Modified NTH Method for Assessing Effective Friction Angle of Normally Consolidated and Overconsolidated Clays from Piezocone Tests

AbstractAn effective stress limit plasticity solution developed at the Norwegian Institute of Technology (NTH) provides a theoretical evaluation of the effective stress friction angle (ϕ′) for soft to firm normally consolidated (NC) to lightly overconsolidated clays (LOC) from piezocone penetrometer tests (CPTu). A modified NTH method using the equivalent stress concept is detailed to allow the evaluation of effective stress friction angle for overconsolidated (OC) clays. The solution requires a knowledge of the soil stress history, specifically, the overconsolidation ratio (OCR). A wide variety of clays have now been calibrated using this approach, including measured CPTu and triaxial data collected from six categories: (1) laboratory chamber tests in NC clays; (2) centrifuge testing in NC clays; (3) centrifuge tests in OC clays; (4) field CPTu in natural NC-LOC clays; (5) field CPTu in OC intact clays with OCR>2.5; and (6) field CPTu in OC fissured clays. Results from compiled databases are statistically shown to validate the CPTu-interpreted ϕ′ values in comparison with laboratory benchmark values obtained from CAUC and CIUC triaxial tests made on high quality samples. Case studies of CPTu soundings conducted in the aforementioned categories are presented to elaborate the application of the NTH solution.