Inflection point method for predicting settlement of PVD improved soft clay under embankments

Abstract Prefabricated vertical drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankments on soft ground. The construction is carried out in stages and the height of each stage construction depends on gain in strength of soft subsoil and target factor of safety. The inflection point method for estimating the degree of consolidation for vertical drainage has previously been published. The degree of consolidation was estimated to be 70% at the inflection point. After 70% consolidation, the rate of consolidation reduces to a minimum value and it is economical and technically feasible to allow the second-stage loading. Additional load at this stage would ensure increase in shear strength of the subsoil. In this paper, the authors have extended the procedure of inflection point method for vertical drainage to a three-dimensional drainage when PVDs are adopted for subsoil improvement. Theoretical graphs have been developed which can estimate the percent consolidation at inflection point (% Ui) for different subsoil thickness, drain spacing ratios and Cr/Cv ratios. Different factors viz. smear, drain spacing, depth of clay thickness and well resistance were also considered in the analysis. The degree of consolidation at inflection point is observed in the range of 61–78%. The inflection point method has been applied to a settlement data from a case history of two sites and the estimated percent consolidation at inflection point has been compared with the values predicted from developed theoretical graphs. The total primary settlement estimated from inflection point has been compared with that of Asaoka method, which is widely used in the field. It is concluded that inflection point method has the potential for field application and provides an alternate method for estimating of total settlement in field applications using PVDs and surcharge, and to determine the appropriate required waiting period for stage loading.

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