The deepest human made excavation is the Tau Tona Mine in South Africa, which is 3900 metres deep. This is accessed by two vertical shafts, each 2000 metres long. At the bottom of the first shaft there is a short tunnel, at the end of which is the top of the second shaft.
The depth limit on the shafts is due to the capabilities of the winding ropes: due to the weight of the ropes, the strength of the steel used, the unavoidable manufacturing flaws in the ropes and the fatigue induced in the ropes over time as the ropes are wound and unwound from the drum.
If we want to dig an excavation for a building we will need to get the unwanted material out of the excavation so we will need a winder and a kibble (the bucket used to hoist the material out of the hole). With current technology, this limit is 2000 metres.
Other important factors which limit the depth to which we can dig and construct anything underground are:
- Ground stresses - if they're too large the excavation for your construction will collapse or be squeezed so its cross-sectional area is reduced.
- Geothermal heat - if the temperature is too hot, people can't work in the excavation for the construction
- Competency of the material being dug - the strength of the material being dug will determine the maximum dimension of the walls of the excavation and of the overlying roof, if applicable.
- The material being dug - dig in rock and you can go down to 2000
metres; dig in sand and you won't get too far, but if you freeze the sand so it can be dug like rock you can dig deeper into it.
Other factors include:
- The proximity of geological structures, such as faults and shears;
how active such structures are and the extent of the fractured zone
of rock surrounding the structure.
- The availability of engineering construction materials such as steel,
concrete, reinforcing bars.
- How the walls of the excavation will be supported during and after