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It’s not just the North Spur we have to worry about

Published on January 19, 2016
Telegram letters

By now most readers will have heard that the North Spur is a naturally formed dam that forces the Churchill River to veer south, around Spirit Mountain and to flow over Muskrat Falls.

On Jan. 2, Jim Gordon, in his letter “Muskrat Falls and the North Spur controversy,” wrote — and rightly so — that “If the North Spur fails, Muskrat Falls will disappear and be left high and dry. The Muskrat Hydro facility would become a stranded asset, with (if feasible) a repair cost well over several billion dollars.”

While it is true that the North Spur helps create and maintain Muskrat Falls, it is neither safe nor cost efficient to have a sole, isolated focus on the North Spur.

As the Churchill River flows eastward, it approaches a very wide U-shaped (200 metres deep) subsurface, sand/silt/clay-filled valley (only two thirds of which is made up of the natural dam called the North Spur). If any part of this much larger sand/silt/clay-filled valley (not just the North Spur) should fail, “... Muskrat Falls will disappear and be left high and dry.”

The entire subsurface valley spans a full 2,500 metres from the south side of the Trans-Labrador Highway to Spirit Mountain and the one third portion that runs from the river’s north bank to the Kettle Lakes gorge (and that is not part of the North Spur) already shows evidence of landslides.

While Nalcor has conducted borehole and other geotechnical studies along the two thirds of the valley that makes up the North Spur (spending many millions of dollars in an attempt to reduce the risk of a North Spur failure), there is little or no evidence that similar extensive geotechnical studies have ever been conducted along the remaining one third of the subsurface, sand/silt/clay-filled valley.

Since there is no evidence that Nalcor is designing a plan to stabilize the remaining one third (800 metres) of the 2,500 metre long sub-surface, sand/silt/clay-filled valley, on what basis has Nalcor been able to make a rational decision that the one third portion of the U-shaped valley running between the upstream north bank and the Kettle Lakes gorge area already has a natural safety factor equal to or better than the North Spur after stabilization?

While there appears to be little or no evidence that the north bank to Kettle Lakes gorge area is already safe, there does appear to be some evidence that the area may be less safe than previously thought.

Before 2013, Nalcor’s stratigraphic interpretations of the upstream north bank to Kettle Lakes area described the subsurface as having two (presumably more stable, coarser) sand layers between the two existing upper and the one lower clay layer — Figure 7 of Nalcor and SNC-Lavalin’s poster presentation to the 1st International Workshop on Landslides in Sensitive Clays at the University of Laval on Oct. 28-30, 2013. ( http://blog.nalcorenergy.com/wp-content/uploads/2013/10/North-Spur-Poster-Presentation-October-2013.pdf).

However, Nalcor’s post-2013 interpretation describes the same subsurface area as follows: “Figure 7 and 8 present a cross section between (the) upstream side and the Kettle Lakes. The intermediate sand strata previously encountered, is now described as a sandy-silt or silty-sand with a fine content greater than 25 per cent” (see previously referenced poster).

Furthermore, it should also be noted that although the area was previously described as having a total of only six layers of sand and clay, and while the area is now described as having a more complex total of 10 layers of sand, clay, sandy/silt or silty/sand (both silt and clay categorized in the fine particles category), and along with the above-referenced more recent 2013 interpretations, where is the analyses that confirms that this area has the same stabilized potential as previously thought?

In addition to stratification issues, will strengthening the North Spur without strengthening the north bank to Kettle Lakes area increase the risk of failure along the north bank to Kettle Lakes gorge?

Historically, the upstream water elevation generally remains below 18 metres above sea level. Accordingly, the river places no horizontal water (infiltration) pressure on any of the 2,500 metre long subsurface sand/silt/clay-filled valley that is at or above the 18 metre mark.

However, once the Muskrat Falls dams are built and the reservoir is filled to its planned 39 metre elevation, the horizontal water pressure at the 18 metre mark will increase according to the square of the increased depth of the water.

That will increase horizontal water infiltration pressure at the 18 metre mark from zero to nearly one half million pounds per metre along the length of the North Spur and the upstream north bank (and approaching almost one million pounds per metre when the reservoir reaches a maximum flood level of 45 metres).

Furthermore, the North Spur stabilization works are designed to keep water in the north bank to Kettle Lakes gorge area from infiltrating the North Spur. Would this not impede the north bank to Kettle Lakes area from keeping its saturation at a safe level?

If the North Spur stabilization fails, Muskrat Falls will disappear and be left high and dry. But if stabilization helps ensure that the North Spur does not fail and instead the north bank to Kettle Lakes gorge area fails, Muskrat Falls will still disappear and be left high and dry.

For these and other reasons, and in addition to Mr. Gordon’s request that the government’s review be “... expanded to include the geotechnical design of the North Spur dam...,” I would also ask that government include in its review any potential risk/safety and cost implications associated with the entire subsurface, sand/silt/clay-filled valley that extends from the south side of the Trans-Labrador Highway to Spirit Mountain, or as a minimum, from the south side of the Trans-Labrador Highway to where Nalcor’s North Spur stabilization works turn northeastward, away from the reservoir’s north bank.

 

Maurice E. Adams

Paradise