Peak strength of overconsolidated clays

The peak strength of overconsolidated clay soil is conventionally related to the effective stress by a linear Mohr–Coulomb criterion in which the parameters c′_p and ϕ′_p depend on the nature of the soil grains, on the water content, and on the range of stress over which measurements are made. For soils that have zero or very small interparticle bonding the failure line must pass through or close to the point τ = 0 and σ′ = 0, and it must meet the critical state line, so a linear Mohr–Coulomb criterion cannot satisfactorily describe the peak strength of soil except over relatively small ranges of effective stress. The paper reports the results of special triaxial tests in which the peak strengths of samples of stiff clays from southeast England were measured at small effective stresses and large overconsolidation ratios. For each soil the peak strength, with a suitable normalising parameter to take account of water content, was related to the effective stress by a non-linear power law criterion similar to that familiar in rock mechanics. In addition, the peak stress ratios were linked to a state parameter through a relationship similar to that for ordinary Cam clay. For each clay tested the material parameters that describe the peak strength were found to be only loosely related to their plasticity.