ENGINEERING GEOLOGY IN UNDERGROUND AGGREGATE MINING AND SPACE UTILIZATION

As consumption of construction aggregates increased in the Toronto (Ca nada) area and as regular source areas faced constraints, attention wa s given to the possibility of underground mining of limestone aggregat e. Following a study of transportation costs and the drilling of sever al, continuously cored drill holes through the Paleozoic strata, Ontar io Hydro and three Ontario government ministries undertook a feasibili ty study of the mining of limestone aggregate in the Toronto area. The geological study showed the suitability of the Ordovician Gull River Formation as a source rock for the aggregate and the structural condit ions suitable for mining operations. A mining operation to produce 3-5 million tonnes/year over 16-30 years was investigated. This involved a decline or a shaft (depending on location) down to a mechanized room and pillar operation (square or rectangular grid) at depths between 2 00-500 m. Pillar stresses were calculated at less than 40 Mpa - well b elow the 100 Mpa unconfined compressive strength of the limestone. Ope rations would be mostly in dry conditions because of the presence of h igh horizontal stresses.