A Study on Strength Reduction Factor of Pile-soil Interface for Evaluation of Pile Pullout Resistance by Soil Condition

지반조건에 따른 말뚝의 인발저항 평가를 위한 말뚝-지반 경계면 강도감소계수 고찰

Abstract

This paper describes the results of finite element analysis (FEA), in order to investigate a characteristics of pile pullout behavior according to the conditions of the relative density and fines content in original ground. In the FEA, a boundary elements and strength reduction factors ($R_{inter}$) on pile-soil interface were applied to simulate appropriately the shear behavior at the pile-soil interface, and then the reliability of numerical analysis method was verified by comparison of FEA results and previous experimental research(You et al., 2018). In addition, a the deformation characteristics at the pile-soil interface and determination method of $R_{inter}$ value was laid out. The results showed that the FEA, based on the analytical model applied in this study simulates appropriately the characteristics of the pile-soil interface by pullout model test of pile. In order to apply the suggested $R_{inter}$ value, it is necessary to consider the condition of the relative density and the fines content in ground.

본 연구에서는 원지반의 상대밀도와 세립분 함유율의 조건에 따른 말뚝의 인발거동 특성 규명을 위하여, 유한요소해석을 수행하였다. 수치해석에서는 말뚝과 지반 경계에서의 전단거동을 원활히 모사하기 위해서 경계요소와 경계면 강도감소계수($R_{inter}$)를 부여하였으며, 그 결과를 기존의 실험적 연구결과(You et al., 2018)와 비교함으로써 본 수치해석 방법의 신뢰성을 검증하였고, 말뚝-지반 경계면에서의 변형 특성과 함께 경계요소에 대한 $R_{inter}$값의 결정방법을 제시하였다. 해석 결과, 본 연구에서 적용된 해석모델을 이용한 수치해석은 말뚝의 인발모형실험에 의한 말뚝과 지반의 경계면 특성을 적절하게 모사하였다. 또한 제시된 $R_{inter}$의 적용에 있어서, 반드시 지반의 상대밀도와 세립분 함유율 조건을 고려해야 할 필요가 있음을 확인하였다.

Fig. 1. Results of direct shear test by fines content (You et al., 2018)

Fig. 2. Pullout test results by fines content and relative density of soils (You et al., 2018)

Fig. 3. FEA model in applied axisymmetric analysis

Fig. 4. Relationship of stress-strain in hardening soil model (Schanz et al., 1999)

Fig. 5. Total yield contour of principal stress space in Hardening soil model (Benz, 2007)

Fig. 6. Comparison of prediction and measurement value

Fig. 7. Relationship of prediction and measurement value on E_inter of pile-soil interface

Fig. 8. Determination of E_inter on pile-soil interface

Fig. 9. Determination of E₅₀ and E_inter value bounds in original ground

Fig. 10. Relationship of E₅₀ and E_inter value

Fig. 11. Determination of R_ínter on pile-soil interface according to soil condition

Table 1. Summary of strength parameters by direct shear test (You et al., 2018)

Table 2. Parameters in applied FEA (You et al., 2019)

Table 3. Comparison of maximum pullout resistance by prediction and measurement