A Short Trip with Multiple Destinations:
Destination 3: Underground Research Center at Tournemire
Part Two: Going Underground
In Part One we introduced the IRSN (L'institut de radioprotection et de sûreté nucléaire) as our host and as the owner, operator of this underground research facility. We will now go underground.
Going Underground
After some very fine refreshments (I recommend the French as hosts, really!), we went underground. The tunnel was built by the military long ago to allow troops to be taken to and from a base on the plateau behind this mountain:
At the entryway are posters giving the geological history of this location:
This translates to a deep clay layer with overlying limestones (argillites) --this poster shows the activity at the time of side-tunnel construction by IRSN:
Since the military wants to maintain access, the IRSN does not obstruct the trail line in any way:
This means IRSN has to do its work in side tunnels. As we approach the side tunnels, there is daylight visible at the other end of the rail tunnel:
The first side tunnel's experimental area looks like this (only one direction shown):
The experimental area in the second side tunnel looks like this:
In the other direction, the tunnels are longer to allow more experiments to be set up:
What sort of experiments? looking for, finding, and age dating the water in the clay and the fracture-filling materials in old fractures, for example:
There appear to be some parallel open fractures in this flow, perhaps due to stress relief because of the excavation, that cut across some filled, ancient fractures.
This is what some of the measuring apparatus looks like:
The idea is to figure out when the last time was that these fractures saw water:
It appears that the water that brought in the whitish materials that sealed these old fractures was from the time that rivers and karst features (openings in limestone terrain made by water) were developing in the rock above. The water is over 20 million years old. No fast pathways are likely to be able to take water from the limestones above and allow them to move through these thick beds of claystone.
The claystone does fracture around excavation openings where pressure is relived. Here is a "window" showing the fracturing parallel to the main rail opening, and that is does not go very far into the intact rock (second window). Similar fracturing can be expected around the drift in which this window is located.
The way stress is relived by the excavation is not simple, however. On one side of these tunnels the walls are very smooth all the way to the bottom, while on the other side they fracture near the bottom (next 2 photos):
This research is being done to inform French agencies about potential issues with using this type of rock to dispose of radioactive waste. So far, what is being found here is giving insights into fracturing and water flow, and is not calling the choice of hard clay as a repository host-rock into question.
GO BACK to Part One: Getting There OR:
OTHER DESTINATIONS FOR THIS MARCH 2009 TRIP
Destination 4: The Gorges of the Tarn (Introduction)
(with 10 Sub-destination Pages)
Go Back to Destination 1: La Grande Motte
Go Back to Destination 2: Millau and Its Bridge
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