OCR Text |
Show THE CAVING SYSTEM IN US AT BINGHAM The application of the caving system in tho extraction of oro from tho mines of Bingham difTors little from tho practlco In tho Lako Superior district, Michigan, says a contributor to tho Engineering and Mining Min-ing Journal. The ore bodies occur In a limestone, which Is fractured and very much broken by llssurcs occurring at right angles to the bedding of tho formation. forma-tion. Tho llssures and cracko aro tilled with tulr, making It practically Impossible Impossi-ble to hold tho hanging wall with timber, especially ne the oro bodies vary In width from ten to llfty feet and llo at an angle of about 40 degrees from the horizontal. Main levels aro driven 100 feet apart; raises aro put up from these levels to tho top of tho oro at convenient distances apart; then sub-lovels jltc driven fourteen fejet apart. Extraction begins at tho top sub-lovel, but whllo this Is being done, tho lowor lovels arc being cut into blocks about fifteen to twenty feot square by small drifts, which arc sometimes timbered tim-bered vhcro occasion requires It. or whero the ore Is soft or broken. The timber used Is round and from six to eight Inches in dlumeter. Owing to the ore bodies being so flat, thcro Is not tho advantage of tho caved ground following the extraction down and crushing tho oro with Its weight na It Is cut Into blocks, an Is the caso In tho Lako Superior district, and at each levol thcro Is a certain portion of tho oro body lying next to tho hanging wall that does not have any cavod ground immediately over It. The hanging wall will not cavo until from twenty to thirty foot of it Is oxposed. It becomes necessary, theroforo, to use props or stulls to support tho hanging wall until the oro Is takon away, and sufflclont area exposed so that the hanging wall will cavo of Its own weight Tho timber used for this purpo?o Is simply sim-ply round poles, varying from six to ton Inches In diameter, and It Is rarely that timbers lonKCr than six to eight feet aro required. Whero tho voln Is narrow, stulls arc used to reach from foot wall to hanging hang-ing wall, but these are only placed In sufficient suf-ficient number to hold tho hanging wall temporarily, until tho oro Is extracted, and If tli w hanging docs not cavo aa soon as the oro Is cut, the stulls aro blasted, so as to allow tho hanging wall to cavo, to prevent any largo openings being loft overhead. It 1b a matter of experience that the caving cav-ing system works at Its best when the oro bodies occur at an angle of 55 to 90 degrees from the horizontal. When such conditions condi-tions exist, the timber and caved ground follow down In a body as tho oro Is extruded, ex-truded, the timbers forming a network which Is convenient to work under, the caved ground making sufficient wolght to crush tho oro as It Is cut Into blocks by the small drifts, thus avoiding tho uno of powder and lessening tho cost of mining by a corresponding amount. |