The first bunker, the entrance bunker was situated under the floor of the shed, had to be connected with a tunnel passing under the weakened foundations. This tunnel had to be properly reinforced to prevent a new collapse. The problem was how to reinforce the tunnel. A steel plate was needed. The ingenuity of the people in danger is difficult to describe.
Necessity is the mother of invention.
An amazing solution was found. It was a heavy oak wood barrel used for collecting of rain water. You cut an opening through the bottom of the barrel and you have an age old tested tunnel. Within one week from the start of the construction, the bunker was completed. Now, the retaining wall removed and the rubble and bricks scattered over the surface, to provide additional masking.