A horizontal circular cavity with a diameter of 2R0=6m is excavated in the rock mass at a depth of 400m below the surfac

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A horizontal circular cavity with a diameter of 2R0=6m is
excavated in the rock mass at a depth of 400m below the surface. It
is assumed that the natural stress of the rock mass is hydrostatic
pressure state, and the natural density of the rock mass is
ρ=2.7g/cm3. Please calculate:
(1) The redistributed stress on the wall and 2 times of the
radius of the cavity
(2) If the strength parameters of the surrounding rock are
Cm=0.4MPa, φ_m=30°, please discuss the stability of the cavity
(3) If the cavity is not stable, please calculate the radius of
the plastic ring (R1)