Evaluation of Lateral Pile Behavior under Cyclic Loading by Centrifuge Tests

원심모형 실험을 이용한 반복하중을 받는 모노파일 거동 평가

Abstract

This study investigated the lateral behavior of monopile embedded in the dry sand through cyclic lateral loading test using a centrifuge test. The sand sample for the experiment was the dry Jumunjin standard sand at 80% relative density and the friction angle of $38^{\circ}$. In the experimental procedure, firstly, it was determined the static lateral bearing capacity by performing the static lateral loading test to decide the cyclic load. This derived static lateral bearing capacity values of 30%, 50%, 80%, 120% were determined as the cyclic lateral load, and the number of cycle was performed 100 times. Through the results, the experiment cyclic p-y curve was calculated, and the cyclic p-y backbone curve by depth was derived using the derived maximum soil resistance point by the load. The initial slope at the same depth was underestimated than API (1987) p-y curves, and the ultimate soil resistance was overestimated than API (1987) p-y curves. In addition, the result of the comparison with the suggested dynamic p-y curve was that the suggested dynamic p-y curve was overestimated than the cyclic p-y backbone curve on the initial slope and soil resistance at the same depth. It is considered that the p-y curve should be applied differently depending on the loading conditions of the pile.

본 연구는 원심모형실험을 이용해 건조 사질토 지반에 근입된 모노파일의 수평 반복하중에 대한 거동을 연구하였다. 실험에 사용된 지반 시료는 상대밀도 80%에서 마찰각이 $38^{\circ}$인 건조 주문진 표준사를 사용했다. 실험 과정은 우선 반복하중의 크기를 결정하기 위해 정적 하중 실험을 수행하여 극한하중을 결정하였다. 이를 통해 도출된 극한 하중 값의 30%, 50%, 80%, 120%을 반복하중의 값으로 결정하였고, 반복횟수는 100회로 수행되었다. 이 결과를 통해 실험 반복하중 p-y 곡선을 산정하였고 도출된 하중 별 최대 지반반력점들을 이용하여 깊이 별 반복하중 p-y 중추곡선을 도출하였다. 이를 기존 p-y 곡선과 비교 결과, 동일 깊이에서 초기기울기가 API(1987) p-y 곡선보다 과소평가 되었으며, 극한지반반력은 과대평가되었다. 또한, 동적 p-y 곡선과 비교하였을 때, 동일 깊이에서의 반복하중 p-y 중추곡선의 초기기울기와 지반반력이 작게 평가되었다. 이는 말뚝이 받는 하중 조건에 따라 p-y 곡선을 다르게 적용해야 할 것으로 판단된다.

Fig. 1. Soil box (a) Static loading test, (b) Cyclic loading test

Fig. 2. Jumunjin sand particle size distribution

Fig. 3. Static loading test cross section diagram

Fig. 4. Cyclic loading system

Fig. 5. Cyclic loading test cross section diagram

Fig. 6. Load-pile displacement curve

Fig. 7. Comparison of static p-y curve with the cyclic p-y curves at 2 m depth

Fig. 8. N_cycle = 100, experimental cyclic p-y curve

Fig. 9. Cyclic p-y curve by load (a) depth = 2 m, (b) depth = 5 m, (c) depth = 7 m

Fig. 10. Cyclic p-y backbone curve with cycle number at depth of 2 m

Fig. 11. N_cycle = 100, cyclic p–y backbone curve

Fig. 12. Comparison with preceded p-y curve (API p-y curve, dynamic p-y curve (Yoo et al., 2013) (a) Depth = 2 m, (b) Depth = 5 m, (c) Depth = 7 m

Table 2. Pile specification

Table 1. Properties of soil

Table 3. Static loading test conditions (Prototype scale)

Table 4. Cyclic loading test conditions (Prototype scale)

Table 5. Suggested dynamic p-y backbone curve by dynamic centrifuge test (Yoo et al., 2013)