Laboratory scale column penetration tests for deep mixing purposes

Strength verification of dry deep mixed columns is almost exclusively performed by column penetration tests (KPS) using a probe with two wings shearing into the column. Guidelines specify a constant bearing capacity factor ( N KPS ) of 10 to determine the shear strength of the column. This factor has, however, undergone little research, and there are considerable research gaps. Results from laboratory scale tests are presented herein, where cement-improved kaolin columns with three different strengths were tested with KPS and cone penetration tests (CPT). The results showed N KPS ranging from 7.3 to 7.8 at low degrees of confinement around the columns, up to 15.4–16.5 at high degrees of confinement. The column strength did not significantly affect N KPS . The results further indicates that extraction to penetration ratio can be used to predict N KPS . Equivalent bearing capacity factors for CPT were 5.4–7.8 for tests only performed in low degrees of confinement. The findings present new knowledge that degree of confinement is crucial for N KPS , which has important practical implications, particularly that current guidelines oversimplify and might yield unconservative column strengths with increased risk of failure.