Introduction
Capital Square is the largest speculative office development in Edinburgh and is located in the heart of the city’s business district. The project is scheduled for completion in 2020.
BAM Construction is the main contractor on the project, with Careys Civil Engineering providing civil engineering services.
Groundforce became involved at tender stage and supplied a wide range of equipment to assist with the groundworks, including their MP150 and MP250 props, complete with 1200mm diameter Supertube, Mega Brace and Super Mega Brace - the strongest proprietary brace in the industry. They also supplied several bespoke items.
Most of Groundforce’s equipment is modular, arriving on site in sections ranging from half a metre up to 12-metre extensions. Most of the time it will fit most shapes and sizes but due to the irregular shape of this particular excavation they had to fabricate some bespoke components to complete the solution.
Groundforce helped to engineer a solution early on in the process, which had a number of different iterations before arriving at the final solution. Originally, a raking propping solution was considered but this did not suit the programme so a horizontal propping system was used.
Challenges
There were several challenges with this project, including the ’D shape of the excavation, which at its widest part measured approximately 33m wide by 29m long. Another was its location, situated right in the heart of the city centre, creating obvious logistical tests. Traffic management plans were put in place to help with deliveries.
Careys Civil Engineering installed in-situ concrete panels to support the basement. Because there was less surface area to prop to, and the load needed to be evenly distributed across the of the king posts, Groundforce had to carefully consider the prop placement and shear forces.
Solution
Groundforce senior engineer (Major Projects) Adam Fletcher said: “The interesting thing about this project was that rather than the usual contiguous piles or secant pile construction, this was a king post system with concrete in-fills. So the method of installation varied a bit from normal. We had to weld the waler to the upright post, then the concrete was then poured in between the king posts as the excavation progressed, to form the wall.”
In one area some extensive packaging and support was required which would normally require gallows brackets. This was changed for hanging chains attached to the king posts to suspend the frame, rather than support it from below.
Both Adam and Careys Civil Engineering’s senior project manager James Purves agreed that early involvement in the project helped avoid any potential issues.
Adam said: “We often work closely with the pile and structural designers at early design stage as well as the main contractor. On this scheme, we collaborated with Careys on a shared Revit model to optimize the solution. It’s gathering momentum and a lot of contractors are using it. It means that we have visibility of all the relevant information – the construction sequence and programme - as well as the exact equipment location and any potential clashes. Groundforce provided the prop Revit blocks which were incorporated into Careys’ model of the structure. It’s all very interactive.”
What the client said
James Purves said: “Groundforce and Careys became involved at tender stage on this project and Groundforce put forward a solution early on in the process. From tender stage, this was planned on raking props. The raking prop solution would have worked but it didn’t provide us with programme assurance, something which was incredibly important due to the amount of work required prior to installing the props.
“If anything had prevented us installing the props, it would have been a challenge to complete on time with so much work ahead of us, plus, there would have been additional work to remove the raking props. The flying props, as I call them, are a great solution.”