抄録
In case of slender buildings and/or tower-like structures, large overturning moment induced by seismic force and/or wind force exerts large cyclic vertical loading on piles supporting these structures. Screw piles are known as an effective means in these cases, and have been used in many projects. These screw piles can be classified into two types of steel pipe piles either with a spiral blade attached near the tip or with spiral wings fixed around a pipe shaft. Although several studies on the behavior of the former piles under cyclic loading have been made, few studies for the latter piles have been made so far.
In order to evaluate the bearing capacity and pull-out resistance of steel piles with spiral wings fixed around a pipe shaft, each of a spiral pile (a pile with spiral wings) and a straight pile (a pile without spiral wings) in a sand tank was tested under monotonic compressive and tensile loading, as well as cyclic compressive, tensile, and reversal loading conditions. The pile models were made of steel pipes having diameter Dp of 48.6mm and closed end, and consisted of double-tubes, enabling us to measure skin and point bearing capacities independently. The sand tank was cylindrical in shape, having diameter and height 1,200mm each. Sand was pluviated into the tank and compacted using a vibrator, and then subjected to vertical and horizontal pressures of 10kPa and 5kPa, respectively, through membranes. After screwing the pile into the model soil, the vertical and horizontal pressures were increased to 100kPa and 50kPa, respectively, to simulate the stresses at depths of about 10m. The stepwise load increment used in the cyclic loading tests was 1/6 of either the ultimate bearing capacity or the pull-out resistance obtained from the monotonic loading tests.
Test results and discussions have led to the following conclusions: (1) Under monotonic loading conditions, the bearing capacity and pull-out resistance of the spiral piles were larger than those of the straight piles by more than the diameter ratio between the two, Dw/Dp (Dw=Wing diameter); (2), The bearing capacity and pull-out resistance of piles in cyclic compressive and tensile loadings were the same as those in monotonic compressive and tensile loadings; (3) The bearing capacity and pull-out resistance of piles in cyclic reversal loading, in contrast, were approximately 0.6 times as large as those in monotonic loading; (4) The decrease in the axial force at the head of spiral piles in cyclic reversal loading was induced by the decrease in the shaft friction, whereas the wing resistance and the tip resistance in the pushing direction did not decrease with increasing number of cyclic loading; (5) The decrease in the axial force at the head of straight piles in cyclic reversal loading was induced by the decrease either in the pile tip resistance in the pushing direction or in the shaft friction in the pulling direction; and (6) The decrease in bearing capacity and pull-out resistance in cyclic reversal loading was probably due to soil disturbance and loosening around the pile.
