Holdfast Training Services Trial Blog

The Woking parking complex has now been demolished to ground level and the strip out of the ground floor slab and foundations has begun.

The current foundations are large shallow strip foundations at approximately 7m in width, 1.5m depth and 70m in length. They are heavily reinforced and approximately 1.5m below the ground force slab.

The structural designer has planned for the entire removal of the foundations to make way for the piles and pile caps that will be replacing them. However, ever the opportunist our demolition subcontract has had the idea to only remove the sections of the strips that would collide with the new pile caps to save time, money and waste. His argument also that the hole in the foundation would save time on having to build and strike formwork.

The drawing above shows in pink the existing foundations with the proposed. It sounds perfect on paper, but the question remained would it really be easier to precision cut out sections of a foundation at depth rather than dig the entire lot up.

The first challenge was the depth. The batter needed to get down to that depth is around 1.5m. The centre point of the proposed pile caps are 7m c/c and the caps themselves are 3m wide. If you excavate this cap with the batter that does not actually save you much in terms of excavated fill. It also means your site is 2/3 hole and 1/3 trackable access.

Secondly was the rebar. Bulk removal with a leading edge would simply mean using a muncher to rip up the rebar. However the process of leaving holes in the foundations requires a breaker machine followed by a bloke hot cutting the rebar instead. A trial foundation was dug on site this week and the whole process took 2 days for one pile cap.

the photo above is the trial attempt to “neatly form a hole in the existing foundation”.

In my eyes value engineering has its place and innovation should be considered, but there is a time and a place and I don’t think it worked this time. As the PC we were all sceptical but allowed the SC to trial it, they can chose whether they persist with this methodology but they will not receive an instruction from us to permit them any further allowances.