 .6a n d 4 8 -0 M P a , t h e r a t i o s o f t h e l a b o r a t o r y s t r e n g t h t o

T A B L E I . S L IM M A R Y O F S T R E N G T H F O R M U L A E P R O P O S E D F R O M L A R G E SC A L E i l l s i I u I E S T S I N C O M P R E S S I O N

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( o a l S q u a r e ( I . 3 0 ( I . 7 4 0 . 4 1 7 S = 2 8 0 0 { \ , w / ~ / h S ) ( l b f / i n ' ) , ~ r d a t i o n e l 5 " = : 9 0 1 1 w h 1 2 ]( P i t t s b u r g h } t o t o t o o r i ll I b ( ' i n : ~ a ~ a l s o h m n d

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[ W i t b a n k l t o m t o w d h w a n d h i rt m e t r e s w t d t h - m q / e i g h t l a h o s < II 2 2 t . 2 2 2 -1 ) a n d s p e c i m e n w i d t h - 1 5 m

C o a l S q u a r e 1 "~ 0 6 1 .0 1 6 S 2 5 + 2 w h I M P a ) V a l i d I b l I t O I( W i l b a n k ) t o t o t o ~ i d t h t o - h c i g h l l a t i o n I

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lec t a l l g t l l a l 2 .11 _ ~ {) 2 2 r i s e 1 i1 ~ , t ] l c d t t t a ~ e l lC o a l S q u a r e 1 4 0 . 4 1 I I -1 1 0 S I 0 .s~ 4 2 u h ( M P a l l l N I

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I n s i t u Tests on Large Rock Specimens 103i n s i t u strength being between 2.0 and 3.2. It wasremarkable, however, that the shear strength as derivedfrom the triaxial tests i n s i t u , which was between 2"35and 3"0 MPa, was as high as 0-72-0"94 of the shearstrength derived from the laborato ry triaxial tests. Thei n s i t u modulus of elasticity was found to be between1"25 and 2"9 GPa which was generally half that of thelaboratory determined modulus, with the exception ofone site where the i n s i t u modulus was 0.16 of the labora-tory modulus. No formulae were proposed.

In 1966, Jahns [4] reported on four large scale in s i tutests in compression conducted in Germany and aimedat determining the strength and deformation character-istics of iron ore. The specimens were cubical in shapeand with a maximum side length of up to I'm; they weredrilled out, exposing five faces with the sixth faceremaining attached to the floor. The specimens weresubjected to a maximum stress of 70 MPa using 49 hyd-raulic jacks and a pyramid shape stack of steel I-beamsto concentra te the load on the specimens. Load defor-mation characteristics were measured up to failure of thespecimens. In addit ion, 13 smaller specimens were testedin the laboratory. The results of the tests showed that thestrength of a specimen depended upon its size. Forexample, a 0.1 m cube was found to have a strength of117 MP a compared with 49 MPa for a 1 m cube.

Gimm e t a l . [5] reported, also in 1966, on two testsconducted in Germany to determine the strength anddeformation characteristics of one iron ore and oneshale specimen which were abou t 2 m 2 in cross-sectionalarea and 1.5 m in height. The specimens, which remainedattached to the floor of a mine, were loaded in uniaxiaicompression by a number o f hydraulic jacks, each of2 MN loading capacity. De formation o f the specimenswas measured by means of dial gauges attached to thesides of the specimens. The results showed that thedeformation characteristics of the specimens were in-fluenced by the bedding planes and joints in the rock.

Hydraulic flat jacks of a very high loading capacitywere designed in 1966 by De Reeper [6] and used fortesting one iron ore specimen i n s i t u . The flat jacks were360 x 360 mm in size and 30 mm thick. At an oil pres-sure of 140 MPa the loading capacity of each jack was15 MN. The maximum stroke of the jacks was 20 mm.The one iron ore specimen (1 m cube in size) which wastested was found to have a uniaxial compressive strengthof about 50 MPa.Lama [7] reported in 1966 on in s i tu large scale testsconducted in Poland to compare the strength and defor-mation characteristics of two different coal seams. Itappears that four specimens were tested (two in eachseam). The specimens were square and had a width ofapproximately 0-65 m and a height of between 1.64 and2.0 m (width-to-height ratio of about 0"4). The speci-mens, cut free on five faces with the bottom face remain-ing attached to the floor, were loaded by means of 16hydraulic jacks (each of 5 MN capacity, stroke 95 mm,load area 330 270 mm) which were capable of apply-ing a to tal maximum compressive stress up to 35.7 MPaat an oil pressure of 60 MPa. The i n s i t u strength of a

0"65 x 0"65 x 1"64 m (height) specimen was found to be6.8 MP a as compared with the laboratory strength of100 mm cube specimens of 24.5 MPa. In another seamthe corresponding values were 11"3MPa ( i n s i t u ) and17.4 MP a (in laboratory).In 1967, Bieniawski [8-10] reported on large scale ins i t u tests in compression conducted in South Africa andaimed at the determination of the strength and deforma-tion characteristics of coal pillars. He presented theresults of 44 i n s i t u tests conducted between 1966 and1968 on square coal specimens measuring from 0'5 to2 m in side length and of various heights. The influenceof width-to-height ratios of up to 3" 1 was investigated bytesting specimens of different heights and with the samecross-sectional area. Specimens of the required dimen-sions were cut from corners of coal pillars by means ofa universal coal cutting machine. The prepared speci-mens remained i n s i t u , attached to the floor and with fivefaces exposed. The specimens were loaded to destructionin uniaxial compression by means of up to 36 hydrauli-cally actuated jacks (1.5 MN capacity each, 300 x300 mm loading area, stroke 125 mm). The jacks wereplaced on the top of the specimens and exerted a loadagainst the specimen and the roof of the seam. To simu-late the constraining effect produced by the roof on acoal pillar, a lateral constraint was introduced on the topof the tested specimens in the form of either wood shut-tering or steel shuttering or reinforced concrete capping(75 mm thick) placed around the upper part of the speci-men. The choice of the type of the lateral end constraintwas arbitrary but it was compatible with the specimensizes, that is, weaker constraint was applied to smallerspecimens and stronger for larger specimens.

The axial and lateral deformation of the specimenswere measured using extensometer units attached to thespecimen surfaces and axial deformation units placed inthe specimen centre. The results of the tests on 44 speci-mens of 16 different sizes showed that, for cubical speci-mens, the strength decreased with increasing specimensize and that the strength remained constant from a 'cri-tical specimen size' onwards, which was about 1.5 m.This meant that these data could be directly applied tofull size pillars. Based upon the test results, the influenceof width-to-height ratio on the strength of specimens,smaller and larger than 1.5 m in width, was expressed bya formula. Two relationships are given in Table 1 andwill be discussed later.

In 1968, Richter [11] reported on four tests on ironore, sandstone and shale specimens which had the fol-lowing dimensions: height 1.25-1.50m; width 1.25-1.40 m; length 1.50-2.15 m. Standard cylindrical hydrau-lic jacks were used to load the specimens in compression(each of stress capacity 2"5 MPa) and deformat ion wasrecorded during loading. Laboratory tests (48 mm speci-men dia) accompanied the i n s i t u tests. It was found thatthe i n s i t u strength of iron ore specimens was 18 timeslower than the laboratory strength (20MPa vs.360 MPa) while in the case of sandstone the ratio was 4(7,5 MP a vs. 30 MPa). No significant differences werefound in the case of the modulus of elasticity.


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1 04 Z . T . B i e n i aw s k i a n d W . L . V a n H e e r d e nG e o r g i e t a l . [ 1 2 ] c o n d u c t e d o n e u n i a x i a l c o m p r e s -

s i o n t e s t o n a l a r g e b l o c k o f g r a n i t e i n s i t u . T h e o b j e c to f th e t e s t w a s t o d e t e r m i n e t h e i n f l u e n c e o f j o i n t s o n t h ed e f o r m a t i o n b e h a v i o u r o f t h e r o ck . T h e s p e c i m e n w h i c hw a s 1 x l m in c r o s s - s e c t i o n a n d 1 .2 m h i g h , w a s l o a d e di n c o m p r e s s i o n b y m e a n s o f h y d r a u l i c j a c k s . A m a x i -m u m c o m p r e s s i v e st re s s o f 13 M P a w a s a c h i e v e d . I t w a sc o n c l u d e d t h a t t h e d e f o r m a t i o n b e h a v i o u r o f t h e in s i tus p e c i m e n f o l lo w e d b a s ic a l ly t h e s a m e m e c h a n i s m a s t h eb e h a v i o u r o f l a b o r a t o r y s p e c i m e n s w i t h d i s c o n t in u i t ie s .

L a r g e s c a l e in s i tu t e s t s o n c y l i n d r i c a l m a r l sp e c i m e n sw e r e c a r r i e d o u t b y C h a o u i e t a l . [ 1 3 ] , t h e p u r p o se o fw h i c h w a s t o c o m p a r e s t r e n g t h s i n u n i a x i a l c o m p r e s -s i o n , t r i a x ia i c o m p r e s s i o n a n d i n d ir e c t sh e a r. T w o sp e c i-m e n s w e r e te s t e d in u n i a x i a l c o m p r e s s i o n , t h r e e i n t r i a x -i al c o m p r e s s i o n a n d t h r e e i n d i r e c t sh e a r . A x i a l l o a d sw e r e a p p l i e d t o c y l in d r i c a l ro c k s p e c im e n s o f 0 '7 m d i aa n d 1 m h i g h b y m e a n s o f t h r e e h y d r a u l i c , ja c k s r e s ti n go n a c o n c r e t e p a d w h i c h w a s c a s t o n t o t h e s p ec i m e n .L a t e r a l lo a d s w e r e a p p l i e d b y m e a n s o f f o u r t h i n c u r v e dj a c k s w h i c h w e r e i n s e r t e d b e t w e e n t h e s p e c i m e n a n d as t ee l c y l i n d e r su r r o u n d i n g t h e sp e c i m e n . I t w a s f o u n dt h a t t h e c o h e s i o n d e t e r m i n e d f r o m t r ia x i al a n d s h e a rt e st s w a s a b o u t t h e s a m e a n d w a s a b o u t I M P a w h i le t h eu n i a x ia l c o m p r e s s iv e s t r e n g t h w a s 3 M P a . B y c o m p a r i -s o n , l a b o r a t o r y u n i a x i a l c o m p r e s s i v e s t r e n g t h o f t h es a m e m a r l w a s 1 0"5 M P a .

