Case Studies: Successful Anchoring in Tunnel Projects
Exploring Anchoring Techniques in Tunnel Construction
Tunnel construction requires materials and components capable of coping with amongst the most demanding environmental conditions. Nowhere must the product selection be more closely scrutinised than where mechanical anchors are concerned. Anchoring techniques must be compatible with each application and the construction materials within the tunnel as there is no one-size-fits-all approach.
In the same way that the tunnel stabilization approach and the drilling methods will require early consideration to ensure they contribute to the success of a tunnel construction project, anchoring techniques must be reviewed in detail and not left to chance at a later stage.
So, what are successful anchoring techniques in tunnel construction? This will depend on numerous factors. But the main influence on the anchoring approach will be the applications – what are the anchors to be used for?
Traditional vs. Innovative Anchoring Solutions
Mechanical anchors can deployed in a number of ways in tunnel construction, but it is important to differentiate between traditional anchors, often used for more structural purposes, and the innovative anchoring solutions such as those developed by LIEBIG and EJOT.
Anchoring is commonly used in tunnel construction where poor ground support may lead to significant deformation of the surrounding rock. The types of anchors used here, however, are very different to those required for post-installation applications within the tunnel, such as for attaching support bracketry for overhead services, jet fans and PA systems.
Regardless of which type of anchor is selected and deemed most suitable for the application, however, key factors in confirming its suitability will include tensioning requirements and its bond strength, which is an overall parameter used for description of connection quality between the steel anchor and concrete.
Key Technical Considerations for Anchoring in Tunnel Construction
Another key considerations is the level of corrosion protection offered by the anchor. Tunnels, particularly road tunnels, present amongst the most demanding environmental conditions possible for construction products, so the grade of the metal used in the manufacture of anchors must be fit for purpose.
HCR (high corrosion resistance) stainless steel, also referred to as grade 1.4529, is widely used for anchor bolts, through-bolts and expansion anchors used in post installation applications. This grade of stainless steel is most appropriate to resist continuous exposure to the moist air present inside tunnels, particularly those under rivers and in coastal locations, as well as the pollutants generated through vehicle exhaust emissions which create very aggressive conditions.
But HCR stainless steel is not the only material used in anchors for tunnels. Innovative solutions from LIEBIG including the Superplus BLS self-undercutting anchor, may also make other high grades of stainless steel – principally A4-80, a more acid-resistant type of stainless steel –suitable for certain tunnel applications.
When selecting any type of anchor, whether it is designed for tunnel applications or something else, it is important to ensure the manufacturer can provide evidence of rigorous quality control procedures, and that there are equally robust processes in place during its installation.
In the manufacture of LIEBIG anchors, for example, the quality of the metals used is routinely checked and tested to ensure it meets the specification. Quality checks are also completed at each stage of its assembly and once complete, the anchors are tested to ensure the pull-out, tensile and shear strength is as stated in the ETA (European Technical Assessment), UKTA (UK Technical Assessment) and any other internationally respected standards.
The Significance of Ground Support
Ground support in tunnelling projects is significantly important in providing structural integrity, particularly given the threat to the safety of construction personnel if unstable surrounding ground conditions are unstable and unpredictable. Hence why rock bolt anchoring is used as an active support technique. This helps to accommodate minor deformations in the tunnel’s surrounding rock and exploits the self-stabilizing capacity of the rock.
Enhancing Tunnel Stability and Safety
Tunnel stabilization is a key part of the tunnel construction process and one of the most important safety measures that needs to be factored into the design. The load from the ground above the tunnel and to the sides must be accommodated, which is why a tunnel lining is often incorporated with materials like steel or concrete. These provide support to the tunnel walls and transfer the stresses in the subsurface around the tunnel.
In addition to providing stability where surrounding rocks and soil is loose, tunnel stabilization addresses breakage in the rock experienced during traditional tunnel boring and it helps to prevent water penetration. This important work provides structural reliability and ensures that public safety is maximised throughout the lifespan of the tunnel.
