Estimation of Lugeon Number at the Abutments of Bakhtyari Dam Site Using Seismic Tomography

In rock engineering, seismic tomography has been applied mainly for rock characterization of the proposed dam sites; detection of fractured, altered, and weathered zones in a rock; evaluation of grouting of a fractured rock and so on. Although, estimated velocity from seismic tomography indicates site rock mass quality, yet it is very desirable to transform velocity values to a criterion which quantitatively shows permeability state and understandable for geotechnical engineers. In studies of seepage from foundation of a dam, Lugeon number is as the most important parameter in determining critical extent of permeability and also a criterion in capability or necessity of rock mass quality groutability. The aim of this study was estimation of lugeon values at the abutments of Bakhtyari Dam site using velocity structures resulted from seismic traveltime tomography. In order to counteract the problem of nonuniqueness with cross-hole (or between gallery) data, 3-D tomographic inversion on Bakhtyari dam seismic data was performed. To extract 3D velocity structure, ray tracing and travel-time computations were carried out using Um and Thurber (1987) method, and traveltimes were inverted using SIRT method. In this study, lugeon values were estimated by the Barton (2006) formula. Extracted 3D images of P wave velocity were compared to the site geology. Detected low velocity anomalies at the site are attributed to the presence of major joints. The results of tomography and lugeon number shows that the trend of weak zones in some parts is approximately parallel to the bedding surface while they follow the main trend of major joints