基于非饱和土极限分析模型的含节理黄土隧道围岩压力计算方法

The stability of loess tunnels is largely affected by the hydraulic conditions because of the joints and water sensitivity of loess. The surrounding rock pressure increases rapidly and even the tunnels collapse when the moisture content of loess increases. The present project aims to develop limit analysis model for unsaturated jointed loess, study the collapse mechanism of jointed loess tunnels and establish methods for calculation of surrounding rock pressures. Based on the theory of limit analysis, the suction stress model is employed to describe the shearing strength of loess and joints and three-dimensional limit analysis model is going to be developed for unsaturated loess through the development of equivalent forces. Efficient numerical limit algorithms will then be proposed by incorporation of simplex tetrahedron element, joint element and semi-definite programming. Model tests and numerical limit analysis will be conducted to study the effect of moistening and joints on the collapse behavior of loess tunnels, analyze the different types of collapse mechanisms and discuss the classification criteria of deep and shallow buried tunnels. Collapse mechanisms are then approximated to establish methods for calculation of surrounding rock pressures of loess tunnels based on upper bound theory of unsaturated soils, providing scientific basis for design of support and reinforcement. The innovative formulation for surrounding rock pressures based on unsaturated limit analysis model has enormous potential in engineering application and can serve as references for tunnel analysis and design.

黄土特殊的水敏性和普遍存在的节理使黄土隧道围岩稳定性易受水文条件影响，增湿后围岩压力显著增加，甚至引发隧道塌方事故。本项目旨在提出含节理非饱和黄土极限分析模型，研究增湿条件下黄土隧道围岩失稳破坏机制，建立围岩压力计算方法。拟以极限分析理论为基础，由吸应力模型刻画非饱和黄土及节理强度特性，基于等效力方法建立非饱和黄土三维极限分析模型。结合单纯形四面体单元、零厚度节理单元和半正定锥规划，提出非饱和黄土数值极限分析方法；借助数值极限分析和围岩失稳模型试验，研究增湿和节理对黄土隧道围岩松动特性影响机理，分析劣化条件下不同类型围岩破坏机制，探讨黄土隧道深、浅埋划分标准；基于破坏模式近似描述并应用非饱和土上限定理，建立围岩压力计算方法，为黄土隧道支护和加固设计提供科学依据。利用非饱和土极限分析模型计算围岩压力具有创新性，在隧道工程设计中具有广泛的应用价值和借鉴意义。

黄土具有显著的水敏性，垂直节理发育，节理的存在增大了黄土体渗透系数，使黄土具有间断性和各向异性，改变围岩结构性损伤规律，其影响已被工程实践证实。本项目借鉴岩体力学方法研究黄土隧道，发展节理黄土隧道松动土压力计算方法，分析增湿和节理对非饱和黄土隧道围岩压力的共同影响。开展了不同含水率原状黄土及黄土节理直接剪切试验，分析了含水率对原状黄土及节理面变形、强度指标的影响。在试验的基础上，基于经典极限分析理论，引入基于吸应力的非饱和土有效应力，建立吸应力等效力，提出了非饱和土极限分析模型；通过有限元方法对求解域进行空间离散，将强度准则分别表示为二次锥和半正定锥形式，建立了非饱和土稳定性分析高效数值方法。引入薄层实体单元模拟黄土节理，通过增大节理饱和渗透系数和赋予节理有效强度参数反映其对黄土宏观渗透性和抗剪强度的影响。建立了规则及随机节理下黄土隧道渗流计算模型及围岩压力计算方法，研究了节理间距、入渗速率、入渗时间等对隧道围岩含水率及围岩压力的影响。项目的实施在一定程度上提高了现有方法的计算效率，计算结果能够为黄土隧道设计和施工提供重要参考。