Prediction of Unloading Failure Strain and Undrained Shear Strength of Saturated Clays by Limit Pressure from Prebored Pressuremeter

Abstract Saturated clays mostly fail in lateral unloading by the reduction of undrained shear strength (Su) because of an increase in lateral unloading strain (εu), which is the most predominant factor affecting the soil mass in excavated structures. An attempt is made to extend the work of Houlsby and Withers (1988) on the self-boring pressuremeter (SBP) and Ferreira and Robertson (1994) for the full-displacement pressuremeter (FDP) and SBP, for unloading curves of the prebored pressuremeter (PBP) test for large strain (up to 41.5 %) tests in a specific type of soil (i.e., saturated lean clay, which so far have been rarely analyzed). This article presents a study to examine and explain the magnitude of strains developed in loading and unloading curves of PBP to predict the ratio of compression to unloading strain (εc/εu) and Su from unloading curves, for which open field experimentation is conducted on saturated clay deposits with a plasticity index of 10 to 16 %. The results of εc/εu and Su that were determined from the undrained PBP tests were compared with anisotropically consolidated undrained triaxial compression tests, which showed reasonable agreement. Correlations are proposed for net limit pressure versus εc/εu and Su from PBP, which offer a new approach to quantifying εu and Su by solely performing compression curves. The proposed correlations are validated by performing PBP tests at two separate sites and are further supported by comparison with the pressuremeter curves of previous studies. The variation in experimental versus predicted values for two separate sites (using the proposed correlation) is within ±10 % for a confidence interval of 95 %.