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Unformatted text preview: Soil Strength Soil strength ◆ Soils are essentially frictional materials – the strength depends on the applied stress ◆ Strength is controlled by effective stresses – water pressures are required ◆ Soil strength depends on drainage – different strengths will be measured for a given soil that (a) deforms at constant volume (undrained) and (b) deforms without developing excess pore pressures (drained) MohrCoulomb failure criterion The limiting shear stress (soil strength) is given by τ = c + σ n tan φ where c = cohesion (apparent) φ = friction angle τ σ n • The parameters c, φ are in general not soil constants. They depend on • the initial state of the soil (OCR or I d ) • the type of loading (drained or undrained) • The MohrCoulomb criterion is an empirical criterion, and the failure locus is only locally linear. Extrapolation outside the range of normal stresses for which it has been determined is likely to be unreliable. MohrCoulomb failure criterion Effective stress failure criterion τ σ φ = + ′ c n ' tan ' are known as the effective (or drained) strength parameters. If the soil is at failure the effective stress failure criterion will always be satisfied. Effective stress failure criterion τ σ φ = + ′ c n ' tan ' are known as the effective (or drained) strength parameters. Soil behaviour is controlled by effective stresses, and the effective strength parameters are the fundamental strength parameters. But they are not necessarily soil constants. If the soil is at failure the effective stress failure criterion will always be satisfied. Total stress failure criterion τ σ φ = + c u n u tan If the soil is taken to failure at constant volume (undrained) then the failure criterion can be written in terms of total stress as c u and φ u are known as the undrained strength parameters Total stress failure criterion τ σ φ = + c u n u tan If the soil is taken to failure at constant volume (undrained) then the failure criterion can be written in terms of total stress as c u and φ u are known as the undrained strength parameters These parameters are not soil constants, they depend strongly on the moisture content of the soil. Total stress failure criterion u u n u s c = + = φ σ τ tan If the soil is taken to failure at constant volume (undrained) then the failure criterion can be written in terms of total stress as c u and φ u are known as the undrained strength parameters These parameters are not soil constants, they depend strongly on the moisture content of the soil. The undrained strength is only relevant in practice to clayey soils that in the short term remain undrained. Note that as the pore pressures are unknown for undrained loading the effective stress failure criterion cannot be used. Tests to measure soil strength 1. Shear Box Test Motor drive Load cell to measure Shear Force Normal load Rollers Soil Porous plates Top platen Measure relative horizontal displacement, dx vertical displacement of top platen, dy ◆...
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 Three '11
 DAirey
 total stress, strength parameters, effective strength parameters, cell pressure

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