Sprayed Concrete Lined Tunnels

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Alun Thomas is an independent tunnel designer and the founder of All2plan Consulting ApS. Having graduated from Cambridge University in 1994, he went on to work for several major design companies with a short stint working on site for contractors. Alun has a broad experience of many types of tunnelling methods from immersed tubes to segmental linings, from closed face TBMs to hand excavation under compressed air. He is a recognised expert in sprayed concrete lined (SCL/NATM) tunnels and numerical modelling, having completed his PhD at Southampton University in those fields. Alun has been involved in promoting the use of innovative technologies such as permanent sprayed concrete, fibre reinforcement, GFRP rock bolts and spray applied waterproof membranes. He has been involved in many of the recent major UK tunnelling projects such as the Jubilee Line Extension, Heathrow Express, Thameswater Ring Main, Terminal 5, Victoria Station Upgrade and the Elizabeth Line (Crossrail), as well as working on design and construction of tunnels internationally. Alun has been an active member of several industry bodies, notably participating in writing guidelines for BTS, ITA and ITAtech technical committees. Considering the title of this book, this seems like the first question that should be answered. An SCL tunnel is a tunnel with a sprayed concrete lining. This generic definition makes no claims on how the tunnel was designed, the ground it was built in or what its purpose is. It simply describes the type of lining used. Modern SCL tunnel construction is described in more detail in Section 1.3. Figure 1.1 and Figure 1.2 show a typical excavation sequence and cross-section for a large diameter tunnel in soft ground at a shallow depth. The arrangement of the excavation sequence is influenced by the geometry of the tunnel, the stability of the ground and the construction equipment. In shallow tunnels, it is important to close the invert as close to the face as possible, in order to limit ground deformations. However, the designer has a fair degree of freedom in choosing the exact arrangement of the excavation sequence. After each stage of the excavation sequence has been mucked out, sprayed concrete is sprayed on the exposed ground surface. The lining is often built up in several layers with mesh reinforcement inserted between the layers. Alternatively, short fibres can be added to the mix to provide some tensile capacity. Once that section of lining is complete, the next stage is excavated, and so the process progresses and a closed tunnel lining is formed. Often, the sprayed concrete lining does not form part of the permanent works and another lining is installed at a later date (see Figure 1.1). In rock tunnels, sprayed concrete works in concert with rock bolts to support the rock (see Figure 1.3). As such, the sprayed concrete is an important part of the support and often forms part of the permanent support (Grov 2011). As with tunnels in soft ground, the degree and timing of support and the excavation sequence are governed by the stability of the ground. A plethora of other terms exist for tunnels with sprayed concrete linings: most famously in Europe, there is NATM – the New Austrian Tunnelling Method; in North America, SEM – Sequential Excavation Method – is often used; while elsewhere no particular emphasis is placed on the use of sprayed concrete as a distinguishing feature, for example, in hard rock tunnelling. The invention of sprayed concrete is generally attributed to Carl Ethan Akeley in 1907, who used a dry mix sprayed mortar to apply a durable coating to dinosaur bones. However, in Germany, August Wolfsholz had been developing equipment for spraying cementitious mortar in tunnels for rock support from as early as 1892 (Strubreiter 1998), and Carl Weber patented a method for spraying concrete in 1919 (Atzwanger 1999). While sprayed concrete
was used on a few engineering projects to repair concrete structures or for rock support in the first half of that century – it was even trialled by the Modernist architect Le Corbusier for one of his projects – this material and method first attracted serious attention after its use on a series of pioneering projects in Venezuela and Austria by Ladislaus von Rabcewicz in the 1950s (Rabcewicz 1969). Sprayed concrete and mortars can have a multitude of uses including architectural purposes, fire protection and even 3D printing. This book focuses on their uses in tunnelling only.

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