A field test has been undertaken to investigate changes to in situ Mercia Mudstone at the soil–pile interface after installation of four 5.5 m long 350 mm diameter continuous flight auger piles. The test investigated whether a remoulded zone exists, the extent of the zone, changes caused by remoulding and the effect of the installation method. The piles and surrounding soil were excavated after installation to a depth of just under 2 m. The sections of pile and surrounding soil were returned to the laboratory, where a variety of detailed observations at both micro- and macroscopic scales were undertaken, together with chemical and mineralogical analysis. It was found that a remoulded zone existed in all piles but that this varied in thickness both laterally and vertically around a pile. Across all piles the maximum thickness observed was 55 mm. The average thickness, fabric and texture were all affected by installation method. A distinctive vertically oriented fabric was observed in which up to four vertically oriented layers could be distinguished. There was evidence of changes in texture and fabric, structure, colour, mineralogy and chemistry.