Effect of spacing of grid PHD on performance of combined PHD–PVD vacuum preloading method for treatment of clayey slurry

A novel ground improvement method that combines grid prefabricated horizontal drains (PHDs) with prefabricated vertical drain (PVD) assisted by vacuum preloading is proposed for the beneficial reuse of dredged clayey slurry for reclamation purpose. To assess the feasibility of this innovative method, physical model tests are designed and conducted using high-water content Hong Kong marine deposits as the clayey slurry material. Furthermore, the impact of the spacing configuration of the grid PHD on the effectiveness of the proposed method is investigated through a series of model tests. A test without the installation of PVD was set, and in this case, two phases of vacuum preloading are applied sequentially through the PHD layer installed in stage. The other three tests involve three phases, with the addition of a vacuum preloading stage through PVD and variations in arrangement pattern of grid PHD layer. Results show that this proposed approach yields a final average undrained shear strength of soil of approximately 30 kPa, meanwhile reducing the average water content to around 50%. Furthermore, it is observed that decreasing the vertical spacing of grid PHDs results in growing final settlement. Reducing the horizontal spacing has less impact on the final settlement.