针对西安地铁一号线盾构隧道下穿朝阳门段城墙及朝阳门门洞具体工程，通过建立三维有限元模型，模拟分析了盾构掘进过程中的沉降机理，对古城墙受力进行了理论分析，预判出古城墙及附近地面的沉降值，并制定出了针对性的加固方案。数值模拟及工程实践表明： 1）在工法、线路和埋深已优化的前提下，采取的加固方案可使古城墙的沉降和变形得到一定程度的控制； 2）数值模拟结果基本符合盾构施工期间地层和城墙沉降的基本规律，计算结果偏于保守； 3）提出的城墙沉降控制标准也满足实际施工和文物保护要求，这一标准可为今后类似工程提供参考。

A threedimensional finite element model is established to study the settlement mechanism of the city wall in Xi’an, with a shield tunnel on Line 1 of Xi’an Metro crossing underneath the city wall of Chaoyang Gate and Chaoyang Gate openings as the background. Force analysis is made on the ancient city wall and the settlement of the city wall and the ground nearby is predicted. Reinforcement measures are determined on basis of the analysis results. The numerical simulation and the engineering practice show that: 1) Under the condition of the optimized construction methods, route locations and route depths, the reinforcement measures taken can bring the settlement and deformation of the city wall under control; 2) The numerical simulation results are basically consistent with the regularity of the settlement of the city wall, although they are some conservative; 3) The settlement control standard proposed, which can meet the requirements of the tunnel construction and the protection of the cultural relics, can be used as reference for similar projects in the future.