Linear Trendlines to Assess Soil Classification from Cone Penetration Test Data

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, Q(t)-F-r, 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 Q(t) against f(s)/sigma'(vu), where f(s)/sigma'(vu) is sleeve friction normalized by effective vertical stress. In Q(t)-f(s)/sigma'(vu) space, CPT/CPTu data for a given soil yields a linear relationship with a slope designated Delta(Q). The linear Q(t)-f(s)/sigma'(vu) relationships from many variably overconsolidated soils converge to a common origin that is offset from zero. When plotted in Q(t)-F-r space, the linear Delta(Q) relationships become hyperbolas, similar to the Schneider hyperbolas, but the position of the individual hyperbolas in Q(t)-F-r space are defined by the offset origin location. The Delta(Q) index provides a numerical value that can be linked to soil index and engineering properties. Using Delta(Q) values computed for uniform-texture, variably overconsolidated soils, and empirical correlations between Delta(Q) and soil index properties, a CPT/CPTu assessment chart is presented to improve soil classification and the generalized Q(t)-F-r framework. (C) 2017 American Society of Civil Engineers.