A while back, I once again read HG Wells’ The Time Machine. The story basically is a narrative told by a person referred to as the Time Traveller who had returned from his trip into the future.
About half way through the book, the Time Traveller described what he concluded was an underground labyrinth:
Beneath my feet, then, the earth must be tunnelled (sic) enormously … The presence of ventilating-shafts and wells along the hill slopes—everywhere, in fact, except along the river valley—showed how universal were its ramifications. … I began to suspect their true import.
My mind drifted away from the book. The true import about underground aggregate mines is that they exist. In fact, in 2007, there were 86 active underground mines in the US that collectively produced 68.6 Mt of crushed stone, more than five per cent of America’s annual crushed stone production.
Many underground aggregate mines are extensions into the walls of open pit quarries, but some consist of inclined haulways driven into quarry floors. Some are completely independent of existing quarries.
Most underground aggregate mining uses the “room and pillar” mining technique that involves the removal of rock from “rooms” while leaving an array of rock “pillars” to support the overlying roof. Rooms are generally 12 to 13 metres wide and in some instances as high as 13m.
Equipment used in underground aggregate mines includes horizontal drill jumbos and down the hole track drills, scaling rigs (to remove loose rocks from the ribs and roof of the mine), roof-bolting equipment (to secure the roof of the mine), ventilation fans (to provide fresh air for workers and remove exhaust fumes from machinery), as well as loading and hauling equipment.
Depending on conditions, the underground mining of aggregate is 50 to 60 per cent more expensive than extracting aggregate from surface quarries with similar rock properties.
Even so, it may be more economical to mine underground than to strip thick overburden or import aggregate from distant locations.
Even where high quality rock is present, the NIMBY syndrome or incompatible land uses near a quarry may prevent an operator from acquiring new land or even expanding from a surface quarry onto land already owned.
Underground mining may be favoured where these conditions exist.{{image2-a:l-w:220}}
There are significant advantages to underground aggregate mining. Working conditions are not affected by rain, snow, wind, or outside temperature—mine air temperatures hover around 10oC to 18oC throughout the year.
The workers have stable employment, stockpiles do not need to be large, rock does not freeze, and responses to unexpected demands of material are possible at any time. Neighbours are isolated from the “nuisances” of mining.
Frequently the space created by underground mining can prove more profitable than the rock removed.
Spent underground aggregate mines have been turned into state of the art business parks with millions of square feet of commercial and industrial space used as warehousing, laboratories, manufacturing facilities, food storage, records and data storage … the list goes on and on.
As the Time Traveller observed:
[I]n the end … the [undergrounders] would become as well adapted to the conditions of underground life, and as happy in their way, as the Upperworld people were to theirs.