Braced excavations: what about the corners?

当一个城市要建造新的地铁站或深地下室时，需要在地面上挖一个大洞。这些洞需要安全且允许进入，通常选择的解决方案是用支撑的嵌入式挡土墙来支撑大洞的侧面。这些都是大量的临时工程，成本相当高。最近的例子是伦敦帕丁顿新Crossrail车站的230长，24米宽，23米深的挖掘。这种建筑形式的一个重要特点是在墙壁之间有大型支柱，以支撑它们。对于那些负责设计道具大小、位置、数量和墙壁的人来说，关键问题是预测挖掘附近的地面运动（这可能会对建筑物产生负面影响）和支撑力（以便使用正确的道具）。尽管英国建筑信息组织CIRIA最近出版了一些指导设计师的出版物，但关于挖掘角落的行为的报道在这两个设计问题上都很差。有大量发表的关于支撑挖掘计算模型的研究，但只是二维的（即通过长墙的s片），其中一些是根据现场数据验证的，然而，在拐角分析所需的3D效果的考虑是罕见的和不实质性的。提高我们对这些角落的行为的理解，以及它是如何受到土壤行为、系统刚度和支柱载荷的影响的，将导致(a)更经济的支撑方案，(b)更确定地预测角落附近的地面运动，潜在地减少防止相邻结构损坏所需的调节工程。这里提出的研究方案包括考虑到几何形状、材料和顺序的差异，对支撑开挖施工进行复杂的计算模拟。这个问题只能使用3D模型来解决（不像岩土工程中的许多其他问题），但是即使在今天，当我们尝试在3D中建模时，我们使用的计算工具也难以快速交付结果。因此，在这个提议中，我们将使用一种聪明的方法，即使用相对较小的复杂3D模型集的结果来制作“降阶模型”（rom）。ROM更容易使用，并且是通过操纵有限数量的高保真3D模拟生成的。从这些rom中，我们将得出结果并为设计支撑挖掘的工程师准备指导，这将使使用更便宜和更简单的方案成为可能。该项目的研究人员来自杜伦大学和邓迪大学，并得到了由英国工业界关键人物组成的项目监督小组的支持。

When a new metro station or a deep basement are to be constructed in a city, a large hole in the ground is needed. The hole needs to be safe to work in and to allow access and very often the chosen solution is to support the sides of the large hole with a braced embedded retaining wall. These are substantial pieces of temporary works of considerable cost. Recent examples are the 230 long by 24 m wide by 23m deep excavation for the new Crossrail station at Paddington in London. An important feature of this form of construction is large props that span between the walls, to hold them up. For those tasked with designing the prop size, location, number and the walls the key issues are prediction of ground movements adjacent to the excavation (which could negatively affect buildings) and propping forces (so that the right props can be used and while guidance exists for designers in some recent publications produced by the UK construction information organisation, CIRIA, coverage of the behaviour at excavation corners as regards both design issues is poor. There is substantial published research on the computational modelling of braced excavations but only in two-dimensions (i.e. s slice through a long wall), some of it validated against field data, however accounting for 3D effects as required for the analysis of corners is rare and insubstantial. Improving our understanding of the behaviour of these corners and how it is affected by soil behaviour, system stiffness, and prop loading will lead to (a) greater economy in propping schemes and (b) more certainty in the prediction of ground movements adjacent to corners, potentially reducing the accommodation works required to prevent damage to adjacent structures. The programme of research proposed here comprises complex computational simulations of the construction of a braced excavation, taking into account differences in geometry, materials and sequences. The problem can only be properly tackled using a 3D model (unlike many other problems in geotechnical engineering) however even today, the computational tools we use struggle to deliver results quickly when we try to model in 3D. So, in this proposal we will be using a clever method where "reduced order models", (ROMs) will be made, using results from a relatively small set of the complex 3D models. A ROM is much easier to use and is generated by manipulation of a limited number of the high fidelity 3D simulations. From these ROMs we will derive results and prepare guidance for engineers designing braced excavations which will enable cheaper and simpler schemes to be used.The researchers on the project come from Durham and Dundee Universities and are supported by a Project Oversight Group comprising key figures from the UK industry.