The Caisson Solution

by Bennie L. Benjamin, Civ. Engr. and Dir.; Detroit Water & Sewerage Dept., Detroit, MI,
Thomas L. Weber, (M.ASCE), Vice Pres.; Metcalf & Eddy, Detroit, MI,
Jose A. Ramos, (M.ASCE), Chief Geotechnical Engr.; Metcalf & Eddy, Wakefield, MA,


Serial Information: Civil Engineering—ASCE, 1992, Vol. 62, Issue 12, Pg. 44-47


Document Type: Feature article

Abstract: Detroit is expanding its wastewater treatment plant to drastically reduce combined sewer overflows. The expansion, however, is being done on an extremely congested 4 acre tract of land underlain by 60 ft of soft clay. Monitoring geotechnical conditions, protecting the structural integrity of existing treatment structures on the site and choosing the appropriate earth-retention and foundation systems for the new plant components were crucial engineering challenges. To meet the geotechnical challenge, engineers relied on the observational method. With this method, the design of the new plant is based on the most probable subsurface conditions, as interpreted from the field and laboratory data, drawn from a comprehensive instrumentation system. Meanwhile, four ground-support systems were selected for new plant components: flexible steel sheet pile systems, a semirigid drilled pier system, tunnel ground support for two soft ground tunnels and two massive self-sinking caissons, measuring 44 ft and 140 ft in diameter. Both caissons were sunk to final grade by constructing the caisson concrete walls in 10 ft lifts. The concrete provided the necessary weight for the caisson to begin to sink through the soft clay. The caisson sinking rate and plumbness were controlled by excavating along the inside perimeter of the caisson wall.

Subject Headings: Water treatment | Wastewater management | Water treatment plants | Facility expansion | Soft soils | Foundations | Caissons | Concrete

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