Bridging the gap

When it comes to bridges, a thorough understanding of every element – from the type of bridge and its location to its height, length, and the project timescales that need to be adhered to – is key to ensuring the right formwork and shoring solution is engineered.

A temporary vehicle bridge may only require a standard ‘off-the-shelf’ solution, such as a quick bridge, bailey bridge or panel bridge, whereas a permanent bridge spanning a live rail line may demand a bespoke temporary works solution to form the finished structure. Integral edge protection will be required too, to stop debris falling below.

It’s important to bear in mind that terrain and wind speeds will have an impact, too. If ground conditions are poor or unstable, or if there’s a likelihood that adverse weather will affect the ground during the construction cycle, then additional support for a shoring structure may be required. This might include trusses that span between pier foundations, or a formwork solution that’s suspended from the permanent works steel of concrete beams spanning between piers. Similarly, high wind speeds may influence crane usage and installation methods.

For permanent bridges that might include multiple piers, which typically run over live roads or railways, a composite construction method is often best. Safe and fast to construct, horizontal steel or concrete beams are seated on bearings, which are positioned on abutments and piers and stitched together, before providing support to temporary or permanent works solutions forming the deck between the deck edge’s cantilevers and parapets.

Alternatively, there’s cast in-situ bridges, which are generally more complex and sometimes unusually shaped. Casting in-situ is often the preferred method of construction for balanced cantilever and cable-stayed bridges, as well as large viaducts. Alternatively, some projects will specify pre-cast segmental box girder bridge decks. These tend to be used for long viaducts and high-speed railway bridges, with spans of between 20 and 50 metres in length and are assembled using a launching gantry traveller. With the pre-cast units fabricated off-site and delivered in modules, space and crane usage are two key considerations when choosing this method of construction.

Indeed, access constraints for labour, plant and materials is another critical consideration. For components and plant being delivered to site, factors such as weight, dimensions – including any protrusions – and the number of vehicles required to transport equipment all need to be considered. Any restrictions on the existing route to site can come into play, too. For instance, width or height restrictions under or through bridges or tunnels.

Stepping up to station demands

More than ever before, stations are incorporating eye-catching and architecturally impressive structures and designs. For instance, exposed concrete is now often a feature, which places greater emphasis on the quality of the concrete finish. This requirement demands a detailed design of the formwork to ensure compliance with the architect’s specifications.

Because stations are often built in a cut and cover excavation, ground shoring support has a critical role to play. Assured temporary or permanent propping are needed to help support perimeter walls, allowing excavation works to safely progress towards the formation level, while higher single sided wall construction is needed when working back out and up the excavation. Consideration has to be given to the size and spacing of equipment, as well as assessing how all temporary and permanent structures interact for fit and clash prevention. We will discuss more ground shoring considerations for rail infrastructure in our next blog.

Wall forming will inevitably be a part of key works on any above or below ground station project. This might be for a load or non-load bearing wall, or a retaining wall. With simultaneous construction and tight programme times, it’s important to look at the simplest, yet safest solution to cast the station walls in the most efficient way.

When it comes to reviewing which temporary works equipment you need, consider a system that will allow for flexibility and versatility, particularly when it comes to walls of varying heights. In many cases, panel systems will be the best choice, as they are easily installed, easily struck once the concrete has cured, and they can be reused as well.

Another consideration is that due to the architectural design of stations, soffits will vary between flat and curved, depending on the structure’s requirements. For instance, certain elements of the ceiling may be part of the station’s overall aesthetic, which means any selected temporary works need to accommodate the desired concrete finish of the station environment.

These are just some of the factors that will need to be considered when building stations for HS2.

In our second blog discussing keeping rail infrastructure projects – such as HS2 – on track, we’ll be considering tunnels and viaducts, as well as below ground excavations.