Experimental study of airlift drainage process in airlift-assisted vacuum preloading

A novel soft ground improvement technique termed airlift-assisted vacuum preloading (AAVP) has recently been proposed. This method cyclically injects pressurized air at the bottom of prefabricated vertical drains (PVDs) to lift and expel water from the drain core, thereby achieving nearly lossless transmission of vacuum pressure along the PVDs. Large-scale model tests have shown that AAVP provides significantly higher consolidation efficiency than conventional vacuum preloading, yet the key airlift drainage process that governs water removal from the PVDs, along with the influence of operating parameters, is not fully understood. To address this, 237 airlift tests were conducted on a 6-m-long hollow-core panel. A variety of pressure combinations, initial submergence ratios, airlift connector geometries, and PVD bending deformations were considered. The results show that larger pressure differentials greatly reduced drainage duration. Increasing the airlift pressure ratio improved drainage rates by >50%, while optimizing the initial submergence ratio enhanced drainage efficiency by about 51.4%. Importantly, bending of PVDs shortened drainage completion time by 30% with only a minor reduction (1.1%) in total drained mass, and optimized connector geometry further increased both drainage mass and rate. The findings clarify the mechanisms underlying airlift-induced drainage and provide valuable insights for parameter optimization and engineering application of AAVP in soft ground improvement.