ABSTRACT:

This paper reports on a series of hydraulic fracturing (HF) tests conducted near a headrace tunnel for the purpose of estimating the stability of the excavations. It is shown that the results although defendable, could have benefited from proposed experiments in the future Deep Underground Scientific and Engineering laboratory (DUSEL) at Homestake Mine, South Dakota. Such large scale experiments, never before undertaken, could significantly enhance the confidence that practitioners and customers have in the ability of methods such as HF to determine the real stress magnitudes and directions in the subsurface.

1. INTRODUCTION

This paper reports a case history involving a series of hydraulic fracturing (HF) stress measurements conducted in conjunction with the construction of a hydroelectric underground powerhouse. The introduction in the early 1970's of HF as the first method of in situ stress measurements that is unlimited by borehole depth has been particularly beneficial to the hydroelectric power industry [1]. Until then the state of in situ stress could not be assessed prior to the excavation of an underground powerhouse and its associated complex of tunnels. These are typically located several hundred meters below the surface, well beyond the capabilities of previously available stress methods. When measurements are not conducted in advance of excavation the design of caverns is by necessity based on guessing the state of pre-existing stress. If stress tests are run after excavation and stability problems are discovered, a re-evaluation of the design may call for needed corrections that are often extremely expensive [2]. The first projects that took advantage of the new HF stress determination technique were the Helms and Bad Creek hydroelectric schemes in California and South Carolina, respectively. In both cases HF stress measurements were conducted as part of the preexcavation design of the underground facilities [1]. Since then, the use of HF ahead of excavation has become routine all over the world. Despite the obvious advantage that pre-excavation stress measurements provide the designer, HF is not always employed at that early stage. The reasons may include a belief that local stresses are predictable, or a decision by the operator to bypass a measurement that is not considered essential to the project. Such a case is the subject of this paper. In situ stress tests were not carried out until after excavation, when some rock falls from the roof of the headrace tunnel, in the vicinity of the powerhouse excavation in progress, prompted the company to consider conducting a series of HF tests in short boreholes drilled from the level of the excavated tunnel. Although the way the results of these measurements were used by the company has not been made public, the actual tests are of interest because, contrary to expectations, they all resulted in the reopening of pre-existing discontinuities.

2. BACKGROUND INFORMATION

For the purpose of conducting HF measurements, two boreholes were drilled vertically down from short alcoves (4x4 m in cross section, 20 m long) excavated off the sidewall of the headrace tunnel as it approached the powerhouse that was still under construction.

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