Uniaxial Compressive test (UCS) results are essential in evaluation the values of
Poisson’s ratio. However, according to the suggestion of the International Society
for Rock Mechanics, Poisson’s ratio can be determined using three alternative methods:
the secant, average and tangent. Applying these methods causes discrepancies in the
results; according to our experiences, the differences can be threefold or more. This
paper aims to study the process of changes of Poisson’s ratio for intact rock during
loading from micro-crack initiation to failure stage. The objective of this theoretical
investigation is to establish a straightforward mathematical formulation between σ/σ
c and intact rock’s Poisson’s ratio value. To outline these changes forty two granite
rocks were investigated from Bátaapáti radioactive waste repository (Hungary) and
the calculation was performed by using the new formula from the beginning of loading
till failure stage at UCS test. In the laboratory test program, Poisson’s ratio derived
from standard tests varies with momentary stress; it steadily increases as stress
rises until reaching the stress level causing unstable crack propagation. Additionally,
the Poisson’s rate follows a linear increase with stress, up to the point of unstable
crack propagation stress. The research demonstrated that the proposed equations provide
competent values for the root mean squared error value (ranging from 0 to 0.04), the
mean absolute percentage error (ranging from 0.6% to 18%) and the mean absolute error
(ranging from 0 to 0.04). Contrary to previous ideas, our results suggest that the
Poisson’s ratio is not a constant for rigid rocks.
