In late 1980, Exxon Minerals Company performed rock mechanics testing on oriented 150 mm drill core from their 75M tonne massive sulfide deposit near Crandon, Wisconsin. Instrumented samples from depths of 300 meters indicate premining horizontal stresses in a north-south direction (parallel to pillar faces) to average 12,300 kPa, with the perpendicular plane yielding an average stress of 14,630 kPa. Rock strength versus sample size was compared using uniaxial compressive strengths from 47 mm, 95 mm and 150 mm core. Massive sulfide averaged 153,300 kPa, 139,845 kPa and 75,330 kPa respectively, while the hanging wall and footwall averaged 92,000 kPa, 76,560 kPa and 57,960 kPa respectively. Point load compressive strengths were calculated using formula suggested by both Hassani et al., 1980 and Broch et al., 1972. Values calculated using the Hassani formula closely resembled preexisting point load and uniaxial compressive strengths, while values calculated using the Broch formula were 50 percent low.
The Crandon massive sulfide deposit is located in northeastern Wisconsin, approximately 480 kilometers north of Madison, the state capital. Crandon, the nearest community, is located eight kilometers north of the proposed project site. The orebody is a tabular deposit about 1,500 meters long, averages 38 meters wide, and dips approximately 80º to the north. The ore grade material penetrates to a depth of approximately 720 meters beneath the surface. Current published estimates of the probable tonnage and grade are 75 million metric tons that average 5.0 percent zinc, 1.1 percent copper, and 0.4 percent lead (Crandon Project team, 1980). It is proposed that the mining method will be sublevel blasthole open stoping utilizing mill tailings as the backfill materials. The stope block size, as dictated by rock mechanics studies, is currently estimated to be 45 meters wide measured along strike, and will extend from hanging wall to footwall with a vertical height of 120 meters.
In late 1980, eight large diameter (150 mm) core holes were drilled to obtain a thirty ton bulk metallurgical sample. Rock mechanics testing was performed on rocks from three of those holes. Intermediate feasibility studies required more rock mechanics data and data of greater reliability than that which was available from earlier rock mechanics programs using NQ size (47 mm) core.
There have been very few large diameter (150 mm) core drilling programs in the base metals industry which involved both angle drilling and drilling to depths below 300 meters. The program was designed and expedited by Exxon drilling supervisors using contract personnel and equipment. The drilling contractor used a tophead drive Schramm T66B, which met operational criteria for size, speed and angle drilling capabilities. Drilling air was supplied by a rig-mounted compressor rated at .283 m³/S @ 1.72 MPa (600 cfm @ 250 psi) and an auxiliary trailer-mounted compressor rated at .212 m³/S @ 1.72 MPa (450 cfm @ 250 psi). After logging, rock mechanics samples were identified and removed for processing.