Seismic loading

[sonamoni]

Dear All,

In Numerical analysis of Tunnels, seismic loading is applied at the final stage of construction.

Is it necessary to apply at intermediate stages of excavation to check conditions such as earthquake occurs during construction.

Generally in deep ground tunnels earthquake loadings are neglected, are there any general rules for depth below ground surafce where the effect of earthquakes diminishes.

Best regards,

Debasis

 

[miningman]

I would have thought that your potential project has MAJOR safety problems if you even have think about eartquake considerations during construction.

However, having said that, I suspect that it will be geological issues rather than depth below surface that will ultimately be the determining factor. I know men who have been working in hard rock mines during an earthquake who werent even aware of the 'quake untill they returned to surface and yet I wouldnt want to be in an underground structure in relatively incompetant ground, at any depth, regardless of the type and amount of ground support that was already in place, during anything other than a very minor quake.

Check out the literature on the recent catastrophic loss of life in the USA where essentially, an man-induced earthquake type coal bump caused major failure of ground support that was otherwise adequate.

 

[msucog]

let me think out load for a moment:

luckily for those underground, your everyday earthquakes are not nearly as destructive as for those standing in a building on top of the ground. in my opinion, if most of the underground structure is in reasonably "hard" material, the major places i'd be concerned would be the upper portions of shafts where the seismic forces are much stronger. i guess i'd be nervous while in the tunnel itself if you're in an area that may experience vertical shifts or displacement or maybe if you're not in rock where hydraulic forces could get jacked way up.

you might consider the seismic forces in a manner similar to the building code's very generalized format. very high velocity or hard material experiences much lower seismic forces than say a low velocity/soft, plastic soil. even then, a lot of the problems typically associated with construction on top of the ground are due to the ground moving and the building trying to follow that ground movement. once you get down in to the ground, i'm fairly certain it's a much different ballgame. using a made up example with the ibc seismic loading considerations, a tunnel in rock would a site classification A or B so the amplification factors would be very low and the ultimate design parameters would not be anything close to those forces that would be felt at the ground surface where the site class may be a D, E or worse. i don't have the table in front of me, but the amplification factor difference between a site class of A and E would be something like a factor of 4 or more.

for seismic loading, i think (and this is purely my assessment) that it is typically not fully considered during construction unless maybe you're in a high seismic area. if you happen to have a project that spans many years, then i would reconsider this concept to account for the possibility of an earthquake some time during the course of the project.

as miningman mentions, the geology would likely govern much more than the potential of an earthquake (unless you happen to in a high seismic area where thrusting is possible).

i'm not sure any of random thoughts will help but i hope their on target with the thoughts of others that have been around much longer than me. now that you've brought up the question and since i'm involved in tunnel projects (not necessarily design), i'll see what i can find on the subject so that i will possibly have a better understanding myself. if i happen to find a definitive answer to your question, i'll post it.

and i'd definitely be interested to hear the thoughts of others since i rarely see folks posting about the underground tunnels or other structures.

 

[sonamoni]

In Tunnelling problems we tell about deep tunnels and shallow tunnels. are there any definite criteria of depth below ground surface where it can be identified as deep/shallow?

 

[msucog]

i am not certain. i will check in to your question and post back anything i find.