Stress measurements have been obtained from within the Norton Mine in support of site characterization activities intended to determine the in situ stress field around the mine. These results together with other measurements in the area permit an estimate of the principal stresses at the mine. Based on the most recent measurements, the maximum (óHmax) and minimum (óHmin) stresses acting in the horizontal plane are oriented nearly east-west and north-south, respectively, and their magnitudes are 5330 psi and 4100 psi, respectively. These values are expected to be essentially uniform within a few hundred feet vertically above and below the mine elevation. The stress acting in the vertical direction has a magnitude of 3270 psi at the mine level. This measured vertical stress is related to the overburden weight according to óv=1.26ñgh (where ñ is the overburden density, g acceleration of gravity, and h overburden depth). The measured vertical stress exceeds the stress calculated from overburden weight by a factor of 1.26. These in situ stresses are assumed to be principal stresses and, as a result, the vertical stress is the minimum principal stress. These measurements are generally consistent in magnitude and direction with two other much older sets of measurements taken in the mine and they are consistent with the east-west trend of the regional in situ principal stress direction. The average of all three sets of measurements, recent and old, in the mine give a maximum horizontal stress of 6110 psi, a minimum horizontal stress of 3630, and a vertical stress of 3030 psi. The directions of the mine excavation development, which normally are oriented according to the principal stresses, are also consistent with the current and past measurements.
In situ stress is generally considered to be the sum of gravitational, tectonic, and residual stresses present in a rock body. While these stresses are naturally occurring through geological processes in all rocks, their magnitudes and directions differ according to the location and geological situation. As a result, a workable knowledge of the local magnitude and directions of the in situ stress is important to the design and performance of any underground facility. This is certainly true of the proposed compressed air energy storage (CAES) facility at the Norton Mine in Ohio (Figure 1).
At this site the in situ stress state is important because: (1) The operational pressure range of the facility must be determined. If in situ stresses are exceeded by operational pressures, the rock could fracture or existing fractures could open. (2) The facility is a room and pillar mine at depth and will undergo cyclic pressurization during operation. In situ stresses will be used to assess stability and performance of the facility during operation. (3) The shaft bottoms will be plugged to facilitate closure of the facility prior to pressurization through wells from the surface. In situ stresses will be used to assess stability and performance of the plugs.
The results and recommendations of this paper can be used to address 1, 2, and 3 from above, however only 1 can be directly addressed by this paper.
