2017 Volume 85 Issue 2 Pages II_103-II_111
In the excavation of soil within a cofferdam, upward seepage flow occurs and the seepage failure of soil becomes a problem. In this study, to examine the appropriateness of several design-manuals' (or standards') calculation methods, safety factors based on these methods were considered. The effects of upstream residual soils on stability against seepage failure, which are usually neglected, were discussed under various flow conditions: two-dimensional (2D), two-dimensional concentrated within double sheet piles (2DC), three-dimensional (3D), and axisymmetric (AXS) seepage flows. The following results were obtained: (1) With respect to the safety factors against seepage failure, the real factor of safety Fs (True) can be expressed as the product of the safety factor included in calculation Fs (Formula), safety factor in design Fs (Design), and safety factor given by upstream residual soil Fs (Upstream residual soil), i.e., Fs (True) = Fs (Formula) × Fs (Design) × Fs (Upstream residual soil). (2) The design-manuals' calculation methods provide an accurate estimation of the critical hydraulic head difference Hc in many cases, but give excessively small or large estimations in some cases. (3) The effects in the case of upstream residual soil on Hc values are as follows in descending order: 2D, 2DC, 3D, and AXS. The effect of 3D is almost the same as AXS.