ABSTRACT:

Rock conditions in the Devonian Mount Waldo Granite and adjacent Precambrian gneiss near Bucksport, Maine, were investigated to assess the suitability of crystalline rocks of coastal Maine as potential sites for oil storage chambers. Factors investigated included in situ stresses, rock structure, strength, deformability, and deformations produced by rockbursts. The orientation and magnitude of in situ stresses in the granite at shallow depths are controlled by sheeting fractures and tectonic joints. Stress magnitudes were three times as high at a mountain location in massive rock as in an adjacent valley underlain by more closely sheeted rock. The sheeting process expended energy derived from residual paleostresses (the predominant component of the in situ stress field), resulting in rock-stress magnitudes that are controlled by the volume of unfractured rock in which the measurement was made. The chemical effects of water may facilitate destressing and fracturing. Stress orientations below levels of sheeting and alteration in granite and gneiss are more consistent with the tectonic framework of the area (right-lateral strike-slip faulting) than are stresses at shallower depths. The inference that the least principal stress is normal to sheet surfaces was supported by hydrofracture measurements. Fracturing associated with a severe rockburst on Mount Waldo in the late 19th century was compared with the laboratory rockburst model of Holzhausen (1977). In both cases, channels cut normal to vertical walls, which were parallel to the maximum horizontal stress component, caused failure by concentrating applied stresses at the tips of flaws. The fractures propagated unstably, roughly normal to the channels. In the laboratory model, development of through-going fractures was preceded by localized crushing of the rock, which produced features similar to those observed at Mount Waldo.

INTRODUCTION

Several sites on the Maine coast have been suggested for underground oil storage chambers.

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