A Comparison Between Field-Measured and Empirically Estimated Rock Mass Modulus Values

Abstract

Rock mass modulus, also referred to as the deformation modulus, is an important input variable for any load-deformation analysis of a foundation, such as a finite-element analysis for a dam. As the representative volume of rock is increased the rock mass will appear weaker and more deformable due to the inclusion of more discontinuities. The rock mass modulus can be measured directly downhole with a variety of devices, such as a uniaxial or radial jacking rig, a flat jack, or more commonly, a dilatometer. Modulus values can be estimated indirectly using site-specific empirical relationships.This paper provides a comparison of field- and laboratory-derived rock mass modulus values in an effort to develop a range of likely parameters and evaluate data quality/confidence levels. The in-situ values were compared to values developed from eight commonly used empirical relationships. Results indicate that the dilatometer-measured and empirically estimated values are similar where the rock is massive and relatively intact but vary significantly where the rock is fractured or weathered. These significant variations appear to be related to how the rock deforms in unconfined/semi-confined conditions (i.e., failure occurs into open space).