A Comparison of Empirically Estimated Rock Mass Modulus Values at a Soft-Rock Site

Abstract

ABSTRACT The rock mass modulus, or deformation modulus, of an underlying rock mass is an important input variable for any load-deformation analysis of a foundation, such as numeric modeling of a dam. The deformation modulus can be determined empirically using a variety of different relationships that incorporate parameters readily determined from geologic logs and laboratory testing. Relying on one value can be fraught, though, as a low rock mass modulus can improve damping of seismic waves while a high rock mass modulus can increase static stability. Therefore, developing a realistic range of values is of paramount importance. In this paper, a probabilistic approach incorporating distributions for input parameters and ten empirical relationships was used to determine a range of anticipated foundation rock mass modulus values for a proposed spillway, thereby allowing for a range of model outcomes to be simulated. The analysis also found that the relationships to determine rock mass modulus that do not incorporate the elastic modulus as determined during laboratory testing all produced significantly higher rock mass modulus values than those that incorporated the laboratory data; relationships using only RQD and/or RMR appear to break down where the rock mass is relatively fracture free, but the intact rock is soft, as is the case for this site. INTRODUCTION Accurately modeling the response of a structure to static or seismic loads is of paramount importance when evaluating critical infrastructure such as dams. Key to the development of accurate models is developing and implementing realistic ranges of input parameters that facilitate the portrayal of a range of possible outcomes. One of the parameters needed in the modeling efforts is the deformation modulus or rock mass modulus. The terms deformation modulus and rock mass modulus are interchangeable, and both imply load-deformations of the rock mass in the elastic and inelastic ranges (Ulusay and Hudson, 2006).