“铁板砂-黏土”地层中桩靴穿刺数值模拟研究

Numerical Analysis of Spudcan Punch-through on Hard Crust Layer Overlying Clay

在我国自升式平台活跃的埕岛海域,"铁板砂-黏土"地层广泛发育。与砂土和黏土不同,"铁板砂"主要由粉砂或粉土组成,因此可能无法直接套用现有的研究手段或理论公式对桩靴穿刺进行分析。基于大变形有限元方法,对已开展的"铁板砂-黏土"地层中桩靴贯入的离心机试验进行模拟分析。数值分析中,将"铁板砂"分别等效为砂土和硬黏土,采用摩尔库伦(MC)、修正摩尔库伦(MMC)及修正Tresca模型进行模拟。结果表明:将"铁板砂"等效为砂土时,采用MC或MMC模型均显著低估桩靴在"铁板砂-黏土"地层中的贯入阻力;将"铁板砂"等效为硬黏土时,可以较好地模拟桩靴在"铁板砂-黏土"地层中发生穿刺时的峰值贯入阻力。对于离心机试验中采用的有效重度约为11kN/m^3的埕岛海域铁板砂,当等效不排水抗剪强度取值约为80kPa时,数值模拟得到的峰值阻力与离心机试验结果基本吻合。

The soil profile of'hard crust-over-clay'is prevalent in offshore area of Chengdao,where mobile jack-up rigs are used frequently.Different to sand and clay soils,the'hard crust'mainly consists of silty sand or silt.Therefore,the existing numerical method and design formula may not be suitable for analyzing the potential for spudcan punch-through in this type of soil profile.In this paper,large deformation finite element method is used to simulate centrifuge tests that have been carried out for spudcan penetration in hard crust layer overlying clay.In the numerical analysis,the hard crust is modelled as sand or stiff clay layer,using Mohr-Coulomb(MC),modified Mohr-Coulomb(MMC)and modified Tresca models.The results show that when the hard crust is modelled as sand layer,the penetration resistance of spudcan in'hard crust-over-clay'is significantly underestimated by MC and MMC models.By contrast,the numerical analyses,where the hard crust is modelled as stiff clay layer,provide a better prediction for the peak resistance in the hard crust as punch-through happens.For the hard crust soil used in the centrifuge tests of this study,which was collected from Chengdao Sea with an effective unit weight of 11 kN/m^3,an equivalent undrained shear strength of about 80 kPa is suggested.