碎石桩传力特性的实验研究及其理论分析

Experimental study and theoretical analysis of the stress propagation in a stone column

应用自行设计制造的二维实验设备,通过杠杆砝码加压与测力环直接测量桩底压强相结合的方法,研究了不同长度碎石桩的传力方式和破坏模式.实验发现,直径和填料一定的碎石桩存在一个临界桩长(临界长径比),当桩长跨越临界值时,碎石桩内部以及与桩周土之间的传力方式要发生改变.桩长小于临界值时,随着桩顶荷载增加,桩底的承载贡献率增大,而桩周土的承载贡献率减小.与之相反,桩长大于临界值时,随着桩顶荷载增加,桩底的承载贡献率减小,而桩周土的承载贡献率增大.实验还发现短桩和长桩的不同传力方式决定了不同的破坏模式.随着桩顶荷载增加,短桩将发生桩底刺入破坏,而长桩将发生顶端鼓胀剪切破坏.借鉴Janssen的"粮仓效应"经典理论,建立了二维碎石桩的传力模型.该模型对长桩定性符合良好,但对短桩不适合.

An experiment apparatus designed by ourselves, which consists of a lever-weight loading part and column-bottom-pressure ergometers, was used to investigate the stress propagation characteristics and failure modes in stone columns. The experiment results show the existence of a critical value in the length of a stone column. For a longer stone column with a length over the critical value, the growth of top pressure would lead to the increase in the contributing percentage of friction force on the side soil for balancing top pressure, as well as leading to the decrease in the contributing percentage of bottom pressure for balancing top pressure. However, it was converse in a shorter stone column with a length under the critical value. Correspondingly, the failure mode of longer columns was also different from that of shorter ones. The shorter stone columns failed as they plunged into the base soil while the longer ones were damaged by the bulge-shear near the top. To interpret the above mentioned experiment results, by referring to Janssen＇s classical mathematical scheme for the pressure screening effect in a silo, we developed a scheme to describe the stress propagation in a two-dimensional stone column, which was appropriate for longer columns but failed to fit shorter columns.