Microstructure study of soft clayey soil under consolidation by vacuum preloading method

Abstract

Vacuum preloading is a new technique for ground improvement. The vacuum drains out the pore-water, thus creating negative pore-water pressure and increasing the effective stress. The microstructure of soil samples after application of vacuum pressure has examined using scanning electron microscopy, and the orientation of soil aggregates and pores has been determined with GEOIMAGE software. The orientation of aggregates in fine-grained soil is an important factor influencing the anisotropy and engineering properties of the soil. Two quantitative indices to assess the orientation of the aggregates of fine-grained soil, alignment entropy () and frequency distribution function of clay aggregate orientation , are introduced in this paper. The difference between the anisotropy index and is discussed. The image analysis reveals that , , and other fabric data change with the method of consolidation; the isotropic and disordered soil fabric changes into anisotropic and ordered fabric. Therefore a quantitative analysis of the orientation of the aggregates in the soil fabric is necessary for an understanding of soil behaviour. In this paper it is shown that a combined vacuum–surcharge preloading method is a very useful and effective technique in geotechnical engineering.