Investigation of Factors Affecting Vertical Drain Behavior

Some influencing factors on vertical drain behavior were investigated by laboratory tests as well as by back-analyses of test embankments on vertical drain improved subsoil at Saga Airport, Saga, Japan. Based on the results from this study, suggestions are made on determining the design parameters for vertical drain improvement. For the discharge capacity test of a prefabricated vertical drain, confining the drain in clay is essential. Also, due to the creep of the filter and the clogging caused by the fine particles entering the drainage channel, the long-term discharge capacity is significantly smaller than the short-term one, and this should be considered in design. For smear effect, a new equation is proposed for determining the ratio of the hydraulic conductivity of the natural subsoil to that of the smear zone, which considers the fact that laboratory tests normally underestimate the hydraulic conductivity of natural deposits. Regarding the effect of the sand mat, the numerical analysis results in this study show that if the hydraulic conductivity of sand is larger than 10−4 m/s, the assumption of a free drainage condition in the sand mat may not result in significant error. Finally, a methodology of predicting the behavior of vertical drain improved subsoil is proposed.