I have read the latest revision of the ASTM D7012 standard, which includes the test procedure for determining the unconfined uniaxial compressive strength of intact rock core specimens. There, I do not see any discussion on what makes the test valid or invalid based on the mode of failure.

From textbook, “Fundamentals of rock mechanics” 4th ed., by Jaeger, Cook, Zimmerman, the authors state that,

Under unconfined compression, a rock tends to deform elastically, until failure occurs abruptly. This failure is accompanied by somewhat irregular longitudinal splitting. With a moderate amount of confining pressure, longitudinal fracturing is suppressed, and failure occurs along a clearly defined plane of fracture. This plane is typically included at an angle less than 45 degrees from the axial direction (direction of compression). This plane is characterized by shearing displacement along its surface, and is referred to as a shear fracture.

As emphasized, there can be two modes of failure for when one conducts a compressive strength test: irregular longitudinal fracturing for unconfined testing, and shear fracturing for a "moderate amount" of confining pressure.

The above is relevant because when reading the textbook, "Core Analysis: A Best Practice Guide" by McPhee, Reed, and Zubizarreta, the authors seem to imply that shear failure should occur for a valid UCS test. The authors preface their section on unconfined compressive strength testing with the following two points:

  1. "Unless otherwise specified, UCS tests are normally performed on vertical (bedding plane perpendicular) samples..."
  2. "In some applications, the 'unconfined' tests are performed at nominal confining stress (150 or 300 psi). Often referred to as quasi-UCS (q-UCS) test, effectively this is a triaxial test but strength test data measured at nominal confinement are much less influenced by plug irregularities and discontinuities."

The quasi-UCS test sounds quite similar to the "moderate amount" of confining pressure test described in Jaeger et al. that produces shear fracturing.

So the ASTM D7102 standard does not address bedding plane orientation with respect to the rock core/plug, nor does it address the mode of failure and if there is a particular mode of failure that needs to occur for the unconfined compressive strength result to be valid.

My question surrounds these points of note. I am looking for an answer that addresses issues surrounding bedding orientation, mode of failure, and if the "quasi-UCS" test is still considered an unconfined compressive strength test.


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