Figure shows a sheet pile wall cofferdam 6 m wide and 60 m long driven to a depth of 5 m in a sand 9 m thick deposit of sand overlying impermeable bedrock. The water level outside the pile is 3 m above the sand and 1 m of sand is excavated from within cofferdam. The properties of the sand are γsat =20kN/m3 and k=7×10−5 m/s. (Take Datum at the level of lower water table level) Calculate the total seepage flow into the excavation. (Hint: The seepage flow will be symmetrical about the center line of the coffer dam. Therefore, the flow net is drawn only for half of the flow domain. Take this condition into consideration when you calculate the total seepage.) Determine the factor of safety against base heave.

Sand 9m Im 4m 6m 3h = 4m A Sand 9m Im 4m 6m 3h = 4 m A