【作者】 魏少伟；

【导师】 郑刚；

【作者基本信息】 天津大学 ， 岩土工程， 2010， 博士

【摘要】 城市地下空间的大规模开发,不可避免地要在地铁隧道附近进行施工活动,这其中就包括在已建隧道的上方进行基坑开挖。目前国内外已有不少相关工程实例的报道,这些报道表明,基坑开挖会使坑底隧道产生上抬位移与不可忽视的自身变形,影响隧道的安全运行;在软土地区,基坑开挖对坑底已建隧道的影响更需重点关注。保护已建隧道在基坑开挖过程的安全运行,已经成为岩土领域的一个热点工程问题。本论文借助有限元方法与离心试验方法,对基坑开挖对坑底已建隧道影响的作用机理进行了深入研究,同时对各种影响因素进行了参数分析。研究的成果有助于深刻认识这一工程问题,同时为隧道的保护措施提供理论依据。本文的主要内容与结论包括以下几个方面:1.软土地区基坑开挖对坑底隧道影响的二维数值分析表明:（1）基坑开挖引起坑底隧道的位移与基坑开挖引起的周边土体位移场相一致。（2）基坑开挖引起坑底隧道的横截面变形主要取决于其周边土体的总应力变化。（3）基坑开挖过程处于基坑中心的隧道其最大自身变形并不一定发生在距坑底面竖向距离最近的隧道中;对于开挖宽度较小的基坑（ 2 B e /he?10/3）,相同埋深下处于基坑中心的隧道与靠近地连墙的隧道其基坑开挖过程最大的变形量比较接近;对于开挖宽度较大的基坑（ ）,相同埋深下靠近地连墙的隧道其相对变形比处于基坑中心的隧道大,工程中需重点关注。（4）基坑开挖过程隧道横截面产生的附加弯矩与其自身变形相一致,且成线性比例关系。（5）基坑的固结效应对坑底隧道的竖向位移影响明显,而对隧道在基坑开挖过程的自身变形影响很小。（6）加大隧道结构的刚度,可以减小坑底隧道在基坑开挖过程的相对变形,同时增加隧道截面的附加弯矩;在隧道的结构刚度达到一定值时,再增加隧道刚度,坑底隧道在基坑开挖过程的相对变形减小很少。2 Be /he?16/32.软土地区基坑开挖对坑底隧道影响的三维数值分析表明,靠近基坑开挖中心面（ =0）的隧道,开挖引起的隧道各截面的位移与自身变形接近二维分析的结果,且在18m范围内隧道各横截面的位移与自身变形变化很小;从h_ty18m开始,隧道的位移与变形开始减小,超过h_ty 54m（地连墙所在位置）,隧道截面位移与变形迅速减小,hty=81m（距地连墙的水平距离为3倍的开挖深度）时隧道横截面基本不受基坑开挖的影响。3.砂土离心试验反映了不同位置坑底隧道各横截面在基坑开挖过程的弯矩变化及竖向直径改变。对离心试验进行数值模拟,并对隧道相对位置、土的弹性模量进行参数影响分析,得出:（1）数值分析的结果较好地吻合了离心试验中隧道横截面弯矩的变化趋势;（2）在开挖宽度为300mm（原型为24m）、开挖深度为150mm（原型为12m）的砂土基坑中,在开挖宽度为300mm（原型为24m）、开挖深度为150mm（原型为12m）的砂土基坑中,若隧道埋深大于地连墙墙底,基坑开挖引起坑底隧道横截面的相对变形将随着隧道埋深的增加而减小。相同埋深隧道在基坑开挖过程的自身变形的数值接近,靠近地连墙的隧道其相对变形有向基坑中心旋转的趋势;（3）增大土体的弹性模量,可以有效减小基坑开挖引起的坑底隧道横截面的自身变形,同时降低隧道横截面的附加弯矩。更多还原

【Abstract】 Rapid urban development in cities often requires basement excavation close to or directly above existing tunnels. Some reports of correlative projects have indicated that an excavation may induce upward displacement and deflection on tunnel lining and affect the serviceability and safety of the tunnel. In soft soil, more attention should be paid on this complex interaction between a basement excavation and an existing tunnel. The research in this theis aims to study the mechanism of this complex interaction and investigate the influence factors by means of the finite element and centrifuge model tests. The research can futher the understanding of this complex interaction and provide reasonable advice to protect the tunnel lining.Based on the research, following conclusions have been obtained:1.A series of two-dimensional finite element analyses were performed to study the influence of basement excavation on underlying tunnel in soft soil. The following conclusions are drawn: (1) Excavation-induced displacements of tunnels are consistent with the soil movement around excavation; (2) Excavation-induced tunnel distortions lie on the soil stress changes around tunnel lining during excavation; (3) For the tunnels located in the centerline of excavation, maximum diameter change does not necessarily occur in the tunnel with smallest vertical distance to the formation level of excavation. For the excavation with smaller excavation width(2B_e/h_e≤10/3),the magnitude of relative deformation of the tunnel located in the centerline of excavation is approximate to that located close to the diaphragm wall with the same tunnel cover depth. For the excavation with larger excavation width(2B_e/h_e≤16/3),the relative deformation of the tunnel close to diaphragm wall is larger than that located in the centerline of the excavation with the same tunnel cover depth; (4)The bending moment changes of the cross sections of the tunnels are consistent with the tunnel relative deformation, and the mangnitude of the bending moment changes are linearly proportional to the relative deformation.(5)The consolidation effect of excavation on the vertical displacement of underlying tunnel is significant due to the dissipation of excess negative pore water pressure aroud tunnel lining. The influence of consolidation effect of excavation on tunnel distortions is trivial. (6)Excavation-induced tunnel distortion will decrease as the increase of the elastic modulus of the tunnel lining. After certain magnitude of the elastic modulus of the tunnel lining ia achieved, the tunnel distortion during excavation will decrease insignificantly as the increase of the elastic modulus of the tunnel lining.2. A series of three-dimesional finite element analyses were carried out to study the tunnel responses during excavation in soft soil. Based on the research, it can be obtained that excavation-induced displacement and distortion of the cross section of the tunnel close to the center face of the excavation( hty=0) is approximate to the result of two-dimensional analysis. The excavation-induced displacement and distortion of the cross section of the tunnel change slightly when 18m and decrease with the increase ofhty when hty≤18m. When exceeds 54m (the location of diaphragm wall), the excavation-induced displacement and distortion of the cross section of the tunnel will decrease significantly with the increase ofhty . The tunnel cross section located outside the excavation with hty≥81m is insignficantly influenced by basement excavation.3.The result of the two centrifuge tests in sand depicts that the bending moment and vertical diameter changes of underlying tunnels with diffirent tunnel location. The numerical simulation of the centrifuge tests were also performed and the following conclusions are obtained: (1)The result of numerical simulation is consistent with the variation tendency of the bending moment of the tunnel cross section in centrifuge tests(;2)For an basement excavation in sand with excavation width of 300mm(24m in prototype) and excavation depth of 150mm(12m in prototype), excavation-induced distortion of the tunnel cross section will decrease as the increase of the tunnel cover depth when the tunnel cover depth is larger than the depth of the bottom of the diaphragm wall. (3) Excavation-induced distortion of the tunnel cross section will decrease remarkly with the increase of the soil elastic modulus around tunnel.更多还原