During the excavation of the Ganntas Tunnel, in the Algerian Atlas mountain area, high convergences occurred slightly after temporary support installation, and re-profiling operations were carried out along more than 100 m of tunnel. The failure occurred when the tunnel face reached a faulted zone in clayey marls with an overburden of about 100 m. The geological investigations proved to be insufficient at design stage since the presence of such material along the tunnel alignment was unknown until the construction stage.
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