Comparing CPTU Q–F and Q–Δu₂/σ_v0′soil classification charts

This paper extends a framework for classifying soil using normalised cone resistance (q_t) and porewater pressure (u₂) data from piezocone tests (CPTU) to include the friction ratio. Analytical predictions of normalised cone tip resistance (⁠$Q=qn/σ′v0$⁠) and friction ratio (F  =  f_s/q_n) for a range of overconsolidation ratios are compared to databases of piezocone measurements during undrained penetration at clay sites, as well as to data from sands, silts and mixed soil types having partial to full drainage. Aspects of soil behaviour that result in data plotting in different zones of the complementary charts are discussed. It is concluded that Q–F classification charts are useful in the absence of high-quality pore pressure data. However, when attempting to separate the effects of yield stress ratio from partial consolidation during penetration in soils that may have some degree of sensitivity, structure or cementation, additional site-specific calibration may be required.