Large-diameter open-ended steel pipe pile have occlusion effect and reduced soil squeezing effect due to soil plugging, and their vertical bearing mechanism is far more complicated than precast concrete driven piles. Based on the test pile results of large-diameter open steel pipe piles in a port project, combined with the current port engineering pile foundation specifications, the vertical bearing mechanism of large-diameter open steel pipe piles is studied, and the large-diameter open steel pipes are analyzed from the force and bearing properties. The influence of pile diameter on the occlusion effect, the reduction of soil squeezing effect on the side friction resistance, and the occlusion effect on the pile tip resistance.
Conclusion on the vertical bearing mechanism of large-diameter open steel pipe piles
1) The vertical ultimate bearing capacity of large-diameter open steel pipe piles includes three parts: the friction resistance of the soil to the outer wall of the steel pipe pile, the friction resistance of the soil plug to the inner wall of the steel pipe pile and the end resistance of the pile end soil to the steel pipe pile wall. Because the large-diameter open steel pipe pile cannot form a complete occlusion, the soil at the end of the pile is locally damaged, which is different from the overall failure of a fully squeezed soil pile.
2) The large-diameter open steel pipe pile with pile diameter ≥1500mm has large pile diameter and thin pile wall. The soil squeezing effect on the soil during the pile settlement is basically the same as the blocking effect of the steel pipe pile, which has a bearing capacity of the foundation pile. The effects are basically the same, and the pile diameter size effect can be ignored in the calculation of bearing capacity.
3) The soil squeezing effect of large-diameter open steel pipe piles can be basically ignored. However, when calculating the friction resistance of large-diameter open steel pipe piles, the ultimate friction resistance of the soil is appropriate according to the current standard. There is no need to consider the reduction of the limit lateral resistance due to the reduction of the soil squeezing effect.
4) The ultimate end resistance of a large-diameter open steel pipe pile is the supporting force of the pile end soil on the ring of the steel pipe pile end, which is not related to the occlusion effect of the steel pipe pile and the pile diameter, but is only related to the soil characteristics of the pile end and the pile end. The area of the ring is related, and the ultimate end resistance can be calculated according to the formula Qpk=kS=kπDδ, where k is a parameter related to the strength of the soil.
Conclusion on the vertical bearing mechanism of large-diameter open steel pipe piles
1) The vertical ultimate bearing capacity of large-diameter open steel pipe piles includes three parts: the friction resistance of the soil to the outer wall of the steel pipe pile, the friction resistance of the soil plug to the inner wall of the steel pipe pile and the end resistance of the pile end soil to the steel pipe pile wall. Because the large-diameter open steel pipe pile cannot form a complete occlusion, the soil at the end of the pile is locally damaged, which is different from the overall failure of a fully squeezed soil pile.
2) The large-diameter open steel pipe pile with pile diameter ≥1500mm has large pile diameter and thin pile wall. The soil squeezing effect on the soil during the pile settlement is basically the same as the blocking effect of the steel pipe pile, which has a bearing capacity of the foundation pile. The effects are basically the same, and the pile diameter size effect can be ignored in the calculation of bearing capacity.
3) The soil squeezing effect of large-diameter open steel pipe piles can be basically ignored. However, when calculating the friction resistance of large-diameter open steel pipe piles, the ultimate friction resistance of the soil is appropriate according to the current standard. There is no need to consider the reduction of the limit lateral resistance due to the reduction of the soil squeezing effect.
4) The ultimate end resistance of a large-diameter open steel pipe pile is the supporting force of the pile end soil on the ring of the steel pipe pile end, which is not related to the occlusion effect of the steel pipe pile and the pile diameter, but is only related to the soil characteristics of the pile end and the pile end. The area of the ring is related, and the ultimate end resistance can be calculated according to the formula Qpk=kS=kπDδ, where k is a parameter related to the strength of the soil.
