Rock Intact Strength- Point Load Index (Is50).
Scope.
The point load strength test is intended as an index test
for the strength classification of rock materials. It may also be used to
predict other strength parameters with which it is correlated, such as
compressive strength.
Rock specimens in the form of either core(the diametral and
axial tests), cut blokes (the block test), or irregular lumps(the irregular lump
test) are broken by application of concentrated load through a pair of spherically
truncated, conical patents.
Apparatus.
The testing machine consists of a loading system(for the
portable version typically comprising a load frame, pump, ram, and platens), a
system for measuring the load P required to break the specimen, and a system
for measuring the distance D between the two platen contact points.
Procedure.
The diameter test.
Core specimens with length/diameter ration greater than 1.0 are suitable for diametral testing. There should preferably be at least 10 tests per sample, more if the sample is heterogenous or anisotropic. The specimen is inserted in the test machine and the platens closed to make contact along a core diameter, ensuring that the distance L between the contact points and the nearest free end is at least 0.5 times the core diameter. The distance (D) is recorded +/- 2%. The load. The load is steadily increased such that failure occurs within 10- 60 sec, and the failure load P is recorded. The test should be rejected as invalid if the fracture surface passes through only one loading point. The above procedure is repeated for the remaining specimens in the sample.
The axial test.
Core specimen with length/ diameter ratio of 0.3 – 1.0 are suitable for axial testing. Long pieces of core can be tested diametral to produce suitable lengths for subsequent axial testing(provided that they are not weak end by this initial testing), alternatively, suitable specimens can be obtained by saw-cutting or chisel-splitting. There should preferably be at least 10 tests per sample, more if the sample is heterogeneous or anisotropic.
The specimen is inserted in the test machine and the platens closed to make contact along a line perpendicular to the core end faces. The distance D between platen contact points is recorded +/- 2%. The specimen width W perpendicular to the loading direction is recorded +/- 5%. The load is steadily increased such that failure occurs with 10- 60 sec, and the failure load P is recorded. The test should be rejected as invalid if the failure surface passes through only one loading point. The above procedure is repeated for the remaining specimens in the sample.
Calculations.
Uncorrected point load strength ls is calculated as P/De2 ,
where De the equivalent core diameter is given by:
De2 = D2 for diameter tests
= 4A/ for axial, block, and lump test ; and
A = WD = minimum cross-sectional area of
a plane through the platen contact points.
Size correction.
Is varies as a function of D in the
diameter test, and as a function of De in axial , block, and irregular
lump test, so that a size correction must be applied to obtain a unique point
load strength value for the rock sample, and one that can be used for purposes
of rock strength classification.
The size-corrected point load strength
index (Is50) of a rock specimen or sample is defined as the
value of Is that would have been measured by a diametral test with
D= mm.
The most reliable method of obtaining (Is50),
preferred when a precise rock classification is essential, is to conduct
diametral tests at or close to D = mm. Size correction is then either
unnecessary (D MM) or introduces a
minimum of error.
Mean value calculation.
Mean value calculation of (Is50) as defend in
below are to be used when classifying samples with regard to their point load
strength. The mean value of (Is50) is to be calculated by deleting
the two highest and lowest values from the
or more valid tests and calculating the mean of the remaining values. If
significantly fewer specimens are tested, only the highest and lowest values
are to be deleted and the mean calculated from those remaining.