A rational design approach to peat ground improvement by vertical drains

Abstract

Use of the vertical drain method (hereinafter called “VD”) in peat ground is spreading to reduce residual settlement and increase strength by accelerating consolidation. In recent years, high embankments of nearly 10 m have been constructed on peat ground by using a combination of VDs, or plastic-board drains (PBDs) in particular, and various ground stabilization techniques. However, the conventional design methods include a number of unclear points. This paper focuses on the significant stress-dependency of coefficient of consolidation in peat, and describes a rational design approach to evaluating the long-term settlement and stability of foundation ground of VD-improved peat ground by modifying conventional design methods. The effect of VDs was evaluated from field observations on the settlement of test embankments constructed in Hokkaido, Japan, and simulations using the finite-element method were performed to interpret the behavior of the ground. A systematic series of parametric analyses demonstrated that, in peat ground with a high natural water content, the permeability decreases drastically as the load increases, and hence the residual settlement increases significantly and the stability is greatly reduced. Therefore, drain pitch needs to be considerably small under such conditions. Despite its common use in the conventional design of vertical drains, the accuracy of Barron’s solution is naturally limited in peat ground with high natural water contents since it assumes a constant value for the coefficient of consolidation.