P r a t t e t a l . [ 1 4 ] u s e d a n i n g e n e o u s te s t in g m e t h o d i n1 9 70 in t h e U , S .A . fo r t h e d e t e r m i n a t i o n o f t h e s t r e n g t hp r o p e r t i e s o f a q u a r t z d i o r i t e r o c k m a ss . T h e r o c k sp e c i-m e n s w e r e p r e p a r e d b y c u t t i n g t h r e e s l o ts o n t h e su r f a c eo f a n o u t c r o p b y a d r il l a n d b r o a c h t e c h n iq u e . T h e t h re es l o ts , t w o a t s l a n t a n g l e s o f 6 0 to t h e su r f a c e o f t h e r o c k ,f o r m e d t h e s i d e s o f t h e sp e c i m e n , w h i l e t h e t h i r d s l o t w a sv e r t ic a l a n d n o r m a l t o t h e s i d e s l o ts a n d f o r m e d t h e e n do f t h e sp e c im e n . T h i s t e c h n i q u e p r o v i d e d a sp e c i m e nw i t h t h e c o n f i g u r a t io n o f a n e q u a l a t e ra l t r i a n g u l a rp r i sm . L o a d s w e r e a p p l i e d b y m e a n s o f a s t a c k o f t ri -a n g u l a r f l a t j a c k s g r o u t e d i n t o t h e v e r t i c al s l o t a t t h e e n do f t h e sp e c i m e n . T h e sp e c i m e n s w e r e u p t o 2 . 75 m i n s id el e n g th . A m a x i m u m s t re s s o f 3 4 M P a c o u l d b e a p p l i e db y t h e f l a t j a c k s . T h e i n s i t u t e s t p r o g r a m m e o f 1 0 sp e c i -m e n s w a s su p p l e m e n t e d b y a s e ri e s o f 1 3 l a b o r a t o r ytests.A m a j o r f i n d i n g o f t h is s t u d y w a s t h a t t h e s t r e n g t h o fd i o r i t e d e c r e a se d w i t h i n c r e a s i n g sp e c i m e n s iz e b y a f a c -t o r o f 1 0 a n d a n a s y m p t o t i c v a l ue o f th e s t r e n g t h w a sa t t a i n e d a t t h e sp e c i m e n s i z e o f 0 .9 m . T h i s c o n f i r m e dt h e t r e n d p r e v i o u s l y r e p o r t e d f o r c o a l b y B i e n i a w sk i [ 8 ]as dep ic t ed in F ig , 1 .C o o k e t a l . [ 1 5 ] i n t r o d u c e d i n 1 9 71 a t e s t i n g m e t h o db a se d o n a p r i n c i p le d e sc r i b e d b y C o o k [ 1 6 ] . T h e a i mo f th i s m e t h o d w a s t o d e t e r m i n e c o m p l e t e s t r e s s - s t ra i nc u r v e s o f c o a l p i ll a rs . E a c h j a c k ( l o a d i n g c a p a c i t y u p t o4 .5 M N ) w a s s u p p l ie d w i t h o il b y a n i n d e p e n d e n t p u m p( m a x i m u m o i l p r e ss u r e 1 00 M P a ) . E a c h j a c k h a d a l o a da r e a o f 3 0 0 x 3 0 0 m m a n d a s t r o k e o f 20 0 m m . B y s e t-t i n g t h e v o l u m e o f o i l d e l iv e r e d b y e a c h p u m p t o t h esa m e v a l u e , th e d i sp l a c e m e n t o f t h e l o a d e d su r f a c e sc o u l d b e k e p t u n i f o r m d u r i n g t h e t e s t . T h e l o a d i n g sy s -

150I0 07O5O

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o

o * - - B i e n i o w s k i ( 1 9 6 7 )

0 5 I 1 5 2 2 5 3S p e c ime n s id e le n g th , m

F i g . I. T h e p h e n o m e n o n o f th e s t r e n g t h a p p r o a c h i n g a s y m p t o t i c a l lya c o n s t a n t v a l u e - - a s o b s e r v e d f r o m l a r g e s c a l e t e s t s o f v a r i o u si n v e s t i g a t o r s .

t e m w a s d e s i g n e d i n su c h a w a y a s t o m i n i m i z e t h ea m o u n t o f h y d r a u l i c fl u i d in t h e sy s t e m so a s t o i n c r e aset h e s t i f fn e s s o f th e sy s t e m . F o r t h i s p u r p o se , f o u r - e l e m e n tB o sc h d i e se l f u e l - i n j e c t i o n p u m p s w e r e u se d f o r h i g hp r e s s u r e o i l s u p p l y t o e a c h j a c k . C o m p l e t e l o a d - d e f o r -m a t i o n c u r v e s o f la r g e c o a l s p e c i m e n s w e r e o b t a i n e d f o rthe f i r s t t ime i n s i t u . H o w e v e r , s p e c im e n s w i t h w i d t h - to -h e i g h t r a t i o s o f g r e a t e r t h a n 2 .2 c o u l d n o t b e l o a d e d t of a i l u re b e c a u se o f i n su f f i c ie n t l o a d i n g c a p a c i t y .

T h e r e su l t s o f th e t e s t s u s i n g t h i s m e t h o d w e r er e p o r t e d b y W a g n e r [ 1 7 ] w h o o n t h e b a s i s o f 1 2 t e st sp r o p o se d a f o r m u l a w h i c h i s l i s te d i n T a b l e 1.

V a n H e e r d e n [ 1 8 ] m o d i f i e d t h e a b o v e u n i f o r m d e fo r -m a t i o n m e t h o d t o a c h ie v e a h i g h e r l o a d i n g c a p a ci t y. Ac o n c r e t e b l o c k w a s i n t r o d u c e d o n t h e t o p o f e a c h sp e c i-m e n , t h e p r o p e r t i e s o f t h e b l o c k b e i n g c h o se n so as t os i m u l a t e t h e c o n s t r a i n t g i v e n b y t h e r o o f o f t h e s e a m i nt h e c a se o f a c t u a l c o a l p i l l a rs . T h e c h o i c e o f c o n s t r a i n tw a s b a se d o n a f i n it e e l e m e n t a n a l y s i s o f t h e s t r es s d i s t r i-bu t io n in coa l spe c ime ns which were 1 .4 x 1-4 m inc r o s s - s e c t i o n a n d o f v a r i o u s h e i g h t s g i v i n g w i d t h - t o -h e i g h t r a t i o s o f u p t o 3 '4 . T h e sp e c i m e n s w e r e l o a d e d t oc o m p l e t e f a i l u r e in s i tu a n d c o m p l et e l o a d - d e f o r m a t i o nc u r v e s w e r e o b t a i n e d . T e n sp e c i m e n s w e r e t e s t e d o n t h eb a s i s o f w h i c h a s t r e n g t h f o r m u l a w a s p r o p o s e d a s li s te din Tab le 1.I t w a s a ls o s h o w n t h a t t h e m o d u l u s o f e la s t ic i ty o f t h ec o a l w a s i n d e p e n d e n t o f th e w i d t h - t o - h e ig h t r a t i o o f t h esp e c i m e n s a n d t h a t t h e p o s t - f a i l u r e m o d u l u s ( d e r iv e df r o m t h e s l o p e o f th e s t r e s s - s t r a i n c u r v e a f t e r f a i l u r e) d e-c r e a se d w i t h i n c r e a s i n g w i d t h - t o - h e i g h t r a t i o t o r e a c h ac o n s t a n t v a l u e o f 5 00 M P a ( 0- 5 G P a ) a t a w i d t h t o h e i g h tra t io o f 3 -5 .T h e l i t e r a t u r e p u b l i sh e d o n l a r g e s c a l e in s i tu testsg e n e r a l l y r e v e a l e d t h a t c o n s i d e r a b l e v a r i a t i o n s i n t h es t r e n g t h o f l ar g e r o c k s p e c i m e n s m a y b e e x p e c t e d f r o mo n e l o c a l i ty to a n o t h e r . F o r e x a m p l e , a l a r g e n u m b e r o fin s i tu t e s ts i n v a r i o u s c o a l - f i e ld s o f t h e U . S . S . R c o n -d u c t e d b y B i c h ( L a m a [ 7 ] ) h a s s h o w n t h a t t h e in s i tus t r e n g t h o f c o a l s e a m s v a r i e d f r o m 2 - 4 5 t o 1 7 . 6 5 M P a .T h e e f f e c t o f l a te r a l c o n s t r a i n t a p p l i e d t o t h e t e s t ed


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In s i tu Tests on Large Rock Specimens 105specimens at their top and bottom was considered ofoverriding influence by Jaeger and Cook [19] whendetermining the compressive strength of coal pillars.Recent investigations [20] show, however, that whilelateral constraint is an important factor, its effect will beovershadowed by the scatter of experimental results dueto geological structure of in situ specimens, even fromthe same locality.

CHOICE OF TEST METHODR e q u i r e me n t s

Large scale in situ tests aimed at determining strengthand deformat ion characteristics of mine pillars necessit-ate a technique for which certain requirements must befulfilled.Three of the more important ones are mentioned here:

(1) Determination of complete s t ress -s t ra in curves oftested specimens including their post-failure data. Thisensures that there is no doubt as to the actual failureload; in some cases it may not be obvious that the speci-men has, in fact failed. It has been found, for example,that attaining the peak of the stress-strain curve is notnecessarily accompanied by extensive cracking of thespecimen, thus the failure point might not be clearlydetected. A complete st ress- strain curve, however, showsthe peak clearly.(2) Determination of in situ data for h igh wid th - to -height ratios. This is important because actual mine pil-lars often feature high width-to-height ratios--in SouthAfrica between 2 and 5. This necessitates equipment ofhigh loading capacity for large scale tests.(3) Reasonable simulation of la teral constraint condi-t ions at the interfaces between pillar top and stope roofas well as between pillar bottom and stope floor. Thisensures that the lateral constraint conditions are takeninto account although, as was shown elsewhere [20], thisrequirement may not be so vital as was previouslythought. Nevertheless, it is advisable to aim at satisfyingthe lateral constraint conditions as closely as possible.Specimen prepara t ion

For coal specimens, a universal coal cutter is recom-mended for specimen preparation. Iron ore specimensmay be drilled out to a required size and shape. Blastingas a means of specimen preparation should be avoided.

