Hi John
I first saw this before it was installed, then after it had been installed and before the area was backfilled.
It has an approx 10 inch diameter corrugated pipe leading into the base of it from a hole at the base of the stonework blocking the adit entrance.
The pipe then heads off under the track and presumably joins the new concrete canvas lined drainage channel at some point.
I will have a closer look this weekend, to tidy up some of the details of what is happening.
When I checked on a number of occasions a few weeks ago there was no air movement at the adit entrance.
Allan