Calibration chamber tests were conducted on both close-ended and open-ended model piles, driven into dry siliceous sands with different soil conditions, in order to clarify (1) the effect of soil plug on the bearing capacity of open-ended pile through comparison of end bearing capacities for both open-ended pile and close-ended pile, and (2) the effect of soil conditions on the load transfer mechanism within the soil plug. The model pile used in the test series was devised so that bearing capacities of both close-ended pile and open-ended pile could be measured out into two components(e.g. end bearing and outside skin resistance) and three components(e.g. outside skin resistance, plug resistance and tip resistance), respectively. In the case of open-ended pile, the plug resistance was estimated under the assumption that the unit shaft resistance due to pile-soil plug interaction varies linearly near the pile tip.
The plug capacity which was defined as the plug resistance at ultimate condition is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug which transfers the load decreases with the decrease of relative density, but it is independent on the ambient pressure and the penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug is calculated. It is gradually reduced with increase in the longitudinal distance from the pile tip. At the bottom of soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Furthermore, the comparison of bearing capacities for close-ended and open-ended piles show that the end bearing of open-ended pile in initial depth is very smaller than that of close-ended pile, but it is nearly equal to the end bearing of close-ended pile due to the increase of plugging rate with increase in the penetration depth. However, the outside skin resistance of open-ended pile is not equal to that of close-ended pile until the pile reaches the last stage of installation.
Based on the test results for open-ended piles, an empirical equation was suggested to compute the earth pressure coefficient. Then, the evaluated earth pressure coefficient is used in the calculation of plug capacity based on the one-dimensional analysis. It has been founded that the equation predicts proper plug capacities for all soil conditions considered in this study.
