Engineers Australia

Separating influences of yield stress ratio (YSR) and partial drainage on piezocone responseWinner 2007 Baden Clegg Award

James A. Schneider


Calculation of failure loads is typically a part of most geotechnical designs. For evaluation of the potential for failure, soil behaviour is essentially governed by the initial yield stress ratio (state), the stress level at failure, and the degree of consolidation during loading. These facets of behaviour will not only influence geotechnical design, but also strongly influence the soil response measured during a cone penetration test (CPT) with pore pressure measurements, or piezocone penetration test (CPTU). This study evaluates a method to separate the influence of yield stress ratio (YSR) from partial drainage during a CPTU, with particular application to soil classification by piezocone. Increases in YSR and degree of consolidation during loading tend to result in an increase in normalized cone tip resistance (Q=qcnet/σ’v0) and decrease in pore pressure parameter (Bq=∆u2/qcnet), which are typically used for soil classification by piezocone. Using theoretical studies, centrifuge experiments, field experiments, and databases of CPTU measurements, this study illustrates that for many cases the influence of YSR and partial consolidation have opposite effects when plotting data as Q against ∆u2/σ’v0 (=Bq⋅Q). Therefore, charts of Q plotted against ∆u2/σ’v0 are more useful for evaluation of soil type than conventional plots of Q against Bq.