  We demonstrated the stability of tunnels made of granular matters is strongly
dependent on the grain size, tunnel diameter, and water content in the
granules. Larger tunnel radius, larger grain size, and too much water content
tend to destabilize the tunnel. We also develop a model to describe such
findings. We identified a phase diagram of stability which greatly controlled
by granular bond order. For granular bond order of larger than unity, we can
always made a stable tunnel. However, for granular bond order of less than
unity, we obtain a general expression for maximum tunnel thickness that can be
made. To best of our knowledge, this is the first exploration regarding the
granular tunnel stability.
