Holdfast Training Services Trial Blog

Demolition on my job at Victoria Square, Woking is now almost complete. The demolition sub-contractor are in the process of removing the existing strip foundations, backfilling and creating a piling mat with the site won crushed concrete from the demolished structure.

We have a few risks regarding the interface with adjoinging structures such as the live shopping centre and adjacent construction but none are more sensitive than the live substation on footprint of our demoliton.

The substation was part of the ToyRUs on the first two levels of the previous structure but feeds a large propotion of the shopping centre and therefore must remain live. UKPN who own the substation have dictated that no vibration above a PPV (Peak Particle Velocity) can be achieved above 5 mm/s and there is a vibration monitor fixed to the substation to ensure this.

The problem is the substation sits on the foundation we need to remove and we are creating a signifficant amount of piles and substructure very close to the extent of the walls.

The yellow designates the foundation to be removed and you should also just be able to see the ground beams, pile caps and piles being installed in the coming months. There was the added complication that the exact location of the substation was slightly off during the design of the new structure (compare the grey lines on the drawing to our overlaid purple lines) which make these tollerances even tighter.

The vibration of installing piles so close to the substation is being mitigated by use of a CFA method (although this was always going to be the chosen method anyway as it was selected for the rest of the site) but it still leaves the problem of breaking out a 1.5m deep reinforced concrete foundation 150mm from the substation which isnt allowed to experience vibration. The solution…

Diamond stitch drilling. By using a 150mm diamond drill bit and drilling along a given line around the substation overlapping the holes creates a seperation of 150mm of the foundation under the substation and the foundation to be removed. This is an incredibly slow process with each driller completing less than a metre a day each. With 4 operators on site this will still be nearly 2 weeks work.

In the photo you can see 4 drill rigs set up as the drill out cores of the foundation. It hasnt been raining that soggy foundation is caused by the amount of water used to keep the drill piece cool.

I still have my reservation on whether this will prevent vibration when it comes to removing this foundation considering I feel it in the site office, but it shows we are taking practicable measures to comply with UKPN targets. Whether UKPN have set unreasonable targets is a question for another time I guess, a quick bit of research told me that structures can normally resist up to 20mm/s but I dont have a full understanding of sensitivites to generators or transformers that may be inside.