Water is a problem. When the hydraulic head difference between the up- and downstream sides, H, is too large, seepage failure occurs at the base of the excavation. In practice, we encounter various types of excavation using single or double sheet pile walls, and various shapes of excavation, e.g., long and narrow, rectangular, square, or circular shapes in a plan view. In recent years, large scale or deep excavation is also encountered. In the design of the cofferdam, the various flow conditions due to the above situations should be specified correctly, and the critical hydraulic head difference against seepage failure, H c, must be estimated exactly. In this paper, the characteristics of seepage flow and seepage failure of soil under various flow conditions are discussed.
On excavating soil in area with a high ground water level, seepage of water is a problem. When the hydraulic head difference between the up- and downstream sides, H, is too large, seepage failure occurs at the base of the excavation. On excavating a large area, as shown in Fig.1, the seepage failure of soil in front of a sheet pile wall is a problem in two dimensions, which is designated as two-dimensional flow (2D flow). Case studies on seepage failure of soil in the 2D flow condition are reported by Terzaghi and Peck (1948), Tanaka and Nagai (1997). In the excavation of soil between double sheet pile walls, as shown in Fig.2, seepage water concentrates into the soil two-dimensionally from the outside, which is called two-dimensional concentrated flow (2DC flow). The two-dimensional concentrated flow of water lowers the safety factor regarding the seepage failure of soil (Bauer, 1984; Tanaka et al., 2002). When the longitudinal length of the double sheet pile walls is small, seepage flow concentrates three-dimensionally into soil surrounded by a rectangular wall.
