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.

At night the grouting machine was lowered into the hole, shown by the yellow circle.

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.

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The sides are taken from the podcast on the Royal Society, where you can watch the entire lecture. Note, the slides wont 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.

I was reading the LordsoftheBlog and posted a comment from there onto another chat site – commenting on the rather polite way the Lords reacted when part of the ceiling collapsed in the 1980s.

Another chap over there then asked if it was true that it was illegal for an MP to die in the House of Commons – and after some googling around, it turns out that while not actually illegal – if an MP did die in the Houses of Parliament, it seems that the Royal Family could, in theory, be called to act as a Jury in a Coroners inquest.

Hence, a principle exists that MPs are not declared dead until outside the Houses of Parliament.

Anyhow, in a roundabout way, that research lead me to learn about the Town Police Clauses Act 1847, which is apparently still in force in English law.

One of the offenses within that bill applies to “Every person who flies any kite, or who makes or uses any slide upon ice or snow”

If convicted, you could end up with a jail sentence of up to two weeks!

If you are Welsh or Scottish, then you are safe, but any English or Irish noticed sliding down snow covered hills this week risked arrest, trial and imprisonment.

Beware!

As one of my hobbies is reading the Early Day Motions in the House of Commons (yes, I am that sad), some of which are serious and some can be quite amusing – I came across this one which was actually quite interesting.

I used to do a little bit of lobbying and still fire off letters to unsuspecting victims to express my views on an issue.

The key thing I was taught when doing letter writing though – is never ever send a “copy/paste” letter to your MP. The MP’s office will get hundreds, if not thousands of identical letters from constituents and will promptly realise that the “concerned voter” is actually an organised campaign by a large organisation.

If I do write a letter, based on an organised campaign I will always state that fact – but then explain why I am personally concerned about the issue and only highlight the research from the organiser which is relevent to my comments.

To his credit, my local MP is quite receptive to these sorts of letters.

I was therefore a bit troubled to read this Early Day Motion this morning:

That this House notes that a campaign by Greenpeace sent around 7,000 largely identical emails to hon. Members who had signed an Early Day Motion about Heathrow; observes that this caused some hon. Members’ mailboxes to become unavailable to constituents who wished to discuss this or other issues or personal problems; further notes that a request to discontinue was not accepted unless hon. Members were willing to commit to vote as the organisation wished; and believes that denial of email service by mass spam is an inappropriate and unpersuasive tactic.

I totally agree with the MP (the audience faints in shock) and think this sort of blunderbluss spamming of MP’s email and postbags is frankly just a waste of time. The MPs are going to, understandably treat 30 identical letters as just one complaint – as it is so obviously from just the one originating source.

Having said that I support the point that a DoS style spamming of their email addresses is a bad idea – I am slightly amused by the campaign to ‘cc every email on a specific date to our illustrious Home Secretary to protest against the government plans to keep a record of every single email transaction I engage in.

Incidentally, having a secure link to my email server – which happens to be in the USA – tends to render that law impotent.

Tragically, only in a virtual reality website. I wonder if we can persuade them to let people for it for real – for charity of course.

However, while bungee jumping is not possible, people do occasionally abseil down the clock face to clean it – and occasionally climb up the outside to protest about something or other.

http://londonist.com/2009/01/bungee_jumping_off_big_ben.php

Although – if you want to visit Big Ben for real, you can go on tours of the Clock Tower and stand next to the infamous bell as it strikes the hours.

http://www.ianvisits.co.uk/blog/2008/11/11/visiting-big-ben/