Quantifying CPT cone factors in clays derived from weathered mudstone

The empirical interpretation of cone penetration test (CPT) cone factors (Nk) can be subject to considerable variability for clays derived from weathered mudstones, leading to significant deviations in the estimation of undrained shear strength (Su). This paper presents a comparison of triaxial and CPT data from a site investigation in clays derived from weathered mudstones in central England. Corrected cone factors (Nkt) were derived from a one-to-one comparison of 94 pairs of unconsolidated, undrained triaxial and CPT data from equivalent depths. The performance of the cone factors was evaluated using a training set (75 pairs) and a test set (19 pairs). A parametric study was used to explore the variability of Nkt, quantified using the coefficient of variation (COVNkt), for varied separation distance thresholds (Ds) between individual triaxial and CPT test data. The absolute deviation between the laboratory shear strength (Su(Lab)) and that predicted from CPT profiles (Su(CPT)) was not sensitive to Nkt values in the range 25 < Nkt < 31. The parametric study showed that Ds could be increased from 50 m to 250 m, to include more data pairs for estimates of Nkt, without substantially increasing COVNkt.