Q$2$:

Two tunnels A and B with circular cross$-$sections are to be excavated parallel to each other and in the same horizon at a depth of $400$ meters below the ground. The stress condition is also assumed to be hydrostatic. The diameter of tunnel A is designed to be $12$ meters and the diameter of tunnel B is $14$ meters. Calculate the minimum center$-$to$-$center distance between the cross$-$sections of these two tunnels so that they do not affect each other and analyze the reasons.