Seepage, Flow Nets & Quick Conditions

Worked: under a sheet pile cofferdam

A flow net drawn for a cofferdam has 4 flow channels and 12 drops, with 20 ft of head difference and k = 2 × 10⁻⁴ ft/s. q = 2 × 10⁻⁴ × 20 × 4/12 = 1.33 × 10⁻³ ft³/s per ft. Along a 100 ft wall that is 11,520 ft³ a day to pump.

The last square at the excavation floor is 2.5 ft long, and it loses 20/12 = 1.67 ft: i_exit = 0.67. With γ_sat = 124.8 pcf, i_c = 62.4/62.4 = 1.0, and FS = 1.0/0.67 = 1.5.