Deformation Based Design Methodology for Excavation Support Systems

摘要：地下建设和改造工程在全国许多城市都很普遍。由于邻近的基础设施很近，这些项目地点的建筑活动空间通常有限。涉及深挖工程的一个主要问题是与挖掘有关的地面移动对邻近建筑物和公用设施的影响。挖掘区域边缘的过度横向移动可能导致相邻结构的显著位移和旋转。因此，控制侧向位移是开挖支护系统设计的主要考虑因素。在许多城市地区，项目场地是软粘土的底层。这需要额外的考虑，因为固结和土壤蠕变的贡献。在许多城市工程中，往往需要严格的变形控制，以尽量减少对相邻结构的破坏。变形控制通常是通过刚性开挖支撑系统来实现的。传统上，开挖支护系统的设计采用视土压力图。使用这种方法，支撑系统的设计成为最大预期土压力的函数，并由整体结构稳定性控制，而不是最大允许的水平或垂直变形。这种方法产生了一个足以防止结构破坏的支撑系统，但可能导致过度的墙壁变形和地面移动。现有的方法考虑了变形，将侧壁运动与开挖支护系统刚度和基础稳定性联系起来。然而，这些都是使用有限数量的墙壁类型和配置开发的，并且不包括挖掘支撑系统的不同材料的考虑；墙体施工的立体效果；不同支护方式的效果；开挖几何形状和顺序的影响；或复杂的场地地质。由于基坑支护系统和开挖过程的复杂性，很容易得出结论，为了真实地分析土体与基坑支护系统的相互作用，需要建立三维有限元模型。本研究的智力价值在于，最近的研究表明，开挖引起的地面运动和复杂的开挖支护系统的土-结构相互作用本质上是三维的。然而，迄今为止，文献中有限的数据已经报道了一个深挖掘的完全三维有限元分析。此外，还没有人提出了一种将建造支撑墙和安装支撑系统的三维影响纳入挖掘支撑系统的设计方法；场地开挖和回填的三维影响（包括基础设施建设的时间延误）；以及三维地面变形的影响。本研究将提供三个案例历史的三维有限元分析，并将开发一种基于变形的设计方法。这项研究的更广泛的影响是，基于变形的设计方法将潜在地节省数百万美元的维修费用，并减轻挖掘对邻近基础设施造成的损害。此外，本研究成果将直接或间接地适用于隧道设计、挡土墙设计、围堰设计和深基坑设计(如：钻孔井、螺旋桩、井箱等)。预计这些研究成果将扩展到评估结构对施工活动（如顶管和施工脱水）引起的地面运动的响应。本研究的另一个逻辑延伸是评估建筑物和公用事业对动态载荷引起的地面运动的反应，如爆炸载荷、建筑振动和地震载荷。这项研究还将促进本科研究人员的参与，特别强调代表性不足群体的参与。

Abstract:Underground construction and rehabilitation projects are prevalent in many urban areas across the country. Space for construction activities is usually limited at these project sites because of the close proximity of adjacent infrastructure. A major concern for projects involving deep excavations is the impact of excavation-related ground movements on adjacent buildings and utilities. Excessive lateral movements at the edge of the excavated area can lead to significant displacements and rotations in adjacent structures. Consequently, control of lateral displacements is a major design consideration for excavation support systems. In many urban areas, the project site is underlain by soft clays. This requires additional considerations because of the contributions from consolidation and creep of the soil. Strict deformation control is often required to minimize damage to adjacent structures for many urban projects. Control of deformations is typically achieved with stiff excavation support systems. Traditionally, excavation support systems are designed using apparent earth pressure diagrams. Using this approach, the support system design becomes a function of the maximum anticipated earth pressure and is governed by overall structural stability as opposed to maximum allowable horizontal or vertical deformation. This approach produces a support system that is adequate with regard to preventing structural failure, but may result in excessive wall deformations and ground movements. Existing methods that do consider deformations relate lateral wall movements to excavation support system stiffness and basal stability. However, these were developed using a limited number of wall types and configurations, and do not include considerations for differing materials of an excavation support system; the three-dimensional effects of the wall construction; the effects of different support types; the influences of the excavation geometry and sequencing; or complex site geology. Due to the complexity of the excavation support system and the excavation process, it is easily concluded that for a realistic analysis of the interaction between the soil and the excavation support system, a three-dimensional finite element model is required. The intellectual merit of this research is that the most recent studies have shown that excavation-induced ground movements and the complicated soil-structure interactions of the excavation support system are three-dimensional in nature. However, to date limited data has been reported in the literature that presents a fully three-dimensional finite element analysis of a deep excavation. In addition, no one has presented a design methodology for excavation support systems that incorporate the three-dimensional influences of constructing the support wall and installing the support system; the three-dimensional influences of excavating and backfilling the site (including time delays for infrastructure construction); and the influences of three-dimensional ground deformations. This research will provide the three-dimensional finite element analysis of three case histories and will develop a deformation-based design methodology. The broader impact of this research is that a deformation-based designed methodology will potentially save millions of dollars typically expended for repairs and mitigation of excavation-induced damage to adjacent infrastructure. In addition, the results of this research will directly and indirectly be applicable to tunnel design, design of earth retaining walls, cofferdam design, and deep foundations design (eg. drilled shafts, auger-cast piles, cassions, etc.). It is also envisioned that these research results will be extended to evaluating structure response to ground movements resulting from construction activities such pipe jacking and construction dewatering. Another logical extension of this research is evaluating the building and utility response to dynamic loading-induced ground movements such as blast loads, construction vibrations, and earthquake loading. This research will also facilitate participation of undergraduate researchers, with special emphasis on participation of underrepresented groups.