In the last few years, the construction industry has been increasingly faced with a demand for expertise in the design and installation of anchors to concrete. This is reflected in several recent publications that form the basis of anchor design, specification, and installation of fixings, such as BS 8539:2012 Code of practice for the selection and installation of post-installed anchors, and the imminent Eurocode 2 – Part 4: “Design of Fastenings for Use in Concrete”.

Tunnels, particularly those for passenger transportation, pose particular safety and performance concerns. This is due to a multitude of factors, including the confined space and density in passenger presence (and consequently the increased life threats), the difficulties in maintenance and repairs, and the socioeconomic relevance of the served infrastructure (with the associated costs of closure or malfunction).

Fatal in-tunnel collapses during operations at the Fort Point Channel (Big Dig) in Boston, Massachusetts, in 2006, and in the Sasago Tunnel in Japan in 2012, were associated with falling anchors, and a near-collapse has been encountered at the Balcombe Railway Tunnel (2011).

These incidents show that suspended, non-structural elements with sustained loads are of the most critical items leading to severe failures and life threats in tunnels in operation. One can identify failure causes at various stages of the tunnel’s life-cycle, i.e., the design principles at planning stage, the load and resistance assumptions, the detailing, and the fixing product specification at the design stage, the installation on site at the construction stage, and the inspection and maintenance as necessary at the operational phase. This paper aims to summarise information from various publications and guidance documents, to convey the author’s experiences from relevant research and design practice, to identify critical issues and to provide a state-of-the-art advice on common practices for fixings used in tunnels.

Typical sytems in consideration

In tunnels, suspended systems are typically ceilings, catenary installations (e.g., CMS/cable trays, pipes), lighting, electrical equipment (e.g., OHLE, cameras, signage) and heavier machinery such as jet fans. Indicative tunnel cross sections with the said installations are given in Figure 1, while some nominal loads are given in Table 1. As seen from the applications, more than 10,000 fixing points per tunnel kilometre may be expected.

Normative status

The design of fastenings, particularly for safety critical applications, is primarily based on the “European Technical Product Specification” (ETPS) for each fastening product. These specification documents have been developed historically since the 1990s, together with the European Union’s strategy toward harmonisation in the construction industry. Essentially, they deliver information about the fabrication, installation, and associated performance for each product under certain conditions (e.g., strength and cracking of concrete, fire or seismic actions, etc.). For concrete fastening products, the ETPS is the “European Technical Assessment” (ETA), which is prepared on the basis of a “European Assessment Document” (EAD). Another option is that the ETPS is a harmonised European standard (hEN); this may be the case, e.g., for construction adhesives or rebar anchorages, and rather not for fasteners.

The EAD (also known as ETAG until recently) typically prescribes a testing campaign for the product assessment. The ETPS also confirms the “Declaration of Performance” by the product manufacturer and allows for the product’s CE marking. The “European Organisation for Technical Assessment” (EOTA) drives this standardisation procedure by endorsing and publishing the assessment principles and by coordinating the organisations carrying out the assessment (“Technical Assessment Bodies”). A similar procedure is followed in the U.S. with the International Code Council (ICC) and the Evaluation Service Reports (ESR). More recently, particular efforts have been invested in establishing a unified design normative platform covering a range of products, mainly cast-in dowels and channels, and post-installed mechanical and chemical anchors.

As such, current normative standards include Part 4 of Eurocode 2, 1992-4 Design of Fastenings for Use in Concrete (EN 1992-4: 2018) for the European regime, and Chapter 17 “Anchoring to concrete” of the ACI 318 “Building code requirements for structural concrete” in the U.S. Also, the International Federation for Structural Concrete (fib) has issued Bulletin No. 58 “Design of anchorages in concrete”, which serves as a thorough design guideline and background information to the codes. The design provisions of EN 1992-4 only apply to anchors with an ETPS. EN 1992-4 then accommodates three design methods with differing degree of simplification and conservatism, which rely on values provided in the ETPS, and on structural calculation models included in the code.

Design and specification considerations Product specification depending on the conditions, actions, and substrate material

Given the multitude of fixing products available in the market, the various versions of each product, and the multitude of applicable conditions of use (cracked concrete, dynamic loading, sustained loading, or fire – to name a few), it is expected that the design engineer in charge of the fixing design, will specify the exact product used for each connection, and indicate it on the design drawings.

The drawings should also indicate the assumptions under which the design was carried out (e.g., concrete properties and environmental exposure), installation instructions, exact positions and dimensions, and tolerances. Further advice on this exercise may be also found in the BS 8539:2012 “Code of practice for the selection and installation of post-installed anchors in concrete and masonry”.

