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# strength4 - 53:030 Class Notes C.C Swan University of Iowa...

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Period #24: Shear Strength of Soils (IV) A. Review and Motivation The Mohr-Coulomb shear strength model for soils is of the form: | τ f | = c + σ′ n tan( φ D ) where: | τ f | is the absolute value of shear stress that causes failure on a given plane; c 0 is the cohesion of the soil; σ′ n is the normal effective stress on a plane; and φ D 0 is the so-called drained angle of friction for a soil. Typically, one does not expect to see a value of φ D greater than about 45 o . Note that in the above shear strength criterion, the normal stresses are always effective stresses. For saturated, fine-grained soils subjected to loads, the effective stresses change over long periods of time as excess pressures dissipate. Therefore, the shear strength of a saturated, fine-grained soil will generally change (increase) with time as excess pore pressures dissipate. This period, we discuss different strength models for fine-grained soils that are based on combinations of total stresses and effective stress. Typically, models based on effective stresses, apply to the long term drained behavior of fine-grained soils. Models based on total stresses typically apply to short-term behaviors such as in the first few weeks/months after a structure is built upon a fine-grained soil deposit. 1 53:030 Class Notes; C.C. Swan, University of Iowa

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B. Drained Shear Strength Behavior of Clays For normally consolidated clays, c 0, and the friction angle is denoted φ D . For over-consolidated clays, c > 0, and the friction angle is denoted by φ D-1 . When the soil is overconsolidated: (or when σ′ < σ′ c ) | τ f | = c 1 + σ′ n tan( φ D-1 ) When the soil is normally consolidated: (or when σ′ = σ′ c ) | τ f | = σ′ n tan( φ D ) Typical values of φ D for silty/clayey soils: 12 o φ D 30 o , φ D 30 o for low plasticity soils (PI = 5-10); φ D 12 o for high plasticity soils (PI = 50-100); 12 o ≤φ D 30 o for moderate plasticity soils (10 PI 50) 2 53:030 Class Notes; C.C. Swan, University of Iowa τ c 1 φ D φ D-1 σ′ c σ′
53:030 Class Notes; C.C. Swan, University of Iowa Example 24.1: 3

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C. Drained and Undrained Strength Behaviors of Fine-Grained Soils For simplicity, we will first discuss this in the context of triaxial compression tests, and then discuss the drained/undrained strength behaviors in terms of geotechnical applications.
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