Performance of a Stone Column Foundation

Vibro‐replacement stone columns were used to support a large wastewater treatment plant founded on up to 48 ft (15 m) of soft estuarine deposits. Support was required for distributed foundation stresses up to 3,000 psf (145 kPa). The basic design requirement was a loading of 30 tons (265 kN)/stone column with a settlement of less than 0.25 in. (6 mm) under that load in a test on a single column within a group. Column spacings ranged from a $4\mathrm{ft}\times 5\mathrm{ft}$ (1.2 m by 1.5 m) pattern under the most heavily loaded areas, to a $7\mathrm{ft}\times 7\mathrm{ft}$ (2.1 m by 2.1 m) pattern under lightly loaded areas. Twenty‐eight single column load tests were done during the installation of the 6,500 stone columns to evaluate load‐settlement behavior. Laboratory tests were done to provide soil property data needed for finite element predictions of both the load test behavior and settlements of the completed structures. Predicted load test settlements were somewhat greater than those recorded during the load tests, but agreement was generally good. The installation of stone columns led to a reduction in settlements to about 30–40% of the values to be expected on unimproved ground. The settlement of a large uniformly loaded area of improved ground was predicted to be about ten times that measured in a load test on a single column within the area. Measured settlements varied from 1.0–2.4 in. (25–60 mm) for a soft sediment thickness of 32–35 ft (10–11 m). A settlement of about 2.5 in. (64 mm) was predicted by the finite element analysis. Settlement predictions using other, simpler methods gave values which agreed reasonably with both the measured values and the finite element predictions.