OS Grid Ref: NH 22393 10846 (access tunnel portal)
Date opened: 1959
Date closed : Operational
There are a couple of construction adits leading to the low-pressure tunnel
at Ceannacroc. Here's Adit 1, not too far downstream from the dam:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
And here's Adit 2, below the surge chamber, on the hill above the power
station. Adit 2 is MUCH more fun:
Illustration: Ceannacroc Adit
Illustration by: Scanned by Mike Ross, from Proceedings of the Institution
of Civil Engineers, Sept. 1958
Adit 2 was much more fun because, thanks to a very nice chap who was working
in it during the refurbishment, I got inside and got the full guided tour!
Inside the adit, looking back out to a small circle of daylight - the railing
is to stop you falling in the 'ditch' which carries seepage water from
the tunnel:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Turning round 180 degrees from the above picture, we see the end of the
adit: a solid concrete plug, with a very heavy bolted cover - don't forget,
99.9% of the time the tunnel beyond this point is flooded with 150ft head
of water:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
A little bird told me it was worth my while coming back the next day...
so I did, and sure as hell the cover was off the end plug of the adit,
and work was going on in the dewatered tunnel inside. Here you can see
the pipes draining seepage water from the tunnel, and discharging it into
the 'ditch' to the right of the railings:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Here follows a couple of drawings of the adit which I photographed (badly)...
in plan:
Photo: Ceannacroc Adit
Photo by: Mike Ross
...and in elevation.You can see it's not quite 'as built' - this shows
the entire adit as lined, whilst there is in fact an unlined section:
Photo: Ceannacroc Adit
Photo by: Mike Ross
A closer view of the end plug - you can see the pipe running through the
thick concrete plug:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Stooping through the steel pipe and the rest of the concrete tube, entering
the terminal section of the adit, turning round and looking back. Beyond
the plug, the adit is unlined. All this section is normally totally flooded
to the roof of course. Looking back through the plug, we can see the circular
section of the steel pipe, the white davit for lifting off the end plug,
the railings in the adit, and, in the far distance, a circle of daylight:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Moving a little further down the unlined section, still looking back -
you can see the circle of the pipe through the plug, the bare rock walls...
and my Trusty Native Guide. Apologies for the poor quality of the shots;
I was handheld, I only had a digital point & shoot with a tiny flash,
and the walls absolutely soaked up what little light there was. If I had
the equipment and experience of underground photography I have now, things
would have been much better. Crying shame, it'll probably be another 50
years before they do a refurbishment like this again:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Turning round 180 degrees from the above shot... the adit ends in a concrete
wall. This is the lining of low-pressure tunnel itself; the small square
hole on the left where the light is gives access to the tunnel:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
Passing through the hole (clearly visible in the photo below), we're now
in the tunnel itself. The dim light from the lamp illuminates the sandbags
which mark the safe route around a sump in the floor of the tunnel. From
this sump the seepage water leaking into the empty tunnel drains into the
adit, and thence outside:
Photo: Ceannacroc Adit
Photo by: Mike Ross
Moving a few feet downstream, and then looking back upstream. The tunnel
is fully lined of course, horseshoe in section, 12ft equivalent diameter
and 14,000ft long. You can just make out the square hole from which we
emerged on the left hand side of the tunnel, and the figures of my Trusty
Guides. Used the (almost useless) camera flash on this shot, and got a
lot of backscatter from the moisture in the air:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
Turning round 180 degrees from the above shot, and looking downstream.
The walls of the tunnel are somewhat clearer, and a short distance downstream
things get a lot brighter. That's the bottom of the surge shaft; the tunnel
transitions to a rectangular section, to accomodate the control gate at
the bottom of the surge shaft, and the prominent metal screen you can see
protects the top of the vertical high-pressure shaft which takes the water
the final drop to the power station below:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
Heading dowstream. You can clearly see the daylight flooding down the surge shaft, illuminating the screen protecting the top of the HP shaft, and the staples in the right wall for climbing into the surge shaft itself. The scaffolding is there as there was refurbishment work going on with the control gate, which allows the flow from the LP tunnel into the surge shaft to be shut off. From here I went on to explore the surge shaft, and you can too, by clicking
HERE:
Photo: Ceannacroc Adit
Photo by: Mike
Ross
