EXPERT COMPARES NORTH SPUR WITH SITE OF MAJOR NORWEGIAN LANDSLIDE

RISSA LANDSLIDE AND THE NORTH SPUR


Guest Post Written by Jim Gordon, P.Eng. (Retired)


On the 29th of April 1978, a marine clay landslide devastated
large areas of the rural district of Rissa in mid-Norway, near Trondheim. One
person died while 13 farms, 2 homes, a cabin and a community centre, were taken
by the clay masses.


By good fortune there were two amateur photographers in the
vicinity with movie cameras who were able to capture the earth movements. One
had to run for his life! These have been included in a very interesting 21
minute documentary entitled “The Quick Clay Landslide at Rissa – 1978 (English Commentary)” available on the Link.

It is well worth looking at, since the soil characteristics
at Rissa are similar to the soil characteristics in the North Spur, as will be
shown. It is a “must see” for anyone interested in the North Spur.



The majority of the Rissa landside occurred within 4 hours. Parts
of the slide flowed out at a rate of 30 km/hour. The slide covered about 1km along
the shore of Lake Botnen and caused small waves to flow 5km across the lake to
inundate the village of Leira, severely damaging all lakeside structures. 200
people were evacuated from the slide area which covered 330,000 square meters
(equivalent to 74 football fields), and displaced about 5 to 6 million cubic
meters of marine clay.


The Rissa documentary contains a chart (at time 16m 20s) showing
the soil properties obtained from drill holes, from which the following data
has been obtained.



For comparison, the North Spur data was obtained from the
Poster Presentation at the First International Workshop on Landslides in
Sensitive Clays, 28-30 October 2013 at the University of Laval, produced by R.
Bouchard, A. Ceballos, N. Dayan, A. Tournier and R. Llett, from the Lower
Churchill Project Geotechnical Delivery Team, Figure 12.


The soil properties are remarkably
similar, except for the undrained sheer strength, which is between 2 and 5
times stronger than the Rissa marine clays. However, from the only drill hole
data available, that for hole d2, drilled in October 1979, the log shows a
sensitive brown silty marine clay of low to medium plasticity between
elevations 10m and 28m, where the sheer strength varies between 25kPa and
37kPa, only slightly above the Rissa sheer strength.


Another measure of the similarity is
obtained from a simple re-moulding to liquefaction test undertaken on a soil
sample, as demonstrated at the beginning of the Rissa documentary, where there
is a comment after the re-moulding experiment that when the salt content drops
below 1 gram per liter, the clay has a tendency to liquefy.

_______________________________________________________________

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_______________________________________________________________

Note that the Spur clay has a very
similar salt content. This is of concern at the North Spur dam, since the salt content
will be slowly reduced as river water percolates through the dam flushing out
the salt, particularly on the clay-silt interface of the marine clay where
reservoir water passing through defects in the cut-off wall will flow along the
marine clay surface. This surface is sloping downstream, facilitating the
development of a massive slide failure.


Roberta Benefiel, President of the
Grand Riverkeepers Association, based in Goose Bay, has undertaken a similar
liquefaction experiment with the same results, but producing a more viscous
fluid similar to cake batter, instead of a fully liquid compound, indicating
the slightly higher strength of the marine clays around Muskrat. Roberta’s
video, which takes about 4 minutes to download, can be viewed here.

If the download time
is considered to be too long, an identical test producing the same results with
about the same viscosity as obtained by Roberta can be viewed on You Tube, here.

The landslide danger
at Muskrat is not new; it was pointed out by the Geological Survey of Canada in
a PowerPoint presentation to the residents of Happy Valley-Goose Bay in 2013,
which contained the comments –


·        
15
large-scale earth flow scars along valley sides of proposed Muskrat Reservoir.
·        
Represent
‘earth slide-flow’ failures, many in excess of 10M cubic meters.
·        
Failures
likely occurred rapidly, in minutes to hours.


In other words, slide characteristics
identical to the Rissa slide characteristics.


This illustrates why it is absolutely
essential to review the geotechnical design of the North Spur engineered dam –
the risk of a landslide is just too high. The review team should include
experts with extensive experience in marine clays.
_______________________________________________________________________
Editor’s Note:
Jim Gordon has authored or co-authored 90 papers and 44 articles on a large variety of subjects ranging from submergence at intakes to powerhouse concrete volume, cavitation in turbines, generator inertia and costing of hydropower projects. He has worked on 113 hydro projects, six of which received awards “for excellence in design” by the Association of Consulting Engineers of Canada. He was also awarded the Rickey Gold Medal (1989) by the American Society of Civil Engineers “for outstanding contributions to the advancement of hydroelectric engineering…”. As an independent consultant, his work assignments have ranged from investigating turbine foundation micro-movements to acting on review boards for major Canadian utilities. He has also developed software for RETScreen and HydroHelp.

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3 COMMENTS

  1. For the non-technical reader, I might point out that the salt in these clays result from when the deposits were formed in ancient times when the ocean salt was part of the process: hence the term "marine" clays. The salt acts as a cement to stabilize the clay, such that the higher the percentage of salt, the more stable it is. But as times goes on this salt gets washed out, and the marine clay gets more unstable, so land slides occur without much if any disturbance. With concrete, one can increase the strength by suitable amounts and type of cement, stone and water, and so obtain low, medium or high strength concrete. We still have existing concrete structures build in Roman times. High strength concrete is used to advantage in the concrete dams being built at Muskrat Falls. But this natural dam, the North Spur, is indeed a risky part of the containment area. The data here by Mr Gordon highlights the similarity of the clays to the Rissa site, such that a non-technical person can appreciate the risk and need for the review of the design safety. Winston Adams