Using blinding as struts in cut and cover excavations

术语“盲板”用于描述用于保护开挖基础的非钢筋现场混凝土薄层。通常不认为或利用盲板作为有益的结构元件，即使它显然为明挖和覆盖工程的墙壁提供了一些临时的横向支撑，直到建造底板。该建议源于Powderham在Mott MacDonald的开创性工作，该工作表明，盲支柱可以消除在开挖和覆盖开挖中对临时钢支撑的需要。莫特麦克唐纳曾在英吉利海峡隧道的开挖和覆盖工程、莱姆豪斯连接线和希思罗机场快车围堰的施工中使用过盲柱。这些项目的特点是从一系列深开挖中消除了临时钢支撑，为进一步节省时间和成本的计划铺平了道路。重要的是要注意，在所有这些应用中，盲板支柱的实际能力及其承载的载荷是未知的。因此，对盲板支柱采用了非常保守的方法，导致支柱比必要的更厚，从而产生成本和环境影响。研究是必要的，因为许多客户不愿意批准使用盲支杆，因为他们的性能还没有最终建立。研究将开发一种数值方法，用于预测盲支杆在切割和覆盖开挖中的反应，并在可能的情况下，用实验室测试和现场数据进行校准。由于屈曲荷载取决于开裂、徐变、收缩等因素，以及相对于混凝土强度的发展，侧向和横向荷载引入板中的速率，因此预测盲板支柱的失效荷载是复杂的。这个问题将通过使用岩土和结构有限元程序进行耦合分析来解决。土工分析中将使用盲板支柱结构响应的简化模型。这将使支柱载荷得以确定，然后将其用于支柱本身的精细结构分析。整个过程将涉及迭代，结构分析将细化土工分析中使用的简化支柱模型，这反过来又将提供对支柱载荷的改进估计。结合实验室试验结果，该过程将产生一个盲板支撑非线性行为的建议模型，该模型可供其他人在考虑特定场地和施工条件的岩土分析中采用。盲板支撑的数值分析将使用Izzuddin开发的非线性结构分析程序ADAPTIC进行。其已经提供了用于评估盲板支柱的时间相关响应的所有必要元件和材料模型。将使用Potts开发的有限元程序ICFEP进行岩土工程分析。该程序是专门为岩土工程编写的，并已应用于各种各样的土-结构相互作用问题。岩土工程分析将调查嵌入式悬臂墙的行为，在其顶部附近有和没有一个单一的支柱。在这两种情况下，将在最终开挖水平对临时盲板支柱进行建模。施工顺序将以莱姆豪斯连接线、希思罗机场快速通道和空侧公路隧道施工所采用的程序为基础。土壤条件也将基于这些场地，并对主要在伦敦粘土和兰贝斯组中进行的挖掘进行分析。将在材料特性、端部条件和横向缺陷变化的盲板比例模型上进行一系列20项实验室试验。测试结果将用于验证和完善的数值分析。

The term blinding is used to describe the thin layer of unreinforced over-site concrete which is used to protect the base of excavations. Blinding is not generally seen or exploited as a beneficial structural element even though it clearly provides some temporary lateral support to the walls of cut and cover works until the base slab is constructed. This proposal stems from Powderham's pioneering work at Mott MacDonald, which shows that blinding struts can eliminate the need for temporary steel propping in cut and cover excavations. Mott MacDonald has used blinding struts in the construction of the Channel Tunnel cut and cover works, the Limehouse Link and the Heathrow Express cofferdam. The hallmark of these projects was the elimination of temporary steel propping from a series of deep excavations which paved the way for further time and cost saving schemes. It is important to note that in all these applications, the actual capacity of the blinding struts and the loads they were carrying were unknown. Therefore, a very conservative approach was adopted for the blinding struts which resulted in the struts being thicker than necessary with consequent cost and environmental implications. Research is required since many clients are unwilling to sanction the use of blinding struts because their performance has not been definitively established.The research will develop a numerical method for predicting the response of blinding struts in cut and cover excavations that is calibrated with data from laboratory tests and field data where possible. Predicting the failure loads of blinding struts is complex since the buckling load depends on factors including cracking, creep, shrinkage and the rates at which the lateral and transverse loadings are introduced into the slab relative to the development of concrete strength. This problem will be solved by carrying out a coupled analysis using both geotechnical and structural finite element programs. A simplified model for the structural response of the blinding strut will be used in the geotechnical analysis. This will enable strut loadings to be determined which will then be used in a refined structural analysis of the strut itself. The overall process will involve iteration, with the structural analysis refining the simplified strut model used in the geotechnical analysis, which in turn will provide improved estimates of the strut loading. This process, combined with the results from the laboratory testing, will lead to a proposed model for the non-linear behaviour of the blinding strut that can be adopted by others in their geotechnical analyses accounting for the specific site and construction conditions.The numerical analysis of the blinding strut will be carried out using the nonlinear structural analysis program ADAPTIC, developed by Izzuddin, which already provides all the necessary elements and material models for assessing the timedependent response of the blinding strut. The geotechnical analysis will be performed using the finite element program ICFEP developed by Potts. This program has been written specifically for geotechnical engineering and has been applied to a wide variety of soil-structure interaction problems. The geotechnical analysis will investigate the behaviour of an embedded cantilever wall with and without a single prop near its top. In both cases, a temporary blinding strut will be modelled at final excavation level. The construction sequence will be based on the procedures adopted at the Limehouse Link, Heathrow Express Cofferdam and Airside Road Tunnel constructions. Soil conditions will also be based on these sites with analyses conducted for excavations in predominantly London Clay and in the Lambeth Group. A series of 20 laboratory tests will be carried out on scale models of blinding slabs with variations in material properties, end conditions and lateral imperfections. The test results will be used to validate and refine the numerical analysis.