BEHAVIOR OF SOIL PLUGS IN OPEN-ENDED MODEL PILES DRIVEN INTO SANDS

Calibration chamber tests were conducted on open-ended model piles dri ven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism i n the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open-ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip r esistance. Under the assumption that the unit shaft resistance due to pile-soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the amb ient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative densit y, but it is independent of the ambient pressure and penetration depth . Under several assumptions, the value of earth pressure coefficient i n the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the s oil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pre ssure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the differ ence in the dilation degree of the soil plug during driving with ambie nt pressures, and (3) the difference in compaction degree of soil plug during drying with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coeffic ient to be used in the calculation of plug capacity using one-dimensio nal analysis, and it produces proper plug capacities for all soil cond itions.