Ground Improvement Using Vacuum Loading Together with Vertical Drains

The use of vacuum as a preload together with vertical drains to reduce postconstruction settlement and increase the shear strength of soft ground is increasingly popular. However, conflicting views concerning preloading by vacuum consolidation continue to be disseminated. A review and interpretation of case histories of vacuum loading together with vertical drains indicates that (1) with a novel definition of excess pore-water pressure, existing theories of consolidation, solutions, and associated computer programs that have been developed for fill loading can be applied without any modification to vacuum loading; (2) vacuum that is available in the drainage blanket remains constant with depth within the vertical drains; (3) for vacuum loading as for fill loading, vertical drains may display well resistance; (4) there is no difference in magnitude and rate of settlement for a vacuum load and an equivalent fill load; (5) all empirical concepts of undrained shear strength that have originated from fill loading of soft ground are equally applicable for vacuum loading; (6) a correlation between vertical settlement and horizontal displacement for vacuum loading is expected because both result from consolidation; and (7) preloading by vacuum is accomplished in a shorter period because there is no possibility of undrained failure during vacuum loading, whereas fill loading may require construction in stages to avoid undrained bearing-capacity failure.