A Numerical Study on the Estimation Method of the Results of Static Pile Load Test Using the Results of Bi-directional Pile Load Test of Barrette Piles

바렛말뚝의 양방향재하시험을 이용한 정적압축재하시험 결과 추정방법에 관한 수치해석적 연구

Abstract

Bi-directional pile load test (briefly called 'BDH PLT') cannot be performed at loading levels where ultimate bearing capacity could be assessed in field, it is not possible to precisely determine both ultimate load and yield load and under loading. Since the load is transmitted separately to the skin and the end unlike the static pile load test (briefly called 'SPLT') and the direction of loading on the skin is opposite, such methods could have a result different from actual movements of shafts. In this study, three-dimensional finite element method (briefly called '3D FEM') analysis was conducted from results of the BDH PLT, made with barret piles, which were large-diameter cast-in-place concrete piles, and the calculated design constants were applied to the 3D FEM analysis of the SPLT to interpret them numerically and then, actual behaviors of cast-in-place concrete piles were estimated. First, using the results of the BDH PLT with cast-in-place concrete piles, behaviors of the piles made by loading upwards and downwards were analyzed to calculate load-displacement. Second, the design constants, calculated by the 3D FEM analysis and the back analysis, were applied on the 3D FEM analysis for the SPLT, and from these results, behaviors of the SPLT through the BDH PLT was analyzed. Last, the results of the 3D FEM analysis of the SPLT through the BDH PLT was expressed in relationships as {A ratio of bearing capacity of the SPLT and of the BDH PLT (y)} ~ {A ratio of reference displacement and pile circumference (x)}, and they were all classified by reference displacement at 10.0 mm, 15.0 mm, and 25.4 mm.

현장에서의 양방향재하시험은 극한지지력을 판단할 수 있는 하중재하 단계까지의 재하시험이 실시되지 않기 때문에 항복하중 및 극한하중을 정확하게 알 수 없는 문제점이 있다. 그리고 정적압축재하시험과 달리 주면과 선단이 분리되어 하중이 전이되고, 주면부의 하중재하 방향이 반대이기 때문에 실제 말뚝의 거동과 다른 결과를 나타낼 우려가 있다. 따라서 본 연구에서는 대구경 현장타설말뚝인 바렛말뚝의 현장 양방향재하시험 결과로부터 3차원 유한요소해석을 실시하고, 재산정된 설계정수를 정적압축재하시험의 3차원 유한요소해석에 적용하여 수치해석을 실시하였으며, 그 결과로부터 현장타설말뚝의 실제 거동을 추정하는 방법을 제안하였다. 먼저, 현장타설말뚝의 현장 양방향재하시험 결과를 이용하여, 상 하향으로의 하중재하에 따른 하중-변위 분석을 실시하였다. 그리고 양방향재하시험을 3차원 유한요소해석을 통해 모사하고 역해석을 실시하여 재산정된 설계정수들을 정적압축재하시험의 3차원 유한요소해석에 적용하였으며, 이 결과로부터 양방향재하시험을 통한 정적압축재하시험의 거동을 분석하였다. 양방향재하시험을 통한 정적압축재하시험의 3차원 유한요소해석 결과를 {정적압축재하시험과 양방향재하시험의 지지력 비(y)} ~ {기준침하량과 말뚝주면장 비(x)}의 관계식으로 나타내었고, 10.0mm, 15.0 mm, 25.4mm일 때의 기준침하량에 따라 각각 구분하여 제안하였다.

Fig. 1. Results of standard penetration test(N-value) by types of barrette piles

Fig. 1. Results of standard penetration test(N-value) by types of barrette piles (Continued)

Fig. 2. Loading device of types of barrette piles

Fig. 3. Comparison of static pile load test and bi-directional pile load test (Osterberg, 1998)

Fig. 4. Method of bi-directional pile load test

Fig. 5. Results of bi-directional pile load test (load-displacement)

Fig. 6. Results of bi-directional pile load test (equivalentload-displacement)

Fig. 7. Modeling of three-dimensional finite element method

Fig. 8. Modeling according to application of interface element

Fig. 9. Comparison of results of field measurement and three-dimensional finite element method analysis of bi-directional pile load test

Fig. 10. Comparisons of load-displacement results of bi-directional pile load test and static pile load test by three-dimensional finite element analysis

Fig. 11. Progressive failure of pile surface element (Randolph and Wroth, 1981)

Fig. 12. Shear stress variation with depth of static pile load test

Fig. 13. Calculation of bearing capacity by ultimate load and reference displacement

Fig. 14. {Bearing capacity ratio(y) of static pile load test and bi-directional pile load test} ~ {ratio of reference displacement and pile circumference(x)}

Fig. 15. An interpretation cross-section for the validation of three-dimensional finite element analysis (Kwon et al., 2006)

Fig. 16. Results of the bi-directional pile load test on field(Kwon et al., 2006)

Fig. 17. Comparison of field test and three-dimensional finite element analysis for validation of estimation method of static pile load test by bi-directional pile load test

Table 1. Types and specimens of barrette piles by bi-directional pile load test

Table 2. Results of bearing capacity of barrette piles

Table 3. Results of initial soil properties applied to three-dimensional finite element method

Table 4. Properties of pile applied to three-dimensional finite element analysis (Gere and Goodno, 2017)

Table 5. Properties of loading plate applied to three-dimensional finite element analysis (Gere and Goodno, 2017)

Table 6. Value of interface according to three-dimensional finite element analysis

Table 7. Results of soil properties by back analysis

Table 8. Soil properties of three-dimensional finite element analysis for validation of estimation method of static pile load test by bi-directional pile load test

Table 9. Pile properties of existing research (Kwon et al., 2006)