A numerical method, PEI (Pile behavior in Expansive soil upon Infiltration) developed at the University of Ottawa can be used as a tool for the prediction and interpretation of the mechanical behavior of piles in expansive soils taking account of the influence of matric suction changes associated with water infiltration. This program is employed in back analyses of the in-situ single pile test results for a case study at the Colorado State University (CSU) field test site, which has expansive soil deposits. Four single piles were drilled into the expansive shale of the Pierre Formation west of Fort Collins, Colorado. The piles were 350 mm in diameter and were installed to a depth of 7.6 m. During installation, slope indicator-type VS embedment strain gauges were embedded on the piers at a depth of 1.8 m, 3.1 m, 4.3 m and 5.5 m below the ground surface. Water content variations of the surrounding soil along the pile depth were measured periodically from 1995 to 2004, in addition to other measurements, which include pile axial force distribution and pile head displacement. Extensive analysis regarding this case study with respect to the mechanical behavior of pile associated with changes in water content is available in the literature. In this paper, this case study is illustrated in an innovative way to understand the influence of matric suction on the mechanical behavior of the pile using the program PEI. In addition, in-situ measurements of both pile head displacements and pile axial force distribution are compared with the results of numerical simulations using the PEI. Analyses of the results show that measured in-situ and numerical results are in good agreement. The results of this study are of significant interest for the practicing engineers who routinely design pile foundations in expansive soils.