A few months ago, I went to a lecture on tunnelling technologies, given by Professor Robert Mair FREng FRS, and specifically on what is known as compensation grouting. Meant to write up about it at the time, but it was not the sort of talk that was easy to write about, as the core of the talk needed the slides to illustrate what was being talked about.
However, the Royal Society – who hosted the talk – put podcasts of their lectures on their website, and today I finally got round to reviewing the details (mainly as the podcast page only works in Microsoft browsers!).
Today, I am going to focus only one aspect of the talk, which was also one of the more famous instances of compensation grouting, and that is the Jubilee line work around the Clock Tower, more famously known as Big Ben. Timely, as the Clock Tower is 150 years old this week.
As a Jubilee Line tunnel was tunnelled by the tunnel boring machines (TBM), despite the best efforts of the workers, there is always a slight gap between the tunnel wall and the soil outside, leading to some subsidence at ground level. The gap is only a few millimetres, but when amplified around the entire tunnel diameter, that actually adds up to quite a bit of missing soil, and can cause significant problems. Before work starts on any tunnel now, ground surveys and measurements are taken to calculate the subsidence risks and effects on buildings.
In some areas, where the tunnels are likely to cause significant problems, compensation grouting is used.
This is basically steel pipes that are drilled into the ground above where the tunnel is due to be dug – before it arrives – that pump in a slurry type concrete mixture into the ground to “compensate” for the missing soil below.
For the Jubilee line, this was complicated by the architecture of the location and the fairly shallow foundations of the infamous clock tower. The big risk, which was gleefully latched onto by the news media was that the tower would start to lean sideways towards the tunnel work and may even start to develop cracks or damage. As the TBM worked its way through Westminster, electronic monitors on the tower checked how far it was starting to topple, and then pumped grouting into the soil to basically push the tower back upright again.
The compensation grouting was only carried out at night – as the hole in the ground where they worked was right in the middle of the road, so covered during the day to allow road traffic.
The pipes drilled reaching under the ground were on average 60 meters in length. The black circles show where the grouting was inserted – and on average about 150 litres of cement grout was pumped in at each point.
The following slide shows the movement of the tower. They knew the tower could withstand about 15mm of movement, measured at the height of the clock face, before action was needed – and you can see here how the tower started tilting, then a period where the compensation grouting was applied and after tunnelling, the period where the tower more slowly (and safely) settled.
Incidentally, the tower was already leaning before the tunnelling work started – by about 22cm to the North-West, which is said to be just noticeable to the eye.
Without compensation grouting, it is expected that the tower would have tilted by some 10cm at the top – which would have been obviously unacceptable.
The sides are taken from the podcast on the Royal Society, where you can watch the entire lecture. Note, the slides won’t display in either Firefox or Chrome web browsers – so I had to use MSie to watch see them.
I’ll later do a write up about the work at Kings Cross as some aspects of that sound quite interesting, but needs more research work to be carried out.
Incidentally, if you want to climb up the Clock Tower, visits are fairly easy to arrange – a review of the details here.