Results of a primary stress measurement investigation conducted on the 1100 copper ore body of the Mr. Isa mine, Mr. Isa, Queens., Australia, are presented in this chapter. The investigation started in September 1965, after discussion with the engineering staff at Mr. Isa, and consisted of both underground or in-situ tests at Mr. Isa and laboratory tests on Mr. Isa rocks and stress measurement techniques at the Australian National University in Canberra.
Underground stress measurements were made in and adjacent to the 1100 copper ore body. The primary stress measurement site was in J 32 west crosscut on 14 Level. Approximate coordinates on the mine grid are 3000 north, 1650 east. The vertical depth below ground surface at this point is approximately 2180 ft. Some preliminary measurements were made in M 30 west crosscut on 13 Level, coordinates3 000 north, 2000 east, depth beneath the surface approximately 1990 ft. Additional flatjacks were installed on 11 Level in L 30 west crosscut, coordinates 3000 north, 2200 east, and depth beneath the surface approximately 1580 ft.
All of the sites were in rock that from a mining engineering viewpoint must be considered extremely sound. The 13 and 14 Level crosscuts were approximately 14 ft on a side and nearly square and the 11 Level crosscut was approximately 10 ft on a side. All of the crosscuts had been standing open for several years and no artificial support of any type was necessary. Extraction openings hundreds of feet on a side have been made in this mine in similar rock and these have remained open also without additional support (see Davies 1 for a recent summary of mining techniques and conditions).
The geology of the Mr. Isa mine has been most recently described by Bennett. 2 Briefly, Mr. Isa is a silver-lead-zinc and copper mine in a series of Precambrian siltstones, shales, and bedded carbonates that strike north- south and dip 60° -65 ° to the west. The dominant structural feature of the area is the Mr. Isa fault. This fault truncates the western edge of the ore bodies, strikes north-south and dips 50 ° -70 ° to the west. It has been traced about 40 miles and the relative movement is thought to have been the west side north and up. The copper and silver-lead-zinc ore bodies are separate and distinct in the mine but both are confined to one formation, the Urquhardt shale. The silver-lead-zinc ore bodies are restricted to relatively unaltered shale while the copper ore bodies occur in zones of highly breeeiated and recrystallized shales. The stress measurements in this investigation were made in and adjacent to one of the copper ore bodies. The recrystallized and deformed Urquhardt shale is known locally as "silica dolomite." This is a general term which is simply meant to cover all of the known copper host rocks. The mine geologists recognize and map four varieties of silica dolomite: medium to coarse-grained crystalline dolomite, irregularly breeciated dolomitie shale in a crystalline carbonate quartz matrix, regularly bedded but partly recrystallized shale, and fractured and breceiated siliceous shales.
