3-D CHARACTERIZATION OF COMPLEX GEOLOGIC MIXTURES

Expensive construction difficulties may result from inaccurate three-dimensional (3-D) characterizations of complex geologic mixtures, hard blocks of rock in matrices of weaker rocks or soils. Geological complex mixtures are ubiquitous, and are encountered at the scales of engineering interest (centimeters to tens of meters), as melanges, decomposed granite, fault gouge, or colluviums. The modeling of 3-D subsurface conditions requires geological and geotechnical practitioners to depend on one-dimensional (1-D) and two-dimensional (2-D) data and geotechnical laboratory and field testing results.

In 3-D modeling it is often assumed that the 1-D and 2-D data can be used to generate “representative” cross-sections and geological maps. However, complex geologic mixtures frustrate accurate 3-D characterizations due to the spatial variability of the blocks and matrix materials, and their disparate mechanical properties. Many practitioners simplify geologic complexity by ignoring it. One common practice is to characterize a mass of geologically complex materials by describing it as “soils with boulders” and assigning the strength of the “weak component” to the entire mass. Yet, blocks in melange bimrocks (such as Franciscan Complex melanges of Northern California) range in size from sand grains to mountains, so using terms such as “soil with boulders” is misleading. Also, because the strength of a geological mixture is directly related to the volumetric proportion of the blocks in the mixture, the assignation of the matrix strength alone to the mixture may be overly conservative.

The authors present examples of construction problems related to inadequate characterization of complex geologic mixtures from sites in Northern California, Europe, and Asia. Further, practical guidelines and caveats are offered for more reliable characterizations of complex geologic mixtures using (1-D) drilling and (2-D) geological mapping.