JOHN CORCORAN | INTERVIEW


not widely known to the public. It is now partly operational and it’s going to make a huge difference to London and to the whole country. A 25-year vision realised.” He spent almost 12 years working on


London’s ‘Super Sewer’ - six years on London Tideway West and six years on the Lee Tunnel. That works out at 32km of large- diameter tunnel, at mostly 7.2m in finished diameter. “Lee Tunnel was particularly challenging.


Above: John with Kevin Keegan


That was one of the deepest tunnels constructed in the UK, certainly in the London area; we had a few surprises along the way,” he says. “It was about 75 metres down at the


“We had planned to take it by road,


from London Docks down the A21; but the TBM was 6.5 metres in diameter and the single carriageway road was just too narrow. Hastings doesn’t really have a harbour, but it has a fishing fleet, of quite big boats, that they pull up on winches across the shingle beach every time they land; and we took the TBM in the same way. We had to negotiate financially with the fishermen. “It was quite spectacular. Big crowds


came out to watch.” He adds that “it went so well that after completing the drive we returned the TBM to Herrenknecht the same way.”


In 1999, he went back to Heathrow. He


worked on the six tunnels required to form the transport links to Terminal 5, together with the Storm Water Outfall Tunnel. “T5 was a hugely successful construction


project, that almost everyone I speak to about thoroughly enjoyed. It was constructed on time, and close to the original budget. It demonstrated what we can achieve in this country, when client,


contractor and designer are fully aligned and focus their energies on the end goal. It’s no coincidence that those jobs people enjoy working on are the most successful.” Next, he was off to Belfast. “After T5, I spent a very enjoyable 18


months in Belfast, where the ground was highly variable, and certainly nothing like the stability of London Clay that I had been used to. It demonstrated to me just how aggressive the ground can be, and how important it is, to ‘not let the ground know you are there.’” He adds - “20 metres of highly fluid


ground has a way of finding its way to the atmospheric pressure within a tunnel, at a remarkably quick rate.” What of how tunnelling is perceived? “I think the perception of tunnelling


could be better, both by the decision makers in Parliament, and the public. We need to promote those projects that go well. Tideway, for example, has been a fantastic project and a great example of efficient construction, which is probably


deepest point, excavated using an 8.8m OD Herrenknecht slurry TBM. We were operating at up to six bar working pressure. The Site Information indicated a full face of Chalk with over 50 metres of competent ground above to the River Thames. Then, early one morning River Terrace deposits were seen by the operators of the Slurry Treatment Plant. We had encountered a Drift Filled Hollow - a relatively narrow funnel of hydrostatically-charged loose granular material. “These Drift Filled Hollows are dotted


all over London and are a huge risk to tunnelling, and obviously the deeper you are the more pressure you’re going to get from them. Thankfully, we handled it well. We were at the right operating pressures, we made the right decision to keep going and get out of the bad ground and back into competent chalk. It reinforced my belief that, in tunnelling, you ignore the fundamental operating rules at your peril. “Whilst working at Lee Tunnel I decided


to put my name forward to join the BTS Committee. I felt that tunnelling had been good to me, and I wanted to put something back. I was fortunate enough to be selected. “I remember at my first Committee


meeting. I was surrounded by all these people I had heard about and admired. I remember I was asked one simple question by the Chair, Bob Ibell, and I went bright red. “But, as I gradually became more familiar


Above: Hammersmith team on London Tideway


with it, I really enjoyed the opportunity to get to know a lot of people, from different areas of the industry. I ended up organising the evening meetings, which I selected. I found that the best presentations were a combination of design and a description of the construction works. The evening lectures are a great way to share learning to the wider industry.”


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