Stability of the North Spur at Muskrat Falls
Stig Bernander and Lennart Elfgren Luleå University of Technology, Sweden
stig.bernander@telia.com lennart.elfgren@ltu.se
Abstract
The paper presents the geotechnical background to one of the stability problems regarding the North Spur dam wall: This land was formed in the regression of the sea during and after the last ice age with deposits of multiple layers of silty sands and silty sandy clays that formed the valleys and plains that are now above sea level.
Some of these layers, deposited thousands of years ago in post-glacial times, are vulnerable to liquefaction when they are disturbed. These conditions have in the past repeatedly caused slides along the banks of the Churchill river.
In the current paper, a specific type of possible progressive failure –the most
dangerous one in respect of the safety of the North Spur – is discussed. This type of landslide development may be caused by the rising water pressure, when - or after - the dam is impounded. As will be explained, such a slide could force part of the North Spur ridge to slide along a failure surface sloping East-wards into the deep river whirlpool downstream of Muskrat Falls.
In the following, we provide a brief overview of the geotechnical background behind our concerns, also discussing methods of mitigating the risk of the kind of slope failure in question. Hence, we propose measures such as compacting the soil by piling or by methods of grouting and drainage. We also suggest the need for an expert Advisory Panel to look further into the long-term safety of the North Spur.
Background
In this section we provide an overview of the main issues concerning the risk of a forward progressive failure in the North Spur at Muskrat Falls.
Stig Bernander’s interest in the question of the North Spur’s stability arose in 2013, when he took part in an International Workshop on Landslides in Sensitive Clays in the City of Quebec in Canada (Bouchard et al., 2013, L´Heureux et al., 2014). He was there approached by Mr. Cabot Martin, (Luca Resources Inc, St. John’s), who had already raised questions about possible risks related to the planned North Spur part of the dam wall at the Muskrat Falls hydroelectric generating facilities in the Lower Churchill River in Labrador/NL, Canada. The works were to be carried out by Nalcor Energy, a provincial corporation (the client) with the use of SNC-Lavalin Group Inc. (SLI) as the main geotechnical consultant.
Bernander was subsequently invited to visit the Muskrat Falls’ area by the Grand
Riverkeeper Labrador Inc. (The claim recently made by Nalcor CEO Stan Marshall,
that Bernander has never actually visited the North Spur, is thus totally false,
(Marshall, 2019)). In fact, Bernander carried out an extensive study of the whole area in October 2014 including aerial surveillance by helicopter, ground surveys by car and riverbank landings by boat. He also gave two lectures, in St. John’s, on the topic of progressive landslide risks, summarizing his earlier works on progressive slope failures (Bernander, 1978 - 2011). Dr. Bernander then wrote a report on the possible hazards associated with the project (Bernander, 2015). In two additional reports in 2016 (Bernander, 2016a, b), he provided further critical comments on the Nalcor – SLI Engineering Report (Ceballos, 2016, early version 21 Dec 2015) regarding the North Spur natural dam wall structural design. He also commented on the
progressive failure analyses made on the downstream Eastern side of the North Spur that had been prepared for Nalcor by SLI (Leahy 2015a, b).
The location of the studied riverbank at Muskrat Falls in Churchill River Valley is given in Figure 1, and a view of the Muskrat Falls and the North Spur is shown in Figure 2.
Figure 1. Map of the Northern Hemisphere with Churchill River in Canada and Luleå in Sweden marked with red circles
.
Both regions were covered by ice during the last ice-age that ended some ten thousand years ago, and have similar problems with landslides.Figure 2. Muskrat Falls with the North Spur. The Spur ends with a massive protuberant granite rock structure close to the falls known as the Rock Knoll. Google Earth Sept 27, 2014.
Downward progressive failures
In this section, we present a brief synopsis of the science behind our concerns.
