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3. A circular cavern is constructed in underground. (1) Strength of rock specimens is 15 MPa. P-wave velocities of rock specimens and rock mass are 4.0 km/s, 3.0km/s,respectively. Estimate strength of rock mass. (2) Find the maximum depth (h) at where no plastic zone is developed around the cavern. Vertical stress sigma can be estimated by sigma = p gh. Where density of rock mass p is 2,600 kg/m³ and gravity acceleration g is 9.8 m/s². Horizontal stress sigma H is assumed to be 1.5 sigma

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