Abstract
During excavation of soil from an area with a high groundwater level, seepage failure of soil such as piping or boiling becomes a problem. In recent years, a new type of accident has occurred with respect to seepage failure, because deep excavation has been proceeding due to the construction of large-scale structures. A case study was carried out on seepage failure of excavated bottom soil in a steel-sheet-pile-wall cofferdam. First, evaluations of safety factors with respect to seepage failure of soil were conducted using standards and correction methods for plan view of excavation. Second, seepage flow and stability analyses were performed and the following results were obtained:
(1) Hc, the critical hydraulic head difference between up-and downstream of sheet piles, that is calculated using standards and evaluation methods for seepage failure of soil in front of a single sheet pile wall in two dimensions, is greater than the actual value.
(2) Correction methods give a good estimation of Hc, where the plan view of excavation is thought to approximate the real shape, such as rectangular or circular.
(3) Using Harza's method considering the exit gradient as a seepage failure criterion, Hc is estimated as larger than the actual value.
(4) According to flow conditions, a safety factor with respect to seepage failure Fs is given as follows: Fs (AXSflow) =0.402 Fs (2Dflow), which indicates the decrease in Fs from the 2Dflow to AXSflow condition is very large. The 2Dflow condition represents a two dimensional seepage flow in front of a single sheet pile wall, whereas AXSflow is an axisymmetric seepage flow.
