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Planting a tunnel portal is easy if it is a new plant. Just glue it down and build the mountain around the tunnel. When I did my first portal at this location I found it was easy to get the divide between the ceiling tile rocks and the portal. The line between rock and portal looked very real.
Click here to build the original rockface
This was the tunnel portal in the "OO/HO in the 1870's" period. It was made from a HO Woodland Scenics portal. The portal was narrow enough that blending the rockface into the tunnel was reasonably easy.
Click here to plant the second portal
I didn't want to lose as much rockwork as in the OO/HO portal so I made the opening about 3/8" bigger than the portal. I was hoping that ceiling tiles could be carved to fill the gap.
I located the tunnel and ran trains thorough to make sure there was clearance. Look at every aspect of clearance. My Bachmann outside frame 2-8-0 pilot just clears by a 1/16 of an inch going into the tunnel. I was more worried about the cab in backing.

I spot glued the back of the tunnel with clear hobby glue and allowed that to dry. Run a train through before it dries as a double check.
A piece of ceiling tile was sliced down to fit between the portal and the rockwork.
The portal and rockwork were white glued to the ceiling tile.
A gray paver sand was used for the ballast. It would bond as a darker gray.
The ballast was sprayed with "wet water". A 50-50 mixture of white glue and water was applied to hold the ballast.
I use a mini-baster I  found at Wal-mart in the kitchen section to apply the white glue mixture. It is just the right size.
The tile piece was roughly carved to match the rockface contour. I applied white glue to the transition and pulled the loose tile fibers over into the glue. This blended the tile into the original rockface.
Bits of ceiling tile were glued to the rockface to build up the contour. These were carved to match the rockface pattern.
The other side was done the same with bits added to cross the gap.
I sprayed the tiles with the household cleaner Awesome. The tiles became mushy and could be carved with a spatula.
The tiles were primed with the Kilz2 primer to seal them.
I painted the primed surface with the base earth color latex paint.
The rocks were colored with tube and craft acrylic paints to match the old rockwork. An black india ink/alcohol wash was added to emphasize the texture.
Paver sand was placed on the top of the portal and glued with white/water mixture.
The pebbles were painted with earth acrylics to match the rocks. The sand was washed with acrylics and India ink/alcohol also.
Foliage will be added in the sand areas and crevices.

The rockwork will be adjusted colorwise over a period of time.