The challenge
The new Scottish galleries at the National, located on The Mound in the centre of Edinburgh, was built in 1859 and boasts a collection of works from across Scotland and Europe. With breath-taking works of art from many of Scotland’s finest artists, the gallery is home to a range of priceless paintings and artifacts. However, as part of a new investment into the gallery, National galleries of Scotland looked to improve ease of access through the gallery spaces by means of a tunnel, staircase and lift beneath the building’s listed façade.
Speaking about the project, Stuart Burgess, Project Manager, at Tilbury Douglas, said: “The primary aim of the project was threefold: to create new world-class gallery spaces, improve access to the building and streamline the flow of visitors through the various parts of the gallery. To achieve this, significant structural alterations were required, as well as issues with asbestos, damp and unchartered obstructions.
“One of the elements designed to improve visitor flow was the creation of a tunnel underneath the 19th century Grade A-listed portico, six metres below ground, creating a link between the original 1850s section of the building and the newer 1970s parts.”
With the gallery still remaining fully open during these works, there were strict restrictions and regulations in place, as Stuart explained: “Due to the fragility and historical importance of the buildings listed porticos, any small movement could result in damage being done to them. To ensure this didn’t happen, there was a contractual agreement in place that meant we were only permitted 6mm of differential movement and 12mm of overall movement when undertaking the tunnelling works. Additionally, with the building still open to the public, we had to ensure that noise, dust and vibrations were carefully monitored so as not to impact on visitors.
“With this in mind, it was vital that we were monitoring the effect the works were having on the building throughout the whole project. This included when the weight of the building was transferred onto the temporary works, including a complex jacking system, and again at the completion of the tunnel, when the loads would be shifted back again.”
The team at Tilbury Douglas also had to tackle poor ground conditions, a result of the original construction of Edinburgh’s ‘New Town’ and the use of excavated material and building waste that was used in the formation of The Mound. This ground created further geotechnical challenges, with limited stability for the works taking place.
As a result of this, Tilbury Douglas enlisted the help of Core Drilling Specialists, a cutting and controlled demolition contractor, with a range of support packages for this project. Scott Murray, Managing Director at Core Drilling Specialists, spoke about this, saying: “We were contracted to provide demolition and structural support operations, with some elements of temporary propping forming a part of these packages – such as slapping through exiting walls and crash decks for the removal of the slabs. As our work progressed, we were offered an additional scope, which entailed underpinning the existing south Portico. To do this, we drilled a number of core holes over 13 metres deep through the foundations – with a tolerance of +/- 5mm – and then installed steel needles by pressure grouting them onsite.
“This was then used as a platform to jack the building up to form the new tunnel, after which point our team installed the full temporary works system to the south end of the building and carried out elements of the groundworks. Due to the scope of this project, we knew that we would require additional equipment to support the building during these works. It was from here that we recommended Mabey Hire, as we were working with them on other projects in Edinburgh at the time. The Mabey Hire team quickly turned around its engineered designs, which were presented and accepted by Tilbury Douglas.”
Scott continued: “Due to our prior experience with Mabey Hire, we knew the quality of its kit and the professionalism of its teams. We had full confidence in recommending them for this project, with the complex nature and strict tolerances associated with it something we knew they would be capable of handling.”
Our solution
With this recommendation, our propping, jacking and monitoring solutions were brought onto site and, with the exception of the monitoring package, installed by Core Drilling Specialists’ team. Stuart Burgess continued: “Mabey Hire provided us with a bespoke real-time monitoring and hydraulic jacking system measuring load and displacement, as well as a range of standard props and bracing. They also provided monitoring support across the construction of this new tunnel, with the team regularly onsite to ensure we weren’t breaching the pre-agreed tolerances.
“Mabey Hire’s monitoring team worked directly with Arup, a sub-contractor to Tilbury Douglas, to come up with the methodology for the monitoring system. There were a variety of sensors installed on each portico outside the building, measuring allowable noise, movement of the building, load and displacement. This system was then used to pre-load the other temporary works, with a zero-movement hydraulic propping and jacking system used to fully secure the building.”
Available space posed another challenge on the project, as Stuart commented: “A major challenge we faced during the course of these works was the space in which we were working in. Due to the confined conditions, any equipment that we used had to be manually handled or lifted in with small machinery.
“This was another reason why Mabey Hire was chosen as the temporary works provider for this project, with its propping and jacking equipment able to be easily assembled and moved into place by hand. Likewise, its monitoring equipment isn’t invasive or heavy and can be carried around in a small handheld box, ensuring it wasn’t getting in the way of our other works.”
Stuart concluded: “Working with Mabey Hire made our role on the project that much easier, with its knowledgeable and helpful team always on hand to help with any issues we encountered. It was particularly valuable that Mabey Hire was able to provide all the equipment to support our works, from the propping and jacking to the structural monitoring support.”
