Rock mass characterization is essential for numerous applications in rock engineering practice such as underground rock excavation design, tunnel design, rock slope design, design of foundations on rock etc. It is therefore necessary to obtain design input parameters such as in-situ deformation modulus (Em) and strength parameters for numerical modeling. These parameters can be obtained from in-situ test such as pressure meter test. However during the preliminary design stage, the detailed soil investigation is often not carried out, leading to unavailable or insufficient in-situ test data. Hence it becomes crucial to evaluate rock mass modulus by indirect means using correlations based on rock mass classification systems.
Over the years, many classification systems, such as Rock Quality Designation (RQD), Modified Rock Mass Rating (RMR), Q-system, Geological Strength Index (GSI) systems, Rock mass index (RMi) etc. have been developed. The deformation modulus of rock mass can be estimated using these classification systems with different empirical correlations proposed by many authors. Other such correlations are based on laboratory test parameters such as elastic modulus of intact rock core or uniaxial compressive strength (UCS).
The comparative study of rock mass deformation modulus evaluated based on in-situ testing and selected correlations is presented in this paper. The in-situ testing consisted of pressuremeter test conducted on Basalt, Tuff and Breccia formations below the seabed. The laboratory testing was carried out on the rock samples drilled and retrieved from the identical boreholes and depths at which pressuremeter tests have been carried out. The comparison demonstrates overestimation of Em using empirical correlations and necessitate more rigorous technique for estimation of Em, which can estimate Em values nearer to in-situ test results.