One of the most common ways to stabilise a tunnel is to use specific types of anchors. These geotechnical engineering elements are designed to be inserted into the rock to match the conditions involved and are commonly known as ‘rock bolts’.
Mechanical anchors would not be used for tunnel stabilization work of this type because they are designed for embedment into hard materials such as solid concrete, natural stone and brick.
A comprehensive risk assessment will inform the correct anchoring decisions to be made.
Understanding the Role and Types
The various roles performed by different types of anchors in tunnelling projects means it is important to understand which anchoring approaches are best suited to the application. As with every civil engineering project, when considering the most effective anchor design for a given purpose, consult the relevant engineering standards to avoid test failures and the costs associated with delays for redesign and testing.
Tunnel construction is covered by a wide range of standards and regulatory compliance, both in respect of the structural design and performance and to maintain health and safety during what is hazardous working environment.
The most notable engineering standards applying to design and build in the UK is the BTS (British Tunnelling Society) Specification for Tunnelling which was originally published in 1997 and is now onto its fourth edition. It is regarded as the standard industry document for tunnelling contracts and the latest edition benefits from recent experiences gained from projects including Crossrail, HS2, and Thames Tideway.
Another important standard for UK projects is CD 352 Design of road tunnels. This provides requirements, advice, and guidance relevant to the planning, design, operation of new works and major refurbishment of all road tunnels on the motorway and all-purpose trunk road network in the United Kingdom.
However, due to the wide variation in the engineering considerations that influence the structure and form of a tunnel, its scope in respect of civil engineering aspects is limited. And detailed aspects are covered by additional documents including the Manual of Contract Documents for Highway Works (MCHW): Volume 5: Section 7: Mechanical and Electrical Installations in Road Tunnels, Movable Bridges and Bridge Access Gantries MCHW Series 7000.
All these standards must be considered when determining the anchoring approach, in addition to the performance standards that apply to the anchors themselves – particularly mechanical anchors.
Anchor Load Testing: Ensuring Structural Integrity
Focusing specifically on mechanical anchors, there are a number of testing procedures that need to be undertaken to ensure they will provide sufficient load distribution and structural integrity in-situ throughout their design life.
One of these tests is known as a ‘determination of resistance test’ which is a tensile or shear test to enable anchor design for unknown or uncertain base materials. It is of particular relevance to tunnel engineers or designers and is usually undertaken when insufficient data is available to determine anchor resistance for safe, efficient and cost-effective design approval, such as for reinforcement or restoration projects.
Another anchor test is an installation quality test. This enables project managers and quality managers to obtain proof loading documentation for post-installed anchors or rebar when they need proof loading documentation of tensile tests. This test is important for the validation of installation quality for quality managers, project owners or external inspectors.
As useful as these site testing processes are, however, they should never be regarded as a substitute for an approval document or overrule one.
Tackling Tunnel Anchoring Challenges
Anchor technology has advanced to respond the many tunnel construction challenges, not least the sustainability of projects and the environmental impact of tunnel through its whole life cycle.
Specifying high quality components that can be trusted to meet the technical specifications, usually those backed by credible evidence and data, will reduce the likelihood or premature repairs or replacement of elements. Unplanned repair and maintenance work carries with it a large carbon footprint so maintenance strategies must be clear from the outset. Hence why tunnel components which offer dependable durability, including mechanical anchors, enable a more accurate assessment of a tunnel’s sustainability to be determined at the outset.
But it is not only environmental impact considerations that present challenges for tunnel anchoring. Ask most designers and engineers ‘what are the main challenges in tunnel anchoring?’ and you will probably find that the responses place equal significance on the practicalities associated with the anchoring installation, including availability of adequately skilled labour.
Cost Efficiency and Project Management
Advances in anchoring technology and design, plus more comprehensive technical support for engineers and installers, has enabled tunnel anchoring processes to be more streamlined, whilst increasing assurance in terms of the integrity of the installed products.
