PIEDMONT GROUNDWATER SYSTEM, PART 2—THE TRANSITION ZONE BETWEEN REGOLITH AND BEDROCK: CHARACTERISTICS

The transition zone (TZ) can be difficult to delineate from the overlying regolith and the underlying bedrock in boreholes. To delineate and describe the characteristics of the TZ, the Recovery/Rock Quality Designation definition of the TZ (REC > 50% and RQD < 50%,) is followed with allowances for geologic judgment and interpretation of the REC/RQD data and the rock core from the boreholes. The Transition Zone (TZ) is comprised of partially weathered rock, open, steeply dipping fractures, and low angle stress relief fractures, either singly or in various combinations below refusal (either auger or roller cone). The TZ has less advanced weathering relative to the regolith and generally the weathering has not progressed to the development of clay minerals that would decrease the permeability of secondary porosity developed during weathering, new fractures develop during the weathering process, and /or existing fractures are opened. The characteristics of the TZ can vary widely based on the interaction of rock type, degree of weathering, degree of systematic fracturing, presence of stress-relief fracturing, and the general characteristics of the bedrock (foliated/layered, massive, or in between). The thickness of the TZ at six sites varies from 0 to 25 meters with average thicknesses of 1.5 to 3 meters. The thicknesses measured in the boreholes have a negative exponential distribution with the means approximately equal to the standard deviation and the median less than the mean thickness. The thicker TZs encountered in the boreholes are primarily related to the presence of high and low angle fractures and to some extent to more advanced weathering associated with the fracturing. 3-D models of the TZ have been developed for four of the sites (2 – foliated/layered; 2 – massive/plutonic). These models show, within the limits of the modeling methodology, the highly variable geometry and complexity of the TZ. The TZ is not a continuous layer between the regolith and bedrock. It thins and thickens within short distances and is absent in places. The absence, thinning, and thickening of the TZ is related to the characteristics of the underlying bedrock as observed in the boreholes.