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Environmental and Engineering Geoscience

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Improved Optimization and Visualization of Drilling Directions for Rock Mass Discontinuity Characterization

¹ Haneberg Geoscience, 10208 39th Avenue SW, Seattle, WA 98146

The first 20% of the full text of this article appears below.

INTRODUCTION

Characterization of planar discontinuities such as bedding, joints, and faults is an essential element of successful rock engineering. Discontinuities exert considerable influence over the shear strength and permeability of rock masses, which are important considerations in the evaluation of slope or tunnel stability, groundwater flow, and contaminant transport problems in rock. Discontinuities also control in situ block sizes that can be important when assessing the adequacy of potential riprap or armor-stone sources, the susceptibility of bridge foundations to scour during floods, and the sizes of rock blocks susceptible to sliding or falling along steep slopes.

The bias introduced by sampling discontinuities along lines, cylinders, and planes has been investigated by such authors as Terzaghi (1965), Kulatilake and Wu (1984), Priest (1994), Mauldon and Mauldon (1997), Martel (1999), Park and West (2002), and Zhou and Maerz (2002). As shown by Terzaghi (1965), discontinuities separated from boreholes by angles of 30° or less fall into a blind zone and are likely to be statistically under-represented or completely missed in subsurface exploration programs. Subsequent authors confirmed her conclusion while investigating complications such as the effects of discontinuities with finite, rather than infinite, extent.

In situations in which there is no knowledge of the discontinuities to be encountered during subsurface exploration, the best strategy is to select a combination of different borehole orientations that minimizes the chances that the average orientation of any discontinuity set falls into a Terzaghi (1965) blind zone. This is illustrated in Figure 1, which shows a lower hemisphere equal area projection with 30° shadow zones for three inclined boreholes. Discontinuities for which dip-lines (defined by a dip and dip direction, as opposed to a strike and dip) fall into the shadow zone for a particular borehole are unlikely to be encountered . . . [Full Text of this Article]