Figure 1. The Point Load Tester.

USING THE POINT LOAD TEST TO DETERMINE THE UNIAXIAL COMPRESSIVE STRENGTH OF COAL

MEASURE ROCK

John Rusnak, Director of Geology

Peabody Group

St. Louis, MO

Christopher Mark, Rock Mechanics Team Leader

National Institute for Occupational Safety and Health

Pittsburgh, PA

ABSTRACT

Point load testing is used to determine rock strength

indexes in geotechnical practice. The point load test apparatus

and procedure enables economical testing of core or lump rock

samples in either a field or laboratory setting. In order to

estimate uniaxial compressive strength, index-to-strength

conversion factors are used. These factors have been proposed

by various researchers and are dependent upon rock type. This

study involved the extensive load frame and point load testing

of coal measure rocks in six states. More than 10,000

individual test results, from 908 distinct rock units, were used

in the study. Rock lithologies were classified into general

categories and conversion factors were determined for each

category. This allows for intact rock strength data to be made

available through point load testing for numerical geotechnical

analysis and empirical rock mass classification systems such as

the Coal Mine Roof Rating (CMRR). 

INTRODUCTION

The point load test (PLT) is an accepted rock mechanics

testing procedure used for the calculation of a rock strength

index. This index can be used to estimate other rock strength

parameters. The focus of this paper is to present the data

analysis used to correlate the point load test index (Is50) with

the uniaxial compressive strength (UCS), and to propose

appropriate Is50 to UCS conversion factors for different coal

measure rocks. The rock strength determined by the PLT, like

the load frame strengths that they estimate, are an indication of

intact rock strength and not necessarily the strength of the rock

mass. 

THE UNIAXIAL COMPRESSIVE STRENGTH TEST

The UCS is undoubtedly the geotechnical property that is

most often quoted in rock engineering practice. It is widely

understood as a rough index which gives a first approximation

of the range of issues that are likely to be encountered in a

variety of engineering problems including roof support, pillar

design, and excavation technique (Hoek, 1977). For most coal

mine design problems, a reasonable approximation of the UCS

is sufficient. This is due in part to the high variability of UCS

measurements. Moreover, the tests are expensive, primarily

because of the need to carefully prepare the specimens to

ensure that their ends are perfectly parallel.

THE POINT LOAD TEST

The PLT is an attractive alternative to the UCS because

it can provide similar data at a lower cost. The PLT has been

used in geotechnical analysis for over thirty years (ISRM,

1985). The PLT involves the compressing of a rock sample

between conical steel platens until failure occurs. The

apparatus for this test consists of a rigid frame, two point load

platens, a hydraulically activated ram with pressure gauge and

a device for measuring the distance between the loading

points. The pressure gauge should be of the type in which the

failure pressure can be recorded. A state of the art point load

testing device with sophisticated pressure reading

instrumentation is shown in Figure 1.
The International Society of Rock Mechanics (ISRM,

1985) has established the basic procedures for testing and

calculation of the point load strength index. There are three

basic types of point load tests: axial, diametral, and block or

lump. The axial and diametral tests are conducted on rock core

samples. In the axial test, the core is loaded parallel to the

longitudinal axis of the core, and this test is most comparable

to a UCS test. 

The point load test allows the determination of the

uncorrected point load strength index (Is). It must be corrected

to the standard equivalent diameter (De) of 50 mm. If the core

being tested is "near" 50 mm in diameter (like NX core), the

correction is not necessary. The procedure for size correction

can be obtained graphically or mathematically as outlined by

the ISRM procedures. The value for the Is50 (in psi) is

determined by the following equation.

Is50 = P/De2 (1)

P = Failure Load in lbf (pressure x piston area).

De = Equivalent core diameter (in).

As Hoek (1977) pointed out, the mechanics of the PLT

actually causes the rock to fail in tension. The PLT’s accuracy

in predicting the UCS therefore depends on the ratio between

the UCS and the tensile strength. For most brittle rocks, the

ratio is approximately 10. For soft mudstones and claystones,

however, the ratio may be closer to 5. This implies that PLT

results might have to be interpreted differently for the weakest

rocks. 

