During the last 3 years, the Cleveland Mine has made significant advances with respect to installing, maintaining, and upgrading roof supports to deter the effects of time dependent ground deterioration in main entries underground. This paper will review relevant mine design, evaluation of the support systems effectiveness, and improvements made in the roof bolt system and its installation.
GEOLOGY
The mine site is located in Cleveland, Ohio on the Whiskey Island Peninsula with workings extending north underneath Lake Erie. We mine a salt bed in what is known as the Salina Group. It is part of the eastern salt basin and underlies sections of Ohio, Michigan, Pennsylvania, New York, West Virginia, and several provinces of Canada. This formation varies from 800' to over 6000' in depth over this region and is composed of several distinct salt beds each varying in thickness and purity. Mining takes place in the "B" salt, 1760' below surface. This salt bed is 40' thick and contains anhydrite bands which vary from 0.13"-3.0" thick. The occurrence of this impurity and our need to avoid it has led us to mining the top 20'-25' of the "B" salt. Directly above the "B" salt is 2'-7' of intermixed anhydrite, shales, and dolomites and above that, 21' of "A" salt. See Figures la-b.
MINE DESIGN
The constant mining pattern over the last 18 years presents a unique opportunity to evaluate roof conditions and correlate their deterioration with mine design and mining practice. Some entries within mining sections must remain open for 5-15 years and main entries for even longer. These entries will be used for conveyor belts, long term travel, and ventilation for the mine and for each mining section.
Conventional mining equipment consisting of LHD's, scalers, face drills, undercutters, and roof bolters are used to mine nearly 2 million tons of salt per year. Production faces are blasted using ANFO. Shaft bottom areas are stable and do not show signs of any deterioration. The nominal extraction ratio around this area is 10%. Roof deterioration has occurred primarily as a result of room and pillar mining which employed large, "stiff" pillars. Standard pillars are 105' square while entries are nominally 45' wide and 20' high. Extraction for these areas is near 51%. Pillars of this size have caused each entry and intersection to act almost independently to form its own stress envelope. This allowed horizontal stress concentrations to build and subsequently be relieved through the creation of voids at varying heights within the mine roof. The age of the mine openings which experience significant deterioration varies between 8 and 18 years. In some areas, massive caves have resulted from extensive ground movement. See Photo 1.
