Holdfast Training Services Trial Blog

As part of the piling proprietary works we were required to remove sections of a reinforced concrete slab with the old Boradgate ticket hall. The slab level that requires breaking sits 108 SSL (street level 112 SSL) and in order to facilitate piling the substation is due to be filled with foamed concrete and clay fill. The purpose of the fill and foamed concrete is to bring the ground level up to street level in order that the piling mattress and piling rig are not working over a suspended slab with a void some 4-5m deep beneath them.

The breaking out of the sections of the 108 slab is to allow remove any obstructions to the piling that may cause the piles to kick and become misaligned. To facilitate the break out we hired a machine called a Brokk which is a small remotely operated breaker and excavator. The great advantage of these machines is that they are able to access confined spaces  and break out areas overhead while removing the risks associated with construction workers (operative) operating within these environments. Although there remains a risk of damage to the machine, considered much less of a risk then injury or death to an operative.

Having successfully remove all but one of the sections of the slab the project manager was pushing to remove the final section but I raised a concern over it appearing to be a canter leaver slab and that we had no way of knowing how the remaining part of the canter lever slab would react to a section being removed. Warnings ignored our project manager continued to push forward with the break out and I found myself with only a limited amount of time to come up with a convincing argument as to why we should not break the slab and an alternative solution.

The area of slab to be removed as a 1.5 x 1.5m section. The section of slab appears to be cantilevered over a small retaining wall with the canter lever wall reaching west to east. The area that appeared to be canter levered is tied into a brick wall at the northern end and then merely sits on top of the connecting flange of a cast iron column at mid-section and then tied into a reinforced concrete slab at the southern end. Between the Column and the northern wall there is only one small supporting strut. Between the column and the southern end the small retaining wall ceases and a vertical shaft opens beneath the slab. My concern was/is that the breaking out of the section would/could result in unpredictable cracking and the failure of the slab presenting a risk of injury or death if loaded. A phone call to the rear guard for some further advice deduced the following:

The area that at first appears to be acting as a canter lever may in fact be acting as a simply supported slab, with hidden cast in beam that spans from the northern wall to the column and then to the southern slab. This would then allow for a one way spanning slab from the retaining wall to the beam. If this is the case then removing a section of the slab would then make the slab work in a way it was not design too with the potential to fail un predictable if loaded. This is of course only a theory, as I have no way of knowing if the edge of the slab is indeed  a simply supported beam and I also do not know the make up of the steel with in the slab.

Initial recommendations:

1. Remove a larger section of the slab to the north of the column. This would then allow the southern section to act in isolation. Any area north of the column that may have been unsupported and pose a risk of unpredictable failure would be removed.
2. Find another way of completing the work.

Final solution:

As a further part of the works we are required to fill the void beneath the 108 slab with foamed concrete. By filling the void first and then breaking out the section of the 108 slab the risk of failure has been completely removed. The order for the works is not important as neither the foamed concrete nor the break out are dependent on the other.

In the words of the great orator “never assume anyone else knows what they are doing” in this case after presenting my theories and recommendations and with no one else having either come up with a recommendation or even being able to prove me wrong we were forced into working to my recommendation. So I hope it all works. May I recommend that should any of you find yourselves in the future be travelling on Crossrail, I recommend that you do not get off at Liverpool street as i have hand in guiding the build…