Borehole elongation and its relation to tectonic stress at the Nevada Test Site

This paper documents the borehole spalling pattern in relation to lithology, geologic structure, stress, and depth of two Nevada Test Site (NTS) areas, Yucca Flat and Pahute Mesa. Downhole movies and stereo photographs from 58 large-diameter test holes at NTS reveal a strong NW-SE elongation due to sidewall spalling. This trend is found in Yucca Flat and Pahute Mesa, which have distinctly different geologies, and is parallel to the direction of regional extension. The directionality is due to tangential stress concentration at the wellbore, which has a peak value along the azimuth of minimum horizontal regional stress. The percentage of borehole length affected by spalling varies from 5% near the surface to 30% at depths over 500 m. The severity of spalling is also a function of lithology; however, geologic discontinuities apparently have little effect on the orientation and occurrence of elongations. Directional spalling was observed in relatively shallow alluvial deposits, suggesting the persistence of horizontal stress differences at shallow depths. If the elongations are truly controlled by the deviatoric stress field, this implies that the regional stress direction is fairly uniform throughout NTS. A simple relation is given for the in situ stress and material strength conditions associated withmore » borehole elongation. 24 references, 13 figures, 2 tables.« less