Using blinding as struts in cut and cover excavations

“遮罩”一词是用来描述用于保护基坑基础的非钢筋混凝土薄层。虽然在底板建造完成之前，百叶窗显然为切割和覆盖工程的墙壁提供了一些临时的横向支撑，但它通常不被视为有益的结构元素或加以利用。这一建议源于Powderham在莫特麦克唐纳的开创性工作，该工作表明，在切割和覆盖挖掘中，盲杆可以消除对临时钢支撑的需求。莫特麦克唐纳在英吉利海峡隧道的切割和覆盖工程、莱姆豪斯干线和希思罗高速机场的围堰的建设中使用了遮光支柱。这些项目的特点是消除了一系列深挖掘的临时钢支撑，这为进一步节省时间和成本铺平了道路。必须指出的是，在所有这些应用中，盲杆的实际承载力和它们所承载的载荷都是未知的。因此，对遮光支柱采用了非常保守的方法，导致支柱比必要的更厚，从而造成成本和环境影响。研究是必要的，因为许多客户不愿意批准使用盲杆，因为他们的性能还没有明确确定。该研究将开发一种数值方法，用于预测切割和覆盖挖掘中盲杆的响应，该方法可能使用实验室测试数据和现场数据进行校准。由于屈曲荷载取决于包括裂缝、徐变、收缩以及相对于混凝土强度发展的横向和横向荷载引入板的速率等因素，因此预测盲杆的破坏荷载是复杂的。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。在岩土力学分析中，将采用一种简化的盲杆结构响应模型。这将有助于确定支撑载荷，然后将其用于支撑本身的精细结构分析。整个过程将涉及迭代，结构分析将改进岩土分析中使用的简化支柱模型，这反过来将提供改进的支柱载荷估计。这一过程与实验室测试的结果相结合，将产生一个关于盲杆非线性行为的拟议模型，该模型可以被其他人在考虑具体场地和施工条件的岩土工程分析中采用。盲杆的数值分析将使用Izzuddin开发的非线性结构分析程序ADAPTIC进行，该程序已经提供了评估盲杆时变响应所需的所有元件和材料模型。岩土分析将使用由Potts开发的有限元程序ICFEP进行。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壤-结构相互作用问题。岩土工程分析将研究在其顶部附近有和没有单一支柱的嵌入式悬臂墙的行为。在这两种情况下，将在最后的挖掘水平上模拟临时盲化支柱。施工顺序将基于莱姆豪斯连接线、希思罗高速围堰和空侧道路隧道施工所采用的程序。土壤条件也将根据这些地点进行分析，主要是在伦敦粘土和兰贝斯集团进行挖掘。一系列的20个实验室测试将在盲板的比例模型上进行，这些模型具有不同的材料特性、末端条件和横向缺陷。测试结果将用于验证和完善数值分析。

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.