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• 3.3 FLUID PRESSURE ASSUMPTIONS Three distinct fluid pressure assumptions were used in the design analysis: • The surrounding rock is dry and cavem fluid does not penetrate the host rock sur- roundingthe cavem but represents a stabilizing hydrostatic internal pressure on the cavem walls. We call this the "dry" case. • The surrounding rock is saturated but the cavem fluid does not penetrate the host rock. The stability of the cavern roof and walls is controlled by the effective stress. We will call this the "operational" case. • The fluid penetrates the formation resulting in the "pore pressure" in the host rock and the cavern fluid pressure being equal. This, we call the "long-term" case. • ,. -- • l The last of these fluid pressure assumptions is the more conservative and represents possible worst case, long-term stability conditions. The operational case could describe the conditions during operations and immediately afterwazds. The fluid in the cavity during solution mining has a specific gravity of 1.1 resulting in a higher pressure in the cavem than the equivalent weight of water. The pore pressure in the host rock was assumed in equilibrium with the fluid above the dissolution surface and equal to the head of water to the water table some 400 ft below the surface. 3.4 IN SITU STRESS The in situ vertical stress was assumed to be in equilibrium with the weigh[ of the overlying strata and the two componenu of the horizontal stress equal to 70% of the vertical stress. This assumed srress field is the samc as that used in our earlier analysis (JFTA, August, 1988) and is consistent with the few stress measuremenu made in the Piceance Creek Basin (Wolff, et al., 1974 and Mitche11,1981). The vertical stress was assumed equal to the weight of the overburden materials at 1 psi per foot of depth. J. F. T. Agapito & Associates, Inc. 15