In coal, specimens of the required dimensions areusually cut from a corner of a coal pillar. The faces ofthe pillar are first trimmed back to remove weathered,damaged and strained coal best to a distance of about1 m. Vertical cuts parallel to the prepared faces are thenmade to separate a column of coal from the remainderof the pillar. A horizonta l cut at the required heightshould be made last. This completes the formation of thespecimen which remains attached to the floor with fivesides exposed. This opera tion generally takes a full shift.After the specimen has been cut, it is cleaned and care-fully inspected and its dimensions are measured towithin 5 mm. At this stage the geological structure of thespecimen should be mapped.

Subsequently, in order to ensure full utilization of theloading capacity of hydraulic jacks, a reinforced con-crete block is cast on top of the specimen. The dimen-sions and properties of the block will be di fferent for dif-ferent specimen sizes and the rock tested. They can beselected on the basis of a finite element analysis of thestress distribution in an actual mine pillar and in an insitu specimen [18], the aim being to simulate the lateralconstraint conditions. In the case of square coal speci-mens 1"5 m in width, the block d imensions were 1'6 1"6 and 1 m in height. The concrete was of 40 MPa un-iaxial compressive strength (very strong concrete), rein-forced with three layers of steel bars 12 mm dia at100 mm centres near the top, in the middle and near thebottom of the concrete block. Such a block had a massof about 6500 kg and contributed a compressive stress ofabout 0.03 MPa in a specimen with a strength of17 MPa.

Once the concrete is cured, a space is left for the load-ing jacks, e.g. 400 mm in the case of the CSIR tests [18],and then the space above the jacks is filled with concreteup to the roof. This serves to avoid loading of jacksagainst the roof coal which may fail before the specimendoes. Filling up the gap with timber, however hard, isnot recommended as this decreases the overall stiffnessof the system and results in much of the stroke of thejacks being lost on compressing the timber.Load ing sys tem

Displacement-controlled loading, as opposed tostress-controlled loading, appears to be the best choicefor large scale in situ tests in compression because itenables determination of the complete stress-straincurves. The recommended loading system is that de-scribed by van Heerden [18]. Basically, this system con-sisted of 25 hydraulic jacks (more may be required forlarger specimens) each connected to a separate pump.The pump unit consisted of seven four-element Boschdiesel fuel-injection pumps (in effect 28 separate pumps)which incorporated a control for accurate setting of thedelivery of each pump. This hydraulic jacking systemwas designed to minimize losses in stiffness normallyfound in hydraulic systems. Connections between thejacks and the pumps were made with high pressure steeltubing (2 mm i.d., 6 mm o.d.).

Each jack had a loading capacity of 4.8 MN at an oilpressure o f 100 MPa. The mass of each jack was 180 kg,its stroke 200 mm and its loading area 300 300 mm.An individual pressure gauge for each jack was mountedin a panel fitted to the front of the pump unit.M e a s u r e m e n t o f d e f or ma t io n

To avoid difficulties encountered underground withprecision electronic measurement equipment, it isrecommended that all measurements be carried out withhydraulic displacement indicators. The indicators usedby the CSIR consisted of a master and a slave cylinderinter-connected with thin bore nylon tubing. The con-stant volume of oil between the master piston and the


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1 06 Z . T . B i e n i a w s k i a n d W . L . V a n H e e r d e ns l a v e p i s t o n w a s k e p t a t c o n s t a n t p r e s s u r e b y a n a i r a c -c u m u l a t o r p r e s s u r i z e d t o 0 . 7 M P a . T h i s a c c u m u l a t o ra c t e d a s a s p ri n g , k e e p i n g t h e m a s t e r p i s t o n i n f i r m c o n -t a c t w i t h t h e s u r f a c e s b e t w e e n w h i c h t h e d i s p l a c e m e n t sw e r e t o b e m e a su r e d . M o v e m e n t o f t h e s la v e p i s to n r o dw a s t r a n s m i t t e d t o a d ia l g a u g e m o u n t e d o n t h e s a m ep a n e l a s t h e p r e s s u r e g a u g e s o f i n d i v i d u a l j a c k s .

I n t h e a c t u a l t e s t s fo u r s u c h d i s p l a c e m e n t i n d i c a t o r sw e r e m o u n t e d b e t w e e n t h e b o t t o m o f t h e c o n c r e t e b l o ck ,r e s t in g o n t h e s p e c im e n , a n d t h e f lo o r o f t h e s e a m . I na d d i t i o n , h y d r a u l i c d i s p l a c e m e n t i n d i c a t o r s w e r e u s e dw h i c h w e r e m o u n t e d o n t h e j a c k s t o i n d i c a t e j a c k p i s t o nd i s p l a c e m e n t f r o m t h e s t a r t t o t h e e n d o f t h e t e s t . T h i ss e r v e d a s a c r o s s - c h e c k s i n ce t h e t o t a l p i s t o n d i s p l ac e -m e n t a t a n y s ta g e o f t h e t e s t c o n s i s t e d o f t h e d e f o r m a t i o no f th e s p e c i m e n p l u s t h e e l a s ti c d e f o r m a t i o n o f th e t w oc o n c r e t e b l o c k s a n d t h e e l a st i c in d e n t a t i o n o f t h e f l o o ra n d r o o f o f t he s e a m .

D e f o r m a t i o n a n d o i l p r e s su r e s r e a d in g s w e r e r e c o r d e dd u r i n g t h e t e st b y p h o t o g r a p h i n g t h e p a n e l w i t h a p o l a r -o i d c a m e r a , a t s u i t a b l e t i m e i n t e r v a ls d u r i n g t h e t e st .T e s t i n g p r o c e d u r e

L o a d i n g o f th e s p e c i m e n s s h o u l d b e s t a r t e d a t a s m a llp r e - l o a d ( a b o u t 1 3 k N ) f o r e a c h j a c k . T h i s i s d o n e t oe n s u r e t h a t a l l j a c k s a r e i n fi r m c o n t a c t w i t h t h e t w o c o n -c r e t e s l a b s . A t t h e s t a r t o f t h e t e s t t h e o i l p r e s s u r e s i n al l. ja c ks w e r e a d j u s t e d t o b e t w e e n 2 .0 a n d 3 -0 M P a a f t e rw h i c h a l l t h e d i a l g a u g e s , m e a s u r i n g d i s p l a c e m e n t , w e r es e t t o z e r o a n d a l l t h e b l e e d - o f f v a l v e s w e r e c l o s e d . T h es p e c i m e n w a s n o w l o a d e d b y p u m p i n g o i l a t t h e s a m es l ow c o n s t a n t r a t e i n t o e a c h ja c k . T h e o i l d e l i v e r y r a teo f a l l p u m p s w a s s e t s o t h a t t h e s p e c i m e n w a s d e f o r m e da b o u t 2 0 m m i n 2 h r . P o l a r o i d p h o t o g r a p h s w e r e t a k e na t a b o u t 5 r a in i n t e r v a l s d u r i n g t h e l o a d i n g o f t h e s p e ci -m e n u p t o f a i l u r e . A f t e r f a i l u r e , i n d i c a t e d b y a d r o p i nh y d r a u l i c p re s s u re , t h e t i m e i n t e r v a l b e t w e e n p h o t o -g r a p h s w a s a d a p t e d t o s u i t th e c i r c u m s t a n c e s . I n s o m eo f t h e s p e c i m e n s w i th l o w w i d t h - t o - h e i g h t r a t i o s w h i c hf a il e d r a t h e r q u i c k l y p h o t o g r a p h s w e r e t a k e n a t a r a teo f a b o u t 2 p h o t o g r a p h s p e r m i n u t e. D e f o r m a t i o n c o u l db e r e a d t o t h e n e a r e s t 0 . 1 m m a n d o i l p r e s s u r e s t o t h en e a r e s t 0 "5 M P a .

T h e s t r o n g e s t s p e c i m e n w i t h a w i d t h - t o - h e i g h t r a t i o o f3 .4 f ai l ed a t 2 5 .0 5 M P a b u t t h e e s t i m a t e d m a x i m u mc a p a c i t y w a s 5 0 M P a , e n o u g h t o b r e a k s p e c im e n s w i t hw i d t h - t o - h e i g h t r a t i o s o f 5 .M o d e q l i l h i l u r e a n d s t r e s s - s t r a i n c u r v e

F a i l u r e i s u s u a l l y a s s o c i a t e d w i t h g r a d u a l o p e n i n g o fv e r t i c a l c l e a t s a n d s p a l li n g f r o m t h e c o r n e r s o f t h e s p e c i -m e n . I n t h e C S I R t e s t s a l l th e s p e c i m e n s t e s t e d f a i l e d ina c o n t r o l l e d n o n - v i o l e n t m a n n e r l e a d i n g t o g r a d u a ls p e c i m e n d i s i n t e g r a t i o n a n d r e s u l ti n g i n a s y m m e t r i c a ld o u b l e p y r a m i d a t t h e c o n c l u s i o n o f th e t e st , as i l lu s -t r a t e d i n F i g . 2 . A t t h i s s t a g e t h e s p e c i m e n w a s c o m -p l e t e l y c r u s h e d a n d s m a l l p ie c e s c o u l d b e r e m o v e d f o r c i-b l y e v e n f r o m i t s c o r e . T h e t o t a l a x i a l d e f o r m a t i o n w a so f th e o r d e r o f 2 0 m m a n d t h e r e w a s a r e s i d u a l s t r e n g t ho f th e o r d e r o f I M P a .

