The paper describes a programme of field experiments with heavily instrumented displacement piles installed in a stiff, lodgement till. Measurements of the effective stresses recorded at various levels along the pile shaft during installation, equalization, and load testing are presented. Shaft capacity is shown to be governed by an effective stress failure criterion, with the mobilized stresses depending on the clay consistency, distance from the pile tip, shaft surface texture, type of loading, installation rate, and degree of equalization.Key words: displacement pile, instrumentation, glacial till.