Fracture characteristics in weathered granites

The variability of weathered materials is an important factor in the geotechnical characterization of rock for engineering purposes. Most engineering rock mass classifications include weathering schemes that separate the weathering profile into zones or grades that depend upon the engineering and geological properties of the rock. Many geotechnical characteristics, including weathering, are controlled by the density and arrangement of fractures within the rock, but the relationships between fracture patterns and weathering grades are typically not addressed. re characteristics were investigated in 13 exposures in five study areas in weathered granite in eastern Asia. All weathering grades were present, but never in the same exposure. Two approaches were used to evaluate the field data: (1) joint spacings were tabulated and examined within each weathering grade (tabulated classification); and (2) each exposure was classified according to the dominant weathering grade (visual classification). Mean and median joint spacings and joint spacing frequency distributions were analyzed and compared statistically for each approach. The box fractal dimensions for joint spacing were calculated for exposures classified visually in each weathering grade. Three-dimensional models of fresh and weathered granite were also generated and sampled for comparison to the field data. oint spacing is usually 25% or more closer in weathered granite than it is in fresh granite, and the difference between the mean spacings for weathered granite and fresh granite tend to be statistically significant. There are no significant differences between any distribution medians. The joint spacing distributions for weathered granite and fresh granite are also not statistically significantly different, and there are no significant differences among the joint spacing frequency distributions for the different grades of weathered granite. Fractal analysis of joint spacings, however, suggests spacing characteristics of fresh and slightly weathered (SW) granite are very different from those in moderately, highly, and completely weathered granite, and sampling of three-dimensional models for weathered and fresh granite supports this. In an engineering context, this suggests that joint spacing relationships in the various grades of weathered granite can be treated as the same regardless of weathering grade and that joint patterns in fresh granite must be evaluated separately. This knowledge could result in significant time and cost savings in the geotechnical characterization of these materials.