Linear Trendlines to Assess Soil Classification from Cone Penetration Test Data

Abstract

The data format developed by Robertson and his coworkers to assess soil behavior type with cone penetration test data using a series of concentric circles can provide an unreliable assessment in overconsolidated soils. With increasing overconsolidation, the normalized net-tip stress-normalized friction ratio, Qt–Fr, data for uniform-texture, but variably overconsolidated soils follow the shape of a hyperbola as shown by the generalized soil behavior type chart developed by Schneider and his coworkers. To improve soil classification from cone penetration test/piezocone penetration test (CPT/CPTu) data, a method is proposed that plots Qt against fs/σvo′, where fs/σvo′ is sleeve friction normalized by effective vertical stress. In Qt–fs/σvo′ space, CPT/CPTu data for a given soil yields a linear relationship with a slope designated ΔQ. The linear Qt–fs/σvo′ relationships from many variably overconsolidated soils converge to a common origin that is offset from zero. When plotted in Qt–Fr space, the linear ΔQ relationships become hyperbolas, similar to the Schneider hyperbolas, but the position of the individual hyperbolas in Qt–Fr space are defined by the offset origin location. The ΔQ index provides a numerical value that can be linked to soil index and engineering properties. Using ΔQ values computed for uniform-texture, variably overconsolidated soils, and empirical correlations between ΔQ and soil index properties, a CPT/CPTu assessment chart is presented to improve soil classification and the generalized Qt–Fr framework.