Predicting fracture properties in weathered granite in denied areas
This chapter describes a model developed to predict fracture properties of weathered granite in denied areas, predictions made using imagery, and a test undertaken to determine prediction accuracy. This project is part of a geotechnical characterization of weathered granites that will be used to predict weapons effects at denied sites. Joint spacings, orientations, and trace lengths were measured in four field areas near the denied sites. Three-dimensional models of field measurements from an analogue area and lineation data derived from imagery over this area and over two denied sites were sampled to determine mean joint spacings in simulated horizontal and vertical boreholes, mean joint trace length, joint intensity, termination percent, mean rock block width and volume, and RQD (Rock Quality Designation) at the denied sites. The simulated data were regressed against scale in log-log space and the equations for the best fit were used to calculate the predictions.
The predictions made for the denied sites are realistic compared to measurements made in the four field areas. They are within the standard deviations of field measurements and generally fall within the range of measurements made in the four field areas. Most predictions are within 25% of field measurements in the analogue area. The accuracy of the predictions was assessed by selecting one of the field areas as a denied area, and then using the same procedures to make predictions for the same set of variables. These predictions are very similar to the field measurements; most lie within 10–15%, indicating that predictions made using these procedures are accurate as well as realistic.