Lots of crack and fracture occurred on the segment of underlying shield tunnel due to the extensive excavation and unloading of ground surface.Taking a shield tunnel in Chengdu Metro as an example,the change laws of the displacement and internal force of underlying tunnel during large scale excavation and unloading of ground surface were analyzed by finite element simulation method from the aspects of formation condition,earth excavation and construction sequence,segment assembling quality as well as the positions of segment crack and fracture.Results show that the overall uplift deformation of underlying shield tunnel presents the rule of decreasing in horizontal diameter and increasing in vertical diameter.The bending moment of tunnel vault and arch bottom decreases and the axial force remains unchanged.The inside tension is substituted by lateral tension in the local area of tunnel vault.The vault joint changes from inside opening to outside opening and inside extrusion.The threat to tunnel safety mainly comes from the inside extrusion of tunnel vault,the increase in the vertical ellipticity of tunnel and the uneven contact (faulting of slab ends) of segments on both sides of the joint,rather than the change in structural internal force.Therefore,the corresponding scheme is put forward for repairing the crack and fracture of tunnel segments with modified epoxy resin grouting,epoxy mortar filling and carbon fiber bonding.The bearing capacity of the repaired segment can fully meet the requirement for metro operation.%针对地表大范围开挖卸载引起下卧盾构隧道管片出现大量裂缝与碎裂掉块现象,以成都地铁某盾构隧道为例,从地层条件、土方开挖与施工顺序、管片拼装质量以及管片开裂掉块情况出发,运用有限元仿真方法,对地表大范围开挖卸载过程中下卧隧道的位移和内力的变化规律进行分析.结果表明:下卧盾构隧道的整体隆起变形呈现水平向直径变小、竖向直径增大的规律;隧道拱顶和拱底的弯矩减小、轴力基本不变,拱顶处局部区域由内侧受拉转变成外侧受拉,拱顶接缝由内侧张开转变为外侧张开、内侧挤压;隧道安全的威胁主要来自隧道拱顶处内侧挤压、隧道竖向椭圆度增加以及接缝两侧管片不平整接触(错台),而非结构内力变化.由此提出采用改性环氧树脂灌注、环氧砂浆填充和粘贴碳纤维对隧道管片的裂缝和破碎处进行修补的方案,修补后管片的承载能力完全满足地铁运营的要求.
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