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Variations in cohesion / friction with strain and significance in design

By K. O’Brien & A. Parrock

This article details findings into variations of cohesion and friction angle throughout the duration of a triaxial test. It is proposed that these parameters are, in fact, not only functions of soil tested, but also of strain. Strains within a structure under serviceability (SLS) and ultimate limit state (ULS) are vastly different. It is thus postulated that cohesion and friction angle values of a soil will also differ at these 2 limits states. A method of determining, via reference to triaxial test results, appropriate values of cohesion and friction angle under serviceability and ultimate limit states, is discussed.

Generally it was determined that, under characteristic SLS strain levels, high cohesion and low friction values are attained, while the reverse is true at high strains, characteristic of ULS conditions. These findings are very relevant to limit state relevant to limit state designs, which are now the norm in geotechnical codes.