Soft soil improvement is an important subject in civil engineering, and searching for an effective admixture is an important research. Silica fume (SF) is a kind of recycled material, it can be used in engineering as a pozzolanic material. The main objective of this study is to assess the effectiveness of industrial waste silica fume (SF) as an admixture to improve the cement stabilized soft soil. The unconfined compressive test (UCT) and scanning electron microscopy (SEM) test of cement stabilized soil with different SF contents and different curing times have been carried out. UCT after 28 days revealed that the addition of SF can effectively increase the strength of cement stabilized soil and reduce the amount of cement, and 1.5% SF content is considered optimum, excessive SF will not further increase the strength. SF helped to accelerate the cement hydration reaction and significantly improve the early-age strength of stabilized soil even at 3 days, which can improve construction efficiency in actual projects. SEM analyses shows that the proper SF content could make the hydration product calcium silicate hydrate gel (CSH) fill the pores and increase the strength of the material, but excessive SF will increase the large pores content of the material and reduce the strength. This provided a basis for application of SF in improving soft soil.

The macroscopic mechanical properties of natural sedimentary soft soils, which are usually linked to their microstructure, are different from those of remolded soils. The interaction between soil structure and mechanical behavior is a manifestation of structural mechanics effects. It is essential to understand the effects of secondary compressibility to predict long-term foundation deformations. The effects of soil composition on secondary compression deformation are little studied, and the soil structure is rarely involved in the compression process. The sedimentary environment creates the initial composition and structure of soft soil, and it also basically determines its grain size and mineral composition, while different depths give soft soil different overburden pressures, and the soil composition and depth directly affect its yield stress during compression. So, natural sedimentary soft soils sampled at different depths and from different sedimentary environments (such as marine-neritic facies, sea shore facies and limnetic facies) were selected to study the influence of structure on the secondary compression coefficient C_α during pressure change and the relationship between soil composition and C_α. One-dimensional compression and consolidation creep tests were carried out on undisturbed and remolded samples. The undisturbed samples were obtained by the thin-wall samplers in rotary wash borings, and the quality of the samples met the test standard. Based on the concept of the void index I_v and the intrinsic compression line (ICL) proposed by Burland, the role of structure in the compression process was studied, and the influence of soil composition and structure on secondary compression characteristics was summarized. The C_α/C_c values are 0.031, 0.034, 0.030, and 0.036 for Shanghai, Tianjin, Suzhou, and Ningbo soft soils, respectively, within the range of inorganic clays and silts (0.04 ± 0.01) given by Mesri. According to the compression index C_c obtained by compression test, C_α/C_c can be used to estimate C_α. The yield stress of normal consolidated soil is near pre-consolidation pressure, while that of structural soft soil is greater than its pre-consolidation pressure. Natural sedimentary soft soils show over-consolidation characteristics due to the action of the structure; the soil structure resists the external load and hinders secondary compression. When the soil structure is almost destroyed, the pressure reaches the structure full yield stress P′. The tests of structural soft soils show that C_α changes with pressure before the structure completely yields, first increasing and reaching peak C_αmax near P′; the value of P′ is approximately 1.6–3.0 σ’_k, where σ’_k refers to the structure yield stress of soil obtained by the Casagrande method. After the structure disappeared, C_α gradually decreased and then stabilized, which is considered to be independent of the load. The C_αmax is positively correlated with the liquid limit, indicating that the peak value that can be reached by the C_α is related to the maximum content of bound water in soft soil, thus the soil composition has a significant influence on secondary compressibility, which contributes to the prediction of long-term foundation deformation.