사질토에 근입된 헬릭스 피치에 따른 헬리컬 파일의 수치해석적 거동분석

Analysis of Helical Pile Behavior in Sands Varying Helix Pitch Based on Numerical Analysis Results

Abstract

캐나다 및 베네수엘라에 주로 분포하는 오일샌드는 원유들이 모래질 흙의 간극에 존재한다. 이러한 오일샌드로부터 원유를 따로 추출하기 위해서는 규모가 큰 플랜트를 건설해야 한다. 일반적으로 오일샌드 플랜트의 기초는 주로 항타말뚝 혹은 현장타설말뚝이 사용되고 있다. 하지만 주로 극지에 위치한 오일샌드는 얼어있는 지반과 표층의 유기토 때문에 항타말뚝의 시공 및 장비 진입에 어려움이 있으며, 현장타설말뚝의 경우 기온이 낮기 때문에 콘크리트 양생에 문제가 있다. 이번 연구의 주제인 헬리컬 파일은 크지 않은 연직력에 기초한 회전력을 중심으로 빠르고 간편하게 시공이 가능하다. 따라서, 접근성이 떨어지는 극지환경에서도 소형장비를 사용하여 간단한 시공이 가능하며, 헬리컬 파일의 두부에 역회전을 가해 말뚝기초의 인발 및 재사용 또한 용이하다. 이번 연구에서는 헬릭스 피치를 변화시켜 헬리컬 파일 및 헬릭스의 거동을 수치해석으로 분석하였다. 수치해석의 검증은 모형 헬리컬 파일의 실내모형실험 결과와 비교하여 수행하였으며, 헬릭스의 피치에 따른 헬리컬 파일의 극한하중, 헬리컬 파일의 축에 부착한 각 헬릭스의 변위, 하중분담률을 분석하였다.

Oil sands, which are largely distributed in Canada and Venezuela, are a mixture of crude oil and sandy soils. In order to extract crude oil from oil sands, construction of massive oil sand plants is required. Generally, the typically-used foundation types of the oil sand plant are driven piles and cast-in-place piles. Most of the oil sand plants are located in cold and remote regions. Installation of driven piles in frozen or organic surface soils is difficult due to high resistance and installation equipment accessability, while the cast-in-place pile has concrete curing problem due to cold temperature. Helical pile can be installed quickly and easily using rotation with a little help of vertical load. As the installation of helical pile is available using a small and light-weight installation equipment, accessibility of installation equipment is improved. The helical pile has an advantage of easy removal by rotation in reverse direction compared with that of installation. Furthermore, reuse of removed helical piles is possible when the piles are structurally safe. In this study, the behavior of helical piles varying helix pitch was analyzed based on the numerical analysis results. Numerical model was calibrated based on the results of model helical pile tests in laboratory. The ultimate helical pile loads, the displacement of each helix attached to the shaft of the helical pile, and the load sharing ratio of each helix were analyzed.

Fig. 1. Schematic of a helical pile [modified after Carol and Roy (2018)]

Fig. 2. Load-Settlement curve of helical pile (modified after Kulhawy, 2004)

Fig. 4. Sand pluviator; (a) Schematic of sand pluviator and (b) Relationship between relative density and falling height of sand (modified after Lee et al., 2017)

Fig. 5. Modeling used in the numerical analysis: (a) integrated model of ground and pile and important dimensions and (b) enlarged helical pile model and its dimensions

Fig. 6. Comparison of laboratory model test and numerical analysis

Fig. 7. Four different pitches of helical piles (units in mm): Helix pitch equal to (a) 25 mm (0.5D), (b) 50 mm (1D), and (c) 75 mm (1.5D)

Fig. 8. Load-settlement curves of helical piles with different helix pitches and the ultimate loads by different criteria

Fig. 9. Linear, transition, and final linear regions from load-settlement curves varying helix pitches: helix pitch of (a) 25 m m (0.5D), (b) 50 mm (1D), and (c) 75 mm (1.5D)

Fig. 11. Load sharing ratio with pitch of helix

Fig. 3. (a)　Soil chamber and　(b) helical pile and their dimensions used for laboratory model test

Fig. 10. Displacement of each helix with load at pile head; pitch of helix is (a) 25 mm (0.5D), (b) 50 mm (1D) and (c) 75 mm (1.5D)

Table 1. Estimation method of ultimate load of helical pile by researchers

Table 2. Physical properties of crushed sands used in this study (Lee et al., 2017)

Table 3. Material properties used for numerical analysis

Table 4. Ultimate loads and the corresponding settlements for different helix pitches

Table 5. Loads and corresponding settlements of turning points from linear to transition regions and those from transition to final linear regions

Table 6. Average displacement of each helix when the equivalent load is applied