基于混凝土损伤模型的卸载作用下盾构隧道损伤机理

Damage mechanism of shield tunnel under unloading based on elastoplastic damage model of concrete

在密集的地铁交通网附近区域进行工程活动会引起下卧盾构隧道产生较大纵向差异变形,对隧道结构服役性能及行车安全造成不利影响。既有研究大多基于弹性、弹塑性理论框架,忽视盾构隧道纵向变形带来的结构损伤的影响。考虑混凝土材料非线性损伤力学特性,首先引入一种基于能量的拉应力和压应力分解,考虑混凝土拉/压力学特性的差异;其次,建立双标量混凝土弹塑性损伤本构模型;最后,基于三维非连续接触模型及多尺度建模技术构建盾构隧道−围岩精细化模型,探究差异变形下盾构隧道损伤劣化机理。研究结果表明:卸载作用下,管片混凝土以局部拉损伤为主而压损伤较小,在管环拱顶和拱底内缘、拱腰外缘以及螺栓连接处拉损伤最严重;卸载作用会引起盾构隧道隆起变形,横截面椭圆度也随纵向发生变化,拐点附近管环椭圆度最大;卸载引起的盾构隧道纵向附加剪力和弯矩最大值分别出现在拐点附近和模型中心处;混凝土材料损伤劣化会降低盾构隧道承载能力、纵向刚度和环向刚度,基于弹性本构的计算模型会高估整体结构的刚度;当前分析工况下,管片钢筋较难屈服,而中间部位管段上半侧纵向连接螺栓较易屈服;加载曲线拐点附近管环受损最严重,且承受较大的附加剪力和弯矩,应特别予以关注。

Engineering activities in the vicinity of dense metro traffic network caused large longitudinal differential deformation of the underlying shield tunnel,which adversely affected the service performance and traffic safety of the metro system.Previous relevant studies were mostly carried out within the framework of elastic or elastoplastic theory,despite the influence of structural damage caused by longitudinal deformation of shield tunnel.The nonlinear damage characteristics of concrete material were considered,and a novel positive/negative decomposition of stress tensor in energy norm was introduced to consider the asymmetric tensile/compressive material behavior of concrete.Secondly,a bi-scalar damage constitutive model of concrete was further developed.Finally,to investigate the damage and degradation mechanisms of shield tunnel under unloading,3D discontinuous contact model in conjunction with multiscale mixed modelling technology was employed to develop tunnel-soil numerical model.The results show that when shield tunnel suffers unloading stress,localized tension damage dominates while compression damage is minor.The severest tension damage is observed at the inner sides of tunnel crown and bottom,outer sides of tunnel waist,and bolt connections.Tunnel heave is induced by unloading process,ellipticity of tunnel cross section varies along the longitudinal direction,and the maximum convergence deformation is observed at the segmental ring near the inflection point.The unloading-induced maximum additional shear force appears near the inflection point,while the peak bending moment is observed at the model center.The damage and degradation of concrete materials will reduce the tunnel bearing capacity,longitudinal and circumferential stiffness,whilst the elastic-based model will overestimate the integral structural stiffness.According to the investigated cases herein,the segmental rebar is hard to yield,while the longitudinal coupling bolts on the tunnel upper half are easier to yield.It should be noted that the segmental rings near the inflection point are most seriously damaged,and bear large shear forces and bending moments.