CPT Evaluation of Yield Stress Profiles in Soils

A generalized methodology for evaluating the yield stress (σp′) of soils from cone penetration tests (CPT) is presented in terms of net cone resistance (qnet=qt−σvo) and material index, Ic. The initial baseline expression is derived from a hybrid analytical solution combining spherical cavity expansion theory and critical state soils mechanics (SCE-CSSM) that gives a first-order estimate: σp′=0.33qnet for intact clays. A commonality is found with an algorithm derived from statistical regression analysis for CPT data from calibration chamber tests on clean quartz-silica sands that gives the approximation: σp′=0.33qnet0.72. A worldwide database of 93 field sites covering a wide diversity of geomaterials including clays, silts, sands, and mixed soil types was compiled to formulate a general relationship of the form: σp′=0.33qnetm′. The exponent m′ decreases with mean grain size (D50) and increases with both fines content (FC) and CPT material index (Ic), specifically for uncemented, insensitive, and inorganic soils. Several case studies are presented to illustrate the application of the method to several well-documented sites.