Exactly in what situations are tunnels used instead of just cutting the rock away?
I have seen a number of layouts lately that had tunnels with only three, four inches of rock above them... example in HO.
To me this seems to short. When modeling a tunnel how much rock should be above it?
Well...4" in HO approximates to 30 feet...so the question is...was it "easier" back then to blast a tunnel rather than blast a 50foot deep cut...also "cheaper" or "quicker"??
I guess if the answer to any of the above is "Yes", then 4" is protypical!!! :-)
Mike Simpson
The question of tunnel vs. cut ultimately depends on which will provide the most stable structure, which in turn is based on a number of geological and engineering considerations. A 40-ft cut may
seem to be preferable to a 20-ft high tunnel with 20 feet of rock overburden. However, if the rock is thinly-layered or jointed and the layers or joints dip toward the planned right-of-way, a cut may
result in chronic slope failure; this would result in higher long-term maintenance costs and possibly catastrophic events. A tunnel in such a situation may have a higher initial cost but lower
maintenance costs and a higher stability.
John Monrad