Friction Angle of Sandy Soils from CPT Using the NTH Limit Plasticity Solution

A closed-form effective stress limit plasticity solution for assessing the peak effective friction angle (φ’) in soils from piezocone penetration tests (CPTU) was developed by the Norwegian Institute of Technology (NTH) over five decades ago with the versatility that it could address all soils, including clays, silts, sands, and mixed soil types. The NTH solution utilizes normalized cone resistance (Q), pore pressure ratio Bq, and plastification angle β. A recently compiled database on 32 well-documented sands and silty sands that were subjected to in-situ CPTU soundings and field sampling with laboratory triaxial compression tests (n = 71) permits a look at the verification and validation of the NTH solution for evaluating φ' in granular soils of low compressibility. The dataset is comprised mainly of sandy soils that are quartzitic to siliceous, albeit more complex mineralogies are also found. It is found that the theoretical plastification angle (β) is directly analogous to the state parameter of sands (Ψ). Both parameters are indicators on the behavior of sand to either be contractive or dilative. Relationships linking β in terms of Ψ are derived using six available CPTU screening methods for the state parameter, Ψ. Results from the triaxial compression test database are used to validate the evaluation of φ’ from the NTH solution utilizing all three readings of the CPTU: qt, fs, and u2.