Voices of Science

So Westminster station, right alongside Big Ben, was a huge engineering challenge because it was a very deep station excavated from the ground surface, involving putting walls into the ground deep down, probably to a depth of about fifty metres, and then excavating between those walls, propping across the whole way as you went down, to a depth of nearly forty metres below ground level.  And this was all happening only thirty-five metres away from Big Ben.  So one could see almost by drawing a forty-five degree line from the bottom of the station up towards Big Ben that the effect of this very big and very deep excavation – the deepest ever constructed in London – right alongside Big Ben that was sitting on pretty fragile foundations – Big Ben was built in the 1860s and in those days they just dug a hole, and it’s right alongside the River Thames, and they went on digging until the water was pouring in and they stopped and they just poured in some, what in those days would be quite basic, concrete, and that was it, those are the foundations of Big Ben. With quite a high pressure on them because the actual tower itself, Big Ben tower is quite high, so the actual stress on its foundation is quite high.  So Big Ben had already shown some sign of leaning, not to the naked eye, but measurements showed that it was actually already leaning, leaning already to the north in the direction of where this station was going to be, which was going to be constructed to the north of Big Ben.  So this very large excavation for the Jubilee Line Extension was undoubtedly going to cause quite a lot of distress to Big Ben, so we had to do a lot of very careful prediction as to what might happen and then come up with a solution to prevent Big Ben from moving.  And we employed a technique called compensation grouting, which was a technique that we, GCG, had coined earlier, just a few years earlier on a project for London Underground at Waterloo.  Compensation grouting essentially involves the installation of tubes in the ground, steel tubes, which are drilled and installed in the ground before any tunnelling. And they’re positioned in the ground at a level between the tunnel and the foundations of the building that one wishes to protect.  And the idea is you then, when the tunnel is constructed, you then inject from those tubes – and I should say that each tube has lots and lots of holes in it which are covered with a tight rubber sleeve, but the point of this is that you can select any one of those holes from which to inject the grout, just one hole, and you can choose the exact position and you can specify the exact quantity of the grout.  So that you inject the grout to compensate for the movements being caused by the tunnel and thereby prevent the movements migrating up to the ground surface and affecting the foundation of the building.  So you’re effectively intercepting the movements that would take place.  And so you combine this with very detailed analysis, very detailed measurement, so you’re measuring all the time the response of the ground and the response of the building you’re trying to protect, and you’re constantly injecting in different holes through different tubes in order to minimise the effect of the tunnel.  When the Jubilee Line Extension project was underway, this compensation grouting technique was adopted pretty widely on that project to protect buildings, the most famous example being Big Ben.  So there was a shaft sunk in the middle of Bridge Street, which is the main street running over Westminster Bridge past Big Ben, and from that shaft in the daytime that shaft was covered over with a steel deck and the traffic just ran, but night-time, the traffic was diverted, the cover was taken off, drilling rigs were put down the shaft and horizontally drilling from the shaft all the way up to a distance of about fifty or sixty metres beneath the foundations of Big Ben quite deep down, such that there was an array of these tubes fanning out underneath the foundations of Big Ben and it was those that were used for injection of the grout.  So as the construction of the station took place, the movements of Big Ben were controlled, over a period of about two years.  It was a very well controlled and very successful operation. If we had not done any compensation grouting, if we’d simply just built the station, then Big Ben, the clock would have moved by probably approaching 150 millimetres, something like that, at the level of the clock, which would have been really very significant.  That would have probably caused cracking, distress to the point where Big Ben is linked to the Houses of Parliament, the actual building.  So I think the world, I think that was a tremendously big technical leap, to actually use compensation grouting in that way, obviously because Big Ben is such a sort of iconic monument, many eyes were upon it.