ABSTRACT
In the course of interpreting strain relief measurements from overcoring very near the ground surface in western Ohio, the authors have attempted to evaluate the magnitude of non-tectonic stresses caused by a variety of surface conditions. Potential effects, specifically from surface topography, were modeled using finite element techniques. In Ohio, diurnal and seasonal temperature variation surface structures that relieve stress (fractures and pop-up structures) and residual strain also clearly play an important role in interpreting strain results. Stable, reproducible strain relief measurements of horizontal stress components were successfully made at depths between .5 and 1 meter using a doorstopper-type strain gauge unit overcored with a 5cm diameter core drill. The rocks were flatlying Silurian carbonates exposed on the floors of shallow quarries. This is the only type of setting in the area in which glacial deposits have been removed to expose bedrock. Surface topography produces its own variable loading pattern, and in irregular quarries or mountainous areas, the magnitude of the effect should be taken into consideration. Finite element modeling techniques are useful for estimating these stresses. The results indicate that tensional stresses develop in the center of the quarry floor due to the low walls at the quarry edge, but in the broad shallow Ohio quarries, the effect is small. However, for hilly or mountainous topography the effects may be substantial. Surface thermal effects were estimated by Hooker and Duvall (USBM, RI 7589) using standard elasticity equations, and were shown to be very large at extremely shallow depths, especially in the case of diurnal variations. Our concern that no effective way may exist to compensate for this factor right at the surface led us to emplace the gauges at depths up to lm beneath the surface. As predicted, the large diurnal temperature variations were almost completely attenuated at depths greater than .5m. However, the deviation from average annual temperature must also be determined and a correction made. In regions of significant topographic relief, the temperature correction may not be straightforward, but a good estimate can be made by superimposing thermal and topographic effects in a finite element model.
INTRODUCTION
Expanded use of near-surface overcoring techniques for determining the ambient horizontal stress field has led to a number of apparently successful measurements in the past few years (de la Cruz and Raleigh, 1973; Engelder and Sbar, 1976; Newman and Clark, 1976). However, uncertainties in the interpretation of the data remain, particularly in the estimation and correction of non-tectonic sources of apparent stress in the rocks (Friedman, 1972; Swolfs et al., 1974; Engelder and Sbar, 1976). These sources include topographic relief, diurnal and annual surface temperatures, and "residual stresses" stored as elastic distortions in grains due to interaction with adjacent grains in the rock's fabric. In the course of estimating the effects of these non-tectonic factors in measurements made in western Ohio, the authors have attempted to develop semi-quantitative models that illustrate the magnitude of the problems associated with the shallow overcoring methods. Major effects have already been expended by others on thermal and residual stress effects, and in this paper we consider in more detail potential problems associated with topographic relief.
