Prediction of double-layer consolidation settlement in floating stone column-reinforced ground

Stone columns, as one of the most effective methods of treating soft ground problems, are commonly fully penetrated to the firm soil underneath. However, partially penetrating or floating types of stone columns are sometimes used. The consolidation behaviour of the treated and untreated layer is considered as a double-layer problem. This chapter presents a design approach to estimate the consolidation settlement and the time rate of consolidation for the double layer. For the treated layer, a simplified homogenization method is adopted where the composite soil with equivalent compressibility and coefficient of permeability is established. The degree of consolidation of the double-layer system is computed from the sum of the settlement for each layer over the total settlement, where the settlement of each layer is calculated based on the degree of consolidation at the specific time. The methods are verified against the finite element method. The design graphs are presented for a case where the coefficient of permeability of columns is five orders of magnitude higher than the surrounding soils. Different area replacement ratios and depth ratios are considered in the design approach. Design graphs and design examples are presented to allow easier implementation of the method in practice.