Early studies (Bieniawski, 1975; Broch and Franklin,

1972) were conducted on hard, strong rocks, and found that

relationship between UCS and the point load strength could

be expressed as:

UCS = (K) Is50 = 24 Is50 (2)

Where K is the "conversion factor." Subsequent studies

found that K=24 was not as universal as had been hoped, and

that instead there appeared to be a broad range of conversion

factors. Table 1 summarizes published results obtained for

sedimentary rocks. Most of the estimates place the conversion

in a range between 16 and 24, with even lower values for

some shales and mudstones.

In studies comparing the PLT with the UCS, it is

generally assumed the UCS test is the standard. In reality,

however, UCS tests provide an estimate of the “true” UCS of

the rock. The accuracy of the estimate depends on the natural

scatter in the UCS test results (indicated by the standard

deviation (SD)) and the number of tests conducted (n). This

relationship is captured by the concept of the “Confidence

Interval” (CI). For normally distributed data, the 95% CI of

the mean is expressed as:

CI (3)

SD

n

95% = 196 .

Table 1. Published comparisons between the point load and uniaxial compressive strength tests for sedimentary rock.

Reference Rock Type Location Number of tests Conversion

Factor Comments

Das, 1995 Siltstone Western Canada,

bituminous coalfields NG1 14.7 lumps, fresh core, old core 

Sandstone/siltstone NG 18

Shale/mudstone NG 12.6

Vallejo et al, 1989 Sandstone Eastern KY, VA, WV 420 PLT, 21 UCS 17.4 Freshly blasted rock,

irregular lump samples

Shale surface coal mines 1,100 PLT, 55 UCS 12.6

Smith, 1997 Dredge material various harbors NG 8 UCS<1000 psi

Dredge material various harbors NG 15 UCS<3500 psi

sandstone/limestone unk NG 24 UCS>6000 psi

Broch and Franklin, 1972 Various UK (?) NG 23.7 11 rock types

Carter and Sneddon, 1977 Coal measure UK 1,000 PLT, 68 UCS 21-22 3 units tested

O'Rourke, 1988 Sedimentary Paradox Basin, US 66 30 samples from one borehole 

Hassani et al., 1980 Sedimentary UK 1,000 29

Singh and Singh, 1993 Quartzite India, copper pit 65 23.4

Read et al, 1980 Sedimentary rocks Melbourne, Australia NG 20 Reference in Choi and

Hong, 1998

Bieniawski, 1975 Sandstone South Africa 160 23.9

Rusnak, 1998 Coal measure Southern WV 386 20 Subset of current data

Jermy and Bell, 1991 Coal measure South Africa NG 14.1 Mainly sandstones

1

NG=Not given in reference
Figure 2. Location of the drillholes from which

the samples were obtained.

In general, the variability in the PLT-UCS relationship can

be attributed to three sources:

1. Inaccuracy in the estimate of the true UCS obtained from

UCS tests.

2. Inaccuracy in the estimate of the true PLT obtained from

PLT tests.

3. Real differences between the two tests.

Many of the studies summarized in Table 1 compared a

suite of point load tests to a single UCS test. With such an

experimental design, much of the scatter in the results might

actually be attributable to the inaccuracy of the UCS tests. 

PEABODY ROCK MECHANICS DATA

Peabody established an in house rock mechanics testing

facility in the fall of 1986. This facility is located in Freeburg,

IL. A full range of equipment was purchased to perform tests

including uniaxial compressive strength, indirect tensile

strength, point load index, triaxial compressive strength, flexural

strength, direct shear strength, long term creep, roof bolt

anchorage capacity, slake durability, ultrasonic velocity,

swelling strain and Atterberg limits. The diametral point load

(DPL) was not initiated until 1996, which resulted from an

interest in utilizing of the Coal Mine Roof Rating (CMRR).

ASTM and ISRM procedures are followed for all rock

mechanics testing.

This testing has been done primarily on core samples

obtained from exploration drilling to provide data for mine

planning and design. The Freeburg facility was eventually sold

to Standard Laboratories in 1991 who continue to operate it.

The majority of the data analyzed in this paper was from the

testing done at Freeburg. After 1997, all West Virginia testing

has been carried out at Commerical Testing and Engineering's

facility in Beckley, WV. Currently, the data base contains rock

mechanics test results from 914 drill holes from the states of

WV, IL, KY, IN, CO, and OH. Most of the core was NX-size

with some 75-mm (3-in) diameter. A map showing the

distribution of the drilling is shown below (figure 2).