The stability conditions in natural slopes, like the North Spur, are closely related to their geological and hydrological history. Slopes of clay (particle size less than 0,002 mm) and silt (particle size 0.002 to 0,63 mm) are made up of glacial and post-glacial marine deposits. These deposits emerged thousands of years ago, after the last glacial period. As the glaciers retreated and the land rose, the sea regressed despite the simultaneous worldwide rise of sea-water levels. The sediments in the bottoms of seas and fjords formed deep layers of clays, silts and sands. These post-glacial deposits may today be found high above the present sea level, in what now are valleys and plains. The North Spur soils were normally deposited in mildly sloping
layers, as winter flows alternated with summer spring floods. The deposition and ongoing erosion and sliding differed year by year due to the variability in flood flows and changing geophysical conditions. This has resulted in layers with mixedproportions of clays, silts and sands thus rendering widely varying soil composition and shear strength.
As the ground gradually rose above the sea level, the strength properties of the
deposited soils have slowly changed, but in different ways. Consolidation and
ongoing creep movement over time have taken place to accommodate increasing
loads, due to changing hydrological conditions, such as the formation of the Churchill
River Valley. However, thick water-saturated layers of extremely porous sands, and
especially very porous silty sands, are – according to Tables 4:1 and 4:3 in Ceballos
(2016), – richly represented in both the Upper and the Lower Clay formations. Highly
porous soils of this kind are denoted as being ‘metastable’ because, when subjected
to additional shear deformation, they radically tend to lose shear resistance, and that
often even into a state of liquefaction, see e.g. Terzaghi & Peck (1976), Terzaghi et al. (1996).
The properties of different soil layers in the dam ridge at the site may vary
considerably from loosely layered highly porous silty sands and clayey silts to clays that, to some extent, may be over-consolidated. Yet, and this constitutes a crucial issue, many of these layers are vulnerable to liquefaction when they are disturbed as is also clearly documented by the many slides that have already occurred on the banks of the Churchill River as a result of all these post-glacial processes.
There are three main types of slides that may be of interest in the current context, see Figure 3:
Figure 3. Three types of landslide failure:
(a) Serial retrogressive failure with debris flow.
(b) Forward or downhill progressive failure.
(c) Uphill retrogressive slide, often called
´spread´.
The figure is based on Locat et al. (2011).
Type (a) Serial retrogressive slides, usually resulting in massive clay flow. They are
normally related to the loss of lateral support due to just previously occurred slides of any sort. They are common in both Scandinavia and Canada as well as in the
Churchill River Valley.
Type (b) Forward or downhill progressive landslides. These are triggered by an uphill
additional force or instability. These landslides are more common in Scandinavia,
where the sensitive clays are normally not highly overconsolidated. Yet, Type (b)
slides may also occur in Canada as, for instance at Saint-Fabien in Québec (2004).
Type (c) Uphill retrogressive landslides (in Canada termed ‘spreads’). They are
common in the highly overconsolidated clays in Eastern Canada but do not seem to be a likely risk agent in the Churchill River Valley because of the lack of highly overconsolidated soils according to SLI’s own assessments.
In the following, the discussion will be focused on slides of Type (b). An analytical method of studying the different phases of this type of landslide failure was first presented in Toronto, (Bernander 1984). Further studies have been made by e.g.
Bernander et al. (1984 - 2016), Quinn (2009), Locat et al. (2011, 2013, 2015) and Wang & Hawlader (2017).
The resulting residual shear resistance of sensitive clays, and especially that of water-saturated ‘metastable’ soils under undrained conditions, is highly dependent on the nature (i.e. magnitude and rapidity) of the disturbing agent. This is illustrated in Figure 4, where the shear stress (local force per unit area, e.g. m
2) is given as a function of the shear deformation (angular change)
for the Upper Silty, Sandy Clays in the North Spur. In the figure, the peak shear strength s
uvaries between 35 and 135 kPa while the residual values s
uR in the remolded condition is as low as 2 and 3,75kPa, as given by SLI, Leahy (2015a) and Ceballos (2016). As no stress/deformation relationships have been provided by SLI, the strain values related to the peak
resistances were assumed to be 3.75% and 7.5 % respectively, as derived from
‘back-analyses’ of Scandinavian landslides that have previously occurred in sensitive clay formations. The values of the deformation related to the residual shear stress s
uRhave been estimated in Figure 4 (Bernander & Elfgren 2017, 2018a, b).
