Evaluation of CPTU Nkt cone factor for undrained strength of clays

The evaluation of undrained shear strength of clays (s_u ) is most often sought using the net cone resistance (q_net = q_t – σ_vo ) and a cone factor (N_kt ) such that s_u = q_net/N_kt . While site-specific calibration of N_{k t} with laboratory reference values (i.e. triaxial compression, simple shear) or field benchmark (i.e. vane) is the best approach, this requires considerable extra time and funding to accomplish. In the approach covered herein, a database involving 407 high-quality triaxial compression tests (CAUC) was used to review strengths from a wide variety of clays ranging from intact soft to firm to stiff to hard and fissured geomaterials. The study considered a total 62 clays, categorized into five groups: soft offshore, soft-firm onshore, sensitive, overconsolidated, and fissured clays. The backfigured N_kt factors ranged from 8 to 25 and found to decrease with pore pressure ratio, B_q = (u₂ – u₀ )/q_net .