I was browsing for Gibraltar strait tunnel where due to greater depth it is not being build. Question popped up as to why tunnel building is difficult at lets say 1000m. we have coal mines which go deeper than that, than why not tunnel?
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1$\begingroup$ So how are coal mines different to tunnels for traffic flow? What conditions will you have to engineer for? $\endgroup$– Solar MikeCommented Oct 1, 2017 at 6:51
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$\begingroup$ for what its worth the most viable ( or least ridiculous ) Gibraltar tunnel concept is only 300 meters depth en.wikipedia.org/wiki/Strait_of_Gibraltar_crossing $\endgroup$– agentpCommented Oct 2, 2017 at 20:09
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1 Answer
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Tunnel Access
To begin, to construct and use a tunnel, it needs to be accessed. Access will be via either a vertical or sub-vertical shaft or a ramp that wheeled vehicles can use.
Shafts may be practical for mines, but for a traffic tunnel access via a shaft is impractical because of the rate of transfer of vehicles from one level to the other. A small number of vehicles will be able to fit into a cage in a shaft & then the cage will be lowered or raised, transporting the vehicles. This is OK for mines but not for a large number of vehicles carrying trade goods or cars with families.
This means that the only practical way of accessing such a tunnel is via a ramp. Irrespective of whether the ramp is straight or spiraled, its overall length will be dictated by the gradient of the ramp.
Assuming a uniform gradient of 1 in 10 (10 percent), for a tunnel 1000 m (1 km) deep, the horizontal length of the ramp will be 10 000 m and its slope length will be 10 050 m (10.05 km). Such a ramp will need to be constructed at both the entry and exit of the tunnel. Thus, the overall length of the tunnel will need to be an additional 20.1 km. This would add significant cost to the construction of the tunnel.
For vehicular traffic, will one tunnel, divided in half length-ways be enough, or will two tunnels be required: one allowing for traffic to flow in one direction and the other tunnel for traffic to flow in the other direction. If two tunnels are needed, then double the cost of construction.
Tunnel Ventilation
Irrespective of the type of vehicles that use the tunnel, electrically powered vehicles or combustion engine powered vehicles, ventilation will need to be provided to the tunnel.
Ventilation can be provided via ventilation shafts or another tunnel dedicated to ventilation and servicing the vehicular transportation tunnel. If the tunnel passes under water, such as the Straits of Gibraltar, then ventilation shafts cannot be used along the route of the tunnel - a dedicated ventilation tunnel must be used.
Then there is the cost of powering the ventilation fans and how the energy will be sourced to do so - solar, wind, gas, nuclear, coal power stations.
Ground Control
Another issue that will need to be considered is that at 1000 m depth ground stresses may be large which will dictate the form of ground control measures and the subsequent cost.
Tunnel Rock Temperature
Additionally, at 1000 m depth the rock temperature will be high. Depending on location, temperatures of 40 C to 60 C are not unusual. To cater for this, will the vehicular transportation tunnel need to be cooled via an air cooling system. This will cost a lot of money to install and operate. Again, the issue of a power source for such a system will need to be addressed.
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$\begingroup$ Nice explanation. Covered most aspects of engineering. $\endgroup$ Commented Oct 1, 2017 at 8:39
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$\begingroup$ With regard to ground control the comparison with coal mines and tunnels at 1000m is an important one. Coal mines only work in the form mentioned because the groundwater is pumped out so that they are dry. For a railway under the sea this wouldn't be achievable using pumping. Grouting used in Norwegian subsea tunnels might be achievable although the geology is very different. If the tunnel adopted a waterproof lining then the water pressure would be very high. At a depth of 1000m the water pressure could be maybe an order of magnitude higher than the highest pressures for other tunnels. $\endgroup$ Commented Apr 25, 2018 at 15:32