Man has used underground voids since the beginning of recorded history. As time has passed underground construction has increased in sophistication, with technological improvements allowing for fast, efficient excavation of large quantities of rock. Underground space is used for subway systems, power supply, water and sewerage reticulation, and storage throughout the world. Australia is making use of underground construction in order to improve our cities and make them more comfortable for people to inhabit.
There are some reasons as to why underground construction may be favoured over surface construction. Environmental and social impact may be minimised by underground construction. Moving infrastructure underground enables more surface area to be allocated to parkland and open space. Underground construction can be used to preserve historical areas of cities. Political reasons also influence the decision to build underground. Statutes may require or encourage certain land uses to be underground, such as car parking. Climate is another important reason, in some locations surface space can only be readily used for part of the year.
Whether for transportation, infrastructure, storage, public works, car parking, or commercial activities, underground space is a valuable commodity. Time has shown that underground space is the best location for infrastructure, and is becoming more popular for storage. European experience shows that storage in rock caverns is less costly than for surface warehouses, in terms of both capital expenditure and operating cost.
Underground space for public utilisation must be carefully designed. The general perception toward underground space is one of apprehension. In order to be amenable, the space needs to be comfortable, functional, and inviting. High quality, interesting, and distinctive interior design will influence people to overcome negative views of underground spaces. Integration with the surface is also important if a feeling of isolation from the 'outside' world is to be avoided.
Underground quarrying of hardrock material is not a new idea, but rather a re-realisation of past experiences. Underground extraction of building stone has occurred at various times through history. Example of past underground quarrying are the catacombs of Paris and the quarries at Bath in England. Underground extraction of quarry material has numerous advantages over surface quarrying. Proximity to market, environmental protection, lower community impact, and void value are all reasons why the apparently more expensive option of underground extraction may be favoured. The income generated by void utilisation may in fact generate more income than product sales, as such stoping voids should be designed for post-excavation utilisation.
Cost modelling has indicated that underground excavation can be competitive against surface quarrying. Large scale sub-level open stoping may be able to produce construction aggregate for AUD9.40/t, which is only marginally above the current general open pit production cost of AUD8-9/t. Smaller scale sub-level open stoping may produceAUD11.00/t, which benching and room and pillar extraction at AUD15.50/t and AUD28.50/t respectively. Benching and room and pillar are however better suited to higher value uses, which may in fact be more profitable once income from void utilisation is considered.
Brisbane and South-East Queensland have strong potential for both underground construction and quarrying. Underground extraction of hardrock reserves, especially of previously sterilised deposits may be possible. This would not only create valuable space but also provide an aggregate source. Central Brisbane, due to its geology, is best suited to cut and cover excavation or underpinning. The soft variable rock of the Neranleigh-Fernvale beds are not suitable for excavation of wide spans, in contrast the Brisbane Tuff may prove to be an excellent underground construction media. The Kholo Creek hardrock resource is another potential underground project. If utilising large scale stoping and being connected to the state rail network, this quarry could produce large quantities of aggregate at low cost for a wide market area while providing voids for disposal of waste which could be railed in from great distance.
Improvements in technology will see the cost of both underground construction and quarrying fall significantly. Comparison of cost models with those produced only two years ago indicate that underground excavation costs are falling already. New technologies such as the Oscillating Disc Cutter will allow for fast, efficient, and community friendly excavation of underground space in hardrock, while competitiveness in both the quarrying and underground contract mining industries will see underground production costs fall further.
Australia has a unique opportunity to embrace underground construction and quarrying while our cities are still uncluttered. Failure to do so will see our cities in the same situation as many larger cities see themselves in now, overcrowded and nowhere to grow. Underground construction and quarrying can improve quality of life through improving surface amenity and isolating us from the 'necessary evils' we don't want to see.