Effective Cohesion for Compacted Clay

When an unsupported chunk of compacted clay is placed in water, it will generally completely slake apart. However, if the sample is compacted over optimum and after the sample is allowed to cure, it may not slake apart. For the unsupported compacted clay sample, the horizontal total stress is zero, and if the sample is sheared at a very slow rate to enable the dissipation of negative pore pressures, then the effective cohesion can be determined. The effective cohesion for compacted silty clay varied from 8 psf (0.4 kPa) for one day of curing to 45 psf (2.2 kPa) for 21 days of curing. Curing allows for the development of bonds, yielding reduced swelling and higher effective cohesion. The effective shear strength envelope for the compacted silty clay at high normal stresses can be described by the linear relationship defined by the angle Φ′=28° and c′=265 psf (12.7 kPa). But in the low normal effective stress range, the strength envelope is curved with a significantly lower effective cohesion intercept.