DEM Study About Influence of Fabric Anisotropy on Formation Disturbance
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摘要:
组构各向异性是影响砂土力学特性的重要因素，进而对盾构隧道开挖过程中地层扰动产生显著影响. 为探究组构各向异性对砂土地层扰动特性的影响，基于二维离散元方法，创建具有重叠的类椭圆簇颗粒，长轴取向被特定排列以形成不同各向异性的砂土试样，长轴取向与水平沉积方向为层理角；进行二维代表性体积单元（RVE）代表单元体双轴实验，并将数值模拟所得到的峰值摩擦角与已有文献中的实验结果进行对比验证；生成具有不同层理角的地层，隧道开挖地层损失依据Park地层损失假定. 模拟结果表明：当在各向异性地层中进行盾构隧道开挖时，隧道中轴线两侧扰动范围出现不对称效应；地表沉降曲线分为影响区、扩展区与削弱区，层理角对地表沉降的影响仅表现在影响区、扩展区；地表沉降槽中最大沉降的最大值出现在层理角为0°的试样中，为0.037 m；层理角为45°的试样中最大沉降最小，为0.011 m；随着层理角增大，45°左侧的最大沉降呈线性上升趋势，右侧呈线性下降趋势；隧道主应力的偏转与位移偏转造成的不对称效应一致；层理角为0°和90°的试样中，接触法向分布的主方向未发生改变.
Abstract:Fabric anisotropy is an important factor affecting the mechanical properties of sandy soil, and the formation disturbance caused by shield tunnel excavation in sandy soil is largely affected by the mechanical properties of the overlying sandy soil. This study utilizes the two-dimensional discrete element method to model particles as overlapping ellipse-like clusters, specifically arranging the long-axis orientation to create sand samples with varying anisotropy. The long-axis orientation and horizontal deposition direction form the bedding plane. A two-dimensional RVE biaxial test shows that the peak friction angle initially decreases and then increases with the bedding angle, aligning with experimental findings. Subsequently, strata with different bedding angles are generated, and tunnel excavation is analyzed based on Park's stratum loss model. In anisotropic strata, the disturbance range exhibits asymmetry about the tunnel's central axis. The surface settlement curve divides into three zones: influence, expansion, and weakening. The effect of bedding angle is evident only in the influence and expansion zones. Maximum settlement occurs at a bedding angle of 0°, while it is minimized at 45°. As the bedding angle increases, maximum settlement trends upward on the left side and downward on the right side of 45°. The principal stress deflection of the tunnel correlates with the displacement-induced asymmetry. In the samples with bedding angles of 0° and 90°, the main direction of the distribution of the contact normal did not change.
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Key words:
- sandy soil /
- fabric anisotropy /
- shield tunneling /
- formation disturbance /
- ground settlement /
- discrete element method
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图 5 不同层理角砂土的偏应力和体应变随轴向应变的变化
Figure 5. Mechanical properties of angular sand with different bedding planes
图 9 地表沉降随距隧道中轴线距离的变化
Figure 9. The variation of surface settlement with distance from the central axis of the tunnel
图 10 地表最大沉降与层理角的关系
Figure 10. The relationship between maximum surface subsidence and bedding angle
图 11 不同层理角地层中应力十字架分布
Figure 11. Distribution of stress crosses in strata with different bedding angles
图 13 不同层理角下接触法向组构的差异
Figure 13. Differences in contact normal fabric under different bedding angles
表 1 试样的微观参数
Table 1. Micro parameter of samples
微观参数 取值 $ r $/mm 2.5～7.5 $ {d}_{1} $ 3r $ {d}_{2} $ 2r 法向刚度$ {k}_{{\mathrm{n}}} $/（GN·m−1） 4.0 刚度比$ {k}_{{\mathrm{n}}} $/$ {k}_{{\mathrm{s}}} $ 1.5 $ \mu $ 0.5 阻尼$ \rho $ 0.7 
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表 2 详细的试样方案
Table 2. Experiment planning in detail
试样 α（°） H/m D/m R/m 地层损失率/% S0 0 1 2 0.165 0.2 S15 15 1 2 0.165 0.2 S30 30 1 2 0.165 0.2 S45 45 1 2 0.165 0.2 S60 60 1 2 0.165 0.2 S75 75 1 2 0.165 0.2 S90 90 1 2 0.165 0.2 随机分布 随机分布 1 2 0.165 0.2
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