Colorado Sinkhole

[GroundquestMSA]

This article deals with a sinkhole along a Colorado hwy, evidently caused by the collapse of an underlying railway tunnel.
http://cavingnews.com/20120713-experts-discover-cause-behind-deep-sinkhole-next-to-colorado-highway-us-route-24-tennessee-pass?utm_source=cavingnewswidget&utm_medium=widget&utm_campaign=cavingnewswidget

Could someone help me understand this particular quote?

engineers, maintenance supervisors, and geological experts examining the hole, now estimated at 30 meters (100 feet) deep, discovered that it was caused by a century-old railroad tunnel that collapsed decades ago.....Experts suspect that the decade long delay in the appearance of the hole is due to the soil deep down remaining frozen until very recently.

Does deep soil freeze? And is this article suggesting that the soil between surface and tunnel has been frozen for decades?

[gindling]

Sounds like they are talking permafrost but i didn't think it was present in Colorado, I wonder what the elevation of the area is. It could happen if the tunnel was similar to an ice cave in a lava field where the cave doesn't allow for circulation of air and becomes a coldtrap which eventually ends up reaching freezing temps and all infeeding groundwater or runoff turns to ice. Then some sort of event causes the cave to start circulating air and the ice begins to melt and then things start moving.

[GroundquestMSA]

In that case wouldn't they have said "due to ice in the tunnel remaining frozen until very recently?"

[NZcaver]

Interesting situation. I assume they are referring to thawing permafrost. The elevation of Tennessee Pass is around 10,000 ft and permafrost does occur in the Rockies.

[GroundquestMSA]

The elevation of Tennessee Pass is around 10,000 ft and permafrost does occur in the Rockies.

Perhaps so. Though the only permafrost that would be present in Colorado seems to be relatively temporary. Wouldn't the average annual temp. have to be below freezing to create permanent permafrost? I suppose I'll look at the yearly temps. for that area.

[GroundquestMSA]

It seems that this area is rather chilly, but it also gets a fair amount of snowfall, which would seem to insulate the ground a bit.

Interestingly, an article in the Denver post says that the hole is a mere 50' - 60' deep and was caused by timber failure.