砂卵石地层工程性质较为特殊，具有渗透性高、自稳性差、受扰动较为敏感以及区域差异性大等特性。盾构长距离掘进该地层时极易引起地层变形，甚至出现隧道失稳塌陷、地面建筑物开裂等事故，而且施工中刀盘磨损严重，增加了带压换刀的风险，影响盾构隧道施工安全和工程进度。因此，本文以西安地铁6号线田穆区间盾构隧道为依托工程，针对长距离砂卵石地层盾构施工风险评估进行研究，并应用于实际工程。主要工作和研究成果如下：

（1）通过分析砂卵石地层特性、盾构穿越该地层的扰动机理以及施工风险的产生机理，提出了一套系统的风险评估方法及体系。首先，采用工作分解结构（WBS）与风险因素分解（RBS）相结合的矩阵方法，对施工过程中可能遇到的风险进行识别和分类。然后，通过集成解释结构模型（ISM）与动态贝叶斯网络（DBN），对识别出的风险因素进行深入分析，从而对施工风险进行量化评价。最后，通过敏感性分析，识别出对系统风险影响大的关键因素。

（2）采用WBS-RBS结构矩阵方法，围绕盾构始发、盾构掘进、盾构到达三个施工阶段，分析风险与施工活动之间的关系。基于4M理论（人、机、法、环），分别从人的不安全行为、机械的不安全状态、管理措施不到位、环境因素不佳的角度进行WBS-RBS施工安全风险因素识别。通过构建WBS-RBS风险耦合矩阵，得出30项具体的安全风险因素，形成了长距离砂卵石地层盾构法施工安全风险清单。

（3）利用解释结构模型（ISM）结合问卷调查法，分析风险因素之间的关联性；将风险因素分为直接因子、中间因子和深层因子并建立风险层析结构图。基于解释结构模型，建立静态贝叶斯网络结构，结合风险的动态特征确定状态转移网络，设置了12个动态节点，进而研究风险的传递规律。采用三角模糊数和Noisy-or gate模型对原始数据进行处理，计算出先验概率、条件概率、转移概率，构建了动态贝叶斯网络模型。

（4）在建立各阶段动态贝叶斯网络的基础上，对长距离砂卵石地层地铁盾构法施工安全风险状态进行评价。利用动态贝叶斯网络的时间属性，揭示了安全风险因素随施工活动的推进而动态演化的过程。运用诊断推理方法，得出施工人员经验及能力不足（R₂A₃₁）--注浆系统控制不当（R₂T₂₃）--管片拼接不当（R₂T₂₁）--盾构掘进阶段风险（R₂）在内的14条关键风险传递路径。通过对地铁盾构法各施工阶段进行敏感性分析筛选出负环管片拼装不到位、盾构参数设置不当、周围建筑物等11项敏感因素。

（5）结合西安地铁6号线的盾构施工项目，对其安全风险状态进行了详细的评估。评估结果显示，整体安全风险等级为II级，其中管理和环境风险评定为II级，人员操作和机械风险为III级。针对评定较高的风险，提出了具体的应对措施，以降低风险并保障施工安全。

Sand and gravel stratum has special engineering properties, such as high permeability, poor self-stability, sensitive to disturbance and large regional differences. It is very easy for shield tunneling to cause formation deformation, even tunnel instability collapse, cracking of ground buildings and other accidents, and the cutter head wear is serious during construction, which increases the risk of pressure tool changing and affects the construction safety and project progress of shield tunnel. Therefore, based on the shield tunnel in Tianmu section of Xi 'an Metro Line 6, this paper studies the risk assessment of shield construction in long distance sand and gravel stratum, and applies it to practical projects. The main work and research achievements are as follows:

A set of systematic risk assessment method and system is proposed by analyzing the characteristics of sand and gravel formation, the disturbance mechanism of shield tunneling through the formation and the generation mechanism of construction risk. Firstly, the matrix method combining work breakdown structure (WBS) and risk factor decomposition (RBS) is used to identify and classify the risks that may be encountered in the construction process. Then, through the integration of interpretive structural model (ISM) and dynamic Bayesian network (DBN), the identified risk factors are deeply analyzed, and the construction risk is quantitatively evaluated. Finally, through sensitivity analysis, the key factors that have great influence on the system risk are identified.

(2) The WBS-RBS structure matrix method was adopted to analyze the relationship between risk and construction activities by focusing on three construction stages: shield initiation, shield tunneling and shield arrival. Based on 4M theory (man, machine, technology and environment), WBS-RBS construction safety risk factors are identified from the perspectives of unsafe behavior of man, unsafe state of machinery, inadequate management measures and poor environmental factors. Through the construction of WBS-RBS risk coupling matrix, 30 specific safety risk factors are obtained, and the safety risk list of long distance sand pebble formation shield construction is formed.

(3) Interpretive structure model (ISM) combined with questionnaire survey was used to analyze the correlation between risk factors; The risk factors are divided into direct factors, intermediate factors and deep factors, and the risk chromatographic structure is established. Based on the interpretive structure model, the static Bayesian network structure is established, and the state transfer network is determined according to the dynamic characteristics of risks, and 12 dynamic nodes are set up to study the law of risk transfer. Using triangular fuzzy number and Noisy orgate model to process the original data, the prior probability, conditional probability and transition probability are calculated, and the dynamic Bayesian network model is constructed.

(4) On the basis of the establishment of the dynamic Bayesian network at each stage, the safety risk state of the construction of the shield method of the long-distance sandy and gravel stratum subway is evaluated. Using the time attribute of dynamic Bayesian network, the dynamic evolution of safety risk factors with the advancement of construction activities is revealed. Using the diagnostic reasoning method, 14 key risk transmission paths were obtained, including lack of experience and ability of construction personnel (R₂A₃₁)--improper control of grouting system (R₂T₂₃)--improper splintering of segments (R₂T₂₁)-- risk of shield tunneling stage (R₂). Through the sensitivity analysis of each construction stage of subway shield method, 11 sensitive factors were screened out, such as inadequate assembly of negative ring tubes, improper setting of shield parameters, and surrounding buildings.

(5) Combined with the shield construction project of Xi 'an Metro Line 6, a detailed assessment of its safety risk status is carried out. The overall safety risk was rated at Level II, with management and environmental risks also rated at Level II and personnel operations and machinery risks rated at level III. In view of the high risk assessed, specific countermeasures are put forward to reduce risk and ensure construction safety.

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