Mobilisable Strength Design for Deep Excavations

岩土工程师经常依靠简单、封闭的解决方案来估计地基的能力，并应用安全系数来粗略地防止过度变形。或者，有限元方法提供了更严格的分析，但需要大量的工作和土壤本构关系的详细知识，这可能是不可用的。因此，有必要提出一种新的合理的设计方法，适用于手持(或袖珍计算器)的使用变形计算。可动强度设计(MSD)方法结合了弹性理论和塑性理论来满足这些标准，并提出了一种从理想化的土体特性中得出的应变场，从而能够预测边界位移。岩土工程师特别感兴趣的是支持城市地区深挖工程的柔性挡土墙。MSD方法已被开发用于悬臂式挡土墙、支撑挖方和狭窄的挖方，但涉及一些限制假设。这些因素包括：假定的在自由面垂直于地面的“塑性”变形机制、用于支撑开挖的完全刚性支撑以及不排水条件，从而将分析限制在粘土中的短期荷载。本研究旨在对MSD方法进行扩展。首先，改变假定的变形机制以满足土层表面的边界条件，并将解扩展到包含支柱变形。然后可以计算沿挡土墙的应力分布，从而实现支撑力和墙体弯矩的计算，从而更有效地进行墙体设计。研究被动桩和超载对地面的影响。最后，将尝试新的MSD解决方案来分析排水条件和动态加载。进行有限元分析，并与新方案进行比较。

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.