Re:The Boring Company FAQ
by jaclaz on Wednesday May 17, 2017 @12:34PM
(#14360102)
JFYT, that is a technique widely used, but normally not on tunnels that are (final diameter) TBM excavated, only or those that are excavaed through more traditional means (explosives/excavators).
Basically if you want to excavate a tunnel (say) 12 m in diameter, you first bore a tunnel with a much more small TBM, typically 3.6 m in diameter.
This pre-bore provides three great advantages:
1) Ventilation of the tunnel during the enlargement is greatly simplified
2) You have a very sensible saving on explosives (if explosives are used) or however faster excavating times (because a large part of the amount of explosive is needed to "start" the demolition of the rock, i.e. pull the center "core" out)
3) From the pre-bore you can (when you encounter problematic terrain) drill radial holes for consolidation of the surrounding terrain (though cement injection or other similar techniques)
If the final diameter is TBM excavated as well, #1 advantage above is only partial, #2 is almost non-existing while of course #3 remains, BUT, you cannot make the pre-bore much smaller than the 3/3.6 m diameter as otherwise it would be only useful as a geo prospection mean, as it wouldn't be practical to use it for anything else, and there would surely be issues (for smaller diameters) with ventilation.
AND the other bad news are that - more or less and as a first approximation - a TBM has an operative tunnelling speed of roughly 400 m/month that is almost independent from the diameter (while a traditionally excavated tunnel is more around 100 m/month without pre-bore and 120/130 m/month with the pre-bore), so while it may make sense (of course a lot of factors are involved), if you have a 4 Km tunnel to make a pre-bore that will add roughly 12 month to your 50-60 months project excavation time (traditional), it won't make much sense unless for other very particular considerations to add those same 12 months to the 12 months a full face TBM would take.
What is often done on full face excavated tunnels with TBM is prospection holes (horizontal, drilled from the face of the TBM) in the 30-50 m range, to explore what lies ahead in the immediate future.
This pre-bore provides three great advantages:
1) Ventilation of the tunnel during the enlargement is greatly simplified
2) You have a very sensible saving on explosives (if explosives are used) or however faster excavating times (because a large part of the amount of explosive is needed to "start" the demolition of the rock, i.e. pull the center "core" out)
3) From the pre-bore you can (when you encounter problematic terrain) drill radial holes for consolidation of the surrounding terrain (though cement injection or other similar techniques)
If the final diameter is TBM excavated as well, #1 advantage above is only partial, #2 is almost non-existing while of course #3 remains, BUT, you cannot make the pre-bore much smaller than the 3/3.6 m diameter as otherwise it would be only useful as a geo prospection mean, as it wouldn't be practical to use it for anything else, and there would surely be issues (for smaller diameters) with ventilation.
AND the other bad news are that - more or less and as a first approximation - a TBM has an operative tunnelling speed of roughly 400 m/month that is almost independent from the diameter (while a traditionally excavated tunnel is more around 100 m/month without pre-bore and 120/130 m/month with the pre-bore), so while it may make sense (of course a lot of factors are involved), if you have a 4 Km tunnel to make a pre-bore that will add roughly 12 month to your 50-60 months project excavation time (traditional), it won't make much sense unless for other very particular considerations to add those same 12 months to the 12 months a full face TBM would take.
What is often done on full face excavated tunnels with TBM is prospection holes (horizontal, drilled from the face of the TBM) in the 30-50 m range, to explore what lies ahead in the immediate future.