Throughout the product selection process it should be clearly documented, in accordance to the design criteria and wider project specifications, which environments and requirements need to be served. In example, it is rarely the case that concrete is uncracked, or that fire resistance is not required.

Quite often, the environmental conditions in tunnels are aggressive for the concrete, steel and possible adhesive materials used for the fixing. Besides, the imposed loads on the fixing point are generally sustained (long term dead weight) loads. This loading situation poses various concerns due to the time related degradation effects of the fixing itself as well as the surrounding concrete. Additionally, the fixings may need to be designed for dynamic actions because of the vibrations imposed on the fixture due to running traffic and air circulation, and due to the attached machinery operation as for example in the case of ventilation fans. Almost in all cases, fixings need to be fit for use under fire situations and to be detailed against some level of corrosive attack.

Finally, the tunnel may need to be designed for seismic actions; whereas the tunnel itself may prove safe against seismic oscillation, a critical seismic frequency can occur on the fixing due to the seismic loads transferred to the fixture’s eigenvalues (see exemplary representation in Figure 2).

In regards to the installation of anchors in steel fibre reinforced concrete (which is more and more often used in tunnel linings), some research has been conducted by various teams worldwide but based on the author’s review and there is no clear consensus on whether steel fibres can have a positive influence on a fixing’s performance. Therefore, it is considered reasonably safe to assume plain concrete as a substrate.

Concerns have been raised in recent years within the industry in regards to the long term performance of anchors. The design norms suggest a reduction factor for bonded anchors under sustained loads, which can also be derived from the products assessment document.

Particularly for tunnels, the embedment depth should be designed so as to allow some drill depth tolerance in the secondary lining, in order to avoid damage of the tunnel waterproofing; the drilling depth should be at least 30- 50mm smaller than the minimum lining thickness, or vice-versa the lining thickness should be designed in order to always accommodate a minimum required anchorage depth.

Robustness and redundancy

Furthermore, it is of vital importance to provide certain robustness to the fitted system for safety critical installations. A typical example is the double system fixation of jet fans in tunnels, where there is a main system of fixture and a secondary system (e.g., safety chain); in case the primary system fails due to erratic design or installation, the secondary can eliminate the risk of complete collapse. In that case, it is advisable to secure the secondary system with a different type of anchor than the primary one, in order to cover for systematic errors.

The same is valid for failure of a fixing in catenary installations. A system redundancy is then advisable, where the load originally intended to be carried by one lost fixing point can be transferred to the adjacent ones. The additional, redistributed load may possibly be attributed an accidental load safety factor, but the design situation must be calculated appropriately, implementing a fixture sufficiently stiff to accommodate the load transfer.

Also, the owner of the asset should maintain an alert system in case of loss of one fixing, in order to avoid failure progression.

Fire

For fixings with additional performance requirements in fire, one should consider the endurance of the materials in very high temperatures over the required period of time (per the fire design curve). In bonded anchors, the weak link can lie in the sensitivity of the resin material under elevated temperatures. Generally, for any type of metal insert in concrete one should also then consider the possibility of concrete surface spalling in the case of fire, which is associated with a loss of material and accordingly a loss of embedment depth (a reduction of typically 20-40mm). Concrete serves as a heat insulator, suppressing the heat transfer to deeper areas of the lining. However, the insert itself can serve as a thermal bridge to deeper layers of concrete and induce loss of anchorage throughout a part of the anchor’s length. Finally, the designer should definitely select a product approved for use in fire, and then also balance out these effects by an appropriate selection of anchor size and anchorage depth, and possibly additional fire insulation measures.

BIM requirements

The implementation of Building Information Models does not leave the design of fixings unaffected. In the author’s opinion, the collection of as much information as reasonably achievable in such models is definitely of benefit during the design, installation, as well as operation and maintenance phases, although the amount of fixings used in a project makes it impractical to record a complete data set for each fixing.

One can classify the information included for fixings by Level of Development (LOD), from generic objects with approximate volumes or geometries, up to a fully detailed dataset of the fixing product, including locations, dimensions, technical specifications, installation date and service life, and sub-contract and procurement information.

Installation considerations

Studies in the U.S. and Europe in the past have shown that there is a particular lack of adequate and consistent knowledge among the construction industry as regards the correct installation methods for anchors. Although one may expect that this is a generic issue, the evidence has been particularly focused on injection bonded anchors, for which the installation tools and procedures can largely affect the short- and long-term performance of the system. In any type of installation, the manufacturer’s installation guidance (or approval/assessment document) must be strictly adhered to.