F i g . 2, A t y p i ca l f a i le d c o a l s p e c i m e n a f t e r c o m p l e t i o n o f ' a n m ~itu t e s t

A n i n t e r e s t i n g o b s e r v a t i o n w a s t h a t , w h i l e a t m a x i -m u m l o a d t h e r e w a s u s u a l ly i n c r e a s e d " t al k in g ' o f th es p e c im e n s o m e t i m e s a c c o m p a n i e d b y h e a v y b u m p i n g ,t h is w a s n o t a l w a y s t h e c a s e a n d e x t e n s i v e s l a b b i n g o n l yo c c u r r e d w e ll a f te r t h e f a i l u r e o f t h e s p e c i m e n . M a x i -m u m s t r es s w a s m a i n l y i n d i c a t e d b y t h e d r o p i n t he h y d -r a u l i c p r e s s u r e w h i c h w a s b y n o m e a n s a s u d d e n p h e n o -m e n o n a l t h o u g h t h i s d e p e n d e d o n t h e r e l a t i v e s t i f f n e s so f t h e l o a d i n g s y s t e m a n d o f th e s p e c i m e n .

A p a r t i c u l a r a d v a n t a g e o f t h e d i s p l a c e m e n t c o n t r o l l e dl o a d i n g i s t h a t t h e r e i s n o t i l t i n g o f t h e s p e c i m e n a n d t h i sr e s u l t s i n a s t a b l e e x p e r i m e n t . U s i n g t h e t e c h n i q u e s d e -s c r i b e d a b o v e c o m p l e t e s t r e s s s t r a i n c u r v e s m a y e a s i l yb e o b t a i n e d f o r s p e c i m e n s w i t h w i d t h - t o - h e i g h t r a t i o s o fu p t o 5 a n d t h e l a t e ra l c o n s t r a i n t c o n d i t i o n s a r e r e a s o n -ab ly s a t i s f i ed .

A t y p i c al c o m p l e t e s t r e ss s t r a in c u r v e o b t a i n e d d u r -ing such t e s t s i s g iven in F ig . 3 .

20 - ' - '~ - MPo

~ E Wid th / h e iq h t ra t i o = 2 ,7 81 5 ~ / " ~ M o d u lu s o f e l a s t i c i t y = 3 , 6 2 0 G P o

I I I 1 I I I0 0 ,01 0 ,02 0 ,03 0 ,04 0 ,05 0 ,06 0 ,07A x i a l stro in

F ig . 3. C o m p l e t e s t r e s s - s t r a i n c u r v e i n u ni a x i a l c o m p r e s s i o n o b t a i n e din situ f o r a s q u a r e c o a l s p e c i m e n o f 1 .4 m w i d t h .


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In s i tu T e s t s o n L a r g e R o c k S p e c i m e n s 1 07P R E S E N T A T I O N O F S T R E N G T H

T E S T R E S U L T ST h e m a i n o b j e c t o f l a rg e s c a le in s i tu t e s ts i n c o m p r e s -

s i o n is t o e s t i m a t e s t r e n g t h b e h a v i o u r o f r o c k s t r u c t u r e ssu c h a s m i n e p i l la r s . F o r t h i s p u r p o se a n e m p i r i c a l f o r -m u l a w h i c h i n c l u d e s so m e c h a r a c t e r i s t i c s o f t h e sp e c i -m e n s i s u s u a l ly d e ri v e d f r o m t h e s t r e n g t h d a t a o b t a i n e df r o m l a r g e sp e c i m e n s .

O r i g i n a ll y , s u c h f o r m u l a e w e r e d e r i v e d f r o m t e s t s o ns m a l l l a b o r a t o r y s p e ci m e n s , u s u a l ly c oa l , a n d w e r e t h e ne x t r a p o l a t e d t o w a r d s l a r g e s ca l e d a t a . U n f o r t u n a t e l y ,o n l y a f ew i n v e s t i g a to r s p r e s e n t e d f o r m u l a e d e r i v e d f r o ml a r g e s c al e te s t s . T h e se a r e l i s t e d i n T a b l e 1 f r o m w h i c hi t w i l l b e n o t e d t h a t t w o t y p e s o f e x p r e s s i o n s w e r e p r o -p o se d t o d e sc r i b e l a r g e s c a l e t e s t d a t a , n a m e l y :

S = A + B ( w / h ) (1)w h e r e S i s t h e s t r e n g t h , w t h e w i d t h a n d h t h e h e i g h t o fa s q u a r e c r o s s - s ec t i o n s p ec i m e n , a n d A a n d B a r e c o n -s t a n t s m e a s u r e d i n t h e s a m e u n i t s as t h e s t r e n g t h S( u su a l l y M P a ) . ( A + B ) r e p r e se n t s t h e s t r e n g t h o f a c u b eo f th e m a t e r i a l t e s t e d .

S = k ( w " / h b ) (2 )(a spec ia l case i s S = k v w / h , that i s , for a = h = 0-5)w h e r e t h e p a r a m e t e r s S , w a n d h a r e t h e s a m e a s f o re x p r e s s i o n (I ), a a n d b a r e d i m e n s i o n l e s s c o n s t a n t s w h i l ek i s a c o n s t a n t h a v i n g t h e d i m e n s i o n I S ] x [ l e n g t h ] ~-b~,t h a t i s b e i n g m e a su r e d i n M P a m ~a-~) i n t h e S I s y s t e m o fu n i ts . O n l y i f a = b , t h e c o n s t a n t k h a s t h e m e a n i n g o ft h e s t r e n g t h o f a c u b e w i t h u n i t s i d e l e n g t h .

E x p r e s s i o n ( 2 ) c a n a l so b e w r i t t e n i n t h e f o r mS = k [ ( w / d ) " ] / [ ( h / d ) h]

wh ere d is t he s ide l eng th o f a cu be (w = h). For (w =" h) = d t h e e q u a t i o n r e d u c e s t o S = k , so t h a t k r e p r e -s e n t s t h e s t r e n g t h o f a c u b e w i t h u n i t s i d e le n g t h o f th em a t e r i a l t e s t e d .

I t w i l l n o w b e sh o w n t h a t e x p r e s s i o n ( 1) is p r e f e ra b l et o e x p r e s s i o n ( 2) .

T h e f i r s t s t r e n g t h f o r m u l a w a s p r e se n t e d i n 1 9 3 9 a n dw a s a sp e c i a l c a se o f e q u a t i o n ( 2 ) . G r e e n w a l d e t a l . [1 ]m e n t i o n t h a t t h i s f o r m w a s p r o p o s e d a s e a r l y a s 1 90 0 f o ra n t h r a c i t e a f t e r l a b o r a t o r y t e s ts m a d e f o r th e S c r a n t o nE n g i n e e r s C l u b . T h e i r s e c o n d f o r m u l a p r o p o s e d i n 1 9 41w a s o f t h e g e n e r a l f o r m o f e q u a t i o n (2 ) b u t i t o n l y f i t te dse v e n o u t o f 1 2 sp e c i m e n s t e s t e d . I n f a c t , G r e e n w a l d e ta l. g a v e t h r e e f o r m u l a e i n t h e i r p a p e r s a n d w h i l e t h e yh a d h o p e d t o c o n d u c t m o r e t e s t s t o c r o s s - c h e c k t h e i rr e su l t s t h i s w a s n o t d o n e .

M o s t o f t h e su b se q u e n t l a r g e s c al e in v e s t i g a t i o n s , c o n -d u c t e d f r o m a b o u t 1 9 65 , d i d n o t y i e ld a n y s t r e n g t h f o r -m u l a e , t h e o n l y e x c e p t i o n b e i n g t e s ts c o n d u c t e d i n S o u t hA f r i ca . T h u s t e s t r e su l ts b y N o se [ 3 ] , J a h n s [ 4 ] , G i m me t a l . [ 5 ] , d e R e e p e r [ 6 ] , L a m a [ 7 ] , R i c h t e r [ 1 1 ] , G e o r g i

* B a s e d o n h i s l a b o r a t o r y s t u d i e s , S k i n n e r p r e s e n t e d a f o r m u l a f o rf u ll s i ze a n h y d r i t e p i l l a r s a s f o l lo w s : S t r e n g t h ( M P a ) = 1 .4 + 0 . 9 (w /h ) ,t h e o r i g i n a l f o r m u l a b e i n g i n i m p e r i a l u n i t s a s S ( l b f /i n 2 ) = 2 01 +12 9 (w/h) .

e t a l . [ 1 2 ] , C h a o u i e t a l . [ 1 3 ] a n d P r a t t e t a l . [ 1 4 ] a r en o t i n c l u d e d i n T a b l e 1.

