灌注桩基础桩底复合式后注浆及承载特性研究

Composite post grouting at pile tip and bearing characteristics of cast-in-place pile foundation

为更好控制灌注桩桩底后注浆过程中的注浆区域、注浆压力及注浆量,提出一种桩底复合式后注浆的技术。结合西凌井公路大桥工程开展2根灌注桩(SZ1、SZ2)的桩底复合式后注浆及静载试验,研究注浆过程中的注浆压力、注浆量及桩顶位移的动态响应规律及其承载变形特性。基于球孔扩张理论结合广义SMP准则推导了桩端浆泡半径的解析式,并建立了复合式后注浆灌注桩荷载–沉降关系的计算方法。研究表明:桩底“开式—闭式—开式”的复合式后注浆是劈裂、渗透和压密注浆交替进行的过程,注浆量均达到设计值,注浆压力较稳定,桩顶抬升位移较小;SZ1与SZ2桩端2.5倍桩径范围内侧阻的增强效应显著,增强系数分别达到3.37,3.45,端阻增强系数分别为3.28,5.37;基于双曲线函数的荷载传递模型,并考虑桩端扩大头效应建立的荷载–沉降计算方法能较准确的预测复合式后注浆灌注桩的荷载–沉降关系。

In order to better control the grouting area, pressure and quantity during the post grouting at the tip of cast-in-place concrete piles, a composite post grouting technology is proposed. Based on the Xilingjing highway bridge project, the composite post grouting and vertical static load tests on two cast-in-place concrete piles(SZ1 and SZ2) are performed to study dynamic response laws of grouting pressure, grouting quantity and pile-top displacement during the grouting process as well as their bearing deformation characteristics. Based on the theory of spherical cavity expansion and the generalized SMP criterion,the analytical formula for the radius of grouting bubble at pile tip is derived, and the method for calculating the load-settlement relationship of composite post grouting cast-in-place piles is established. The research shows that the composite post grouting process of "open-close-open’’ at the pile tip is an alternating process of splitting, penetration and compaction, the grouting quantity reaches the design value, the grouting pressure is stable, and the lifting displacement of the pile top is small. The strengthening effects of the lateral friction of SZ1 and SZ2 within the range of 2.5 times of pile diameter of pile tip are significant when the strengthening coefficients are 3.37 and 3.45 respectively, and the strengthening coefficients of tip resistance are 3.28 and 5.37 respectively. The load-settlement method based on the load transfer model of hyperbolic function and considering the enlarged head effects of pile tip can accurately predict the load-settlement relationship of composite post grouting cast-in-place piles.