This includes the appropriate drill type (hammer and in some occasions diamond core drill), the borehole brushing through a proprietary tool, and further removal of the drill dust by blowing and suction equipment, although some recent technologies introduced to the market enable a simplification of this procedure. Specifically for chemical anchors, the guidance typically includes the preparation of the dispenser (as applicable), the insertion of the anchor rod with a twisting action (to allow a uniform distribution of the bonding agent around the steel part), and finally the applicable curing and load application timing.

In the U.S. and in Germany there is also the requirement of certified anchor installers so as to reduce installation defects due to human errors, a practice that is occasionally adopted by contractors and project owners worldwide.

Another practice often requested in large scale projects, as well as in the case of non-standard applications is an on-site pre-testing campaign of the installed anchors in order to confirm the installation quality, but also the allowable load. In linear projects, such as tunnels, it is often a cost-efficient choice to procure a large scale setting machine which performs the whole installation cycle (locating-drilling-cleaning-setting-testing) in an automated routine. In any case, it is meaningful to keep an installation protocol, where details of each fastener installed are confirmed between the installer and the contractor or owner’s inspector.

Typical errors associated with the anchor installation are provided below in form of a checklist:

  • 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 was not appropriate for the substrate *
  • The anchor was not appropriate for cracked concrete
  • The resin is allowed to flow before the anchor is inserted (no seal plug), or the product is not suitable for overhead installation *
  • Inadequate rounds of blowing/brushing
  • The brush is too small and doesn’t adequately remove the drill dust
  • The resin was stored above/below the appropriate temperature *
  • The two-component mortar is not evenly mixed *
  • The cartridge, dispenser, and nozzle are not compatible *
  • The anchor 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

(* referring only to chemical anchors)

Post-construction life-cycle considerations

The importance of maintenance in large structures is widely acknowledged, and it forms part of all modern operator and owner organisations. The condition of fixings may be directly linked with the operation and safety performance of the infrastructure asset. Typically, this includes physical inspections at predefined intervals. However, modern technologies allow for robotic inspections and continuous monitoring. The information retrieved from such exercises can directly assist in the health-plan of the asset by signifying the level of serviceability and remaining service life, and by allowing for an adequately planned repair, thus optimising the structure’s whole-life costs. In physical inspections, one should check that bolts are tightly locked and in position, without deformation from the intended state, and that there are no signs of deterioration in the concrete (cracking, carbonation, etc.) and in the fixture (corrosion or cracks). The inspection should also ensure the loading conditions and that these have not changed from the intended state, as for example, a result of changes in the attached items. Otherwise, a more precise condition assessment should be carried out in order to evaluate the stability of the structure.

Some automated scanning technologies may be well applicable to detect even very small deformations or the fixture, or change of texture as result of deterioration, while automated monitoring devices, may allow for a dense recording of any environmental fluctuations, changes in the fixture, as well as the underlying structure itself.

Based on this concept, the operator becomes aware of issues with the structure well before the structure reaches a low performance age, and there is time to improve it, e.g., by upgrading or change of use (see also Figure 4). In particular, the whole-life optimisation can be briefly described in the following steps:

1. define the fixings’ service life requirements

2. design and specify the individual fixings with given qualities (fatigue and ageing resistance, component durability, etc.)

3. plan in advance any repairs/upgrades on the components based on 2.

4. design planned inspections and condition monitoring (timing and type of intervention)

5. improve these maintenance cycles, as experience of the system and similar assets is gained and as technology advances

Depending on its use, and the operator’s life-cost assessment input and maintenance appetite, the attachment may have a service life demand of only a few years and up to several decades. According to the fundamental studies for the Brenner Base Tunnel, electronic equipment should have a service life of minimum 30 years, mechanical and energy installations should deliver a service life of minimum 50 years, while structural installations are required to provide a service life of up to 200 years (all the above with allowance for inspections and monitoring). Fixings for the above applications should comply with the above requirements.

Most current assessment documents for usual fixing products are calibrated toward a service life of 50 years (without inspection or monitoring). In order to allow for a consistency among these requirements, a plan for inspection or monitoring and if necessary a replacement after the fixing’s service life should be at the operator’s hand.

In any case, an initial prediction of the long-term performance and accordingly the engineered selection of components and properties to be monitored is of paramount importance.

Conclusion

Regarding the long-term life cycle performance of tunnel structures, installations and fit-outs in tunnel structures are historically shown to be very sensitive structural items. As such, fixings in tunnels are critical links in the chain of operational safety and performance, while failure thereof may lead to very high post-failure costs and more importantly to injuries and life threats.

This paper presents main aspects of fixings in tunnels, and it attempts to convey an outlined but as possible comprehensive set of considerations concerning the specification, design, installation, and maintenance of fixings in tunnels throughout the asset’s life-cycle.