EJOT UK is one of the companies ideally placed to help tunnel project managers to optimise cost efficiency. Firstly, it has developed a portfolio of anchors suitable for a vast range of construction applications, many of which are ideally suited to tunnel projects. This range includes EJOT branded through-bolts, concrete screws and chemical anchors, along with LIEBIG branded mechanical anchor bolts, a brand it acquired in 2017.
In terms of tunnel anchoring project management and efficient project planning, EJOT and LIEBIG anchors offer benefits because they are designed to be straightforward and efficient to install in general. But one product in particular, the LIEBIG Superplus BLS, has additional advantages for tunnel projects.
Superplus BLS is self-undercutting and requires no setting tool during installation, and it allows for two approved embedment depths per diameter for added versatility. The result is that installation is highly efficient which contributes to a lower installed cost overall compared to traditional undercut anchors.
In addition, anchors like the Superplus BLS, as well as products such as the LIEBIG Ultraplus, have a modular design. This means it is possible to create bespoke anchor lengths without the extended lead times and high unit costs usually associated with such products, helping to keep the project schedule on track and within budget.
It is not only innovative product design that makes for tunnel anchoring project efficiency, however.
Workforce training and support for the contractors on-site is also crucially important. Hence why EJOT provides installer training through toolbox talks on-site to allow the fine tuning of anchor installation, and at its Applitec Centre in Yorkshire before the project commences.
This state of the art R&D, testing and training facility provides the resources to enable EJOT UK’s anchoring specialists to train installers by giving them experience of using its anchors in the exact substrates they will face in the tunnel. This facility also gives site operatives the opportunity to learn how to conduct pull-out tests on EJOT and LIEBIG anchors.
Case Studies Highlighting Anchoring Success
One of the easiest ways to understand the benefits of advanced anchoring technology is to see how products have actually been used in tunnelling projects. In one tunnel anchoring case study regarding a new road tunnel project in the south east of England, the advantages speak for themselves.
Here, a tunnel anchoring solution was required for use throughout the new tunnel’s main bore and separate under-tunnel, beneath one of the UK’s most significant waterways, to meet the demanding performance specification required for the highly corrosive environment. Project management priorities had to be met too, which is why the solution also had to enable the main contractor to keep the project on track and on budget by avoiding lengthy lead times and high unit costs.
EJOT provided the anchoring solutions here to the project’s sub-contractor, BISON UK, a nationwide market-leading supplier of fixings, support systems and bracketry to the M&E and building engineering services sectors. More than 6,000 high performance mechanical anchors were supplied for the installation of the electrical services and public address (PA) system in what is one of the country’s most high profile new road tunnel projects.
EJOT and LIEBIG branded HCR (high corrosion resistance) anchors were supplied to meet the requirements of the brief with maximum efficiency. Two products from the company’s portfolio were specified: the EJOT BA-E Plus HCR through-bolt manufactured in 1.4529 high corrosion resistance stainless steel, and the LIEBIG Superplus BLS-P self-undercutting heavy duty expansion anchor manufactured in A4 stainless steel.
The BA-E Plus 10/10 HCR through-bolt anchors were used to securely attach the bracketry for the electrical system trays throughout the tunnel. These were supplied by EJOT in a bespoke version which featured an additional washer to offset the brackets from the tunnel wall and prevent moisture collecting. A special tool was also supplied by EJOT to enable easier overhead installation of the anchors.
Bespoke LIEBIG Superplus BLS-P anchors in two different lengths were specified for use in stand-off applications. These included speakers and other components associated with the tunnel’s PA system. These anchors met a slightly different brief in that they were also required to demonstrate a high level of vibration resistance resulting from vehicle flow through the tunnel. The PA system bracketry then utilised the anchors two lengths to accommodate the size and shape of the speakers in the context of the tunnel’s curved walls.
