Consolidation of composite ground improved by granular columns with medium and high replacement ratio

As pointed out in the Technical Standards and Commentaries for Port and Harbour Facilities in Japan (2009), the consolidation rate of subsoil improved by the sand compaction pile with high replacement ratio tends to be delayed compared to that predicted by Barron’s equation (1948). Moreover, the time delay in consolidation becomes greater with an increase in the replacement ratio. This finding cannot be interpreted by the conventional theories. In this context, this paper attempts to interpret this finding by employing two analytical models previously developed by the authors, and by taking the consolidation of column itself into consideration. Moreover, these two analytical models are further extended to predict the consolidation rate under multiple instantaneous loading and multiple ramp loading. Then they are compared to a conventional model with no consideration of column itself. The results show that the conventional model, which does not consider column consolidation, will overestimate the average degree of consolidation, and that the overestimated value increases exponentially with the increase in the replacement ratio. Especially in the case of medium and high replacement ratios, the maximum overestimated average degree of consolidation may increase from 12% to 43% when the replacement ratio increases from 0.4 to 0.8. The time delay observed when using these two analytical models in predicting the consolidation rate is in good agreement with the finding revealed in the Technical Standards and Commentaries for Port and Harbour Facilities in Japan (2009). In particular, a simple relation is derived to calculate the time delay in a simple and explicit way. Finally, the present models are applied to a case study to verify their feasibility in predicting settlement during consolidation.