As part of the Yucca Mountain site characterization studies for under-ground nuclear waste disposal, we conducted five hydraulic fracturing in situ stress measurements in a 30 m deep test hole, drilled from the main drift of the Exploratory Studies Facility, approximately 250 m below the Yucca Crest. The objective of the in situ stress measurements was to define initial stress conditions for coupled geomechanical analysis used in the design and performance analyses of the candidate repository site.

The vertical stress, Sv, was estimated from the densities of the different rock units overlying the test hole to be 4.7 MPa. The hydraulic fracturing tests yielded estimates of the two principal horizontal stresses as follows: Shmin = 1.7 (±0.1) MPa acting at N75W ; SHmax = 2.9 MPa acting at N15E .

The measured stress regime reveals that the horizontal stresses are moderately differential, but are both clearly smaller than the vertical stress, suggesting normal faulting stress regime. This inference is supported by the local predominance of generally N-S trending normal faults related to the Basin and Range Province. Stress indicators such as fault slip and earth-quake focal mechanism data around Yucca Mountain also indicate normal faulting stress regime with the minimum horizontal stress generally oriented at E-W.

Collaboration with Prof B. Haimson, UW-Madison.