为研究上软下硬复合地层中新建盾构工程下穿既有隧道时，引起的地层沉降变化及开挖面极限支护压力，提出一种考虑地层变异性和既有隧道刚度的改进楔形体力学解析模型。以青岛地铁15号线一期工程为背景，基于工程勘察资料，采用ABAQUS建立复合地层数值模型，分析下穿隧道全过程中各分析步及各地层的沉降曲线，同时研究此过程中既有隧洞拱顶与拱底变形的动态变化规律，以及弹性模量与泊松比对地表沉降的影响，在一般控制措施之外提出了针对下穿既有隧道的控制措施。最后，分析不同内摩擦角及黏聚力下的水平位移曲线，获取最大水平位移随支护应力比的变化规律，进而求得极限支护压力。研究结果表明：新建盾构隧道对既有隧道及地层变形的影响不容忽视，相较于地表位移，下穿前后夹层土及既有隧道时的拱底沉降更为显著，最大沉降值为16.5 mm；既有隧道对新建隧道拱顶沉降及地表沉降具有一定的变形抑制作用，下穿隧道时地表沉降值及沉降槽宽度均显著减小。

In order to study the settlement change of the stratum and the ultimate support pressure of the excavation surface caused by the new shield tunnel project penetrating the existing tunnel in the upper-soft and lower-hard composite stratum, an improved wedge shaped dynamic analytical model considering the variability of the stratum and the stiffness of the existing tunnel was proposed. With the background of Qingdao Metro Line 15 Phase I project, based on the engineering investigation data, the composite stratum numerical model was established by ABAQUS to analyze the settlement curves of each analytical step and each stratum during the whole process of tunneling, and at the same time to study the dynamic change law of the deformation of the arch top and arch bottom of the existing tunnel in the process, as well as the influence of elastic modulus and Poisson's ratio on the ground settlement, so that control measures for tunnel underpassing the existing tunnel were put forward in addition to the general control measures. Finally, the horizontal displacement curves under different internal friction angles and cohesion were analysed to obtain the variation rule of maximum horizontal displacement with support stress ratio, and then the ultimate support pressure was obtained. The results of the study show that the impact of the new shield tunnel on the existing tunnel and the deformation of the ground should not be ignored, compared with the surface displacement, before and after penetration of the sandwiched soil and the existing tunnel when the bottom of the arch settlement is more significant, with a maximum settlement value of 16.5 mm. The existing tunnel has a certain deformation inhibition effect on the new tunnel arch settlement and surface settlement, the surface settlement value and the width of the sinkhole when penetrating the tunnel are significantly reduced.