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.