T h e n e x t l a r g e s c a l e t e s t f o r m u l a e a f t e r G r e e n w a l d e ta l. w e r e t h o se r e p o r t e d b y B i e n i a w sk i [ 9 , 1 0 ] . H e f o u n dt h a t t h e s t r e n g t h o f c u b i c a l c o a l sp e c i m e n s d e c r e a se sw i t h t h e i r i n c r e a s i n g s i d e l e n g t h a n d t h a t f r o m a c e r t a i ns iz e o n w a r d s , a b o u t 1 .5 m , t h e s t r e n g t h o f su c h sp e c i-m e n s r e m a i n s c o n s t a n t i n sp i t e o f i n c r e a s i n g s iz e . T h i si s i l l u s t r a t e d i n F i g . 1 w h i c h i n c l u d e s s i m i l a r f i n d i n g sw h i c h h a v e a l s o b e e n o b t a i n e d b y J a h n s a n d b y P r a t te t a l . T h e r e su l t s a r e n o t su r p r i s i n g s i n c e s im i l a r o b se r v a -t io n s a p p l y t o o t h e r m a t e r i a l s a s w el l. D e n k h a u s [ 2 1 ]p o i n t e d o u t t h a t t h e s t r e n g t h o f s te e l r e m a i n s c o n s t a n tf o r si ze s o f a b o u t 2 0 m m u p w a r d s w h i c h is w h y s t ru c -t u re s s u c h a s b r id g e s a n d s h i p s a re d e s i g n e d o n t h e b a s iso f s t r e n g t h t e s ts o n s t a n d a r d s te e l sp e c i m e n s o f m u c hsm a l l e r s iz e s. F i g u r e 1 m e a n s t h a t s t r e n g t h d a t a o b t a i n e do n sp e c i m e n s o f a t le a s t t h e c r i t ic a l s i ze a r e a l so a p p l i c -a b l e t o f u l l s iz e s t r u c t u r e s o f l a r g e r s c a le a n d t h a t t w or e l a t i o n sh i p s a r e n e e d e d t o d e sc r i b e t h e s t r e n g t h o f l a rg esp e c i m e n s ; b o t h a r e g i v e n i n T a b l e 1 . T h e f i r s t o n e i s o ft h e g e n e r a l f o r m o f e x p r e s s i o n ( 2) a n d i s v a l id , i n t h e c a seo f co a l , f o r w i d t h - t o - h e i g h t r a t i o s o f l e ss t h a n u n i t y a n dfor spec im en s i zes o f l e ss t ha n 1 '5 m in s ide l eng th . I tm e a n s t h a t , f o r c o n s t a n t h e i g h t , t h e s t r e n g t h o f a sp e ci -m e n i n c r e a se s w i t h i n c r e a s i n g w i d t h a n d f u r t h e r t h a t , f o rc o n s t a n t w i d t h , t h e s t r e n g t h o f a s p e c i m e n d e c r e as e sw i t h i n c r e a s i n g h e i g h t .

B i e n i a w s k i [1 0 ] a l s o p r o p o s e d a s e c o n d t y p e f o r m u l ao f th e f o r m S = A + B ( w / h ) a s o b t a i n e d f r o m t h e d a t ao b t a i n e d o n sq u a r e c o a l sp e c i m e n s 1"5 a n d 2 m i n s id el e n g t h a n d w i d t h - t o - h e i g h t r a t i o s o f b e t w e e n 1"0 a n d 3" I .T h i s l i n e a r e q u a t i o n ( 1 ) t h u s d e sc r i b e s t h e i n f l u e n c e o fw i d t h - t o - h e i g h t r a ti o s o n t h e s t r e n g t h o f s p e c i m e n s h a v -ing s izes l a rge r t h an th e ' c r i t i ca l s i ze ' o f 1 .5 m an d i sh e n c e a p p l i c a b l e t o f u l l s i z e p i l l a r s . T h i s f o r m o f l i n e a re q u a t i o n a l so f i t t e d t h e d a t a f r o m o t h e r c o l l i e r i e s a so b t a i n e d s u b s e q u e n t l y b y b o t h W a g n e r [ 1 7 ] a n d v a nH e e r d e n [ 1 8 ] . W a g n e r , h o w e v e r , p r e fe r r e d a sp e c i a l c a seo f e x p r e s s i o n ( 2 ) (s e e T a b l e 1 ) b u t , a s w i ll b e s h o w n l a te r ,a l i n e a r f i t t o h i s d a t a w o u l d h a v e b e e n m o r e a p p r o -pr i a t e .

I n o r d e r t o c h e c k t h a t t h e e x p r e s s i o n S = A + B ( w / h )i s t h e m o s t a p p r o p r i a t e s t r e n g t h f o r m u l a , a t t e n t i o nsh o u l d b e t u r n e d t o t h e r e su l t s o f sm a l l s c al e i n v e s t ig a -t i o n s . L a b o r a t o r y s t u d i e s o f t h i s a s p e c t d a t e b a c k t o1 90 0 a n d s o m e o f th e s e i n v e s t ig a t i o n s in c l u d e d w o r k b yB u n t i n g [ 2 2 ] , S t e a r t [ 2 3 ] , G a d d y [ 2 4 ] , S k i n n e r [ 2 5 ] ,H o l l a n d [ 2 6 ] a n d B i e n i aw s k i [ 2 7 ]. B u n t i n g f r o m 6 47 a n -t h r a c it e s p e c i m e n s a n d S k i n n e r f r o m 2 0 7 a n h y d r i t es p e c i m e n s r e p o r t e d t h a t a f o r m u l a o f t h e f o r m S - - A + B( w / h ) w a s m o s t s u i ta b le . * G a d d y f o u n d t h a t t h e s t r e n g t ho f c o a l i s in v e r se l y p r o p o r t i o n a l t o t h e e d g e d i m e n s i o no f a c u b e w i t h a s i d e D a s f o l l o w s : S = k D ' . S t e a r t f r o m. foul" tests(!) o n s q u a r e c o a l s p e c im e n s (2 3 0 m m i n w i d t h )a n d f r o m o b s e r v a t i o n s o f m i n e p i ll ar s , f o r m u l a t e d t h ef o l lo w i n g r e m a r k a b l e s p e c u l a t io n s f o r th e s t r e n g t h o fl a r g e c o a l p i l l a r s : ( l) t h e s t r e n g t h o f p i ll a r s w i t h c o n s t a n tw i d t h v a r i e s i n i n v e r se r a t i o t o h e i g h t ; ( 2 ) t h e s t r e n g t ho f p i l la r s o f c o n s t a n t h e i g h t v a r i es a s t h e s q u a r e r o o t o f


8/13

1 08 Z . T . B i e n i a w s k i a n d W . L . V a n H e e rd e n25O

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In S itu tests~'ts ~ , ~ . . . . . . -h:~"m-- SolomOn'S. . . . ' W - - - T - - I 2 " ~ ,2 1 ~ w " ~ , n ~ I I I ~o. . . , ~c ,~I 2 3 4 5 6 7 8 9_wW i d t h t o h e i g h t r a t i o h

F i g. 4. T h e r e l a t i o n s h i p b e t w e e n t h e s t r e n g t h a n d t h e w i d t h - t o - h e i g h tr a t io o f s q u a r e r o c k s p e c i m e n s - - p u b l i s h e d d a t a .

t h e ir w i d t h ; a n d (3) t he s t r e n g t h o f p i l la r s o f c u b e fo rmvar ies in inverse ra t io to the square roo t o f the i r d imen-sions.

In 1968 , B ien iawsk i [2 7 ] re por te d t es t s on 145 sand-s tone spe c imens wi th 125 125 mm cross -se c t ion andw i t h w i d t h - t o -h e i g h t r a t i o s o f 0 . 3 3-1 0 . H e fo u n d t h ere la t ionsh ip S = 10 + 10 ( w / h ) (M P a ) fo r w i d t h - t o -h e i g h t r a t i o s b e t w e e n 1 a n d 5 , F ro m t h e re o n , a r a p i di n c re a se i n th e s t r e n g t h w a s o b s e rv e d u p t o a r a t i o o f 1 0,a b o v e w h i c h t h e s a n d s t o n e s p e c im e n s c o u l d n o t b eb ro k e n e v e n a t 1 0 M N (1 0 0 0 t o n s ) l o a d .

S t u d i e s o n s a n d s t o n e b y C ru i se [2 8 ] , B a b c o c k [2 9 ]a n d b y S h e o re y a n d S i n g h [3 0 ] r e v e a l e d t r e n d s s i m i l a rt o t h o s e o b s e rv e d b y B i e n i aw s k i . Th e s e r e s u l t s a r e p l o t -ted in Fig . 4 . I t wil l be clear from this f igure that , s t r ict lys p e a k i n g , t h e r e l a t i o n s h i p b e t w e e n t h e s t r e n g t h a n dw i d t h - t o -h e i g h t r a t i o c o u l d b e s t b e d e s c r i b e d b y a p o l y -nomia l f i t , bu t fo r p rac t i ca l pu rposes and fo r the ra t io sb e t w e e n 1 -5 , w h i c h a r e m o s t c o m m o n l y fo u n d i n m i n e s ,t h is r e l a t io n s h i p m a y w e l l b e a p p ro x i m a t e d b y a s t r a i g h tl in e e q u a t i o n o f th e fo rm S = A + B ( w / h ) . F i g u re 4 , h o w -e v e r , a l s o d i s ti n c t ly r e v e a l s t h a t a n e q u a t i o n o f t h e t y p eS = k x / w / h i s n o t ju s t i f i e d b e c a u s e i t p r e d i c t s a t r e n d n o ts u b s t a n t i a t e d b y t h e e x p e r i m e n t a l r e s u l t s . H o w e v e r , i tm u s t b e a d m i t t e d t h a t i n th e r a n g e o f w i d t h - t o -h e i g h tr a t i o s o f I - 5 s u c h a n e q u a t i o n w i l l b e a c c e p t a b l e fo rp ra c t i c a l p u rp o s e s . Th i s i s w h y a n u m b e r o f i n v e s t ig a t o r sw e re m i s l e d i n t o u s i n g t h i s f o rm fo r w i d t h - t o -h e i g h tra t io s o f over 5 , the in f luence o f wh ich , in fac t , i s under-e s t i m a t e d b y t h e fo rm u l a .A s a fu r th e r p ro o f o f t h is a rg u m e n t , t h e a u t h o r s c o n -d u c t e d t h e i r o w n i n v e s t i g a t i o n s o n 4 2 s a n d s t o n e s p e c i -mens, 100 x 1 0 0 m m i n c ro s s - s e c t io n a n d o f v a r i o u she igh t s . The resu l t s a re g iven in F ig . 5, f rom wh ich i t wil lb e g le a n e d t h a t n o d o u b t e x i s ts a s t o t h e t r e n d o f th eresul ts .

Data f rom sandstone specimens~ 2 C ( , OO x lO O m m S qu are ~ /

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Width to height rat io W_hF i g . 5 . R e s u l t s o f c o n f i r m a t o r y t e s t s d e p i c t i n g t h e s t r e n g t h v s . w i d t h -

t o - h e i g h t r a t i o s o f s p e c i m e n s .

In F ig . 6 the resu l t s o f la rge sca le in s i tu t es t s con-d u c t e d i n S o u t h A f r i c a a r e p l o t t e d a n d i t i s o b v i o u s f ro mt h e g r a p h t h a t t h e r e s u lt s m a y b e r e p re s e n t e d w i t h c o n f i-d e n c e b y a l in e a r e q u a t i o n .

