深厚软土区桥梁基桩水平静载试验与m取值分析

Horizontal static load tests on bridge pile in deep soft soil area and m value analysis

我国沿海地区广泛分布深厚软土,其软土地基水平抗力系数的比例系数m值的合理取值直接影响桩基设计的经济性和桥梁等上部结构的安全性。为了解决软土m值取值的合理性问题,采用单向多循环加载法和慢速维持荷载法分别对深厚软土地区4根单桩进行水平静荷载试验,开展单桩的水平承载变形特性和m值取值研究。研究结果表明:相比于单向多循环加载法,慢速维持荷载法呈现的弹塑性分界点(拐点)更明显,对应的水平位移大,获得的淤泥质黏土m值小。桩在地面处位移超过6.5 mm(慢速维持荷载法)和5 mm(单向多循环加载法)时,桩侧淤泥质黏土进入弹塑性阶段。单桩水平临界荷载和极限水平承载力分别为189 kN和253 kN。淤泥质黏土的m值随水平位移的增大而呈幂函数衰减。在同一荷载等级下,m值随循环次数增加呈线性衰减,相比第1次循环,第5次循环m值的折减系数均值为0.84,荷载条件对循环折减系数影响有限。试验获得淤泥质黏土层m值的标准值为6226 kN/m4,大于铁路和公路规范中m值的上限值,为下限值的2.08倍。研究提出了宁波淤泥质黏土及与其物理状态和力学指标相近的软土m取值建议。研究成果可为m的合理取值和深厚软土地区桩基的优化设计与安全评估提供重要参考和依据。

Soft soil is deep and widely distributed in coastal areas of China,and its proportional coefficient m value directly affects the economy of pile construction and the safety of superstructures such as bridges.The horizontal static load tests of four single piles in deep soft soil area were performed by one-way cycle loading method and slow maintenance loading method.The horizontal bearing characteristics and m value were studied.The results are drawn.Compared with one-way cycle loading method,it is found that elastic-plastic cut-off point is more obvious.The corresponding horizontal displacement is larger,and m value of the muddy clay is smaller from maintenance loading method.When the horizontal displacement of pile exceeds 6.5 mm from maintenance loading method and 5 mm from cyclic loading method,the muddy clay of pile side will work in the elastic-plastic stage.The horizontal critical and ultimate bearing capacity of single pile are 189 kN and 253 kN,respectively.The m value decreases as a power function with the increase of horizontal displacement.Under the same load level,the m value attenuates linearly with the increase of the number of cycles.The mean reduction coefficient of m value in the fifth cycle is 0.84,It is shown that the load has insignificant influence on the cyclic reduction coefficient compared with the first cycle.The standard value of m value of muddy clay layer is 6226 kN/m4,which is larger than the upper limit and 2.08 times the lower limit from the railway and highway codes.The m value of Ningbo mucky clay and soft soil with similar physical state and mechanical indexes to Ningbo mucky clay is suggested.The research results can provide reasonable m value of soft soil for the optimal design of pile foundation in deep soft soil area.