SYNOPSIS:

Determination of rock strength and its anisotropy is important in rock mechanics and engineering geology investigations. Destructive tests such as measurement of uniaxial compressive strength and/or point load strength index are commonly employed for this purpose. In the present investigation, an attempt is made to explore the possibility of using anisotropy of magnetic susceptibility (AMS) data as a gauge of rock strength anisotropy. For this, 10 blocks of quartzites that do not show mesoscopic foliations were sampled from a quarry in the Ghatshila area (eastern India). AMS analysis was performed on these blocks to identify the magnetic foliation in them. This identified foliation direction was then marked on the blocks and subsequently cores in directions parallel as well as perpendicular to the foliation were extracted. Subsequently the point load strength index in cores perpendicular to the foliation (SP) and parallel to the foliation (SL) was determined on the basis of which the strength anisotropy (RS = SP/SL) was calculated. It is noted that rocks with high SP also have a high SL. It is also noted that in rocks with a degree of magnetic anisotropy (P′) greater than 1.045, RS tends to increase with the increase in P′ value. It is therefore concluded that the P′ value of 1.045 is the threshold value for the quartzites beyond which the RS increases with P. These initial results provide the possibility of using AMS data for gauging rock strength anisotropy and thus hold promise for future studies in rock mechanics.

INTRODUCTION

Rocks containing well-developed mesoscopic foliations show strength anisotropy which is crucial in rock engineering (1). Many rocks in the earth are foliated (e.g., granite gneisses, mica schist etc.) and it is well established that rocks with well developed foliations have strong magnetic susceptibility anisotropy too (2, 3, 4). Therefore, in strongly foliated rocks, the strength and magnetic susceptibility anisotropies are expected to be related. Several studies have focused on determination of strength anisotropy of various foliated rocks (5, 6, 7, 8, 9,10,11). But when a rock is devoid of mesoscopic scale foliations, any strength anisotropy in that rock is not immediately apparent. An important usefulness of anisotropy of magnetic susceptibility (AMS) is that weak planar fabric defined by preferred orientation of minerals (magnetic foliation) can be identified even if the rock is devoid of mesoscopic scale foliations. In the past, AMS analysis has been used to carry out petrofabric studies in variety of rocks including those that lack visible foliation (2, 12, 13, 14, 15, 16, 17). Hence, although on the mesoscopic scale a rock may not contain a foliation, fabric elements can be identified from AMS data. As rock strength tests are of destructive nature, this investigation is also meant to explore the feasibility of using AMS data obtained from non-destructive measurements as an index of rock strength anisotropy. The present investigation aims to explore this issue for quartzite rocks (from Ghatshila area, eastern India) that do not contain mesoscopic foliations.

This content is only available via PDF.
You can access this article if you purchase or spend a download.