A tunnel measures how much light gets through its own air, and a drop means smoke, and smoke means people are about to die if nobody acts. Bay 7 drops to fifteen percent obscuration at four in the morning with no vehicle in the tunnel.
I am a tunnel systems engineer. I look after the ventilation, the detection and the control system in two road tunnels.
What is measured and how.
Along the ceiling there are opacity meters: a light source at one side and a receiver at the other, measuring how much of the beam gets through the air. Clean air passes nearly all of it. Smoke blocks it, and the reading is given as percentage obscuration per metre.
There is also carbon monoxide monitoring for the ventilation control, and a linear heat detection cable along the whole length, which is a cable that reports a temperature at every point on it.
The tunnel is divided into bays for all of this, so an alarm tells the control room where.
An obscuration alarm at the fire threshold does a great deal at once: it closes the tunnel, drops the barriers, reverses or boosts the fans to a preset plan, and turns on the escape lighting. That is a serious intervention and it is exactly the intervention you want at a real fire.
Bay 7 in the eastbound bore reads fifteen percent obscuration for between two and five minutes at around four in the morning.
Perhaps twice a month.
Fifteen percent is not a fire alarm. Our fire threshold is higher and there is a rate of rise condition as well, which is deliberate, because diesel haze on a busy morning will lift a reading and you do not close a motorway tunnel for that.
Fifteen is the pre alarm. The control room sees it, the fans step up a level, and nothing else happens.
The tunnel at four in the morning is not empty of traffic but it is close: eight or ten vehicles an hour.
These readings happen with no vehicle in the bore. I have the loop data for every one of them, because vehicle detection is on the same system, and there is nothing in the tunnel for the two minutes either side.
What has been checked.
The meters. Both the source and the receiver in bay 7 replaced, twice, and the optics cleaned on the maintenance schedule like the rest.
An opacity meter has a self check. It moves a reference filter into the beam and confirms it reads what it should, and it does this every hour and logs it. Bay 7's self checks pass, including the ones taken during the event.
That is the part I keep coming back to. During a two minute obscuration event the meter dropped its reference filter in, read the filter correctly, and then went on reading fifteen percent of something that is not a filter.
The heat cable. Nothing. The temperature in bay 7 during these events is the temperature of the tunnel.
The carbon monoxide. Nothing above background.
So there is something in that air that blocks light and is not warm and is not combustion.
What we did in 2023, which was a proper job and cost real money.
Two portable particulate monitors were installed in bay 7 for a month, of the kind used for air quality work, measuring particle counts by size.
They caught six events.
The particle count does not move. Not at any size band, not by a percent, while the obscuration meter beside them reads fifteen.
The report from the company that did it says the two instruments cannot both be right and that the optical path is the more likely to be in error.
What we do about it.
The pre alarm is logged and the fans step up, which costs us a small amount of electricity twice a month and does nobody any harm.
I have not asked for the threshold to be raised or for bay 7 to be excluded. Somebody suggested it and I said no in a meeting and I would say it again: the day you tell a tunnel to ignore a bay is the day that bay has a fire in it.
What I do.
I walk the bore on the maintenance closure once a month and bay 7 is a bay like the others: white tiles, a light, two cameras, a hydrant, a cross passage door.
The one thing I have changed is that I do not do the walk in the direction of travel any more.
You normally walk with the traffic direction so that anything that comes through a closure comes at you from behind and is going the way you expect.
I walk against it now, from the far end back, so bay 7 is in front of me for four hundred metres rather than behind me.
The reason is April 2023, on a closure at half past three, when I was at bay 9 and the obscuration meter at bay 7 went to fifteen, and I looked back down the bore.
A tunnel under maintenance lighting is lit its whole length and you can see the far end.
There was nothing in it. The reading held for four minutes and I watched the whole thing from four hundred metres away with my own eyes on clean air, and the light at the far end of the tunnel, which is a bright rectangle of the outside world at the portal, went dim in the middle of it for as long as the reading lasted.