Abstract: Static loading tests on pile groups with prototype size were carried out in order to study the behaviour of the cap-pile groups-soil in the pile foundation. The soil resistance under the cap, the shaft resistance and the tip resistance of pile were measured by installing various measuring gauges. Based on these test results, clamping action was shown to play an important role. Cap is possessed of weakening effect to pile shaft resistance and strengthening effect to pile tip resistance. Clamping is a complex action. The pile groups efficiency is larger than 1 in the condition of the test. The coaction of pile-soil-cap should be considered in optimum design of pile foundation.