Tunnel vision: temporary solutions support upgrades to historic gallery

EARLIER this year, Tilbury Douglas wrote in Project Scotland about some of the challenges presented by what was described as ‘one of the most complex engineering projects undertaken in a heritage building in Scotland’.

When new and improved access routes linking existing and new spaces were to be installed underneath the National in Scotland’s capital city, it was critical that the building was kept structurally safe and fully open to visitors during the works.

As part of investment into the gallery, National Galleries of Scotland looked to improve ease of access through gallery spaces by means of a tunnel, staircase, and lift beneath the building’s listed façade.

The ground created geotechnical challenges, with limited stability for the works taking place.

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, MD at Core Drilling Specialists, said, “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.”

Mabey Hire’s 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, project manager at Tilbury Douglas, stated, “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 the 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.”

Limited space posed another challenge on the project, necessitating the need for equipment that 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,” Stuart added. “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.

“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.”