Stress Field at Yucca Mountain, Nevada

Hydraulic fracturing stress measurements performed in four holes (USW G-1, USW G-2, USW G-3, and Ue25P1) indicate that at Yucca Mountain, the least horizontal stress S_h is less than the vertical stress S_v. Values of the greatest horizontal stress S_H are intermediate between S_h and S_v, corresponding to a normal faulting regime with values of Φ = (S_H-S_h)/(S_v-S_h) between 0.25 and 0.7. Drilling-induced hydraulic fractures seen on borehole televiewer logs indicate an S_h direction of N. 60° W. to N. 65° W. in USW G-1, USW G-2, and USW G-3. The same S_h direction is inferred from breakout orientations in USW G-2 and Ue25P1. The S_h values in the upper parts of the three USW G holes are less than the pressure of a column of water filling the borehole to the surface. Thus, the long drilling-induced hydraulic fractures in the shallow parts of these holes could have been formed in attempts to maintain circulation during drilling. These low S_h values may be intimately related to the low water table and fracture-dominated hydrology of Yucca Mountain.