I t i s t h u s r e c o m m e n d e d t h a t t h e s t r e n g th o f la rg e s c a lein s i tu t es ts , fo r spec im en s izes showing no s ize e f fec t andfo r w i d t h - t o -h e i g h t r a t i o s o f u p t o 5 , b e r e p re s e n t e d b ye q u a t i o n (1) .

I t s h o u ld , h o w e v e r , b e a d d e d t h a t i n o rd e r t o m a k e t h ei n s i t u t es t resu l t s genera l ly app l ica b le ( i .e . no t on ly to thel o c a l i t y w h e re t h e a c t u a l t e s ts w e re c a r r i e d o u t ) , e q u a t i o n(1 ) shou ld be expre ssed in d imens ion less fo rm. In the caseo f t h e in s i tu t es t s conducted in Sou th Afr ica , th i s i sd e m o n s t r a t e d i n F i g . 7 f ro m w h i c h i t w i l l b e s e e n t h a tthe s t reng th resu l t s f rom d i f fe ren t co l l i e r i es fa l l c lose toa s t ra igh t l ine . The e qua t ion o f th i s line i s:

~ /a l = 0"64 + 0 .36 ( w / h )w h e re a i s t h e st r e n g t h o f t h e s p e c i m e n a n d a l i s t h es t reng th o f a cub ica l spec imen o f the s ize tes ted .

Th e a b o v e fo rm u l a h a s t h e a d v a n t a g e t h a t t h e i n fl u en c eo f t h e v a r i o u s p ro p e r t i e s o f d i ff e r e nt c o a l s t r a t a i s in c l u d e di n t h e v a l u e o f a l - Th i s f o rm u l a c a n t h u s b e u s e d fo r th edes ign o f fu ll si ze coa l p i l l a rs in S ou th Afr ica p r ov ide dt h a t ~ 1 i s d e t e rm i n e d f ro m t e st s o n c u b i c a l s p e c i m e n swi th a s ide l eng th equ a l o r g re a te r th an the "cr i t ica lsize" of 1"5 m.

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F i g . 6 . R e p r e s e n t a t i o n o f s t r e n g t h d a t a f r o m l a r g e s c a l e in s i tu t e s t s o nc o a l c o n d u c t e d i n S o u t h A f r ic a .


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w / h r a t i oFig. 7.E X P E C T E D D A T A

W h e n u n d e r t a k i n g l a rg e s c a l e in situ t e s ts i n c o m p re s -s i o n , i t i s u s e fu l t o k n o w wh a t t y p i c a l d a t a m a y b eexpec ted f rom the tes t s . These wi l l now be l i s ted .Expected s trenoth data

(1) A l l i n v e s ti g a t o r s [1 -1 8 ] a g re e t h a t t h e s t r e n g t h o fl a rge c u b i c a l s p e c i m e n s i s le s s t h a n t h a t o f c u b i c a l s p e c i -m e n s o f s m a l l e r s i de l e n g th s .

(2) Th ose inves t iga to rs w ho h ave tes ted a su ff ic ien t lyl a rg e n u m b e r o f c u b i c a l s p e c i m e n s o f v a r i o u s s i z e s h a v efo u n d t h a t t h e s t r e n g t h d e c re a s e s a s y m p t o t i c a l l y w i t hs ize un t i l a s ide l eng th i s ob ta ined a f te r wh ich there i sno longer a chan ge in the s t ren g th [10 , 14 ]. Th is ' c r i ti ca ls ize ' i s 1-5 m fo r coa l and 0"9 m fo r d io r i t e . By co m par i sonwi t h s m a l l s c a l e d a t a (5 0 m m c u b e s ) , th e s t r e n g t h o f t h e s ecr i t i ca l size spe c ime ns i s sma l le r b y a fac to r o f 7 fo r coa land 10 fo r d io r i t e .

(3) T h e s t r e n g t h o f l a rg e r e c t a n g u l a r s p e c i m e n s m a y b ee x p e c t e d t o b e t h e s a m e a s t h a t o f l a rg e s q u a re s p e c i-,mens wi th s ide leng th equ al to an e f fec tive wid th o f rec-t a n g u l a r s p e c i m e n s w,, = 4A/C where A i s c ross -sec-t i on a l a r e a a n d C i s c i r c u m fe re n c e (W a g n e r [1 7 ] ). B yc o m p a r i s o n , s m a l l s c a l e d a t a s u g g e s t e d [2 7 ] t h a t t h es t re n g t h o f r e c t an g u l a r s p e c i m e n s w a s g o v e r n e d b y t h ei rm i n i m u m w i d t h d i m e n s i o n , r a t h e r t h a n t h e s q u a re ro o to f t h e r e c t a n g u l a r s p e c i m e n c ro s s - s e c t i o n (Ho l l a n d[26]).

(4) De p e n d i n g o n i ts w i d t h - t o -h e i g h t r a t i o , a l a rges c a le s p e c i m e n c a n p ro v i d e c o n s i d e ra b l e r e s i s t a n c e e v e nwh e n i t s m a x i m u m l o a d b e a r i n g c a p a c i t y ( st r e n g th ) h a sb e e n e x c e e d e d .

t h e o rd e r o f 4 . 0 GP a . S m a l l s c a le d a t a (5 0 m m c u b e s )g iv e v a l u e s o f a b o u t 6 ,0 G P a .

(3 ) T h e d e fo rm a t i o n a t t h e m a x i m u m s t r e s s ( s t r e n g t hfa il u re ) i s n o t l a rg e , g e n e ra l l y o f t h e o rd e r o f 5 m m o n l y ,fo r a s p e c i m e n o f o ri g i n a l h e i g h t o f u p t o 2 m .(4 ) The s t ra in a t fa i lu re o f la rge spec imen s increasesa p p ro x i m a t e l y l i n e a r l y w i t h t h e w i d t h - t o -h e i g h t r a t i o . I tdoe s no t ex ceed E = 0"01 fo r coa l .( 5) S o m e a u t h o r s [1 5 , 1 7 ] d e f i n e d a m o d u l u s o f d e fo r-m a t i o n a s t h e q u o ti e n t o f t he m a x i m u m s t re s s a n d t hec o r r e s p o n d i n g s t ra i n . T h i s m o d u l u s is l o we r t h a n t h em o d u l u s o f e la s t ic i t y a n d d e c re a s e s s l i g h tl y w i t h i n c re as -i n g w i d t h - t o -h e i g h t r a t i o . A m o d u l u s o f d e fo rm a t i o n o f3 "0 G P a m a y b e e x p e c t e d fo r c o a l p i l la r s w i t h w/h = 3 .

(6 ) T h e p o s t - f a i l u r e m o d u l u s i s m a rk e d l y e f f e c t e d b yt h e w i d t h - t o -h e i g h t r a t i o . I t wa s fo u n d [1 8 ] t h a t t h ep o s t - f a il u r e m o d u l u s o f c o a l d e c re a s e d w i t h i n c re a si n gwi d t h - t o -h e i g h t r a t i o t e n d i n g t o a n a s y m p t o t i c v a l u e o f0 " 5 G P a a t w /h = 3 " 5 . T h i s p h e n o m e n o n wa s f i r s to b s e rv e d [3 1 ] o n s a n d s t o n e i n 1 9 6 9 .(7) T h e P o i s s o n ' s r a t i o o f la rg e c o a l s p e c i m e n s d o e sn o t d e p e n d u p o n t h e w i d t h - t o -h e i g h t r a t i o , t y p i c a lva lues fo r coa l be ing 0"25. Smal l sca le da ta (50 mm cubes )y i e ld a P o i s s o n ' s r a t i o o f a b o u t 0 " 35 .

(8) C l o s e r e l a t i o n s h ip e x i s t s b e t we e n l a t e r a l d e fo rm a -t ion o f a coa l p i l l a r and micro -se i smic ac t iv i t i es [17 ] .T h e re i s a r a p i d i n c re a s e i n t h e l a t e r a l d e fo rm a t i o n c l o s et o t h e p o i n t o f s p e ci m e n f a il u re .Expected mode o f a i lure

I t s h o u l d b e e x p e c t e d in l a rg e s c a le c o m p re s s i o n t e s tst h a t t h e re w i l l b e g r a d u a l s p e c i m e n d i s i n t e g ra t i o n o f th es p e c i m e n u n d e r i n c re a s i n g l o a d r a t h e r t h a n a s u d d e nc o l l ap s e . No t o n e s p e c i m e n o f t h e 6 6 la rg e s c a l e t e s tsc o n d u c t e d b y th e a u t h o r s f a i le d in a v i o l e n t m a n n e r . F a i l -u r e w i ll s ta r t a t t h e c o rn e r s o f t h e s p e c i m e n a n d p ro p o -g a t e t o wa rd s t h e c e n t r e . T h i s m a y b e e x p l a i n e d a s fo l -l o ws : U p t o t h e p o i n t o f s p e c i m e n f a i lu r e h i g h s t r e ss c o n -c e n t r a t i o n e x i s t s n e a r t h e c o rn e r s a n d l o w s t r e s s l e v e l si n t h e c e n tr e . T h e c i r c u m fe re n t i a l p o r t i o n s o f th e s p e c i-m e n wh i c h a r e i n i t i a l l y h i g h l y s t r e s s e d b u t u n d e r l o wc o n f i n e m e n t y i e l d f i rs t a n d t h e z o n e s o f h i g h s t re s sm i g ra t e t o wa rd s t h e c e n t r e . S i n c e t h e c e n t r a l p o r t i o n i sc o n s t r a i n e d b y t h e s u r ro u n d i n g m a t e r i a l, i t is c a p a b l e o fw i t h s t a n d i n g e x t r e m e l y h i gh s t r e ss e v e n wh e n t h e p i l la rh a s b e e n c o m p re s s e d b e y o n d i t s m a x i m u m re s i s t a n c e .

