This was also a polyurethane foam closure with a concrete cap. It also had a corrugated steel ventilation pipe. These were constructed to blend in with the natural environment and protect the wildlife (endangered bats) living underground in the mine.
Shown here are other mine closure techniques to allow for the irregular shapes of the mine openings. The steel supports were attached to the Adit walls by drilling and installing steel dowels to weld the angle iron structures to.