Nội dung text 18 Shear Strength.pdf
Geotech 06: Shear Strength of Soil Shear Strength - The internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it. 1. Mohr-Coulomb Failure Criterion Mohr (1900) presented a theory for rupture in materials that contended that a material fails because of a critical combination of normal stress and shearing stress and not from either maximum normal or shear stress alone. For most soil mechanics problems, it is sufficient to approximate the shear stress on the failure plane as a linear function of the normal stress. Mohr-Coulomb Failure Criterion τ = c + σ tanφ Where: c = cohesion φ = angle of internal friction σ = normal stress on the failure plane τ = shear strength The effective stress σ′ is carried by the soil solids. The failure criterion expressed in terms of effective stress, τ = c ᇱ + σ ᇱ tan φ ᇱ Where: c′ = cohesion based on effective stress σ′ = effective stress φ′ = friction angle based on effective stress / drained angle of friction
Figure 1. Mohr-Coulomb Failure Criterion Notes: The value of c′ for sand and inorganic silt is 0 For normally consolidated clays, c ᇱ ≅ 0 For overconsolidated clays, c ᇱ > 0 2. Inclination of the Plane of Failure caused by Shear Angle of Failure θ = 45 + φ ᇱ 2 Figure 2. Inclination of failure plane in soil with major principal plane Relationship of the principal stresses: σଵ ᇱ = σଷ ᇱ tanଶ ቆ45 + φ ᇱ 2 ቇ + 2cᇱ tan ቆ45 + φ ᇱ 2 ቇ
Figure 4. Diagram of triaxial test equipment Connections to measure drainage into or out of the specimen, or to measure pore water pressure, also are provided. 4.1. Consolidated-Drained (CD) Triaxial Test In this test, the saturated specimen first is subjected to an all-around confining pressure, σଷ, by compression of the chamber fluid. As confining pressure is applied, the pore water pressure of the specimen increases by u (if drainage is prevented). If the connection to drainage is opened, dissipation of the excess pore water pressure, and thus consolidation, will occur. With time, u, will become equal to 0. Because the pore water pressure developed during the test is completely dissipated, σଷ = σଷ′ σଵ = σଵ ᇱ = σଷ + (∆σௗ )
English