Construction of the first stage of Carousel Mall, a multi-level shopping mall covering a plan area of about 65,000 m² with a six-story central tower, was completed in 1990 in Syracuse, NY (Stewart et al. 1994). The soil conditions at the site consisted of a sequence of fill, peat and organic silt, loose sand and silt over soft normally consolidated varved clay. These compressible strata extend to depths of 25 to 55 m and rest on a dense sand and gravel stratum. The ground water level at the site was 0.3 to 1.5 m below original grade.

Deep end bearing piles were considered for foundation support with over 50 percent of the pile capacity to be devoted to down drag loading expected to be induced by post construction settlements. This option was found to be too expensive. A load compensated mat foundation was selected. The fill, peat and organic silt were excavated and removed from the building plan area to a depth of about 5.5 m. The weight of the removed material compensated for the building load to result in net unloading of about 24 kPa. The excavation spoil was to be used for site regrading and to raise elevations adjacent to the foundation by an additional 2.7 m above the original ground surface. This would have implied a net loading outside the foundation of up to 53 kPa for a total abrupt stress change of 77 kPa thus creating conditions for severe differential settlements between edge and interior column positions. To extend the possible abrupt change in the settlement profile sufficiently away from the building perimeter, the mat area was extended by 2.1 m beyond the superstructure perimeter to create dead space. Over 28,000 m³ of EPS geofoam was placed over a bed of light-weight aggregates as backfill against the basement and along the entire outside perimeter of the foundation. The water table is maintained at a level 1.5 m above the mat subgrade. Jacking and shimming fixtures were installed at the top of basement columns for future re-leveling, if necessary.

The completed facility has been performing well with only 20 out of a total of 1700 columns requiring jacking adjustment for settlements by 1992 (Stewart et al. 1994). Geofoam was also used as perimeter fill in the foundation system of an additional 5,600 m² expansion that was completed in 1994. The basement wall of this new addition was instrumented to monitor in service lateral pressures induced by the geofoam fill over time. Results to date show that lateral pressures against the basement wall are very low. (Negussey and Sun, 1996). Carousel Mall is now planned to expand and become one of the largest shopping malls in the world.

References :

Stewart, J.P., Lacy, H.S., and Ladd, C.C. (1994) "Settlement of Large Mat on Deep Compressible Soil, Vertical and Horizontal Deformations of Foundations and Embankments." Proceedings of Settlement '94, ASCE Geotechnical Special Publication No.40, pp.842-859

Negussey, D. and Sun M., (1996) "Reducing Lateral Pressure by geofoam (EPS) Substitution." Proceedings of the International Symposium on EPS Construction Method, Tokyo, Japan, 1996.