INTERVIEW | ALASTAIR BIGGART
sure the root cause was a burst
Below: TBMs from tunnel projects in Alastair’s career have included those from Priestley - bentonite (Top), Channel Tunnel early works (Middle).
Bottom: He also worked on LA metro
hydraulic main and a cigarette-end: although hydraulic fluid is proof against fire because it is an emulsion, when it comes out of a broken pipe at huge pressure – they work at about four hundred bar – it atomizes and the two substances in it split and become very flammable. When it happened we got every man out quickly. No man was injured because of the fire but it completely destroyed the TBM. We had 16-inch thick concrete segments and they
were burnt down to six inches. The fire raged for about 19 hours, as I remember. If it had gone on any longer the segments might have burned through and we would have had an inundation. “The problem then was how to fix
the damaged segments. Fortunately, I remembered that on the Channel Tunnel we had some very thin cast iron linings left over. They were only about 60mm thick. They weren’t quite the right diameter, but by machining the joints and putting one extra segment in we produced a cast-iron lining that would fit inside the concrete one: we just erected it inside then grouted it in place. And because it was so thin it didn’t affect the clearance for trains. Segment linings can now be made fire resistant by the very simple use of polypropylene (PP) fibres mixed in with the concrete. This prevents the spalling that nearly destroyed our tunnel. “A really fascinating thing at Storebælt
was that we dewatered the ground ahead of us. We called it Project Moses. We were excavating through glacial tills and the hydrostatic pressures were as high as 5 bar. To reduce it we put wells down through the seabed. The power to these wells was supplied by floating barges; there were six or seven of them across the 8 kilometres, so about one every kilometre, and the wells went down through the glacial till that we were tunnelling through into the marl below and extracted the water out of the marl. That, therefore, reduced the hydrostatic pressure in the marl and that in turn had the knock-on effect of reducing the pressure in the glacial tills. “Some clever guy in the contractor’s
side had been doing some dewatering somewhere and had seen a reaction about a kilometre away and put two and two together and applied it subsea at Storebælt, and that enabled us to reduce the pressure at the face by two atmospheres. So if we were in a depth of water that would have needed five atmospheres of face pressure we could reduce that to three - which was a huge benefit because that is an acceptable pressure for men to work in. So that allowed the personnel to enter the cutterhead under compressed air and maintain the TBM cutters. “Cross passages were a particular
problem on Storebælt and virtually every known ground treatment was used to consolidate the ground. This included vacuum wells, freezing with brine, chemical grouts and silicates and microfine cements.
44 | February 2025
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