Application of vacuum pressure with prefabricated vertical drains in soft clays is a popular and effective ground improvement method. Application of vacuum pressure via vertical drains generates a negative excess pore water pressure (PWP) resulting in an immediate increase in effective stress. This paper summarises the recent advancements in vacuum preloading based on laboratory studies, using the conventional and modified Rowe cells. Location and the magnitude of the average PWP and degree of consolidation during vacuum preloading are investigated. Based on the laboratory experiments a new radial consolidation model is proposed for vacuum preloading incorporating non-Darcian flow.