Tunnelling enjoys a wide choice of generic and industry-specific construction products for the connection and assembly of various elements. These items play an important role as regards the construction phase, as well as the safety, quality, and durability in the operational phase of the tunnel.
Due to the linear nature of tunnels, such elements are repeatedly applied along the alignment. As a rough estimate, one metre of running tunnel can require some 10-30 fixing points. Taken over the total tunnel length on projects such as Crossrail and HS2, this will amount to a considerable number of fixings, most of which will be related to critical elements of the tunnel’s operation.
Typical fixings can be for suspended ceilings, catenary installations (e.g. CMS/cable trays, pipes), lighting, electrical equipment (e.g. OHLE, cameras, signage), walkways, and heavier machinery such as jet fans. In addition, many connections between the concrete internal structures rely on anchorages.
Fastened installations form a critical part of the construction and operation of any tunnel. This article aims to clarify various aspects of the design, specification and installation of fixings in commonly-encountered tunnel conditions.
RULE OF THUMB 1: UNDERSTAND THE LOAD-BEARING MECHANISMS
In general, three load-transfer mechanisms can be identified (Figure 1):
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By GlobalData1. Mechanical interlock: when the fastener is engaged in the surrounding material through simple contact pressure.
2. Friction: which is developed through expansion of the fastener outwards onto the walls of the borehole.
3. Bond: achieved by means of an appropriate chemical mortar that connects an anchoring rod to the borehole, accomplished through a combination of adhesion and micro-keying.
RULE OF THUMB 2: UNDERSTAND THE PRODUCT
Depending on the installation type, two main distinctions are made between cast-in-place and post-installed fixings; for the latter, further distinctions can be made between undercut, expansion, bonded, plastic, power-actuated fasteners and concrete screws.
Cast-in-place fasteners: these can be inserts of several different shapes (ribbed bars, threaded sleeves, headed studs, etc.) that are installed in advance during the assembly of the formwork. Anchor channels constitute a special type of cast-in-place fastening system which comprises C-shaped steel profiles with headed anchors on the backside for fastening into the concrete. Once in place, hammerhead screws can be inserted and tightened into the slot of the profile. The benefit of anchor channels is that the screws can be moved along the longitudinal channel axis and allow adjustments of their final position.
Undercut fasteners: in most cases, to install these requires that either the borehole is formed appropriately by means of special drilling for an expanding part of the fastener to nest, or that the fastener can itself form an appropriate tooth in the concrete.
Expansion fasteners: the functional concept of such systems is to expand a section (bolt type) or a longer part of the body (sleeve type) so that they can develop compression against the walls of the boreholes and activate frictional forces. These expansion systems can be either torque-controlled (in which expansion forces are mechanically driven through screwing), or they can be displacement controlled, where expansion is achieved by hammering a steel cone-shaped part through the outer steel sleeve of the fastener or vice versa. Plastic fasteners employ mainly the same installation and functional principle, by means of a plastic expansion sleeve.
Bonded fasteners: these are usually divided into two systems: capsule systems (the components of the bonding mortar are enclosed in separate compartments of a capsule and are mixed in the borehole by driving in the fastener); and injection systems, where mortar fills the borehole in a liquid state before setting the fastener.
Another classification of bonded fasteners can be made with respect to the bonding material, which is usually a two-component mortar consisting of organic substances (polyester, vinylester, epoxy); inorganic substances (cement-based); or even a combination. Special bonded fixing systems (usually intended for use in cracked concrete) may employ the functional principles of expansion fasteners because of their particular cone-shaped body; or of undercut fasteners by means of specially shaped boreholes.
Concrete screws: the installation of concrete screws demands first drilling a borehole in the concrete surface and then driving in the screw by use of a drilling machine. These screws have properly prepared threads in order to form indentations in the borehole walls and provide a firm interlock throughout their length.
Power actuated fasteners: are a special and increasingly-used type of fixing, primarily for lower load applications but with many logistical and occupational health benefits. These are high-strength steel nails, which are directly installed in concrete at a speed in the order of 100–200m/sec by use of a battery-, pneumatic- or gunpowder-powered setting device. In this case, the heat and pressure generated by this setting process causes the two materials – steel and concrete – to sinter and bond together. Furthermore, the concrete is displaced by the driving of the nail, which has the effect of adding another load-transmission mechanism due to clamping.
