Failure: Tunnel collapse, City of Serres, Greece

A series of tunnels with a diameter of 1m intended to collect and direct groundwater were located below existing residential buildings. The tunnel face had previously undergone several local instabilities, and the municipality regraded the slope and removed the debris. Grading increased the face slope from sixty degrees to 90 degrees, i.e. vertical. The face of the tunnel is shown in Figure 1 prior to failure and Figure 2 after the failure.