Niagara Tunnel: Doing It Right - Part 2

My first boreholes indicate high horizontal stresses, but I need to get a handle on this. Shale is a very different animal under high stress. On the north shore of Lake Ontario, most of the stresses have been relieved, so it is more like a soil. You excavate with the knowledge that air and water will deteriorate it further, so you cover it up as soon as possible.

This leads to a bit of a corundum when you get to higher stresses, since our experience shows that you must let the rock 'relax' for at least 6 months before you pour in the final lining. Thus, it is vital to beef up the science of this.

I am using my existing boreholes and going with hydrofracturing to determine the magnitude and polarity of the horizontal stress. We had done that successfully with our Darlington deep hole. There, we had drilled quite deep, right into the Precambrian, and had done stress measurements. This was a far contrast from the kiddy-holes they drilled at Bruce for the radioactive storage.

Hydrofracturing involves packing off a section of the hole, and pressurizing it with a fluid. At some point the rock fractures, and two cracks propagate away from the hole. With careful measuring of the pressure, you get a measurment of the stress that is holding the cracks together. At Darlington, we got moderate stresses in the Paleozic, with no strong polarity. On the other hand, the Precambrian was quite surprising, with a high value of stress, and a strong NE orientation. In fact the degree of polarity affected the results which can only measure oriented stress, to a maximum of twice the minimum stress. My suspicion was the Precambrian stress was much higher than measured.

But I digress...

My measurements now show me that we are in a band of very high, oriented horizontal stress, much like the Darlington Precambrian (remember this is a fantasy!). I am now in a pickle. We have handled high stresses before. I have visited uranium mines in the north with a very high horizontal stress. They have blasted out huge openings in an elliptical shape that were remarkably stable. I, therefore, am going with a flat ellipse for the opening.

On the other hand, we have ventured into 'horrendous' stress levels. This was encountered in the lower levels of the Underground Research Laboratory (URL) up in Pinawa, Manitoba. You can imagine solid granite virtually exploding into disks the moment you start to drill. Nobody hung around that tunnel very long, and it was soon closed. If the shale was that bad, we would have to adopt extreme measures.

(to be continued)