Laboratory scale column penetration tests for deep mixing purposes

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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 (NKPS) 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 NKPS ranging from 7.3–7.8 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 NKPS. The results further indicates that extraction to penetration ratio can be used to predict NKPS. 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 NKPS, which has important practical implications, particularly that current guidelines over-simplify and might yield unconservative column strengths with increased risk of failure.