Combining NSP- and CPTu-based Nkt to evaluate undrained shear strength

Abstract

A procedure for estimating reliable ranges of Nkt for direct simple shear (DSS)-based and anisotropically consolidated triaxial compression (CAUC)-based undrained shear strength is presented. The procedure is based on a combination of three Normalized Soil Parameter (NSP) methods and two CPTu (piezocone penetrometer) methods and are related to the ratio of net cone resistance to hydrostatic effective vertical stress. The NSP and CPTu data used were acquired from seven sites: these sites have stress histories ranging from very underconsolidated to very highly overconsolidated. The stratigraphies of the seven sites consist of clays that are moderately plastic to highly plastic. This proposed procedure was then applied to 22 published sites with a wide range of fine-grained soil types and index properties. The procedure yielded very consistent and reasonable envelopes of Nkt, which in turn, provide sound DSS-based strength profiles as well as CAUC-based strength profiles. The proposed procedure also clearly demonstrates that the range for Nkt factors is not limited to 15 to 20 for normally consolidated to lightly overconsolidated cohesive sediments, which is frequently employed by practitioners.

Keywords

• in situ testing
• normalized soil parameters
• soft to hard clays
• soil sampling effects
• stress history
• undrained shear strength