2G4-3: POINT LOAD TEST

INTRODUCTION
Point load test is carried out on core rock specimens or irregular rock fragments to obtain the
point load strength index (Is(50)) and unconfined compressive strength. This test does not
require costly specimen preparation and is a quick simple test. The failure load P and the
distance between platens D are measured to obtain the uncorrected point load strength P/D2.
A correction is applied to account for the specimen size and shape, and the unconfined
compressive strength is obtained from a correlation equation. Depending on the specimen
geometry, three types of tests can be performed: diametral, axial, and irregular lump Figure 1.
Testing procedures as suggested by ISRM (2007).

Figure 1: Specimen shape requirements for (a) the diametral test, (b) the axial test, (c) the
block test, and (d) the irregular lump test (ISRM).

OBJECTIVE
To determine the compressive strength of rock specimens from irregular lump test.

THEORY

When first introduced, the point load strength test was used mainly to predict uniaxial
compressive strength which was then the established test for general-purpose rock strength
classification. Point Load strength now often replaces the uniaxial compressive strength in

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this role since when properly conducted it is as reliable and much quicker to measure. Is(50))
should be used directly for rock classification, since correlations with uniaxial compressive
strength are only approximation. On average, uniaxial compressive strength is 20 – 25 times
point load test. However, test on many different rock types the ratio can vary between 15 and
50 especially for anisotropic rocks, so that errors of up to 100% are possible in using an
arbitrary ration value to predict compressive strength from point load strength. The point load
strength test is a form of indirect tensile test, but it is largely irrelevant to its primary role in
rock classification and strength characterization. Is(50)) is approximately 0.80 times the
uniaxial tensile or Brazilian tensile strength.

PROCEDURE

1. Based on the figure 1d, rock blocks or lumps of size 50 ± 35mm of the shape are
suitable for the irregular lump test. The ratio D/W was between 0.3 and 1.0.
2. At least 10 tests per sample being conducted where we needed more if the sample is
heterogeneous or anisotropic.
3. The specimen was inserted in the test machine and the platens closed to make contact
with the smallest dimension of the lump or block, away from the edges and corner.
4. The distance D between the platen contact points is recorded ±2%. The smallest
specimen width W perpendicular to the loading direction is recorded ±5%. If the sides
are not parallel, then L is calculated as (W + L)/2 as shown in Figure 1d.
5. The load was steadily increased such that failure occurs within 10 – 60 sec, and the
failure load P was being recorded. The test was rejected as invalid if the fracture
surface passes through only one loading point (figure 2).

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 Figure 2: Typical modes of failure for valid and invalid test, ISRM (2007).

Part 3 Point Load Test

Sampl Rock Type & W. W1(mm) W2(mm Wave(mm D Load
e Grade ) ) (mm) (kN)

1 Sandstone – Grade 3 39.12 62.12 50.62 26 0.825

2 Sandstone – Grade 2 48.18 80.20 64.19 36 1.003
3 Sandstone – Grade 6 37.42 81.14 59.28 27 0.055
4 Sandstone – Grade 3 40.12 62.42 51.27 30 0.855
5 Sandstone – Grade 4 45.48 66.18 55.83 20 0.480
6 Sandstone – Grade 4 62.58 87.02 74.80 26 0.505
7 Sandstone – Grade 2 35.52 64.72 50.12 27 0.870
8 Sandstone – Grade 6 46.32 55.00 50.66 30 0.200
9 Sandstone – Grade 2 64.02 101.68 82.85 30 1.520
10 Sandstone – Grade 2 42.38 95.86 69.09 32 1.120
11 Sandstone – Grade 6 39.32 72.46 55.89 18 0.045
12 Sandstone – Grade 4 58.60 73.68 66.14 44 0.430
13 Sandstone – Grade 3 57.88 88.78 73.33 49 0.725
14 Sandstone – Grade 5 39.64 54.16 46.90 33 0.325
15 Sandstone – Grade 1 49.12 60.78 54.95 24 0.865
16 Sandstone – Grade 6 49.98 103.46 76.72 44 0.025
17 Sandstone – Grade 2 51.72 86.50 69.11 27 1.155
18 Sandstone – Grade 3 47.54 82.42 64.98 22 0.640
19 Sandstone – Grade 3 46.16 68.78 57.47 30 0.765
20 Sandstone – Grade 2 40.70 49.42 45.06 38 1.385

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 Sampl A(mm2) De2(mm2) Is(Nmm-2) F IS(50)
e =(De/50)0.45 Is x F
1 1316.12 1675.74 0.4923 0.9139 0.4499
2 2310.84 2942.25 0.3409 1.0373 0.3536
3 1600.56 2037.90 0.0270 0.9551 0.0258
4 1538.1 1958.37 0.4366 0.9465 0.4132
5 1116.60 1421.70 0.3376 0.8807 0.2973
6 1944.80 2476.20 0.2039 0.9979 0.2035
7 1353.24 1723.00 0.5049 0.9197 0.4644
8 1519.80 1935.07 0.1034 0.9440 0.0976
9 2485.50 3164.64 0.4803 1.0545 0.5065
10 2210.88 2814.98 0.3979 1.0271 0.4087
11 1006.02 1280.90 0.0351 0.8603 0.0302
12 2910.16 3705.33 0.1160 1.0926 0.1267
13 3593.17 4574.97 0.1585 1.1456 0.1816
14 1547.70 1970.59 0.1649 0.9479 0.1563
15 1318.80 1679.15 0.5151 0.9143 0.4710
16 3375.68 4298.05 0.0058 1.1297 0.0065
17 1865.97 2375.83 0.4861 0.9886 0.4806
18 1429.56 1820.17 0.3516 0.9311 0.3274
19 1724.10 2195.19 0.3485 0.9712 0.3385
20 1712.28 2180.14 0.6353 0.9697 0.6161
∑ IS(50)=5.9554

Discussion:

1) From the experiment, the point load strength obtained after been corrected is 5.9554.
It is approximately 0.80 times the uniaxial tensile or Brazillian tensile strength.
2) Therefore the average point load strength for sandstone is 5.9554 and approximately
4.7643 of uniaxial tensile strength.

Comments:

1) The sandstone used during testing the point load strength is chosen randomly and
some of it a part of the uses rock that had been used from the previous experiment.
Therefore, to get an accurate result, the rock must be carefully chosen from the
sample given.
2) The rock that had been chosen must have slightly flat surface so that it can give more
accurate result during the experiment.

Conclusion:

The compressive strength of rock specimens is determined from the irregular lump test that is
5.9554.

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