Holdfast Training Services Trial Blog

One of the less helpful outcomes of the acrimonious handover of my site from Mace to McAlpine has proven to be the lack of accurate site condition data. Without the ability to survey the site before taking possession we have been left playing catch up on a number of important issues.

The 14 large-diameter bored piles, installed by Mace, are to receive base plates with pre-fabricated column starters and reinforcement cages. These units are limited in size by the capacity of our tower cranes but still come in at up to 18T per piece. This single unit is then to be lifted into place within the pile casing without clashing with any of the existing pile reinforcement. The plan is then to top up the concrete/grout beneath the base plate. The many risks in process can wait for another time because the first problem came with the site handover.

Revit model of the column base   plate in the pile casing

On inspection of the pile heads the existing reinforcement on one of the piles was visibly wonky (technical term). This raised concerns over the global positioning of the pile, was it the rebar or the pile sleeve that was wonky, what is the design tolerance for position and verticality, which was in the right position, who surveyed it for the as-builts and how, etc, etc but the key question was simply can we get the base plate in in the right place without clashing with what is already there.

Having satisfied ourselves that the base plate would fit inside the pile sleeve we set out to identify any clashes with the rebar. In collusion with our geospacial team and BIM manager we superimposed the structural model onto a point cloud survey and ran clash detection between the two. This quickly showed the extent of our problems and has allowed for mitigation measures.  

Model vs point cloud clash detection, red indicating clashes.

Of the clashes, in red on the image above, some could be removed on site (e.g. the anchors you can make out beneath the main steel shear key plates were cut down) but there was to be no damage to any of the existing pile reinforcement (the outer ring of vertical H50 bar, in grey on the image). Instead the structural engineer instructed that the base plate be re-worked off-site by cutting and re-welding the base re-bar.

Diagram showing  base plate rebar to be cut and move inboard to avoid clashes.

This use of 3D modelling and clash detection software was able to identify a serious clash before the key components were delivered to site. Being as the fabricator is in Bury and the work required could not be easily done on site, this early notification prevented an expensive, time consuming and awkward clash on site.