What?? Mass Water In My Shaft & A Little Base Heave
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.
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I’m interested in the base heave. The shaft is 25m deep so there is a vertical stress reduction of 500kPa
There are two ‘forms’ of heave
One is elastic rebound
The othere is swelling consequent upon effective stress reduction ( you remember that e -v- stress virgin curve and unload curve
Two Q’s
1 Do you have any data on what heave you were seeing and how quickly it occurred?
2 Are there any measured bing taken for long term arresting of heave against any long term base to the shaft?
John,
After a afternoon of searching yesterday, i cant really find any data that will support any full argument of base heave. There is a general desire to find all other excuses and add as many factors together such as; construction tolerances, engineers settin gout erros, datum error etc. All of these have been blamed for what i believe to be base heave, if only a very small amount. As for whether it is Elastic rebound or swelling I belive it is elastic rebound as i dont believe the reduction in effective stress to be vastly different from the porepressures. I should recive a new set of pore pressure readings tomorrow from which i will be in a better position to form my opion. In the meain time, my recolection of the soil stress stain cure i believe the soil is overconsolidated, and while plastic defoemtion of the soil has taken place due to a reduction in the voids ratio following periods of loading and unloading. The subsequent unloading during our excavation would have seen a reduction in th eeffective stress and the soil would behave elastically and strain. If the pore pressure readings tomorrow are vastly greater thenthe change in the effective stress redcution then this would suggest swelling.
One other observation i would like to make is that i cant help but feel that the engineers are riding on luck rather then sound judgement. When incidents such as this occur there is no time spent on refelction and gain a full understanding of the issue at hand, but the emphasis is on ciorrecting the problem. In this case rather then identify if this poses a real engineering risk there was a rush to dig out the blinding and relay in rder to commnce fixing steel rebar. I can say im overly impressed by the level of engineering it all seems rather ‘agricultural’ like somthing my old man ( who is a farmer) would knock up or in your own words ‘somthing your granny would say’
Steve,
Just to quickly add to your final comment and agree with the point Whilst I am fairly busy again with a number of issues in the ground on my sub station site, i’m still trying to get to the bottom of my concrete investigation as mentioned in my blog. As you say everyone wants to effectively sweep it under the rug by conducting the remedial action without understanding why it occurred and thus minimising the risk of similar incidents occuring at a later date.