地层自重应力场中桩基荷载对隧道开挖塑性区影响的一种理论预测方法

A theoretical approach for predicting the influence of pile foundation load on the potential plastic zone induced by tunnel excavation in gravitational field

城市环境的地铁隧道往往不得不从既有结构基础附近穿过。在基础荷载和隧道开挖的共同作用下,地层会发生应力重分布,甚至可能在临时支护施作之前出现塑性区;为了保证隧道施工安全和周边既有结构的安全和正常使用,需要有效地预测隧道开挖引起的地层潜在塑性区的形状和范围。除了采用数值方法以外,现有的隧道开挖塑性区理论预测方法一般都是以自由地层为前提假设的,没有反映承载地层隧道开挖影响的特殊性。考虑桩基荷载的作用特点,提出一种预测地层自重应力场中邻近桩基隧道开挖塑性区形状和范围的理论方法,通过算例,与数值模拟进行对比,并分析不同桩基荷载的位置、大小和形式对隧道开挖塑性区的影响。计算表明:①理论计算结果与数值模拟结果相似;②达到一定量值的邻近桩基荷载会使隧道开挖塑性区的形状和范围明显的不同于自由地层隧道开挖塑性区,具体与桩基础荷载的位置、大小及分布形式有关;③邻近桩基荷载会使隧道开挖塑性区向下移动,影响程度随桩基与隧道之间水平距离的减小而增大,随桩基荷载量值的增大而增大;④对于不同桩基长度的情况,当桩底位于隧道侧上方或侧下方附近时,桩基荷载的影响最显著,当桩底位于隧道中部附近时影响最小,说明隧道侧上方与侧下方附近地层对桩基础荷载的敏感度较高。

In urban environment, it is often unavoidable for metro tunnels to be constructed in the vicinity of existing structure foundations. Under the combined action of foundation load and tunnel excavation, the near field ground would experience stress redistribution, or even yield prior to support installation; it is thus needed, as part of the effort to guarantee the safety and normal operation of tunneling and the nearby existing structures, to effectively predict the shape and range of the tunneling induced potential plastic zone. Generally underlying in almost all existing theoretical methods, excluding numerical ones, is the green field assumption. Taking into account the characteristics of pile foundation loads, a theoretical approach is developed for predicting the potential plastic zone induced by tunnel excavation in the vicinity of pile loads in gravitational field. Comparisons with numerical simulations have been oonducted for verification, and examples are provided for analysing the influences of various locations, magnitudes m ~ 9atterns of pile loads on the tunneling induced plastic zones. The following observations can be summarized as a result of the calculations： （~） theoretical calculation results closely resemble the corresponding numerical modeling results; ~） Pile loads of sufficient magnitudes, with variable locations and distribution patterns, may significantly affect, with variable extents, the shapes and ranges of the plastic zones induced by nearby tunnel excavations. （~） Pile loads tend to drag the plastic zone downwards, and the degree of such influence increases as the horizontal offset between pile and tunnel decreases and as the pile load increases. （1） As for different pile lengths, the effects of pile loads are more clearly felt if the pile tip is close to the upper or lower parts of the tunnel, whereas the effects are minimal if the pile tip is near the middle part of the tunnel.