Tower Foundations Bearing Above Weak Soils

A 13-level reinforced concrete structure was constructed on Marco Island in southwest Florida. The tower is located 200 feet from the Gulf of Mexico and has plan area dimensions of 115 by 170 feet. The field testing revealed the site was mantled with a 17-foot thick layer of firm sand. The sand stratum was underlain by a compressible 9-foot thick layer of silty sand which had an average SPT N-value of less than 2. Various methods of engineering analyses estimated total tower settlements to range from 1 to 8 inches. Actual measured settlement following the application of dead load was about 1 3/4 inches. An engineering inspection following construction revealed diagonal shear wall cracks