Holdfast Training Services Trial Blog

Mass water in the shaft.
Although the UK has had what has been dubbed a mini heatwave the bottom of the shaft has only this week been dried out after a mass water ingress into the site.
Given that I have argued that mass water ingress was not the main risk on site how had nearly 6inches of water entered the site. Well this turned out to be quite a simple. Given that we have open a hole in the ground 25m deep but in a soil with very low permeability the source of the water was in fact the sky in the form of rain. Having not factored Mother Nature’s other elements into my engineers risk assessment we have been faced with removing the water over the height f the shaft. Without having planned for such an incident we did not have a suitable pump in site with sufficient lift to remove the water. In the short term I resorted to using puddle pumps to pump the water in to the depressurisation well heads and using the depressurisation wells to remove the water. This however required negotiations with Crossrail and the monitoring team as the excess water would and did affect the ground water and ground pore pressure readings. After three days of using the depressurisation well system we were able to install a more suitable system that will remain in place over the reminder of the build to deal with future anticipated down pours as we head in to the winter months.
Base heave and a ‘little I told you so’
Issue. After this small embarrassment I was able to redeem myself with sound reasoning for what I recognised as base heave and not incorrect levelling by the junior engineers. The blinding had been poured in sections and over the course of the current basement level (84.60SSL) there were found to be variations in the levels of between 25-100mm and in places severe cracking. The cracking was initial assumed due to the drilling rig moving over the blinding layer and the incorrect levels was blamed on the junior engineers incorrectly setting the level for the concrete. On inspection of the EDMs the levels set were confirmed as correct and consistent at 83.260SSL.
Cause. Given the high pore pressures and the requirement to reduce the pore pressure below the limits set by the design authority (C138, Motts MacDonald) it would seem reasonable that we have experienced a level of base heave. This has shown that the temporary state of the soil in this case can be as little 4 weeks.
Result. The end result has been the removal of the blinding layer and the re-pour of the blinding working platform before construction of the next slab can commence. Having been behind the programme by 2 weeks due to the requirement to install further de-pressurisation the project will now be a further week behind once pouring has been completed.