Aggregate piers have been used for centuries, with increasing occurrence in the last few decades. Their usage has been driven by land development demand and enabled through equipment and engineering improvements. Understanding the principles controlling bearing capacities and serviceability performance has been an ongoing area of interest for structures supported by aggregate piers. Analytical models, laboratory model tests, and full-scale research have been utilized by various investigators to develop bearing pressure-displacement prediction models. The majority of models to-date have relied on transforming two dimensional unit cell models and analogy methods, such as stiffer and weaker springs, to represent a three dimensional response. These models haven’t proven to adequately capture the non-linear behavior of field load tests. Multiple linear regression (MLR) modeling using actual load test data and test site specific soil properties have produced good prediction estimates. Uncertainties associated with site specific soil conditions and variations in aggregate gradations and sources have led to conservative implementation of current models.
Research of full-scale performance of footings resting on clay and aggregate piers was the underlying framework for this research. The research program included an extensive subsurface investigation of the test site and pier aggregate engineering property characterization. Baseline footing bearing pressure-displacement data enabled estimation of mobilized undrained shear strengths which were used as MLR input values. The baseline footing bearing pressure-displacement data was also used to develop bearing pressure improvement factors, BPIF, for the aggregate pier supported footings at varying slenderness ratios and bearing pressures. As a group, the virgin loaded footings BPIF changed with increasing displacement in similar charting patterns to previous research by others. Reloading of previously tested pier supported footings exhibited slightly stiffer bearing pressure-displacement response than the initial loading. For the reloaded footings, the BPIF at large displacements, were similar to BPIF of the initial footings at large displacements. However up to large displacements, the reloaded footings exhibited different BPIF versus displacement paths than the initially loaded footings. For all of the load tests, the modulus of subgrade reaction, ks, decreased at similar rates with increasing displacement. Greater slenderness ratios tended to produce greater ks at the same displacements. Using the load test bearing pressure-displacement results from the individual and group pier footings, which included differing aggregate pier slenderness ratios and test seasons, bearing capacity and serviceability models were evaluated. This research provided additional confirmation that the recently developed MLR models produced more reliable results than empirical or other models reviewed.