THE UCS AND PLT DATA BASE – DESCRIPTIVE

STATISTICS

The methodology used in the selection of core samples for

testing was as follows:

1. Within each hole, rock units were identified where a

sequence of tests had been performed with an identical

geologic description and no major gaps in elevation.

2. Rock units with at least 3 UCS and 3 PLT results were

selected for inclusion in the data base.

3. The mean UCS, mean Is50, the standard deviations, and the

CI95 were calculated for each rock unit.

The samples were also categorized into rock type based on

the geologic description. These rock types correspond to the

Ferm Classification numbers. Ferm and Weisenfluh (1981)

developed a number system classification for coal measure rock

types using color photographs of rock core in an index guide.

It as been widely accepted as a means for consistent rock

identification and for use in computer data bases of drilling

information.

The categories of rock types and their Ferm classification

numbers used in this data analysis are as follows:

Rock Type Ferm Series

Shale and Claystone 100 - 200

Silty or Sandy Shale 300 - 400

Sandstone 500 - 700

Of the original 36,000 tests, more than 10,000 from 908

rock units were retained for the analysis. The distribution of the

units and representative values are reported in Table 2. The

median of the means is reported, rather than the mean of the

means, to reduce the influence of outliers. All the statistical

analysis was performed using SPSS version 10.

Figure 3 compares UCS determined for different rock types

in different regions. One striking observation is that the rocks

from the Midwest (IN, IL, and western KY) are significantly

weaker than their counterparts from WV (a fact confirmed by

ANOVA).

Figure 4a shows the range of standard deviations for the

UCS tests, expressed as a fraction of the mean UCS. The

standard deviations are greatest for limestones and shales, with

the median SD about 35% of the mean. The median standard

deviations for siltstones and sandstones are about 19% of the

mean.

Table 2 also reports the median Is50 and SD of Is50.

Expressed as a percent of the mean, the median standard

deviation for the Is50 ranges from 20% for sandstone to 35% for

shale. As figure 4b confirms, the variability of the PLT is

similar to that of the UCS test, as has been reported elsewhere

(ISRM, 1985). 
SHALE SILTSTONE

SANDSTONE LIMESTONE 0

25

50

75

100

Count

0.50 1.00 1.50 0

25

50

75

100

Count

0.50 1.00 1.50

Standard Deviation of Is50

(Compared with the mean)

Figure 4b. Histograms of standard deviations, 

Point Load Test.

Figure 4a. Histograms of standard deviations, 

Uniaxial Compressive Strength.

Table 2. Summary of the data used in this study.

Rock type Location N units N UCS

tests

Median

UCS

Median SD of

UCS N PLT tests Median

Is50

Median SD

of Is50

Shale

All states 289 1,541 529 1,664 1,719 269 94

Midwest 209 1,072 4,367 1,314 1,246 175 73

West Virginia 68 389 10,752 2,680 431 488 149

Siltstone

All states 296 1,557 6,286 1,166 1,518 296 77

Midwest 236 1,275 5,931 1,027 1,258 261 69

West Virginia 51 226 13,332 2,699 224 611 146

Sandstone

All states 228 1,591 10,931 2,096 1,459 446 87

Midwest 99 586 6,773 1,601 546 291 77

West Virginia 113 833 14,574 2,450 832 591 87

Limestone Midwest 95 450 18,752 6,614 407 730 241

All states All states 908 5,139 7,040 1,796 5,103 322 92

N - Number of tests.

UCS - Unconfined compressive strength (psi).

SD - Standard deviation (psi)

Is50 - Point Load Index.

The Is50 values obtained from this study are compared

with data reported by Molinda and Mark (1996) in table 3.

Molinda and Mark’s data was obtained from throughout the

US and includes a high percentage of tests from southern WV.

Table 3 reports means of the mean Is50 values to make the

results comparable. It can be seen that the two studies found

similar results. 

ROCK TYPE

SHALE SILTSTONE SANDSTONE LIMESTONE

UNIAXIAL COMPRESSIVE STRENGTH (UCS,psi)

35000

30000

25000

20000

15000

10000

5000

0

REGION

Midwest

W.Va.

Figure 3. Range of uniaxxial compressive strengths found in the

data base.

