Instrumentation placed at three - levels in the concrete lining of the 14-ft I.D. shaft at the Mt. Taylor Mine permitted checking which lining pressure equation best fit the particular geologic conditions. Talobre''s (1957) lining pressure prediction equation for clastic rock most closely approximated the measured results. The Mohr-Coulomb strength criteria appeared to best describe the strength/confinement conditions outward into rock adjacent to the 14-ft Mt. Taylor shaft.
Traditional shaft support has been with either timber or steel sets or a cast concrete lining. Steel and timber sets are much more flexible than concrete. This results from (1) the low compressive strength, typically 250 psi across the grain, of the timber blocking which braces the sets against the rock, and (2) the high elastic flexibility of steel or timber sets. The stiffness of concrete, after curing, is much higher than any composite structure of timber blocking and steel sets. It is this feature of concrete which complicates the understanding of concrete lining design. Irrespective of whether steel, timber or concrete is used for shaft support, the very nature of the shaft sinking cycle generally results in the support being installed fairly close to the shaft bottom. In North American sinking practice concrete lining is seldom more than 20 ft away from the shaft bottom. A zone of "stress shielding" occurs near the shaft bottom which benefits sinking in weak ground. The elastic analysis of Galle and Wilhoit (1961) demonstrated that the elastic stresses in the wall "as close as 2.9 radii from the bottom agreed well with those calculated by the plane strain solution". They also show that the tangential stresses at the shaft wall within 2.9 radii above the shaft bottom are less than predicted by the plane strain solution.
