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 have a small hobby of collecting old copies of the Illustrated London News – a venerable newspaper which was first published n 1842. I acquired a few more copies last week and scanning through the collection this weekend, came across a short news article about the infamous Big Ben bell and its arrival at Parliament. Although the news item was small, it was accompanied with one of the newspapers iconic large drawings of the event.

While the history of how the bell got its nickname of Big Ben is not entirely clear, the bell was never formally named and it is thought that the bell was to be called the Victoria Bell – and indeed, the caption accompanying the picture does call it the Bell Victoria. Proof at last?

The news item is dated Saturday June 5th, 1858 – and the bell was first chimed in position the following year, in July 1859. Just two months later though, the bell cracked as the clapper was too heavy and was taken out of commission and the hours were struck on the lowest of the quarter bells until it was reinstalled. To make the repair, a square piece of metal was chipped out from the rim around the crack, and the bell given an eighth of a turn so the new hammer struck in a different place.

If indeed the bell could be rung with a mere rap of the knuckles as was claimed, then it seems odd that the clapper was so heavy. Personally, I tend to suspect the news item follows the trend of the time, which is to over-state engineering efforts and make them sound better than they really were in truth.

In contradiction to the glowing report in the news item below, Big Ben has chimed with an odd twang ever since that repair and is still in use today complete with the crack.

I’ve taken a photo of the illustration from the newspaper (click for larger version) and transcribed the article below. My long term aim is to scan the newspapers with a decent scanner and archive them somewhere. Incidentally, note how the road and bridge names are spelt with a hyphen and lower cased second word – somewhat different to how we tend to write them today.

If you want to watch Big Ben being struck live, read Visiting Big Ben.

A somewhat speeded up video of the climb up the Clock Tower to stand next to Big Ben when it bongs the hours. I suspect the sound of feet on the steps is not real.

The video was mentioned by the Parliament Twitter manager.

The video goes a bit further than the public tours, as it goes above the bells to the lantern on the top of the Tower. Incidentally, this is lit whenever the House of Commons is sitting, so you can look up and always know if the MPs are nattering at that very moment in the Commons.

For details of how to climb the Clock Tower yourself – click here.

Last night, I wandered along to the South Bank to see what was being touted as a fairly major PR stunt for the forthcoming Watchmen movie. The event was billed as:

“Dr Manhattan, the blue skinned, super-powered being beloved of all Watchmen fans, will rise above the murky depths of the Thames to a height of over 70 feet and tower over all those who dare to attend.”

Wow I thought – and while not overly excited about the movie, this just had to be seen.

Double checked with the Port of London website to see where the barge would be moored (a handy tip if ever attending a river event!) and wandered along to have a watch.

Got there a bit before half past seven and various people were already securing prime spots on the Hungerford Bridge – which seemed to be the best viewing point and I quickly ran off to grab a coffee and returned to get a good location. There was a barge blowing a huge screen of water into the air and projecting various images onto the wall of mist – along with some spotlights whizzing around for dramatic effect.

Looked like it could be quite good!

The bridge filled up with various types, but the majority had that stereotypical look of the sort of person who will like the movie – and as Big Ben tolled the 8 O’Clock for the event, the bridge went quiet in anticipation.

Nothing happened. Hmm, wait a bit – 8:05, nothing going on; 8:10 and still nothing; 8:15 and yet nothing is happening – just the same video on a wall of water about 20 ft high. Ohh, spotlights started blinking, so excitement rose – but nothing happened. I knew the warning about the barge on the PoL website said it finished by 8:30, so as the crowd started to wander off disappointed I hung around a while longer.

Suddenly, a huge yellow smile was beamed onto the Shell Building – gasps of delight as the event was about to start – then it was switched off.

8:30 came and went, with no sign that anything was going to happen.

It was a freezing cold night and despite constantly putting alternate hands inside jackets to try and keep them warm, by now I was so numb from the cold that I could barely use the camera – and was desperate to use the loo.

At 8:40, the bridge was manned by just a few die-hard fans and random people who stopped to have a look as they walked over the river – but I decided to give up by then.

What could have been a really impressive spectacle turned into a PR disaster – which could have been mitigated if the PR firm had some staff on the bridge explaining what was happening. But no PR people were around, no information was provided, and after over an hour of freezing on the bridge, I had had enough.

Not impressed – and no chance to get the photo of Dr Manhattan rearing up over Big Ben that  I had hopped for.

I’m joking – although frankly, I wouldn’t rule it out. Anyhow, enjoy this outstanding picture from the members of the b3ta photoshopping website.

The Day After Today – from b3ta productions.