Mobilisable Strength Design for Deep Excavations

岩土工程师通常依赖于简单的封闭形式的解决方案来估计基础的能力，并应用安全系数来粗略地防止过度变形。或者，有限元法（FEM）提供了一个更严格的分析，但需要大量的努力和土壤本构关系的详细知识，这可能是不可用的。因此，需要一种新的合理的设计方法，适合手算（或袖珍计算器）的正常使用变形。可动强度设计（MSD）方法使用弹性理论和塑性理论的组合，以满足这些标准，并提出了一个应变场来自理想化的土壤行为，使边界位移的预测。岩土工程师特别感兴趣的是柔性挡土墙支持城市地区的深基坑。MSD方法已开发用于悬臂挡土墙，支撑开挖和狭窄的开挖，但涉及一些限制性假设。其中包括：假定的“塑性”变形机制垂直于自由表面处的地面，支撑开挖和不排水条件下的完全刚性支柱，限制了对粘土中短期荷载的分析。这项研究的目的是提供扩展的MSD方法。首先，改变假定的变形机制，以满足土壤表面的边界条件，并扩展解决方案，以包括支柱变形。然后可以计算沿挡土墙沿着的应力分布，从而能够实现支柱力和墙弯矩的计算，以用于更有效的墙设计。研究被动桩及堆载对地面的影响。最后，新的MSD解决方案将尝试分析排水条件和动态载荷。将进行有限元分析与新的解决方案进行比较。

Geotechnical engineers often rely on simple, closed-form solutions to estimate capacity of foundations and apply safety factors to crudely prevent excessive deformation. Alternatively, finite element method (FEM) provides a more rigorous analysis but requires significant effort and a detailed knowledge of the soil constitutive relationships, which may not be available. Hence, a new rational design method is warranted, suitable for hand (or pocket calculator) calculation of serviceability deformations. The mobilisable strength design (MSD) method uses a combination of theory of elasticity and theory of plasticity to satisfy these criteria and proposes a strain field derived from idealised soil behaviour enabling prediction of boundary displacements. Of particular interest to geotechnical engineers are flexible retaining walls supporting deep excavations in urban areas.The MSD method has been developed for cantilever retaining walls, braced excavations and narrow excavations, but involves some limiting assumptions. These include: an assumed "plastic" deformation mechanism normal to the ground at the free surface, perfectly rigid props for braced excavations and undrained conditions, limiting the analysis to short term loading in clays. This research aims to provide extensions to the MSD method. Initially, altering the assumed deformation mechanism to satisfy the boundary conditions at the soil surface, and extending the solution to encompass prop deformations. The stress distribution along the retaining wall can then be calculated enabling prop forces and calculation of wall bending moments for more efficient wall design. The effects of passive piles and surcharge on the ground surface will be investigated. Finally, new MSD solutions will be attempted for analysis of drained conditions and dynamic loading. FEM analyses will be carriedout to compare with the new solutions.