Exp ected deformation data(1 ) I t is rema rkab le tha t l a rge sca le t es t resu l t s

d e m o n s t r a t e i n m o s t i n s t a n c e s a n e l a s t i c b e h a v i o u r e v e ni n su c h m a t e r i a l s a s c o a l. O n l y a t s t re s s l e ve l s g r e a t e rt h a n a b o u t 7 0 p e r c e n t o f t h e s t r e n g t h d o t h e s t r e s s -s t r a in c u rv e s b e c o m e m a rk e d l y n o n - l i n ea r . W h i l e, wh e nl o a d c y c l i n g , h y s t e r e s i s i s g e n e ra l l y a p p a re n t i f i n t h e

e l a s t i c p o r t i o n o f t h e c u rv e t h e l o a d i s r e l ea s e d , t h ed e fo rm a t i o n o f t h e s p e c i m e n r e t u rn s t o a l m o s t z e ro .(2) T h e m o d u l u s o f e l a s t i c it y o f l a rg e s p e c i m e n s i s i n-

d e p e n d e n t o f g e o m e t r i c a l d i m e n s i o n s s u c h a s s iz e, h e i g h to r w i d t h - t o -h e i g h t r a t io . I t s a v e ra g e v a l u e fo r c o a l i s o f

A P P L I C A T I O N O F L A R G E S C A L E T E S T D A T AT O E N G I N E E R I N G D E S IG N

Large sca le in situ t es t s , l ike any o ther t es t s , a re on lyo f v a lu e i f t h e i n fo rm a t i o n d e r i v e d f ro m t h e m c a n b e d i -r e c t l y a p p l i e d t o e n g in e e r i n g d e s i gn . T o b e a b l e t o d ot hi s, o n e s h o u ld k n o w b r o a d l y w h a t r e q u i r e m e n t s m u s tb e ful fi ll e d b y t h e t e s ts a n d wh a t t y p i c a l d a t a a r e n e e d e dby the des igner .Requi remen t s

( I ) The num ber o f l a rge sca le t es t s m us t be su ff ic ien t lyl a rg e t o a l l o w m e a n i n g fu l c o n c l u s i o n s . A t l e a s t t wo


10/13

11.0 Z . T . B i e n i a w sk i a n d W . L . V a n H e e r d e ns p e c i m e n s o f a n y t y p e o r d i m e n s i o n s h o u l d b e t e st e d t oc r o s s - c h e c k t h e r e su l t s . P r e f e r a b l y a t e s t s e r i e s sh o u l dc o n s i s t o f a m i n i m u m o f l 0 s p e c im e n s .

( 2) S p e c i m e n s o f d i f f e r e n t w i d t h - t o - h e i g h t r a t io s , i n -c l u d i n g c u b e sh a p e , sh o u l d b e t e s t e d .

( 3) T h e m a x i m u m sp e c i m e n s iz e sh o u l d b e su f f i c ie n t l yl a r g e t o c l e a r ly e x h i b i t t h e a sy m p t o t i c s t r e n g t h , i . e . t h ec r i ti c a l s iz e f r o m w h i c h t h e s t r e n g t h r e m a i n s c o n s t a n tw i t h i n c r e a s i n g s i z e . M a x i m u m sp e c i m e n s i z e s sh o u l d b eo f th e o r d e r o f a t l e a s t 2 m i n m a t e r i a l su c h a s c o a l . F o rh a r d r o c k , t h e m a x i m u m s p e c i m e n s iz e m a y b e s m a l le r .

( 4) D e t a i l e d g e o l o g i c a l e x a m i n a t i o n o f e a c h t e s t s i t e ise ssen t i a l .

( 5) T e s t s o n sm a l l sp e c i m e n s , c o n d u c t e d u n d e r -g r o u n d , s h o u l d a l w a y s a c c o m p a n y l a rg e s c a le t e st s.

( 6) A c o m p a r i so n o f la r g e s c a le t e s t d a t a w i t h t h eb e h a v i o u r o f t h e f ul l s i ze s t r u c t u r e s t o b e o b t a i n e d f r o ml o c a l e x p e r i e n c e o r su r v e y s i s n e c e s sa ry .D e s ign da taT h e p r o b l e m o f e x t r a c t i n g d e s i g n d a t a f r o m l a rg e s c a let e s t s r eso l v e s i t s el f t o p r o v i n g w h i c h t e s t d a t a a r e d i r e c t l yappl i cab le to fu l l s i ze s t ruc tures , t ha t i s , which da ta a renot sub jec t t o s i ze e f fec t s .

T h e e x p e c t e d d a t a , d i s c u s se d p r e v i o u s l y , sh o u l d t h u sb e r e - e x a m i n e d w i t h t h i s p o i n t i n m i n d . I t i m m e d i a t e l yb e c o m e s o b v i o u s t h a t t h e f o l l o w i n g l a r g e s c a le t e s t d a t a ,i f a v a i l a b l e , m a y b e a p p l i e d d i r e c t l y i n t h e d e s i g n o f f u lls i ze s t ruc tures :

( 1) T h e d a t a i n d e p e n d e n t o f w i d t h - t o - h e i g h t r a ti o s , int h e c a se o f m i n e p i l l a r s : ( a ) m o d u l u s o f e l a s t i c i t y ; ( b )P o i s so n ' s r a t i o .( 2) T h e d a t a d e p e n d e n t o n t h e a s y m p t o t i c s t r e n g t h o fc u b i c al s p e c i m e n s h a v i n g b e e n r e a c h e d a n d d e p e n d e n to n w i d t h - t o - h e i g h t r a t i o s o f sp e c i m e n s o f s i z e s l a r g e rt h a n t h e c r i t i c a l s i z e a t t h e a sy m p t o t i c s t r e n g t h : ( c )m o d u l u s o f d e f o r m a t i o n . ( d) p o s t - f a il u r e m o d u l u s . ( e)a x i a l a n d l a t e r a l d e f o r m a t i o n d a t a n e a r a n d a t f a i l u re . ( f)c o m p l e t e s t r e s s - s t r a i n c u r v e s . ( g ) in s itu s t r e n g t h .

T h e f o l l o w i n g p r a c t i c a l b e n e f i t s a re d e r i v e d f r o m t h ea b o v e d a t a :

( 1) M o d u l u s o f e l a s t i c i t y a n d P o i s so n ' s r a t i o a r e i m -p o r t a n t i n th e d e si g n o f r o o m - a n d - p i l l a r m i n i n g o r l o n g -w a l l c o a l m i n i n g . S t re s s a n a l y s i s t e c h n i q u e s f o r su c h p u r -p o se s , i n c l u d i n g t h e f i n i t e e l e m e n t m e t h o d , w i l l r e q u i r et h e se d a t a .

( 2) M o d u l u s o f d e f o r m a t i o n a n d p o s t - f a i lu r e m o d u l u sa r e r e q u i r e d f o r t h e d e s i g n o f r o o m - a n d - p i l l a r l a y o u t s in -v o l v i n g y i el d in g p il l ar s [ 3 2 ] . M o d u l u s o f d e f o r m a t i o na l l o w s a n a s se s sm e n t o f t h e e n e r g y s t o r e d i n t h e m i n i n gsy s t e m a t t h e i n s t a n t o f p i l la r f a i l u re . T h e p o s t - f a i l u r em o d u l u s e n a b l es c o m p a r i s o n s o f t h e s t if f ne s s o f p i ll a rsw i t h t h e s t if f n es s o f t h e r o c k s t r a t a .

( 3 ) D e f o r m a t i o n d a t a a t f a i l u r e p r o v i d e i n f o r m a t i o na s t o t h e o r d e r o f m a g n i t u d e f o r p il la r c o m p r e s s i o n a tm a x i m u m s tr es s, w h i le t h e a m o u n t o f la t e r a l d e f o r m a -t i o n w i ll b e u se f u l i n p r o v i d i n g a n e a r l y w a r n i n g o f p o s s -ible pi l lar fa i lure .

( 4) C o m p l e t e s t r e s~ s t r a i n c u r v e s p r o v i d e a v e r y v a l u -a b l e i n s i g h t i n t o th e b e h a v i o u r o f m i n e p i l l a r s w h i c h i s

m o s t i m p o r t a n t t o d e s i g n e r s i l l u n d e r s t a n d i n g t h e p r o -ccss o f fa i lu re in these s t ruc tures .

(5 ) In s i tu s t rength da t a a re e ssen t i a l l i ) r t he des ign ofm i n e p i l l a r s a n d o b t a i n i n g t h e se d a t a i s o n e o f th e m a i nobjec t ive s o f l a rge sca l e in situ t e s ts i n c o m p r e s s i o n .

I t w i l l be seen f rom the above tha t t he bene f i t s t o bed e r i v e d f r o m l a r g e s c a le t e s t s a re c o n s i d e r a b l e . I t m u s t .h o w e v e r , b e r e a l i z e d t h a t t h e t e s t s h a v e o n e sh o r t -c o m i n g , n a m e l y , t h a t t h e y a r e a p p l i c a b l e m a i n l y t o t h el o c a l i t y a t w h i c h t h e y w e r e c o n d u c t e d . Y e t , a s p o i n t e do u t b e f o r e , c o n s i d e r a b l e v a r i a t i o n s m a y b e e x p e c t e df r o m t h e d a t a o b t a i n e d a t d i f f e r e n t c o l l ie r i es e v e n f r o mthe same coa l f i e ld .

