Even though pillars were 88 feet square and only 8 feet high, with entries 32 feet wide to give an extraction ratio of only 46%, the standard room-and-pillar layout at depths in excess of 2,500 feet resulted in undesirable roof conditions. A plan was conceived for using small pillars which would yield and shed load onto massive abutment pillars, thus reducing roof stresses in the mining area. Two test areas were mined, the latest giving 90% extraction in a panel 600 feet long and 200 feet wide. Pillar stubs were as small as 12' x 28'; entries became 46 feet wide. Nominal pillar load was 32,000 psi, giving a "safety factor" of 0.16, but there are no signs of roof or pillar failure. The test panel was very productive, largely because of short equipment moves and low-cost pillar robbing. Simple instrumentation showed that the pillars d yield, the load s shed onto abutments, and an arch must have formed. Further instrumented tests are being run to adapt the principle of the yielding-pillar/pressure arch to the various requirements of both development and production panels.
The mining horizon is one of several beds of rocksalt interspersed with beds of shale and dolomite. They are part of the Late Silurian Syracuse formation. Although some of the upper beds are severely distorted, the #6 salt is fairly consistent in thickness, and it dips gently to the south, at about 100 feet per mile. Thickness of cover varies from 2,300 at the shaft near Lake Cayuga, to more than 3,000 feet under the hills to the east of the lake.
1: Geological Cross Sections(available in full paper)
Conventional equipment is used in a room-and-pillar layout, Seven foot and eight foot roofbolts are installed on 4 to 5 foot centers, then the salt face is undercut with a Joy 15 RU undercutter to a depth of 14 feet. A Fletcher face jumbo is used to drill 24 1-3/4" holes in entries 32 feet wide and 8 feet high, then the holes are loaded with either Troja-mite "C" or Tovex 90, and they are blasted. Broken salt is hauled to a Sramlet feeder-breaker with Wagner ST-SA LHD vehicles, then it is conveyed to a preparation plant where it is crushed and screened, then hoisted to surface. Daily production is around 3,300 tons.
Mining in the #4 salt simply followed the thick rolls of that contorted salt bed, so both pillars and stopes varied greatly in width.
2: Map of part of the #4 level, showing how both pillars and stopes varied in width.
The layout in the #6 salt was more systematic. Entries 32 feet wide and pillars 88 feet square gave about 46% extraction, which, at a depth of 2,400 feet, gave an average pillar load of about: (mathematical equation)(available in full paper)
This gave a nominal pillar safety factor of about 1.0, since the compressive strength of rock salt is generally assumed to be around 5,000 psi.