The
load measuring system should indicate failure load to an accuracy of
+
2%. It should incorporate a maximum-indicating device, so that the
reading is retained and can be recorded after the specimen failure.
Theory:-
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 .
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. Is
50
= 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
95% =
1.96SDn
Test
specimen preparation:-
Rock
samples are grouped on both of the rock type and estimated strength.
At least 10 specimens are selected for testing each sample if core
samples are used.
Specimens
in the form of core are preferred for accurate classification.
Acceptable minimum and maximum core sizes are AX and HX,
respectively.
Procedure:-
Diameter
test:-
The
core sample with a length to diameter ratio greater than 1.4 is
suitable for diametric testing.The inclination of bedding, foliation
or other plane of weakness, if present, is recorded with respect to
the line of loading.The diameter “D” of the specimen is measured
to the nearest 0.005 inches by averaging two diameters measured at
right angles to each other at about the upper height, mid-height and
lower height of the specimen. The diameter D is then the average of
the three diameters obtained at the upper height, mid-height and
lower height of the specimen.The specimen is inserted in the test
machine and the platens advanced to make contact along a core
diameter, entering the distance, L, between the contact point and the
nearest free end is at least 0.7 D .
The
load is increased to failure and the failure load P is recorded. The
fragments are retained for water content determination which is
performed after all specimens of the sample are tested for point-load
strength.
