桩基尺寸对砂土中水平受荷桩-土作用力的影响

Dimensional Effect on Soil Resistance for Laterally Loaded Monopiles Driven in Sand

p-y曲线法在水平受荷桩的设计中得到了广泛应用,然而近年来在海上风机超大直径单桩基础设计中受到了学术界和工程界的质疑,当前研究表明,桩基尺寸是产生这一问题的主要原因。本文通过三维数值仿真技术,研究了砂土地基中水平受荷桩-土作用特征的演变规律,探讨了当前超大直径单桩设计方法存在的主要问题及产生的原因。计算结果显示:超大直径单桩基础因其桩径大、长径比(入土长度与直径的比)小的尺寸特征,在水平荷载作用下桩土作用具有明显的三维空间效应,即除受水平向土反力外,还受到桩端水平向的摩擦力、沿桩身不均匀摩阻力产生的抵抗力矩及桩底不均匀土反力提供的抵抗力矩;进一步研究表明,后三类抗力对桩基水平承载力的贡献占比与桩基尺寸有关,即桩基直径越大、长径比越小,这些抗力的贡献越大。因此,在水平受荷超大直径单桩承载特性计算与分析的过程中,除当前水平向p-y弹簧提供的抗力外,还应考虑桩底的水平剪力、桩身摩擦力和桩底反力不平衡产生的抵抗力矩。

Although p-y method has been widely used in the design of laterally loaded monopiles,it has been questioned in the design of super large diameter monopiles used in offshore wind turbine.Current researches suggest that this question is caused by the dimensional effect.Based on the three-dimensional numerical simulation technology,this paper carries out the development of the soil resistance on the laterally loaded monopiles driven in sand and discuss the main disadvantages and reasons in the design of super large monopiles.The numerical simulation results convince that the super large monopile foundations’soil-monopile interaction has obvious three-dimensional effect,owing to large monopile diameter and little length-to-diameter ratio(the ratio of the embedded length to the monopile diameter).In addition to the soil lateral resistance,super large monopiles bear the horizontal shear force developed at the base of monopile,the friction force and the resistance moment generated by the unbalanced reaction force along the monopile shaft.The latter three resistances’contribution to lateral bearing capacity is related to the monopile size.The large diameter and little length-to-diameter ratio increase the contribution of these three resistances.In the process of calculating and analyzing the bearing characteristics of laterally loaded monopile,in addition to the resistance provided by the soil lateral reaction(represented by p-y springs),the horizontal shear force developed at the base of monopile,the friction force and the resistance moment generated by the unbalanced reaction force along the monopile shaft shall also be considered.