The evaluation of the pile–soil adhesion plays a fundamental role in the estimation of the side resistance for total stress analysis. Over the years, researchers have presented proposals for adhesion factor formulations even though only a few of them have shown a certain agreement in numerical and (or) methodological terms. Hence, several real-size experimental analyses have improved the understanding of the pile–soil adhesion phenomenon and mechanism. Nevertheless, the undrained shear strength (cu) values depend on the experimental technique employed. Such results force engineers to make a difficult choice among various formulations. A reliability analysis is performed in this paper to take into consideration the variations in formulations and values of the side resistance of bored piles in clayey soils. This study involves piles having different lengths and diameters, which are supposed to be bored in Matera clays. Such soil is characterized by means of laboratory investigation campaign, and its mechanical and stochastic main features are reported here. Values of reliability index β are calculated by means of the first-order reliability method.