The use of aggregate pier reinforced ground to support medium to heavy loads has increased in construction practice to meet the industry demand for transmitting larger loads through spread footings. The ability to predict load-displacement behavior of isolated spread footings resting on aggregate pier improved ground, however, has been slow to follow. Prediction models for aggregate pier improved ground based on the unit cell concept are not applicable for isolated spread footings resting on floating aggregate piers. Existing models that have been proposed for isolated spread footings either use a transformation to adapt the unit cell model to three-dimensional conditions or use simplified spring analogies which do not adequately capture the observed load-displacement behavior of footings loaded in field load tests.

The overall research program undertaken herein aims to address the shortcomings of currently accepted performance prediction methods. A field load test program was performed to investigate construction variables. The load-displacement performance of single and groups of aggregate piers in clay and the improvement in displacement performance over the unreinforced ground were evaluated. A probabilistic site characterization was performed to assist in the interpretation of the load-displacement behavior of the footing load tests. The results of the test program were combined with high-quality load test data found in the literature to form a database of load tests. The database of load tests was used to evaluate existing methods for bearing capacity and displacement performance of footings resting on aggregate pier reinforced clay. New regression-based and semi-empirical methods were developed to predict the ultimate bearing capacity and displacement performance of isolated spread footings. These new methods were found to be superior to those currently available to engineers and contractors. A reliability-based assessment of the regression-based prediction models was performed to characterize reliability indices as a function of load factor, distribution and variability of load, and resistance factor. These new tools will help engineers and contractors assess the short term ultimate bearing capacity and displacement performance of isolated spread footings on aggregate pier reinforced clay.