Engineering geology in underground aggregate mining and space utilisation

As consumption of construction aggregates increased in the Toronto (Canada) area and as regular source areas faced constraints, attention was given to the possibility of underground mining of limestone aggregate. Following a study of transportation costs and the drilling of several, continuously cored drill holes through the Paleozoic strata, Ontario Hydro and three Ontario government ministries undertook a feasibility 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 conditions 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 200–500 m. Pillar stresses were calculated at less than 40 Mpa — well below the 100 Mpa unconfined compressive strength of the limestone. Operations would be mostly in dry conditions because of the presence of high horizontal stresses.