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Rock strengths and rock failure criteria 91

3.1.3 Triaxial compression test and rock peak strengths
The triaxial compression test has proved to be the most useful test in the
study of the mechanical properties of rocks under a wide range of stress
values (Jeager, Cook et al., 2007). In a triaxial compression test, the major
principal stress (s 1 ) is applied along the axis of a cylindrical rock specimen,
and a minor principal stress (s 3 ,and s 2 ¼ s 3 ) is applied to the curved
surfaces of the specimen by fluid-confining pressure (confining stress).
Notice that the confining pressure and confining stress are interchangeably
used in the triaxial test. Fig. 3.4 shows the axial stresses, confining stresses,
and strains (both axial and lateral) in a typical triaxial compression test in
the Middle Bakken formation. The rock strength is the peak value of the
load (applied axial stress) of each curve of Fig. 3.4. It shows that the rock
strength increases as the confining pressure increases. Therefore, the rock
has a higher strength when it is subjected to three-dimensional
compressions. It should be noted that the UCS (i.e., when the
confining stress is zero) is the lowest strength, but it is an important
parameter used in rock failure criteria for geomechanical analyses. The
triaxial tests also show that Young’s modulus (the slope in each stress-axial
strain curve in Fig. 3.5) is not a constant but increases as the confining
pressure increases.

Figure 3.4 Middle Bakken stressestrain curve from triaxial compression test with
various confining stresses (P c )(Amendt et al., 2013, SPWLA).