深厚软土区超长桩基压缩变形测试及分析

Measurement and Analysis of Compression Deformation of a Super-long Pile Foundation in Deep Soft Deposit

提出了一种联合运用沉降计和应变计观测超长桩基桩身压缩变形及桩底土压缩变形的新方法,详细介绍了所涉及元器件的安装方法和施工工艺;实践表明该方法的监测深度可达地表以下100 m,有效完善了深厚软土区超长桩基沉降变形测试技术.基于实测数据和数值仿真,系统研究了上海地区超长桩基的压缩变形规律及特性.主要结论如下:1)桥梁施工过程中,桩基竖向压缩变形随其受载历史呈“阶梯”式增长变化;群桩基础工后沉降约为5.0 mm,小于相关规范规定的20 mm限值;桩基承台的微小变形易导致不同位置基桩的桩顶沉降和桩身轴力存在一定差异.2)桩底以下20~30 m范围内土体的总压缩变形量和单位厚度土层压缩量已很小,可初步判定上海地区高速铁路桥梁超长桩基的桩底土层压缩影响范围约为20 m.3)沉降计法测试所得桩身压缩变形的精度通常高于应变计法;对于超长桩基,桩身压缩变形占桩基总沉降的比例可达30%~40%,在计算桩基沉降时应予以考虑.4)桩身浅层范围内存在负摩阻力,距桩顶向下11 m处出现中性点,且中性点位置随桩基荷载增加有逐步上移的趋势.

A new method combined with using single-point settlement gauges and strain gauges was developed to monitor the compression deformation of pile foundation,the compressions of pile shafts and soil layers beneath the pile bottom.The measurement depth of the new-developed method can reach more than 100 m below the ground surface,which effectively replenished the settlement monitoring technology of super-long pile foundations in the deep soft soil.The compression deformation behaviors were then systematically analyzed by field experimental and numerical methods.The main conclusions show that the vertical compression deformation of the pile foundation increaseds in a ladder shape during the construction of the bridge,and the post-construction settlement is approximate to 5.0 mm,which is less than the normative limit(20 mm)of a related design code;small deformation of the pile foundation cap may lead to differences in the settlements and axial forces of piles at different positions;then,the total compression deformations as well as the unit compression of the subsoil layers within 20~30 mbelow the pile bottom are quite small,hence it can be preliminarily determined that the influencing thickness of the compressible layers below the bottomof super-long pile foundations of high-speed railways in this tested area is approximate to 20 m;moreover,the accuracy of pile compression measured by settlement gauges is generally higher than that measured from strain gauges,and the pile compression can take a proportion as large as 30%~40% of the total pile settlement for a super-long pile foundation,hence it should be considered in calculating the pile foundation settlement;in addition,negative friction resistance appearis in the shallow depth,and there is a neutral plane at 11 m below the top of the pile;furthermore,the position of the neutral planegradually moves up with the increase of the pile foundation loads.