Dewatering System: Poor Drawdown Near a Sensitive Building
A contractor is dewatering a 20-ft-deep excavation, 100 ft by 60 ft, in silty sand with groundwater at 8 ft. The design requires drawdown to 22 ft, or 14 ft below static, to keep the base dry. A wellpoint system is installed with 20-ft points spaced 5 ft on center around the perimeter, connected to a 6-inch header and a 500-gpm vacuum-assisted pump. After 24 hours, observation wells inside the excavation show only 3 ft of drawdown, and the base is still wet. The nearest building is 30 ft away and founded on shallow footings. The soil report gives k = 1 × 10⁻³ cm/s and notes a 2-ft silt layer at 12–14 ft depth. The contractor wants to add a second pump and increase vacuum. The header vacuum gauge reads 5 inHg, and several points are pulling air.
1. Evaluate whether the 500-gpm pump capacity is the limiting factor. Use the soil k and excavation dimensions to support your reasoning.
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First convert k: 1 × 10⁻³ cm/s ≈ 2.8 ft/day. For a 100-ft by 60-ft excavation with 14 ft of drawdown, a Darcy-type estimate (Q ≈ k i A) with i on the order of 0.3–0.5 gives only tens of gallons per minute; even a generous safety factor is well below 500 gpm. So pump size alone does not explain only 3 ft of drawdown. The 2-ft silt layer at 12–14 ft is a low-permeability barrier that can limit vertical flow and make the wellpoints ineffective if the screens sit above or within it. The header vacuum of 5 inHg and air pulling at several points indicate a vacuum leak or an unsealed system, not a lack of pump capacity. Diagnose the system before adding a second pump.
2. Diagnose why drawdown is only 3 ft after 24 hours. List the most likely causes and the field tests you would run.
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Likely causes: vacuum leaks in the header or risers (5 inHg is low), wellpoints not deep enough or screened in the silt layer, clogged points from fines, undersized header for the number of points, pump not holding vacuum, or a recharge boundary. Field tests: check vacuum at the pump and at the farthest point, isolate sections of header, measure flow from individual points, pull one point to inspect the screen, check water level in the points, verify the pump curve and discharge pressure, and do a dye test to find short-circuiting. Also check whether the silt layer is being penetrated; if the points stop above it, the system cannot drain the sand below.
3. Recommend a revised dewatering and settlement-monitoring plan that protects the building 30 ft away.
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Do not simply add a second pump; that could increase drawdown near the building and worsen settlement. First repair the vacuum leaks and clean or reinstall the points so they penetrate the silt layer. If the building is sensitive, install a cutoff wall or sheet pile between the excavation and the building, or use recharge wells to maintain the water level at the building line. Stage the drawdown and monitor settlement, tilt, and groundwater levels with transducers. A pre-construction condition survey and vibration monitoring are prudent. Control the pumping rate so the drawdown cone does not extend under the footings. If settlement occurs, stop pumping, recharge, and revise the support system.