Many ground improvement techniques are subjected to the limitations of cost, safety and construction time. Innovations in industry are resulting in new technologies and construction methods to overcome these limitations. Displacement Aggregate Pier (DAP) technology developed by Geopier^TM Foundation Company is one such technology and is the focus of this research. Specifically, the influence of pier installation on matrix soil densification is addressed based on evaluation of several full-scale field studies. Cases histories are presented describing the use of cone penetration test (CPT) and standard penetration test (SPT) to investigate matrix soil densification for a range of ground conditions. Additionally, full scale load tests were studies for single piers and pier groups to confirm the current design approach. Data from sixteen of sites were analyzed. Although site specific analysis reveals the unique behavior of IRAP elements, an effort was made to combine data from multiple sites to investigate general relationships between matrix soil densification and soil type, depth, initial relative density, pier spacing, radial distance, groundwater table locations and soil strata. Key findings from this study show that ground densification is highest for matrix soils with less than 20% fines and that the relative density increases for groups of piers and in particular for sandy matrix soils. Evaluations of group effective factor, improvement index, ground modification and settlement are also presented in this research. Simplied design table presenting change the CPT tip resistance for individual DAPs and group of DAPs are present as one outcome from this study.