Hear from Our Experts: Fixing Points & Lifting Anchors

1. Why are fixing points required in the construction of a tunnel?

Within tunnels, a wide range of permanent services are often installed, including high voltage cables, signals, walkways, fire mains and communication cables. These services need to be supported by brackets which in turn must be connected back to the concrete tunnel lining. In addition to these permanent services, the tunnel boring machines also require power and ventilation services that need temporary supports and therefore fixings points during the tunnel construction.

These fixing points can be provided using Halfen DEMU Bolt Anchors on the inner concrete surface of the tunnel. These bolt anchors are typically positioned in a  configuration to suit the bracket fixing positions on the inner curved surface of each precast segment.

2. What testing and accreditations need to be met for construction products in tunnel and bridge projects? 

For tunnel and bridge projects, fixing anchors need to meet a very stringent specification for corrosion, fire and fatigue resistance. With a required design life of 120 years, the anchors need a high level of corrosion resistance to ensure they continue to perform over the lifetime of the structure. This necessitates the use of A4-80 stainless steel. In terms of fire, Leviat is able to demonstrate a product’s suitability using the bolt anchor’s European Technical Assessment (ETA-13/0401), a document issued by DIBt, an internationally recognised, independent, third-party technical assessment body. The ETA illustrates the product’s class A1 reaction to fire and the R120 resistance to fire.

Finally, tunnel projects, especially those involving high-speed trains, require additional tests to be carried out to meet the very high fatigue requirements. The piston effect of trains passing through the tunnel travelling at 200mph imposes forces on the brackets supporting the services and therefore the anchor fixing points. Working on an average of 18 trains per hour, 19 hours a day, 365 days a year and for a 120 year design life, Halfen DEMU Bolt Anchors are fatigue tested up to 30 million cycles, which is far in excess of normal construction requirements. 

3. Precast construction elements are used more and more to speed up the construction process. What factors should be considered when choosing the correct lifting anchor for installing these huge concrete panels?

When designing any precast unit, it is always best to consider as early as possible how you are going to lift and install the element as this can impact the design, safety and cost of the overall solution. The design parameters include how the units will be lifted, whether they need to be pitched or turned, the maximum cable angle on the lifting chains, and the dynamic load factor (which can depend on the type of crane to be used). In addition, the concrete strength at the time of lifting and any potential need for additional reinforcement local to the anchor should also be taken into account.

On a recent viaduct project, early stage discussions between the Leviat technical team and the main contractor resulted in a thorough review of both the lifting method and the lifting anchors. Using the precast experience of the team, Leviat were able to propose a more efficient lifting solution for installing the thin, large area, precast deck units. By using a lifting beam, the lifting anchors could be located in the top of the deck rather than into the sides of the unit, removing the need for localised reinforcement at the lifting locations. The reduced cable angles and the anchor locations particularly suited the high capacity threaded Halfen HD Lifting Anchors. These smaller and more economic anchors resulted in initial savings of around 60% compared to the original design.

In addition to this considerable saving, the contractor had also budgeted for separate secondary fixings for the support frame, which was used to temporarily hold in place the precast parapet units whilst the insitu concrete strip reached full strength. Leviat were able to utilise and reuse the cast-in Halfen HD Anchors for the application, entirely omitting the need for secondary fixings, resulting in further savings for the project.

4. In tunnels, important elements such as emergency exits are under fatigue related stresses caused by passing trains. How do you ensure these stresses are effectively absorbed?

To enable the installation of important safety features within a tunnel, such as the steel door emergency exits, there will always be a need to accommodate the differences between the construction tolerances of the concrete tunnel lining and the manufacturing tolerances of the steel doors and frames. The use of Halfen Cast-in Channels within the concrete tunnel lining provides the necessary adjustment and tolerance at this interface, in order to ensure the steel doors can be correctly positioned and securely fixed back to the concrete.   
 
However for any application, consideration needs to be given to the type of Halfen cast-in channel to be used, beyond ensuring the product has the required capacity for the job. There are two possible manufacturing methods to produce the C-shaped profiles that form the main body of Halfen channel. The first is cold rolling, where a strip of steel passes through a series of rollers that shape it into the C-profile used in Halfen channels. This process is carried out at room temperature and whilst it will produce good channels for general use, this method can lock in residual stresses into the finished product. 

The second method is hot rolling, where, as the name implies, the steel is formed into the channel profiles at high temperature, above the recrystallisation point. This has the advantage of enabling different wall, back and lip thicknesses on the channel profile, putting the correct amount of steel at the locations where it is required; something that is not possible with cold rolling. The hot rolled channel profiles also have a low stress profile, with reduced residual stresses. Importantly this improves the resistance to dynamic loads, making these particular channels ideal for applications such as the tunnel emergency exits. The fatigue load benefits of hot rolled channels are demonstrated in our approval document (ETA-09/0339) with published resistance capacities given for up to and beyond 100 million load cycles.