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

Abstract

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). 1. INTRODUCTION The deformation modulus of the underlying rock mass is an important input variable for any load-deformation analysis of a foundation, such as a finite-element analysis for a dam. The deformation modulus of a rock mass is subject to the scale effects typically found in any rock mechanics analysis. An in-situ rock mass may have a modulus that is a fraction of the intact modulus of specimens tested in a laboratory due to the heterogeneous nature of the mass and presence of discontinuities (fractures, joints, faults, etc.) that serve as planes of weakness. Variability of these rock mass properties can result in wide ranging modulus values for discrete test intervals (in-situ and laboratory). In general, as the scale of the rock mass being analyzed is increased, the number of weaknesses increases and the rock mass will appear weaker and more deformable (Wyllie and Mah, 2004). Both deformation modulus and rock mass modulus terms imply load deformations in the elastic and inelastic ranges (Ulusay and Hudson, 2006). Developing a range of values for model inputs is important as a lower foundation modulus is not necessarily more conservative; the "softer" foundation may allow more transfer of dam load to the abutments and can also lead to increased damping during an analysis of seismic stability. Performing the load-deformation analysis with a variety of rock mass modulus inputs allows a reduction in uncertainty regarding a dam’s reaction under static or seismic loads. The sensitivity of any design to changes in the inputs is likely more critical than any discrete calculation of anticipated deformation or stability (Hoek and Diedrichs, 2006).