Figure 4. Variations in the relationships between the shear stress τ (horizontal force per area) and the shear strain (angular deformation) for a small element as the square in the top right of the figure. The curves are valid for the Upper Silty Clays in the North Spur according to Leahy (2015a) and Ceballos (2016). The blue curves relate to stiff soil layers, while red curves relate to weak soil layers. As no deformation properties have been
presented in the SLI reports, the shear strain values have been derived by back analyses of other slides.
Applying the soil properties in Figure 4, the probability for a progressive failure of Type (b) is estimated to be inherent – the Safety Factor (SF) being less than 1. The Safety Factor is here defined as the ratio of the resisting force R of the soil to the active force N, potentially causing failure, i.e. SF = R/N (Normally in Soil Mechanics a reliable safety factor of SF = 1.5 to 1.6 would be considered as a necessary
requirement.)
Figure 5. A possible downhill progressive failure of Type (b) would start at the upstream Western slope (to the left) when the water- level in the dam is raised (dark blue area). The enormous horizontal thrust due to impoundment (red thin arrows) will induce a radical change of shear deformations in the Western edge of the Spur soil structure. This constitutes a precarious condition in very sensitive - or metastable - soil structures, often leading to massive landslide failures (red thick arrow). In the current case, failure may progress in the downstream direction, and finally, the whole ridge may slide into the 70 m deep downstream river basin (dashed red contour of thick arrow). (From
http://muskratfalls.nalcorenergy.com with added red arrows).
We believe that Nalcor’s research of the soil conditions at Muskrat Falls has been
insufficient, as no stress/deformation relationships related to totally undrained soil
conditions of the saturated very porous soils seem to have been applied in their
studies. This indicates to us that, as regards stress/deformation properties under
undrained conditions, the standard of the geotechnical investigations has been very poor.
The importance of these kinds of studies have, for instance, been underlined by the Mount Polley Dam failure in British Columbia on August 4, 2014 (Mount Polley, 2015). The Marino dam failure (November 5, 2015) in Brazil, and worst of all the Brumadinho dam disaster (January 25, 2019) also in Brazil, show that the LEM method is, by far, not able to model progressive slides correctly in metastable and sensitive soils. At Brumadinho some 300 workers lost their lives.
An example of how a progressive failure may be triggered in the North Spur is briefly given in Figure 6.
Figure 6. The figure illustrates the possible initiation of a progressive downward failure with a depicted deformation at the foot of the upstream Western slope of the North Spur. The figure is based on Leahy et al (2017) but to it has been added the force N caused by the rising water level and the resulting acting shear stress condition τ (red arrows) that has to be balanced by the shear resistance suR (green arrows) in the disturbed soil along a possible inclined slip surface (red dotted line). Now, if the residual shear resistance suR – related to the deformations due to the raised water pressure (dashed blue lines) – in any of the metastable soil layers falls below the currently active shear stress τ (red arrows) – a progressive failure is likely to initiate along the shown inclined slip surface (or along another one with less resistance. (Nature will find the weakest link in the chain). During the continued progressive failure process, massive kinetic energy is accumulated, eventually forming an irresistible effect on the measures for stabilizing the Eastern slope of the spur.
There are also metastable, highly sensitive, soil layers under the Upper Clay, in the in
the Lower Clay. This signifies that there is potential risk of progressive Type (b)
failures also in this soil structure, especially since the slope of failure planes may
there develop much more steeply than in the thinner Upper Clay sediments. This is
further discussed in Bernander (2017), section 4.2, p.9, and in Bernander-Elfgren
(2018a), section 2.3, p.12.
By way of analogy, one might consider a row of cars parked during the night on an icy road sloping downwards. If the top car has smooth tires, it may start sliding at sun rise when the temperature rises and the ice melts. It may then hit the next car,
sending it into a sliding motion. This event may render a domino effect, resulting in a long row of cars sliding downward with massively increasing kinetic energy.