75 %th

25 %th

Median
Table 3. Comparison between PLT data from this

study and that presented by Molinda and Mark (1996).

Rock Type

Is(50) (psi)

Average,

this study

Midwest,

this study

WV, this

study

Molind

a &

Mark

Shale 268 193 510 420

Sandy

Shale

473 275 610 515

Sandstone 476 313 646 600

SUMMARY OF THE REGRESSION ANALYSIS

A linear regression between the mean Is50 and mean UCS

values determined for all 908 units yielded the following

equation:

UCS = 1970 +17.6 Is50 (4)

The correlation coefficient (r2

) obtained for equation 4 is

0.68, which is not bad for rock mechanics. The regression

equation simply uses the pairs of means, without regard to the

associated standard deviations. Therefore, the r2

 for equation

4 implies that the point load test explains two-thirds of the

variability in the UCS, with one-third attributable to the three

potential sources of scatter listed earlier. 

Equation 4 contains an intercept, however, unlike the

traditional form of the UCS-PLT relationship (equation 3).

Unfortunately, the r2 obtained from a zero-intercept regression

is not comparable to the standard r2

. Therefore a different

measure of the validity of the result must be used.

For this study, the validity measure was defined as the

percent of mean Is50 values that fall within the 95% CI of the

corresponding UCS. In addition, the percent of mean Is50

values falling 10 and 20 MPa (1,500 and 3,000 psi) from the

95% CI of the mean UCS were also calculated.

The zero-intercept regression equation obtained from the

entire data set is:

UCS = 21.0 Is50 (5)

With this equation, 50.4% of the predicted UCS values

fall within the 95% CI of the measured UCS. Only 8.5% of

the predicted values are more than 20 MPa away from the 95%

CI (figure 5). For equation 4, the regression equation with an

intercept, 49.0% of the predicted values were within the 95%

CI and 7.6% were more than 20 MPa away. It seems that the

zero-intercept equation is just as accurate as the standard

regression equation that includes an intercept.

DISTANCE F R O M 9 5% ci (p si)

10,00012,00 0

2,0004,0006,0008,000

0

-8,000-6,000-4 ,000-2,000

-1 0,000

ACCURACY O F REGRESSION EQUATIO NS

NUM BER OF UNITS

500

400

300

200

100

0

Figure 5. Histogram showing hte difference between UCS predicted by the PLT using equation 5, and the 95% confidence

interval of the measured UCS. The zero bar includes all cases that fell withing the 95% CI.
Table 4 shows the conversion factors (K) obtained from

zero-intercept regression analyses on various subsets of the

data. Nearly all the K values lie between 20 and 22, regardless

of rock type or geographic origin. Figures 6a-6e shows the

regression equation (equation 5) mapped on the different data

subsets.

Table 4. Conversion factors (K) obtained from

zero-intercept regression analysis.

Rock type Location K

Shale

All states 21.8

Midwest 22.4

West Virginia 20.2

Siltstone

All states 20.2

Midwest 19.6

West Virginia 20.8

Sandstone

All states 20.6

Midwest 20.2

West Virginia 20.4

Limestone Midwest 21.9

All states All states 21.0

Finally, the relationship between rock strength and K was

explored. There was a slight tendency for the UCS/Is50 ratio

(Ki

) to decrease for low UCS rocks. The median Ki for the

entire data set was 22.7, but that decreased to 16.9 for the

subset of rocks whose UCS was less than 20 MPa (3,000 psi).

Unfortunately, a similar trend was not evident in the PLT data,

so efforts to adjust K for low Is50 rocks did not improve overall

accuracy. 

DISCUSSION

Several factors are relevant in comparing this study with

previous ones. First, the present study involves the largest

number of tests, nearly 10 times as many as the next largest

study. It also includes a wide variety of rock types from

several mining regions, and it explicitly addresses the

variability associated with the UCS tests. One disadvantage of

the study was that the number of PLT tests averages about 5

per unit. Therefore it was not possible to follow the ISRM

(1985) suggested method for determining the mean value,

which involves deleting the two highest and two lowest Is50

values from a suite of at least 10 tests. Finally, it should be

noted that in these tests the average moisture content varied

from 0.79% for the shale to 0.49% for the sandstone. Vallejo

et al. (1989) found that K values were greater when the

samples were saturated.

