竖向循环荷载下单桩的累积沉降特性模型试验

高速公路、铁路桩承式路基中桩基的累积沉降发展对路基变形影响很大，为有效控制路基变形并利于后续运营，有必要深入研究竖向循环荷载下单桩的累积沉降发展规律。通过新研发的土体加压装置对模型试验中桩侧土体施加额外的侧向压力，开展了竖向循环荷载下双层地基中单桩模型试验，基于模型试验结果分析了循环荷载下侧向压力对桩顶累积沉降的影响，并进一步探究了侧向增压地基中循环荷载频率、循环荷载比（CLR）、静偏荷载比（SLR）对桩顶累积沉降的影响。研究结果表明：增加侧向压力不仅提高了单桩的极限承载力，还对循环荷载作用下桩顶累积沉降的影响明显，尤其循环荷载较小时增加侧向压力可能会改变桩顶累积沉降的发展模式。循环荷载下单桩的桩顶累积沉降明显受到循环次数、SLR、CLR及循环荷载频率的影响，桩顶累积沉降随着循环次数、CLR和SLR的增加而增大，且在循环加载前期的桩顶累积沉降发展较快，CLR对桩顶累积沉降的影响比SLR的大；荷载频率的增大同样会导致桩顶累积沉降增加，并朝着不利于桩基安全的方向发展；发展型的桩顶累积沉降与循环次数的关系可用对数函数进行表述。当前仅以荷载大小作为桩顶累积沉降发展类型的判定标准存在一定的局限性，应在工程设计及运营中特别注意循环荷载频率、CLR、SLR等因素的影响。

Model Test of Accumulated Settlement Characteristics of Single Pile Under Vertical Cyclic Loading

The development of cumulative settlement in pile-supported subgrades of highways and high-speed railways has a significant impact on subgrade deformation. To effectively control the subgrade deformation and facilitate subsequent operation, it is necessary to study the cumulative settlement development law of a single pile under a vertical cyclic load. Additional lateral pressure was applied to the soil around the pile in the model test using a newly developed soil pressurization device. A few single-pile model tests were performed in a double-layer foundation under vertical cyclic loading. Based on the model test results, the influence of lateral pressure on the pile top cumulative settlement under cyclic loading was analyzed, and the effects of cyclic load frequency, cyclic load ratio (CLR), and static eccentric load ratio (SLR) on the pile top cumulative settlement in a laterally pressurized foundation were further studied under cyclic loading. The test results showed that the lateral pressure increment not only increased the ultimate bearing capacity of a single pile but also influenced the pile top cumulative settlement under cyclic loading. In particular, when cyclic loading is small, the lateral pressure increment may change the development mode of the pile top cumulative settlement. The single pile top cumulative settlement under cyclic loading is affected by the cyclic cycle number, SLR, CLR and load frequency. The cumulative settlement of the pile top increased with the number of cycles and developed rapidly in the early stage of cyclic loading. The cumulative settlement of the pile top increased with an increase in SLR and CLR. The influence on the pile top cumulative settlement of CLR is larger than that of SLR. The increase in load frequency also leads to an increase in the pile top cumulative settlement, which is not conducive to the safety of the pile foundation. The relationship between the progressive pile top cumulative settlement and the cycle number can be expressed by a logarithmic function. There are some limitations in using the load as the judgment standard for the development type of pile-top cumulative settlement. These influencing factors, such as cyclic loading frequency, CLR and SLR should be given special attention in engineering design and operation.