“硬-软-硬”三层黏土上圆形基础的承载力分析

STUDY OF BEARING CAPACITY OF SURFACE CIRCULAR FOOTING ON STIFF-SOFT-STIFF CLAYS

海洋浅基础设计所遇地层条件通常由于基础大尺寸的特点而涉及多土层剖面,其中“硬-软-硬”三层黏土是最常见的地层条件之一。现行海洋浅基础设计常用的API和ISO规范尚无针对“硬-软-硬”黏土竖向承载力的计算方法。本文基于自升式平台基础设计规范推荐的“bottom-up”预测模型,假定“硬-软-硬”黏土上基础的承载力为“硬-软”黏土冲剪破坏产生的侧向摩阻力与“软-硬”黏土挤压破坏提供的承载力之和。通过有限元模拟分析,验证了“bottom-up”方法所假定破坏模型的合理性,但由于规范推荐的相关公式忽略了上覆土层对挤压破坏模式的影响,使其严重低估了“软-硬”黏土挤压破坏产生的承载力。基于变参数有限元分析结果,本文对挤压破坏承载力公式进行了修正,提出了“硬-软-硬”三层黏土上圆形基础竖向承载力的预测方法。

The stratigraphic configurations for the design of offshore shallow foundations usually involve multi-layer soil profiles due to their large sizes.The stiff-soft-stiff clay deposit is one of the most prevalent configurations.Design methods are not available in the current API and ISO standards for shallow foundations to estimate the bearing capacity of a footing on stiff-soft-stiff clays.Based on the“bottom-up”approach recommended for spudcan foundations of jack-up rig,this paper assumes an analytical model that sums the resistances from the punching shear of the stiff-soft layering system and the squeezing of the soft-stiff layering system.The rationality of this predictive model is verified based on comprehensive FE analyses.The comparison between the predictions from“bottom-up”approach and FE analyses indicates that the assumed model is reasonable and realistic.However,the resistance due to squeezing response is significantly underestimated by the current design formula.Detailed investigations are carried out.It is revealed that this is mainly because the effect of the overlying stiff clay on the squeezing mechanism is neglected.Based on the findings,an improved design formula is proposed for the squeezing response and hence for the design of circular foundations on stiff-soft-stiff clays.