Laboratory study of consolidation of marine soft soil using flocculation-vacuum preloading-electro-osmosis

To solve the technical issues in vacuum preloading method, including the sediment clogging of drains and the limitation of drainage conditions, this study investigated vacuum preloading combined with flocculation and electro-osmosis consolidation for marine soft soil. A series of column settling tests was conducted to determine the optimal organic flocculant for the combined method. With the selected organic flocculant, laboratory tests were performed on the soil samples using the combined consolidation method, in which the electro-osmosis was added at different time moments. The tests considered three representative cases at different time moments: (i) at 48 h, i.e., the initial stage of vacuum preloading with the consolidation degree of 0; (ii) at 60 h, i.e., the obvious reduction in the dewatering speed with the consolidation degree of 60%; and (iii) at 84 h, i.e., the dewatering speed of 0 with the consolidation degree of 80%. The effectiveness of the combined method was evaluated using the tested results, including water discharge, soil vane shear strength, water content, and pore water pressure, along with the determination of the best time for adding the electro-osmosis. The results indicated that the combined method effectively delayed the decrease of dewatering efficiency and significantly increased dewatering duration. Also, the shear strength and load-bearing capacity of the consolidated soil were clearly improved, with the evenly dissipated pore water pressure. In addition, the cationic polyamide was the optimal flocculant for the combined method, which enhanced the initial dewatering speed and improved the permeability of the soft soil to solve the sediment clogging in the plastic board during the vacuum preloading. This research demonstrated the effectiveness of vacuum preloading combined with flocculation and electroosmosis consolidation for soil improvement.