Page 96 - Applied Petroleum Geomechanics

88 Applied Petroleum Geomechanics

Figure 3.2 Schematic Brazilian test and the specimen.
Brazilian method often overestimates the tensile strength of the poorly
consolidated rock because the line load applied during the test typically
widens for a soft material (Peng and Zhang, 2007). In such a case, a line
load is no longer applied and the force is distributed over a larger area.
Consequently, a higher tensile strength is calculated.

3.1.2 Uniaxial compression test
Uniaxial compression test (Fig. 3.1A) is used to determine uniaxial
compressive strength (UCS), Poisson’s ratio, and Young’s modulus.
Uniaxial compression test is also called unconfined compression test. In this
test, a circular cylinder of rock sample is compressed parallel to its longi-
tudinal axis. It is the oldest and simplest test and continues to be one of the
most convenient and useful ways for determining the properties of rocks.
During the test, the loads in the axial direction, displacements (normally
measured by linear variable displacement transformers), and strains (can be
measured by extensometers) in both axial and radial directions are recorded
for rock strength analyses.
In the uniaxial test, the following equation is used for calculating the
UCS for the ISRM suggested samples.

P max
UCS ¼ (3.2)
A