复合地基中褥垫层的极限下限分析

作为复合地基的重要组成部分，褥垫层影响着路堤下复合地基的桩、土承载力的发挥，所以有必要对其承载与变形特性进行研究。根据极限分析下限法，在满足应力边界条件和摩尔一库伦屈服条件下，通过构建应力场、应力叠加计算等步骤，分别推导出褥垫层在厚度无穷大和厚度较小时的极限承载力下限解，以及桩土应力比计算公式。通过对算例进行分析，揭示了褥垫层存在压密和塑性流动两个受力变形阶段；桩土应力比随着内摩擦角的增大而增大，随着褥垫层厚度的增大而减小等规律；以及桩土应力比试验值与理论计算值偏差的原因。

Lower bound limit analysis of cushion in composite foundation

Cushion, an essential component of the composite foundation, exerts influences on the bearing capacity of the pile and soil in composite foundation under embankment, so it is necessary to make a study on the cushion＇s bearing and deformation char- acteristics. According to the lower bound limit analysis method, through steps like stress filed construction and stress superposi- tion calculation while the stress of boundary was accorded with the fact condition and cushion＇s yield condition was met with mo- hr-eoulomb criterion, the lower-bound solutions of the cushion＇s ultimate bearing capacity were derived respectively when the cushion＇s thickness was infinite and small, and formulas to calculate the pile-soil stress ratio could also be deduced. Through ex- amples and analyses, two phases in the cushion＇s deformation are revealed, namely compaction and plastic flow process. Further- more, it is also found that the pile-soil stress ratio increases as the internal friction angle increases, but decreases along with the enlargement of the cushion＇s thickness. Besides, reasons for the deviation of the experimental value from the theoretical value of the pile-soil stress ratio are explored.