On combined method of dry mixing with vertical drain and its practice in soft ground improvement

A remarkable combined method of the dry mixing with vertical drains (2D method) has been innovated and successfully practiced in a highway project on very soft clay in Jiangsu, China. The mechanism, construction, in-situ test and monitored results on dry mixing columns and soils between the columns are presented. It is shown that the pore water pressure in the surrounding soft clays of the dry mixing column increases significantly during the jet mixing, resulting in some problems, such as the slope failure, non-uniform mixing, and poor quality in the deeper part of the column. With the vertical wick drains placed between the columns before dry mixing, the pore water pressure caused in the construction dissipates fast, not only improving the quality of the dry mixing column but also enhancing the soil strength. Then, the bearing capacity of the composite foundation improved with 2D method increases obviously compared with the traditional dry deep mixing method. The monitored results including settlements, pore water pressure, and stress on the column and ground surface conducted in a 100 m pilot project of nighway verify the innovation method much valuable.