为了研究基坑开挖导致旁侧既有盾构隧道产生偏心卸载而带来的不利影响，采用MIDAS/GTS
NX软件建立精细化的三环盾构管片模型。基于修正惯用法计算的盾构衬砌环初始围压和Mindlin公式计算得到的基坑开挖卸载产生的附加围压，通过将总围压施加在盾构管片模型上，模拟计算得到盾构衬砌的横向变形和内力；采用椭圆度作为评价隧道安全状况的指标，研究基坑开挖偏心卸载对盾构隧道的影响规律。结果表明：
在基坑开挖偏心卸载过程中，衬砌结构会产生斜向“压扁”的效果，呈现“斜椭圆”变形；由于应力集中的原因，在管片与管片的接缝处应力较大，混凝土应力最大值处在邻接块与标准块之间；混凝土和螺栓的最大应力值会随着椭圆度的增大而增大；随着基坑围护结构位移增大，基坑侧壁应力释放系数β值逐渐增大，隧道的水平位移值也随之增大；在基坑开挖过程中要严格控制围护结构的位移量，以保证盾构隧道安全。
In order to study the influence of eccentric unloading
induced by excavation of foundation pit on the existing shield tunnel, a
refined threering shield segment model is established by using software MIDAS/GTS
NX. The initial confining pressure of shield lining ring is calculated based on
the modified routine method, and the additional confining pressure generated by
excavation and unloading of the foundation pit is calculated based on the
Mindlin formula. By applying the total confining pressure on the shield segment
model, the transverse deformation and internal force of the shield lining are
simulated and calculated. Additionally, the ellipticity is adopted as an index
to evaluate the safety of tunnels, and the influence of the eccentric unloading
of the foundation pit on the shield tunnel is studied. The results show that:
(1) During the eccentric unloading process, the oblique "flattening"
effect would be occurred to lining structure and showing "oblique
ellipse" deformation. (2) Due to the stress concentration, the stress is
large at the joint between two segments, and the maximum stress of concrete is
between the adjacent block and the standard block. (3) The maximum stress of
concrete and bolts increases with increasing ellipticity. (4) As the
displacement of the retaining structure of the foundation pit increases, the
stress release coefficient β of the sidewall will
increase gradually, and the horizontal displacement of the tunnel will also
increase. Therefore, during excavation of foundation pits, the displacement of
retaining structure should be strictly controlled to ensure the safety of the
shield tunnel.
