The Overburden of tunnel construction is increasing recently. Rocks in the depths have high strength and high stress condition. With increasing confinement pressure

brittle rock will become more and more ductile and eventually behave like an ideally plastic medium. However

radial pressure of tunnel is rapidly reduced during excavation. The rock masses surrounding the tunnel are actually in low confinement condition. Tunnel deformation is mainly dominated by rock’s brittle properties. Brittle rock exhibits a progressive loss of strength when they are compressed beyond failure. After reviews of various parameter estimation methods of post-peak strength

the strength loss experiment method

which combines Hoek-Brown failure criterion with rock core experiment

is proposed. According to an actual case analysis

the tunnel deformation at 600 m overburden is about the same whether the post-peak strength degradation occurred or not. Nevertheless

the tunnel deformation has an obvious difference when the overburden exceeds 2 000 m. The difference of deformation is expected 3-4 times. The strength degradation of brittle rock beyond failure; therefore

it plays an important role in deep tunnel deformation.