Thickness of weathered mantle in Piedmont: variability and survey techniques : Segovia, A V Transp Res RecN919, 1983, P16–20

Abstract

The thickness of overburden measured at construction sites is, in many instances, different from that predicted by regional geomorphologic models. Variations in the thickness of residual soils and weathered rock, on the order of 10 m, have been found to occur over distances as short as a few meters. Problems, ranging from tilting and fracturing of structurs to spots that require chronic maintenance on roads, can result from differential settling induced by such variability of the weathered mantle. Geologic and soil maps help to predict problem areas where the variations result from changes in underlying rock facies, such as soluble to insoluble rock. However, sudden variations are common in areas where little or no change in either lithologies or soil types is apparent in such maps. In these areas thickening of the weathered layer is commonly associated with the presence of fracture zones (or zones of closely spaced jointing), which provide preferential loci for water precolation and weathering of the underlying rocks. Such fracture zones can vary in width from one meter to tens of meters. Methods effective in locating fracture zones in the Piedmont include: (a) analysis of aerial photographs, topographic maps, and satellite images to map possible fracture traces and (b) field check carried out by one or more of the following techniques: shallow seismic, magnetic or electric surveys, and trenching or augering. Expeditious field evaluations, based on the examination of samples obtained with a hand auger to determine variations in pedologic characteristics associated with varying rates of water infiltration, can provide a useful guide to the more expensive and time-consuming geophysical surveys. (Author)