Primary and Secondary Compression Behavior of Soft Clays

Abstract

During the construction of civil engineering structures on soft clayey deposits, engineers must have to consider the consolidation behavior of the deposit. Due to the natural viscosity of clayey soils, their consolidation behaviour is strongly influenced by vertical stress as well as by organic content. As more and more constructions are concentrated in densely populated urban areas, there is an increasing need to construct buildings and geotechnical structures on soft clay materials, which usually produce significant creep deformation. Although a lot of research work has been carried out related to the secondary consolidation behaviour of a natural clay material, there are still many questions about this phenomenon. Detailed laboratory studies on soft soil are necessary for the better understanding and, consequently, better prognosis of this behavior. Some of these studies were performed and are presented in this project. One dimensional consolidation tests were conducted on ten numbers of plastic clays of plasticity index (Ip) ranging from 13% to 102%. The samples were prepared by mixing different percentages of silt with commercially available Na and Ca bentonite to have representative samples of different plasticity. Six incremental consolidation pressures were given on each soil sample of different plasticity index starting from 50 kPa, 100 kPa, 200 kPa, 400 kPa, 800 kPa and 1600 kPa. A detailed study of the results obtained from incremental loading test was investigated. From the consolidation test data, the values of different parameters like primary compression compression index (Cc), secondary compression index (C), time rate of consolidation (Cv), coefficient of volume compressibility (mv), permeability (k) were determined. The parameters were studied for factors like stress range, plasticity index and void ratio. It was found that Cc values increases with increase in the plasticity of the soil. The present experimental values of Cc were compared with those values by using available empirical relations. Correlation was established between Cc and Ip. C decreases with increase in stress range but increases with increase in plasticity index. For low to medium plastic clays, C is independent of consolidation pressure but for highly plastic clay, C is stress dependent. Correlation was established between C, Ip, and consolidation pressure (σ׀). Correlation was also established between C/(1+ep) with Ip. Other consolidation parameters like coefficient of consolidation (Cv), coefficient of volume compressibility (mv) and permeability (k) were studied with respect to plasticity index (Ip) and consolidation pressure (σ׀). Coefficient of consolidation (Cv) is less dependent on stress for highly plastic soil. Coefficient of volume compressibility (mv) is high for soft clays at low stress level. Permeability (k) is inversely proportional to stress level.