串珠状溶洞影响下桩基竖向承载特性离心试验

Centrifugal test on vertical bearing characteristics of pile foundation affected by beaded caverns

为探明串珠状溶洞对桩基竖向承载特性的影响,采用离心模型试验,研究桩基穿越两层溶洞且置于下伏溶洞之上时,不同溶洞顶板厚径比下桩基荷载-沉降特征,竖向极限承载力变化规律,桩身轴力、桩侧阻力及分项荷载变化规律,与下伏无溶洞时进行对比,提出合理顶板厚径比取值方法。结果表明:桩基穿越溶洞后,下伏溶洞顶板厚径比对其竖向承载特性影响较大,与下伏无溶洞相比,厚径比由0.5增大到3.0时,桩基竖向极限承载力影响度由57.4%降至4.0%,厚径比大于2.5后,竖向极限承载力影响度小于5.0%;厚径比增大时,桩基所穿越的溶洞范围内桩身轴力几乎不衰减,上、中层溶洞顶板范围内桩身轴力衰减速度减慢,传递至溶洞顶的荷载较大;在溶洞内桩侧阻力几乎不发挥,随厚径比增大,在上层和中层溶洞顶板范围内桩侧阻力减小;厚径比增加时桩侧阻力和桩端阻力占比分别呈逐渐减小和增大的趋势,逐渐向摩擦端承桩转化,厚径比大于2后,与下伏无溶洞时相比,桩侧阻力减幅小于10%。建议串珠状溶洞区的桩基穿越两层溶洞且置于下伏溶洞之上时,顶板厚径比大于2.5即可忽略下伏溶洞对桩基竖向承载特性的影响,可根据桩基承载需求确定合理的顶板厚度。

The effect of beaded caverns on vertical bearing characteristics of pile foundation was investigated.The load-settlement characteristics,variation rule of vertical ultimate bearing capacity,distribution rule of pile axial force,side resistance,and partial load of piles under different roof thickness-pile diameter ratios were studied by centrifugal model test,when the pile passed through two layers of caves and there was an underlying cave.A reasonable method for calculating roof thickness-pile diameter ratio was proposed by comparing with the test results of the case without underlying karst cave.Results show that when the pile passed through the cavern,the roof thickness to pile diameter ratio of the underlying caverns had a great influence on the vertical ultimate bearing capacity of the pile.When the roof thickness-pile diameter ratio increased from 0.5 to 3.0,the influence degree of vertical ultimate bearing capacity of the pile decreased from 57.4%to 4.0%compared with the case without karst cave.When the roof thickness-pile diameter ratio was greater than 2.5,the influence degree of vertical ultimate bearing capacity of the pile was less than 5.0%.When the roof thickness-pile diameter ratio increased,the axial force of the pile was almost not attenuated in the cavern.The decay rate of the axial force of the pile in the upper and middle cave roof was slow,and the load transmitted to the cave roof was large.The side resistance of the pile barely had influence in the cave that the pile passed through.With the increase in the roof thickness-pile diameter ratio,the side resistance of the pile decreased in the upper and middle cave roof.As the roof thickness-pile diameter ratio increased,the proportion of pile side resistance and pile tip resistance gradually decreased and increased,and the pile gradually transformed to frictional end bearing pile.When the roof thickness-pile diameter ratio was greater than 2,the side resistance of the pile decreased less than 10%compared with the condition without underlying karst cave.In beaded karst cave area,when the pile passes through two-layer karst caves and is above an underlying karst cave,it is recommended that the influence of the underlying karst cave on the vertical bearing characteristics of the pile can be ignored if the roof thickness-pile diameter ratio is great than 2.5,and a reasonable roof thickness can be determined according to the bearing requirements of the pile foundation.