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I think the millimeters is the optimum accurate
Based on your question I would like to ask you that are your working which type of tunnelling. Is it micro tunnelling or other open cut (by blasting or other method) it should be cleared?
For any construction of tunnelling the tolerance in both vertically or horizontally mentioned in the project specification. And we try to flow that.
Here I would like to say you that practically it is very difficult to achieve the tolerance. Why I say like that?
During construction work our TBM machine to go through different soil criteria, so there is chance to misleading the actual alignment, but we should control it near our tolerance to achieve the correct/target break through. Same happing in blasting method.
During tunnelling we try to mention the alignment in a transition curve alignment so we reach the target point.
The accuracy should be according to project specifition. And we try for it ,if the middle of the path diverted in little amount the break through part should be reach in the target point.
I am working usually in express way but i think that in tunnel surveying radius accuracy is needed in millimeters.
i believe if we are working with latest surveying instruments there should be accuracy% or withen centimeters