RULE OF THUMB 3: KEEP THE REQUIREMENTS IN MIND, AND CHOOSE THE RIGHT PRODUCT
The standard requirements of a structural engineering design can be a challenge to fulfil but tunnel projects can be particularly demanding. The chosen product should be suitable for the requirements, be they resistance to a tunnel-specific design fire, blasting and impact, unusual dynamic load cycles, or prolonged design service lives.
Given the multitude of fixing products available, the various versions of each product, and the multitude of applicable conditions of use, it is expected that the design engineer in charge of the fixing design will specify the exact product to be used for each connection, and indicate it on the design drawings. Throughout the product selection process, this should be clearly documented, in accordance with the design criteria and wider project specifications, as well as which conditions and requirements need to be met.
In order to be able to match the project requirements with the specified fixing, specifiers can refer to each product’s technical specification. In Europe, this would be ‘European Technical Product Specification’ (ETPS), also known as the ‘European Technical Assessment’ (ETA) sheet. This specification document delivers information about the fabrication, installation and associated performance for each product under certain conditions, and is a guarantee of the ‘Declaration of Performance’ by the product manufacturer; it also allows for the product’s CE marking.
RULE OF THUMB 4: THE SUBSTRATE MATERIAL IS KEY TO THE DESIGN
Fasteners typically perform differently if they are installed in cracked or uncracked concrete, or fibre-reinforced concrete. If fixings are installed in existing, aged and deteriorated tunnel linings, the concrete substrate strength needs to be well documented. Currently, research is ongoing regarding the influence of steel-fibre reinforcement on the performance of fixings in concrete, but for the time being, it is safe to neglect the influence of fibres. However, the load capacity of fixings may be reduced due to rebar reinforcement in concrete unless this is specially designed for the purpose of transmitting concentrated forces. This might seem counter intuitive on reflection, but it has to do with the fact that reinforcement already induces some local tensile stresses in concrete which affect the fixing’s loadbearing performance.
RULE OF THUMB 5: ATTENTION TO THE INSTALLATION
Readers take note: care should be devoted not only to the design of fixings but also to their installation. Recent studies have highlighted that a defective installation can impair or even diminish a fixing’s loadbearing capacity. The ETA sheet indicates how each product should be installed and it must be strictly adhered to. Cast-in fastenings are typically more robust when it comes to installation defects.
Below is a checklist of frequent errors concerning post-installed fastenings:
? Labour force is not certified / adequately trained.
? Inappropriate drill-bit size.
? Inappropriate drilling method (i.e. non-hammer drilling or diamond core).
? The drill hole is not roughened as necessary.*
? Excessive drilling depth.
? The resin used is not appropriate for overhead installations.*
? The resin used is not appropriate for the substrate.*
? The product is not appropriate for cracked concrete.
? The resin is allowed to flow before the rod is inserted (no seal plug), or the product is not suitable for overhead installation.*
? Inadequate rounds of blowing/brushing.
? The borehole brush used to clean the hole is too small and does not adequately remove the drill dust.
? The resin was stored above/below the appropriate temperature.*
? The two-component mortar was not evenly mixed.*
? The cartridge, dispenser and nozzle are not compatible.*
? The fixing is disturbed/loaded before the curing time is completed.*
? The bond or surrounding concrete is damaged due to overtightening of the nuts.
? The fixture is overloaded.
(*refers only to chemical fixings)
RULE OF THUMB 6: KEEP AN EYE OUT FOR NEW DEVELOPMENTS
There have been some significant efforts for guidance and standardisation of fastenings across the civil engineering sector lately. Important references which form the basis of fastening design, specification, and installation of fixings, include:
? BS 8539:2012 Code of practice for the selection and installation of post-installed anchors.
? The recently published Eurocode 2 – Part 4: ‘Design of Fastenings for Use in Concrete’.
? The design guides from the International Federation for Structural Concrete (fib).
? The excellent, free online advise documents offered by the Construction Fixings Association.
Yet, there are certainly more developments to come. Fastenings engineering is a very vivid and innovative field, so more inputs from ongoing research flowing into updates in guidelines and standards in the field are expected. Meanwhile, increasing digitalisation and automation tools are also booming in this particular field of our industry.
