Answer Happy

An excavation in this ground must be dewatered. The excavation is 35 ft x 60 ft in plan and will extend to 60 feet below the ground surface. The GWT should be drawn down to 5 ft below bottom of excavation, i.e., 65 ft below ground surface. Perform an equivalent well design analysis (1) to determine how many wells at what depth are required to dewater the excavation. Show all calculations in detail (including units) and prepare plan and profile view plots of well locations and groundwater conditions so that we clearly understand your assumptions of the situation. Some important notes: • Use a pumping time of 30 days. • Assume a well loss (well friction) fwl = 10 feet. Note also that the length of well exposed to saturated aquifer (w) is incorrectly shown in the ppt file and in chapter 6 of the book. Below is the corrected view. • Estimating the curvature of response at well location vs. between wells is challenging. You may assume that this line is flat, i.e., that the design drawdown h applies at the wells too. • Wells should be placed 5 ft outside the excavation. • Assume 12-inch diameter well boreholes. The total length of dewatering wells (and therefore the ly) is a design variable. One could extend the dewatering wells all the way to impermeable ground OR stop them at a depth short of the impermeable ground. The decision is driven by economics. Assume the cost for each well is $10,000 + $50/ft. (1). In general, equivalent well design provides a reasonable approximation instead of cumulative drawdown analysis) when Ro > 10rs and when the rectangular excavation length < twice the width
Original water table H Subgrade of excavation Predrained water level ** T w