ALL DATA

0

5000

10000

15000

20000

25000

30000

35000

0 200 400 600 800 1000 1200

POINT LOAD INDEX (Is (50), psi)

UNIAXIAL COMPRESSIV

E

STRENGTH (UCS,psi)

ROCK TYPE

UCS=21 PIV

SHALE

SILTSTONE

SANDSTONE

LIMESTONE

Figure 6a. Regression between uniaxial compressive strength and point load index. All rock types (a random selection of 40%

of the data points is shown).
SILTSTONE

5000

10000

15000

20000

25000

200 400 600 800 1000 1200

POINT LOAD INDEX (IS(50),psi)

UNIAXIAL COMPRESSIVE 

STRENGTH (UCS,PSI)

s

Figure 6c. Regression between uniaxial compressive strength and point load index. Siltstone.

SHALE

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000 1200

POINT LOAD INDEX (Is (50), psi)

UNIAXIAL COMPRESSIV

E

STRENGTH (UCS,psi)

Figure 6b. Regression between uniaxial compressive strength and point load index. SHale.
LIMESTONE

0

5000

10000

15000

20000

25000

30000

35000

0 500 1000 1500

POINT LOAD INDEX (Is (50), psi)

UNIAXIAL COMPRESSIVE 

STRENGTH (UCS,psi)

Figure 6e. Regression between uniaxial compressive strength and point load index. Limestone.

SANDSTONE

0

5000

10000

15000

20000

25000

30000

0 200 400 600 800 1000 1200 1400

POINT LOAD INDEX (Is (50), psi)

UNIAXIAL COMPRESSIVE 

STRENGTH (UCS,psi)

Figure 6d. Regression between uniaxial compressive strength and point load index. Sandstone.
he conversion factors (K) determined in this study fall

in the middle of the range of the studies reported in table 1.

They are somewhat greater than the K factors currently used in

the CMRR (Mark and Molinda, 1996). Those values, 12.5 for

shales and 17.4 for sandstones, were originally proposed by

Vallejo et al. (1989). Many of the samples that Vallejo tested

came from the same geologic formations included in this

study. However, Vallejo tested lump samples rather than the

core samples used here, and he compared the results from

single UCS tests with suites of PLT tests.

Like some other researchers, Vallejo et al. (1989)

concluded that K should be smaller for shales than for

sandstones. While the current study found some evidence that

K was less for the weakest rocks (not just shales), it was

difficult to identify those rocks from the PLT measurements

alone. Therefore, using different K values may not be justified

for axial PLT tests. However, diametral tests often give Is50

values that are much lower than those from axial tests.

Converting diametral test results using the same K value is

likely to be inaccurate. It is also unnecessary, because

diametral tests are used as an indirect measure of bedding

plane cohesion, not rock strength (Mark and Molinda, 1996).

It is probably more sensible to report Is50 from diametral tests

directly. 

CONCLUSION

The PLT is an efficient method to determine intact rock

strength properties from drill core samples. It has become an

accepted test in geotechnical evaluations. 

This study found that a conversion factor K=21 worked

well for a variety of rock types and geographic regions. The

variability of the PLT, as measured by the standard deviation,

was no greater than that of the UCS test. There is some

indication that K decreases for lower strength rocks, but the

tendency was not very pronounced. Geologic and engineering

judgment should be used when converting PLT results to UCS.

It must be remembered that both tests can only be used to

estimate intact rock strength and not rock mass strength.

The point load test provides for full utilization of data that

can be gained from exploration drilling programs. Intact rock

strength information can be acquired for use in geotechnical

evaluation and design work through numerical modeling and

rock mass classification systems. The cost of point load

testing is minimal when compared to the overall exploration

expense. Point load testing of roof and floor rock core of coal

seams that are to be mined by underground methods should be

standard procedure in any exploration program.

ACKNOWLEDGMENTS

The authors wish to thank Peabody Group for permission

to utilize the data and present this paper. The conclusions

contained here are solely those of the authors and not those of

Peabody Group. Also, the authors would like to thank Colin

Henkes, Geologist for Peabody Group, who oversees

exploration drilling and testing programs in West Virginia, and

Angela Abruzzino, statistical assistant at NIOSH in Pittsburgh,

who helped prepare the database. 

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