Uniaxial Compressive Strength (UCS) empirical equations

The compressive strength is probably the most widely used and quoted rock engineering parameter. Under uniaxial load conditions (confining stress zero) the maximum stress that a rock sample can sustain is referred to as uniaxial compressive strength (UCS). Because of the lack of core sample, many studies have been done to estimate UCS and rock properties from geophysical logs.

Note that none of the following UCS equations fit all the data, but each did a good job fitting the data reported for those authors. Therefore, you are more likely to get good results if you are using an equation that is appropriate for your lithology.

A number of different empirical equations are offered for defining the Uniaxial Compressional Strength for various lithologies. The applicable lithologies for each model are listed in parentheses after the model name. Models with GMI in their names either were developed internally, based on published data (for instance, GMI-Literature-Phi), or are from core data from specific areas (for instance, GMI-Browse).

References are provided for equations derived by other investigators. The units are as follows:

• UCS psi.
• Young’s modulus (E) psi
• P-wave modulus (M) psi
• Neutron Porosity (NPHI) volume ratio (fraction)
• Porosity (f) volume ratio (fraction)
• Clay volume (V_clay) volume ratio (fraction)
• Acoustic transit time (Dt) µs/ft
• P-wave velocity (V_P) km/s