地下水位循环变化时隧道开挖对群桩的长期影响研究

为解决地下水位随季节升降变化时隧道开挖对邻近桩基的长期影响难题，通过三维离心模型试验，研究了地下水位循环变化时隧道对群桩的长期影响。主要分析水位循环变化时地表长期沉降、桩顶长期附加沉降、桩身长期附加弯矩和附加轴力的变化规律。试验结果表明：在隧道附近，地下水位循环变化尤其是降水对地表长期沉降影响更为明显。地表长期附加沉降随着地下水位循环次数增加而增大，且呈衰减式变形，即使经过3次水位升降循环也不能稳定；桩基长期附加沉降显著，其附加沉降量占总沉降量的50%以上；前、后桩的长期附加轴力基本为正值，桩总轴力增加，对既有受压桩极为不利，附加轴力拐点位置及最大值有所区别；经过3次地下水位循环变化后，前、后桩桩身弯矩反弯点个数减少，但桩身最大附加弯矩均明显变大，当达到极限弯矩，桩身出现塑性铰，这对穿越厚软弱层地基中的既有受压柔性桩极为危险。

A study of the long-term influence of twin tunneling on the existing pile group under cyclic variation of groundwater level

The aim of this study is to explore the long-term influence of tunnelling on the adjacent pile foundation under cyclic change of groundwater level. A three-dimensional centrifugal model test is conducted to investigate the long-term effect of tunneling on a pile group while the groundwater level changes cyclically. The long-term surface settlement, additional pile head subsidence, additional bending moment and additional axial force of pile are examined. The results show that the long-term surface subsidence is influenced more significantly by the cyclically varying groundwater level induced, particularly, by precipitation. The long-term surface settlement increases with the number of cycle, showing an attenuation-type deformation; it is not convergent after three times of water level fluctuation cycle. The long-term additional subsidence of pile foundation is remarkable, accounting for more than 50% of the total settlement. Generally, the values of the long-term additional axial forces of front pile and rear pile are positive; if the total axial force increases, it can adversely influence the existing bearing pile. The inflection point and the maximum of the additional axial force are different. The inflection point number of bending moment of the front pile and rear pile decreases after three times of groundwater level cycles. However, the maximum additional bending moments of pile become significantly larger. The plastic hinge of pile body appears when the maximum additional bending moment of pile reaches the ultimate bending moment, which is extremely dangerous to the existing bearing flexible pile that passes through thick layer of weak soils.