Tentative First Steps

What do porcupines and listed arches have in common?

There’s an old joke about the mating habits of porcupines where the punch line suggests whatever they get up to…they do it ‘carefully.’

Well, that very sage advice is certainly the approach required when piling in close proximity to Grade II listed masonry arches of significant historic importance – the challenge faced by the renaissance engineering team on the ‘Viadux’ project when designing the foundation for a 40 storey tower within the constraints of a listed undercroft.

Now, 40 storeys is tall. But the engineering challenge was further exacerbated because of the historic viaducts. That is, we couldn’t adopt a basement construction to alleviate load to the existing ground…. a common tall building design trick. Coupled with a transfer structure at level 2 and load amplification due to localised eddy shedding left us with columns loads more akin to a 60 storey structure*

The piled raft solution our team developed was driven by the size of rigs that could safely operate within the restricted height space as well as ensuring the piles installed could be subjected to a static load test. The rig needs a bit of space when you’re going up to 6MN per pile. In the end 268 No 600mm diameter piles were installed.

The pile design and calibration was dependent on a number of things. Firstly, extensive SI was required with the risk of ‘down the hole hammers’ or other percussive techniques a technical no-go. Load Test data from the neighbouring tower of light scheme, recent Manchester projects (piling into the same sandstone formation) and the use of test piles within the proposed tower crane base, allowed the capacity of the piles, (and significantly their spring stiffness), to be derived and fine-tuned.

Working collaboratively with the team at A-Squared, 3D soil-structure FEM analysis models helped us derive suitable pile spring stiffnesses, refine geometrical layouts and finalise design through an iterative process, whilst managing the risk of differential settlement in the proximity of the arches.

So here are some pictures of the listed arches and the piles as they were installed, excavated and cropped. They show some clever / smart engineering which is enabling a new structure to grow from within an historic one.

*To put it into context, the columns on The Shard accommodate 90 MN at foundation level and we are supporting 96MN

written by Gary Robinson