摘要
基坑开挖引起的深层位移场一直是工程的难题,尤其在开挖面下方存在运营地铁等对变形要求严格的建(构)筑物时。由于此类问题涉及土层固结与回弹、围护结构内力与变形、基坑开挖与稳定机理、隧道变形与保护、施工顺序与施工力学、反馈分析与施工控制等诸多问题,是一个集岩土工程与结构工程于一体的系统工程,目前尚无一套较系统的理论。本文以上海高灵敏性软土地区大范围、高比例卸载工程实践为基础,通过理论研究和反馈回归,分析基坑开挖卸载引起下方运营隧道回弹的机理并提出了回弹预测方法。
文中引入软硬土体的归一化模型、隧道等效刚度等方法,对运营隧道上方基坑开挖进行了模拟和数值实现,分析了各种因素对运营隧道回弹的影响,并根据信息化反馈数据进行回归分析,总结了影响隧道回弹的主要因素。在此基础上,建立了半近似预测公式,并根据工程监测数据进行了参数修正。本文的主要研究内容及结论如下:
1.为控制隧道的回弹,上海软土往往需要大量的土体加固措施。通过建立归一化的土体模型,便于利用有限元进行数值分析。一方面,利用上海典型软土的流变试验结果,采用半经验修正理论来描述软土的非线性流变特性。另一方面,从水泥搅拌加固土的物理力学特性着手,通过关联度分析方法确立了影响水泥搅拌加固土体弹性模量的主要参数;同时对加固土体的试样进行室内实验,得到加固土体的卸载模量。最后,将上海地区主要土层及加固土体采用了归一化的模型,通过选择合适的参数,可以演变为不同土体的流变方程式。
2.研究了实现数值模拟中的几个问题。利用ANSYS的二次开发功能,将归一化的土体模型作为用户子程序输入。同时为便于模拟隧道结构刚度,建立了盾构隧道的“等效连续化模型”。该模型利用纵向变形特征相似的均匀连续梁模拟隧道全长或某一区段,梁单元具有与隧道相同的等效刚度。利用ANSYS中材料特性设置各向异性材料,来模拟隧道的纵向、横向刚度。
3.有限元分析中调整相关参数进行对比分析,得出以下结论:对隧道回弹影响最大的因素为坑底土体加固,不仅抗拔桩的作用随土体加固强度提高,且土体加固后,原本软土地区对基坑变形有明显影响的施工时间基本对回弹没有太大影响。其他的主要影响因素有分块开挖和基坑底部土体应力释放率。另外,隧道横向刚度要比土体大很多,而纵向刚度又比较小,隧道的回弹对隧道刚度值的变化不敏感。
4.采用分离参数法对反馈数据进行回归,即通过对监测数据中的单个因素进行分离,并进行回归分析得出一些结论:在土体加固后,土体变形基本近似弹性变形;隧道的回弹变化与影响距离大致呈抛物线型关系,但可近似为回弹量与距离呈线性关系;隧道的回弹值与开挖宽度呈倒指数关系。
5.根据土力学经典理论,结合有限元分析和监测数据回归得到的重要影响因素,建立回弹量的半经验性的近似计算公式。计算中充分考虑了隧道处等效土层模量、开挖性状对卸载应力分布的影响、土体加固对坑底应力释放率的影响。并通过近似公式计算结果与监测数据的比较,对应力释放系数进行了修正。
The soil displacements are always the difficulties faced by the engineers during excavation of foundation ditch, especially which is strictly controlled while existing structure such as running tunnel beneath the excavation. These problems are related to soil consolidation, upheaval, stiffness of retain wall, method of excavation, soil stable mechanism, tunnel deformation and protection, construction plan & sequence, monitor, feedback analysis and construction controls etc,so there are not any systematic theory to be applied, but still in the phase of engineering practice. In this paper, integrate the engineering practice, based on the classical theory and feedback analysis, upheaval mechanism of running tunnel due to top excavation in the soft soil area is researched and semi-experience formula is proposed.
This paper focuses on the normalization model of the soft and improved soil which influence the tunnel upheaval, and tunnel equivalent stiffness etc. Through FEM simulation, analyses all possible factors on the tunnel upheaval. By information-based feedback construction and data regression, the main factors affecting the tunnel upheaval is concluded. Thus summarize the main factors and establish the semi-experience formula, revise the parameters according to the projects. The followings show the main contents and the conclusions of the research:
1. To control the soil displacement and the tunnel upheaval, in Shanghai area the soft soil usually be improved. Through establish the normalization soil constitutive-model, it’s more convenient to use finite element for data analysis. On one hand, refers to the rheologic experiment date of the typical Shanghai soft soil, semi-experience modified theory is used to describe the non-linear rheological behaviors, On the other hand, based on the test of physic behaviors of the cement treated soil, through relationship analysis establish the the main parameters which affect elasticity modulus on the cement mixed soil; meanwhile, attained unload modulus of the improved soil through experiments on the soil sample. Lastly, adopt the normalized model for the common soil layer in Shanghai and the improved soil, only need to select suitable parameters separately, different soil’s rheology can be simulated from this model.
2. Study few questions in the FEM simulation. Make use of the secondary development function of ANSYS, the normalized soil model simulated as a sub- program input. Meanwhile, for the convenient of simulate the tunnel stiffness, the“equivalent continuous model”had constructed for bored tunnel. This model base on the similarity of the vertically deformation and the uniform continuous beams to simulate the whole tunnel or a certain section, beam unit and the tunnel have the equivalent stiffness. Use the anisotropic material properties setting in ANSYS to simulate the tunnel’s horizontal and vertical stiffness.
3. In the finite element analysis, adjust the related parameters to contrast analysis, some conclusion worked out: the main factor that affected the tunnel upheaval is the improvement property under the ditch. Not only the effect of the anti-pull pile increase with the improvement property; but the construction duration do not affect the soil deformation anymore which affect the soil deformation obviously in soft soil. The other main factors are part-excavation and stress release rate of the soil of the excavation face. Besides, the tunnel upheaval is insensitive to the change of the stiffness of the tunnel, because horizontal stiffness of tunnel is much larger than the soil, but the vertical stiffness is much less than the soil.
4. Adopt the method of separate parameters, carry out regression on the feedback data, separate a single factor from the measured data, carry out regression analysis and obtain these conclusions: soil deformation is similar to elastically deformation after soil improvement; the tunnel upheaval and the distance of influence can be estimated as linear relationship; the value of tunnel upheaval and the excavation width is in inverse exponential proportion.
5. According to the classic theory, integrate the major factors obtain from the finite element analysis and monitor date regression; establish the semi-experience formula to estimate tunnel upheaval. In computation, take into consideration of the equivalent modulus of tunnel, the affect of excavation character on unload stress distribution, the effect of soil improvement on the stress release rate under the excavation. Comparing the computation results from the finite element and the formula, revise the stress release ratio.
引文
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