The new London Power Tunnels

[dan_b]

Channel 4 feature on the National Grid and the new London Power Tunnels

[resybaby]

Always amazes me how all these new tunnels under cities dont get in each others way, the mapping of them must be a monster headache, i do have a tale of old on the same subject.

Back in my mid 20's i had a team of Water Utility contractors digging out some existing pipework in the old Masshouse Circus roundabout right in the centre of Brum for me, the main needed replacing, i was on site supervising.

Anyway the kango was bashing away at the pavement foundations, around 250mm down and suddenly it disappeared from view, the guy operating it only just about letting go in time, the yellow air hose snapped.
Gingerly the rest of us crept up to the scene to find that the kango had gone through the roof of an old tunnel and was lying on the floor some 50 odd feet below clearly visible in a dust cloud! The tunnel was so huge and you couldnt see anything but its base, easily drive an artic in it, easy.

The tunnel we could see when peering through the kango sized hole was running back under the road we were standing on, as well as heading towards an adjacent multi storey office building.

Got the lads to quickly block the traffic on the road (a major bus/traffic route) that the tunnel ran towards and called the city engineers out in a more than slight panic. My boss, Ken, had twins i swear when i contacted him.

More yellow flouresent jackets than a lollipop person convention appeared rather quickly with lots of folks scratching their heads looking at various maps pointing at all and sundry with worried looks on faces.

Turns out it was the deadend of a short tunnel nobody had any records of at all.

Fortunately the engineers found it stopped short of the office building once they had specialists enter it from the by then enlarged kango hole with ropes, but it did go underneath the adjacent highway. Somebody took the decision traffic could continue once assessed some weeks later until then it was prevented and caused chaos.

The road layout has since those days been relevelled and the area remodelled, but whenever i pass that place on my travels i recall the mayhem landing in my lap that day, and the difficulty in keeping accurate details about such things.

The adjacent office building is still there, and more than likely Severn Trents kango is too, i cant remember if we ever got that back

[dan_b]

The location of, and sheer quantity of, other tunnels criss-crossing under London was also a huge challenge for digging Crossrail. I remember watching a documentary about that and how they literally had to snake in between two other tunnels and also not undermine the foundations of the many expensive high rise buildings above - they had margins of several inches or something ridiculous like that.

[resybaby]

Its crazy to think of the accuracy needed for such a hugely complex operation Dan.
Think when they joined up the ends of the channel tunnel, which was dug from both ends, the boring machines were only a few inches off being 100% aligned, and that was yonks back without any of the tech we have nowadays. Serious achievements.

[dan_b]

Yeah the Channel Tunnel was seriously impressive with its "laser guided" TBMs:

"Once the service tunnel TBMs stopped at 100 m distance, measured lateral offset was 358 mm; levelling : 58 mm; chainage :75 mm. Readjustment over the final 50 m, hand excavated, produced a smooth junctioning of the two drives. Running tunnels all came out within the 15 cm tolerance."

[Oliver90owner]

Yeah the Channel Tunnel was seriously impressive with its "laser guided" TBMs:

"Once the service tunnel TBMs stopped at 100 m distance, measured lateral offset was 358 mm; levelling : 58 mm; chainage :75 mm. Readjustment over the final 50 m, hand excavated, produced a smooth junctioning of the two drives. Running tunnels all came out within the 15 cm tolerance."

Yes, on likely a similar scale to the tunnels now feeding water to Rutland Water.

Lesser distance but smaller diameter tunnels. These tunnels were constructed back in the ‘70s.

While lasers most certainly helped, theodolites were accepted as extraordinarily precise, many decades previously. I have read that after 1500 miles, an expedition in India(?) returned to base with about a 150 yard discrepancy. Disappointed by that ‘excessive error’ they eventually explained the deviation was caused by a mountain that very slightly altered the gravitational force direction, which in turn affected the instrument’s vertical alignment!

[AE-NMidlands]

While lasers most certainly helped, theodolites were accepted as extraordinarily precise, many decades previously. I have read that after 1500 miles, an expedition in India(?) returned to base with about a 150 yard discrepancy. Disappointed by that ‘excessive error’ they eventually explained the deviation was caused by a mountain that very slightly altered the gravitational force direction, which in turn affected the instrument’s vertical alignment!

That's a good story, reminds me of the comment on QI that Everest was originally surveyed as an exact round number of feet. It seemed implausible and so a different number was put in... Wikipedia says

Peak XV (measured in feet) was calculated to be exactly 29,000 ft (8,839.2 m) high, but was publicly declared to be 29,002 ft (8,839.8 m) in order to avoid the impression that an exact height of 29,000 feet (8,839.2 m) was nothing more than a rounded estimate.[30] Waugh is sometimes playfully credited with being "the first person to put two feet on top of Mount Everest".[3

so maybe that is true!