Effective Stress Friction Angle of Normally Consolidated and Overconsolidated Intact Clays from Piezocone Tests

The effective stress friction angle (phi') is an important fundamental property for all soil types. A modified effective stress limit plasticity solution is presented in this paper for the evaluation of phi' of normally-consolidated to overconsolidated clays from piezocone penetration tests (CPTu). The solution takes account of stress history effect by introducing the equivalent stress concept from critical state soil mechanics (CSSM). Values of phi' obtained from laboratory triaxial compression tests on high quality samples are taken as the benchmark reference. The method is applicable to clays that are intact, insensitive, and inorganic. Example results of piezocones performed in normally consolidated kaolin in the laboratory and field tests on overconsolidated clay from Alaska are presented to elaborate application of the solution. A compiled database from 132 piezocone soundings in intact clays at field sites, 1-g chamber tests, and centrifuge series is compared with triaxial tests to show the full range of 18 degrees < phi'< 45 degrees of natural and artificial clays.