Dewatering Practice for Deep Basements in Hawaii-PDF

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The paper reviews dewatering methods for deep basements in Hawaii, where highly permeable coral and alluvial soils lie close to sea level. Although deep excavations were once avoided, rising land values and parking needs have made them more common.<br /><br />Methods include sumps and ditches, cutoff walls, and deep wells. Sumps and ditches collect water but generally achieve only limited drawdown; in coral, pumping can wash out fine material and increase water flow. Sheet piling can reduce seepage when it reaches an impermeable layer, but such a layer may be absent or unsuitable. Grouting has generally failed to create reliable cutoffs in natural coral because the formations are heterogeneous and often lack an impermeable base. Deep wells can lower groundwater in gravity-drained soils and are especially effective in Hawaii’s permeable deposits.<br /><br />Early projects illustrate the risks of inadequate planning. At one Honolulu pump station, sump pumping reached 30,000 gallons per minute without achieving the required drawdown, while eroding soil into the excavation. At a Kakaako site, failure to depressurize a lower water-bearing layer caused heaving and boiling, forcing abandonment of the basement. Inadequate dewatering at the Federal Office Building contributed to major construction delays, cost claims, difficulties with piling and concrete work, and muddy discharge.<br /><br />Later practice increasingly used site investigations, testing, and planned systems. At Ewa Beach, extensive grouting did not form an effective cutoff, and massive open pumping was ultimately required. By contrast, deep-well systems at Century Square, Executive Center, and Bishop Square were successful. They kept excavations dry, reduced delays and costs, simplified material handling and construction, and produced cleaner water for disposal.<br /><br />The paper concludes that properly designed perimeter deep wells are generally technically feasible and economically preferable for deep basement construction in Hawaii. Open pumping can work, but carries substantial risks; grouting is usually unsuitable for natural coral formations.
Keywords
dewatering
deep basements
Hawaii
coral soils
alluvial soils
deep wells
sump pumping
cutoff walls
grouting
groundwater control