Impacts of organic content and pH on consolidation of clayey dredger fill by vacuum preloading method

Abstract

The vacuum preloading method combining prefabricated vertical drains (PVDs) and surcharge preloading from vacuum pressure is widely used in the improvement of dredger fill foundation. However, a clogging effect arises within the soils around the PVDs during the vacuum preloading, which significantly affects the soil consolidation. Previous studies indicated that the organic matter and soil pH circumstance influence the soil-water interaction and soil microstructure. In order to investigate the influences of organic matter and soil pH on the consolidation of clayey dredger fill, a series of small scale vacuum preloading tests was conducted on soils with different organic contents and soil pH values. Subsequently, Scanning Electron Microscopy observation test and vane shear test were carried out to evaluate the effect of vacuum preloading consolidation. The results showed that a low organic content or a high soil pH is beneficial for soil consolidation by vacuum preloading method, resulting in a more compact microstructure and a greater vane strength. There are two actions on the organic matter during the vacuum preloading consolidation, namely, material migration and organic matter dissolution. The organic matter dissolution effect is predominate at soil pH > 9.1, so that the consolidation effect is improved. However, the material migration effect is predominate at pH = 8.1, clay particles and organic matter can be transported with the water flow and redeposit near the drain pipes, resulting in a clogging effect. Moreover, a higher pH can induce the change of microstructure from flocculated association to deflocculated and dispersed association of clay particles, which is positive to soil consolidation. The results in this paper may offer helpful references for engineering practice.