When number of tunnels are excavated parallel to each other it will change the underground stress field which will affect the stability of the tunnel. In the Q-System the value assigned is for single tunnel excavated through an area.
It is depended on using blasting method or TBM method for construction work. You can find out in Barton textbook.
The SRF is not intended for conditions with mining-induced stress change. It has not been developed or tested for such conditions. Thus, if nearby tunnelling significantly affects the behaviour of a tunnel. Q and SRF are not directly applicable. As a general statement it can be said that it must be higher but no guidance is available on how much higher.
Numerical modelling may assist in understanding the related stress path which then can be used to asses whether Q is or is not applicable.
In such cases it would be prudent to consider a higher SRF value which must be same as meant for multiple shears. The perturbations caused by multiple openings are definitely of higher scale.
Kaiser, is right, you need numerical analysis for this condition, but for initial and rough estimation, you can use the highest value for SRF as you hit the more conservative answer.
Use of Q-index for the case of multiple tunnels is not reliable because you should be using a single SRF value. If you encounter a single weakness zone in one of the tunnels, you will be taking the corresponding SRF value for both the tunnels, which is not realistic. If you can consider the spatial variation of SRF value through reliability analysis and apply it in the numerical model, you might end up with a solution closer to reality.
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