CPT Evaluation of Yield Stress Profiles in Soils

A generalized methodology for evaluating the yield stress (${\sigma }_{\mathrm{p}}^{\prime }$) of soils from cone penetration tests (CPT) is presented in terms of net cone resistance (${\mathrm{q}}_{\mathrm{net}}={\mathrm{q}}_{\mathrm{t}}-{\sigma }_{\mathrm{vo}}$) and material index, ${\mathrm{I}}_{\mathrm{c}}$. 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: ${\sigma }_{\mathrm{p}}^{\prime }=0.33{\mathrm{q}}_{\mathrm{net}}$ 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: ${\sigma }_{\mathrm{p}}^{\prime }=0.33{\mathrm{q}}_{\mathrm{net}}^{0.72}$. 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: ${\sigma }_{\mathrm{p}}^{\prime }=0.33{\mathrm{q}}_{\mathrm{net}}^{{\mathrm{m}}^{\prime }}$. The exponent ${\mathrm{m}}^{\prime }$ decreases with mean grain size (${\mathrm{D}}_{50}$) and increases with both fines content (FC) and CPT material index (${\mathrm{I}}_{\mathrm{c}}$), specifically for uncemented, insensitive, and inorganic soils. Several case studies are presented to illustrate the application of the method to several well-documented sites.