F o r t h i s r e a so n i t i s e s s e n t i a l t h a t t h e r e q u i r e m e n t s ,l i s te d e a rl i e r a s i t e m s 4 - 6 u n d e r R e q u i r e m e n t s , a r e s a t i s-f i ed . T h i s m u s t b e d o n e f o r t h e f o l lo w i n g r e a so n s :

( i) G e o l o g i c a l e x a m i n a t i o n s a r e e s se n t i a l f o r c l a s si f ic a -t i o n o f th e r o c k m a ss a n d f o r i d e n t i fi c a t i o n o f a l l i ts i m -p o r t a n t f e a t u r e s w h i c h c a n h a v e a b e a r i n g o n t h e te s tre su l t s .( i i ) Smal l sca l e t e s t s a re e ssen t i a l t o de t e rmine d i f fe r -e n c e s i n r o c k m a t e r i a l p r o p e r t i e s a t v a r i o u s l o c a l i t i e sa n d t h u s a s se s s p o s s i b l e a p p l i c a b i l i t y o f la r g e s c a le t e s tre su l t s ou t s ide the ma in loca l i t y . Smal l sca l e t e s t s a l sose r v e t o u n d e r s t a n d b e t t e r t h e s c a l e e f f e c t i n t h e r o c kb e i n g t e s t e d a n d so m e t i m e s a l l o w d e r i v a t i o n o f p o s s i b l ei n d e x r e l a t i o n sh i p s w i t h l a r g e s ca l e t e s t d a t a . B e c a u se i ti s e c o n o m i c a l t o h a v e a l a r g e n u m b e r o f sm a l l t e s ts , t h e yc a n p r o v i d e u se f u l t r e n d s i n t h e m a t e r i a l b e h a v i o u r , p r o -v i d e a n u p p e r l im i t in t h e s t r e n g th a n d d e f o r m a t i o n d a t aand g ive the enginee r an exce l l en t " fee l ' o f h i s ma te r i a l .

( ii i) A c o m p a r i so n o f p r e d i c t i o n s b a se d o n l a r g e s c al et e s t d a t a w i t h t h e b e h a v i o u r o f t h e a c t u a l f u l l s iz e s t r u c -t u r e s i s n e c e s sa r y t o t e s t t h e v a l i d i ty o f su c h p r e d i c t io n s .

O n e a p p r o a c h f o r a c ro s s - c h ec k o n t h e b e h a v i o u r o ff u l l s i z e s t r u c t u r e s i s t o c o n d u c t a su r v e y o f f a i l e d a n ds t a b l e m i n e w o r k i n g s a n d d e r i v e a p i l l a r s t r e n g t h f o r -m u l a w h i c h w o u l d s e r v e t o e s t a b l i sh w h e t h e r l a r g e s c a l et e s t p red ic t ions a re rea l i s t i c .

I n F i g . 8 , t h e s t r e n g t h e q u a t i o n s d e r i v e d f r o m l a r g esc a le t e s t s in S o u t h A f r i c a a re p l o t t e d t o g e t h e r w i t h a p i l -l a r s t r e n g th f o r m u l a d e r i v e d b y S a l a m o n [ 3 3 ] f r o m asu r v e y o f a c t u a l c o a l p i l l a r s i n S o u t h A f r i c a i n v o l v i n g

g 2C:-: '7::j>

~ -e s~ S $ = 2 ' 5 + 2

2 3 4P i l la r w id fh to he igh t ra t io W_._h

F i g . 8 . C o m p a r i s o n o f l a r g e s c a l e te s t s f o r m u l a e w i t h t h e p i l la r f o r -m u l a f r o m a s u r v e y o f c o a l p i l l a r s i n S o u t h A f r i c a .


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I n s i t u Tests on Large Rock Specimens 111stable and collapsed cases. The Salamon formula is plot-ted for a range of width and height dimensions becausethe formula is of a 'Pittsburgh type' after Greenwald e ta l. [2] while the large scale test strength formulae in-volve directly width-to-height ratios rather than indivi-dual values of pillar widths and heights. It is interestingto note that a glance at Fig. 8 immediately reveals thatthe full size pillars formula could also have been repre-sented by a straight line equation similar to the largescale test formulae--for example, S = 4.5 + 3 ( w / h )would fit well.It will be seen from Fig. 8 that the full size pillars for-mula falls between the results obtained from large scaletests at the three collieries. This is understandable since,as was shown earlier in this paper, coal st rata from somecollieries will yield higher strength values than from theothers while the full size pillars formula by Salamonrepresents average colliery data.It may be concluded from Fig. 8 that large scale testdata make realistic strength predictions for full size coalpillars. The great value of large scale tests is that theycan make mining more economical by allowing a higherextraction at collieries where stronger coal strata wasfound during in s i tu tests by comparison with the aver-age data applicable to the whole country.To summarize, in s i tu tests in compression on largerock specimens do provide valuable information of di-rect use in the design of engineering structures such asmine pillars. It must be emphasized, however, that inorder to be able to have full confidence in the test resultsa number of requirements must be fulfilled and thosewhich were listed in this section must all be observed.

COST AND TIME CONSIDERATIONS. Having demonstrat ed the many benefits of large scalein s i tu tests, it is appropriate to discuss what costs areinvolved in such tests in terms of both money and time.Large scale in s i tu tests have always been considered asexpensive and time consuming but no detailed coststudies have as yet been published. The authors havekept full records of the costs involved in their tests andcan, therefore, present reliable figures.

It is believed that the costs of large scale tests mus t beconsidered in the light of the percentage turnover spentby an industry on a research programme involvinglarge scale tests and in the light of the financial gains de-rived from the tests.The research programme described in this paper in-volved a total expenditure of R213,000" in 10 years. For66 tests the average cost of one test was about R3250which is a high figure by most research standards. Fur-thermore, an average of R21,300 per year for 10 years isalso a handsome amount. Yet, if it is considered thatduring the same period of time the industry has earnedsome RI030 million from coal sales, then the costs oflarge scale tests emerge as being of little financial burdento the sponsor. In fact, the average production of the

* O n e r a n d ( R ) = U . S . $1 . 5.

smallest of the three collieries at which the large scaletests were conducted is at present 123,000 tons of coalper month which, at R2.50 per ton, is worth R308,000per month. The cost of the test programme at that col-liery was about R20,000 (R2000 per specimen) whichrepresented about 6-5 per cent of its I month productionvalue and could, in fact, be earned by tha t colliery in 1"5days.In addition, there are direct financial gains from largescale tests. Cont inuing with the example of the same col-liery, the large scale tests conducted there have shownthat the pillars in this colliery are about 50 per centstronger than as predicted by Salamon's formula for allpillar workings in South Africa. It was calculated thatthe mine could, for the same safety factor of 1"32 as usedat present, increase percentage extraction by 9-3 per cent.In economical terms this would mean a monthly in-crease of 11,457 tons of coal or an additional income ofabout R28,600 per month which would more than off-set the total cost of large scale tests at this colliery.This clearly demonstrates that large scale in s i tu testsare indeed economical and that the costs involved arevery modest indeed compared to the production incomein the mines concerned. In addition, this example onlyquoted the benefits related to the pillar strength deter-mination while, as was shown before, many other techni-cal data could also be expected.To summarize, the following distribution of costs,time and labour force were applicable to the authors'research programme:

Development costs: R50.000Capital costs: R50,000Running costs: R2000 per specimenTime factors: specimen prepara tion 8 weeks for10 specimens; testing of specimens5 weeks for10 specimens; duration of one test series--6months to report submission

Labour force: one research engineer, two techni-cians, five labourers.It should be noted that the research programme de-scribed in this paper, which was spread over 10 years, in-cluded 2'5 years of development and preliminary trials

and after 5 years of tests the research objectives werechanged which necessitated further development of tech-niques. Today , with all the techniques established and allequipment available the cost per specimen would beR2000 (U.S. $3000), depending, of course, upon the realvalue of money.CONCLUSIONS

(1) The best technique for large scale in s i tu tests incompression is one involving displacement, rather thanstress, controlled loading. F or full utilization of the load-ing capacity of hydraulic jacks, a reinforced concreteblock should be placed on the specimen top to simulatereasonably the natural end constraint in s i tu .(2) Although simulation of the interface constraintbetween pillar and roof and floor is important, the


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112 Z. T. Bien iawski and W. L. Van Heerdenmagni tude o f such cons traint i s not as cr i t ica l as wasprevious ly be l i eved . The in f luence of end cons traint mayeas i ly be overshadowed by the s cat ter of exper imentalresults due to geological s tructure of i n s i t u spec imenseven in the same local i ty.

(3) It sho uld be expec ted tha t due to structural differ-ences in rock strata, the s trength data from large scalei n s i t u tests may differ substantial ly from on e mine to theother.

(4) T he num ber of large scale tests m ust b e suffic ientto a l low for meaningfu l conc lus ions . At l eas t tw o spec i-mens of any type or d imens ion should be tes ted to cros s -check the results . General ly, one series of tests shouldcons i s t of a min imum of 10 spec imens .

(5) Specimens cubical in shape as wel l as of differentwidth-to-height ratios must be tested.

(6) The m axim um spe cime n s ize should be suffic ientlylarge to enable reaching the asy mp totic s trength, i .e . oneindependent of the spec imen s ize . M axim um spec imensizes of the order of 2 m should be expected in materialssuch as coal .

(7) Both strength and deformation data should bemo nitor ed and comp lete s tress s train curves should beobtained .

(8) The fo l lowing parameters should be determined:ma xim um strength, res idual s trength, m od ulus o f e lasti -c i ty , mod ulus of de format ion (max imum s tres s /s tra in atfai lure) , post-fai lure modulus and Poisson's ratio.

(9) Representation of the s trength results from largescale tests should be of the form:

Strength = A + B ( w / h )where A and B are cons tants and w / h i s the width-to-height ratio, for specim ens larger than the crit ical s ize atwhich the asymptot i c s trength was reached . This equat ionshould be expres sed in d im ens ion les s form.

(10) Tests on sm all specim ens, cond ucte d under-ground, should a lw ays accom pany large scale tes ts .

(1 I ) A cross-check o n the behaviour of the pr ototyp eis always necessary.

(12) Exc lud ing deve lopment cos ts and cos ts of theequipm ent, the running costs of large scale i n s i t u testsare about R 2000 per spec imen .

(13) One series of t0 large scale tests wi l l take about6 months to comple te (as suming a l l the equ ipment i savai lab le ) and wi l l involve a team cons i s t ing of anengineer, two technicians and a few unski l led labourers .

(14) Th ese seeming ly expensiv e tests wi l l be m orethan offset by f inancial returns and technical ga ins de-rived fro m an i n s i t u test programme; in any case, thesecosts are a very small percentage by comparison with thevalue of productio n from a mine. In fact, the total c ostso f o n e i n s i t u test series may be recovered in 2 produc-t ion days of

1975_ significance of in-situ tests on large rock specimens.pdf

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