Geotechnical and Geological Engineering, 1993, 11,269-272

T E C H N I C A L N O T E

Correlation between point load index and compressive strength for quartzite rocks V.K. S I N G H 1 and D.P. SINGH 2 1Slope Stability Division, Central Minin 9 Research Statmn, Dhanbad, Bihar, India 2Rock Mechanics Divlsion, Department of Mining Engineering, Institute of Technology, Banaras Hindu University, UP, India

Received 18 August 1992

Summary

The relationship of the point load test with uniaxial compressive strength was examined using quartzite rocks to substantiate the existing correlations.

Keywords: Point load text; compressive strength; quartzite rocks.

Introduction

The point load test is widely used to determine the uniaxial compressive strength indirectly because of its ease of testing, simplicity of sample preparation and possible field application.

The work of D'Andrea et al. (1964), Broch and Franklin (1972), Bieniawski (1975), Hassani et al. (1980), Gunsallus and Kulhawy (1984), and Panek and Fannon (1992) is well acknowledged with regard to point load testing and has resulted in widely used correlations between point load index and other geotechnical parameters. However, more experimental work helps to substantiate the existing correlations.

NX size core samples of quartzites from an open pit for extracting copper in Rajasthan State were used for testing. The testing was done using a point load tester and a universal testing machine to establish a relationship between the point load index and uniaxial compressive strength.

Experimental work

The point load test was performed on NX size core samples of amphibole feldspathic quartzite (AFQ) and feldspathic quartzite (FQ). A T-500 model point load tester

0960-3182 �9 1993 Chapman & Hall

270 Singh and Singh

(Terrametrics Inc., USA), was used for testing. The cores were taken from 10 different locations of a copper pit. Six or seven samples were tested at each location.

The diametrical point load test was performed on NX size core samples because of the convenience in sample preparation (Bieniawski, 1975). The sample preparation and testing procedure were in accordance with ISRM (1985).

The specimens of both the rock types (AFQ and FQ) were also tested in a universal testing machine. These samples were taken from the same 10 locations from where point load tests were carried out. The sample preparation and testing procedure conformed to the requirements of ISRM (1981).

The mean and standard deviation of test results from point load and uniaxial compressive strength tests were computed for both the rock types (Table 1). The relationship between uniaxial compressive strength (ao) and point load index (Is) for both the quartzite rocks, is shown graphically in Fig. 1.

Table 1. Comparison of uniaxial compressive and point load test results

Point load index (MPa) Uniaxial compressive strength (MPa)

No. of No. of samples Standard samples Standard

Rock type tested Mean deviation tested Mean deviation

Amphibole feldspathic 60 3.32 0.008 30 77.52 0.161 quartzite

Feldspathic quartzite 70 3.28 0.010 35 76.61 0.151

0 . J

C

g

334

332

330

328

326

329

Amphibole Felspathic Quartzite ~ o

o ~ 8

o ~ Is o ~ (for NX(S4.mm) core)

Correlation coefficient=O.98 0 0

0 0

' c Quor*.zile IMPa = 102 t/m 2

I I I I l I I I I 7600 7620 76/*0 7660 7680 7700 7720 77/*0 7760 77~0 7800

Uniaxial compressive strength, 6" c ( t /m 2)

Fig. 1. Relationship between point load index and uniaxial compressive strength for the quartzite rocks

Point load index and compressive strength

Results

271

It is clearly evident from Fig. 1 that there is a straight line correlation between uniaxial compressive strength and point load index. Table 2 shows complete information regarding the correlation between ac and Is.

Table 2. Correlation between cr c and Is

Ratio between Correlation Rock type ao and Is coefficient

Amphibole feldspathic quartzite 23.36

Feldspathic quartzite .23.38

Amphibole feldspathic quartzite and feldspathic 23.37 quartzite together

0.802

0.800

0.980

Discussion

Based on the present study the ratio between a c and Is for the quartzites of the copper mine in Rajasthan is 23.37. This value relates well to the findings of Broch and Franklin (1972) and Bieniawski (1975) (Fig. 2). Broch and Franklin found this ratio to be 23.7~ rounded offto 24, by extrapolating the test results on 38 mm core diameter samples to 50 mm core. Bieniawski obtained a ratio of 23.5, rounded off to 24, for 54 mm core diameter samples.

~ 2s I

Z o

tlJ

7> 0 t J

z 2c

Z w I1;

6 i

W a Z

15 20

USE OF POINT-LOAD TEST ~ q R ~ IN THIS RANGE IS NOT RECOMMENDED 1~

J o

Franklin 1972

D'. Andrea et a l . , 1965 [] O"c: K] s

i , ~ i I J i I I I I I I [ I I 1 r 30 4.0 50

CORE DIAMETER (rnm) (After Bienlowskl,1975)

Fig. 2. Size correlation graph for point load index to uniaxial compressive strength conversion

J 6 0

272 Singh and Singh

References

Bieniawski, Z.T. (1975) Point load test in geotechnical practice, Engineering Geology, 9(1), 1-11. Broch, E. and Franklin, J.A. (1972) Point load strength test, Int. J. Rock Mech. Min. Sci., 9(6), 669-97. D'Andrea, D.V., Fisher, R.L. and Fogelson, D.E. (1964) Prediction of compression strength from other

rock properties, Colorado School of Mines Quarterly, 59(4B), 623-40. Gunsallus, K.L. and Kulhawy, F.H. (1984) Comparative evaluation of rock strength measures, Int. J.

Rock Mech. Min. Sci. Geomech. Abstr., 2(5), 233-48. Hassani, F.P., Scoble, M.J. and Whittaker, B.N. (1980) Application of point load index test to strength

determination of rock and proposals for new size correction chart, Proc. 21st Syrup. Rock Mech., Rolla, pp. 543-64.

ISRM (1981) Suggested method for determining the uniaxial compressive strength of rock materials, Rock Characterization, Testing and Monitoring (ISRM Suggested Methods), 113.

ISRM (1985) Suggested method for determining point load strength, Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., 22(2), 51-60.

Panek, L.A. and Fannon, T.A. (1992) Size and shape effects in point load tests of irregular rock fragments, Rock Mech. Rock Eng., 25, 109-40.