Abstract: Systematic field tests on pile groups with near prototype size have been carried out in order to research into the behavior and bearing capacity of bored pile groups in silt. The soil reaction on the cap, shaft resistance, point resistance and displacement of pile and soil have been measured by installing various measuring elements. Based oa a part of these testing results, the cap-pile-soil interaction characteristics were analysed. It was found that, under certain conditions, "settlement-hardening" and "settlement-softening" occur in the shaft resistance and the pile cap brings "weakening effect" to the side resistance and "strengthening effect" to the point resistance. No matter the pile spacing is large or small and the cap is in contact with ground or separated from ground, "block failure" does not occur. On the basis of these, a method, taking into consideration of the cap-pile-soil interaction for calculating vertical ultimate bearing capacity of pile group has been proposed.