Assessment of the efficacies of correction procedures for multiple thin layer effects on Cone Penetration Tests

Abstract

Multiple interbedded fine-grained layers in a sand deposit have a “smoothing” effect on the measured Cone Penetration Test (CPT) tip resistance (qc), resulting in a significant underestimation of the predicted liquefaction resistance of the sand layers. Trends identified by De Lange [14] through calibration chamber tests on stratified sand-clay profiles are used herein to develop a new thin-layer correction procedure for qc (the “Deltares” procedure). The efficacies of the Deltares and the independently-developed Boulanger and DeJong [6] procedures are both directly assessed using CPT data from calibration chamber tests and indirectly inferred from CPT-based liquefaction case histories in Christchurch, New Zealand. The results highlight limitations of the assessed thin-layer CPT qc correction procedures for layers less than 40 mm thick. Multiple, interbedded thin layers also influence the measured CPT sleeve friction (fs), but in a more complex way than they influence qc. To-date, no procedures have been proposed to address all the thin-layer-effects phenomena on the measured fs, with errors in properly characterizing the fs of a layer inherently influencing the accuracy of predicting the liquefaction susceptibility and potential of the layer. In totality, the thin-layer-effects correction procedures proposed to-date generally result in slightly less accurate predictions of the observed liquefaction severity for cases having highly stratified profiles, opposite of what would be expected and desired.