We may, of course, be so fortunate that no ‘slip surface’ actually proves to be weak enough to let a failure be initiated during the impoundment of the dam. However, waves due to a large upstream landslide may suddenly result in a heavy load increase. Massive slides – having occurred many times before – may cause a flood wave that will travel down-stream, hitting the North Spur Dam and initiate a failure of Type (b).
Another serious disturbance could be earthquake activity. The study made by Leahy (2015b) uses average stress values neglecting the low resistance values in Figure 4.
Moreover, as it is not known where the weak soil lenses are actually located, there is an obvious risk that one of them close to the Western rim may initiate a Type (b) downhill progressive failure.
About efforts to enlighten the client, the consultants and the authorities
In this section we describe how hard it can be to get new knowledge accepted.
Bernander’s work on the stability of the North Spur was the focus of a poster panel at the 2
ndInternational Workshop on Landslides in Sensitive Clays in Trondheim in June 2017 (Bernander et al. 2017, Dury et al. 2017, Thakur et al. 2017). Robin Dury, a graduate student at Luleå University of Technology presented his MSc research on the risk of progressive failure initiated at the up-stream West side of the natural dam wall. Stig Bernander also presented results from a few investigatory analyses of the same type of progressive failure based on stress/strain relationships determined by back-analysis of large landslides in Scandinavian sensitive clay formations (App. IV in Bernander & Elfgren, 2018a)
At the same workshop, a paper on the North Spur stabilization works (Leahy et al.
2017) was presented by Regis Bouchard, SLI. However, all the SLI results have been based on the Limit Equilibrium Mode of analysis (LEM), which has little relevance in the current context as it does not consider the softening, disturbed, part of the stress- strain diagram in Figure 4. During the discussion of the paper, it was pointed out that possible downwards progressive failures ought to have been considered.
During the years 2014-2016, when Bernander’s concerns were first raised, the response given from SNC-Lavalin, has merely reaffirmed that everything is fine regarding the stability of the North Spur. In 2017, Bernander and Elfgren published two further reports discussing mitigation measures to prevent dam breach
(Bernander & Elfgren 2017, Bernander Oct. 2017). These reports were sent to Regis
Bouchard at SLI and others, and were also published on the web (http://ltu.diva-
portal.org/smash/).Yet, no response was ever received from SLI or Nalcor.
Finally, in 2017, Nalcor commissioned a scientific Geotechnical Peer Review Panel (GPRP) to investigate the issues raised by Bernander’s and Elfgren’s research, and to address their concerns about the safety of the natural dam wall structure.
However, the peer-review panel did not correctly respond to any of the modern Research and Development (R & D)-methods, or even to basic corrective measures in the SLI analyses, recommended by Bernander and Elfgren. Instead, they merely recapitulated the previous SLI methodology that Bernander and Elfgren had seriously questioned. This approach was clearly evidenced by the introduction to the GPRP- Report (2018).
Quotation: “The report is based on information made available to the Geotechnical
Peer Review Panel (GPRP) by SNC-Lavalin Inc. (SLI) and the Client. The GPRP has not performed any calculations verifying the accuracy, completeness or validity of the results obtained by SLI. The opinion of the GPRP is solely based on a review of available data and on the concept and methods used by SLI and the client to assess stability issues at the North Spur. Therefore, the GPRP makes no representation regarding the accuracy and hereby disclaim any liability in connection therewith.”
This is understandable. Considering that all SLI stability analyses have been based on the Limit Equilibrium Mode (LEM), their results, as has already repeatedly been stressed, has little relevance in the current context.
As regards possible progressive failures of Type (b) – related to the massive
hydraulic pressure on the Western slope – the GPRP did not even comment on the obvious error in the SLI analyses, namely the fact that they were solely based on
horizontal failure planes. Moreover, this hydraulic pressure cannot just vanish butwill be transmitted to the metastable soils near the Western rim - either directly by the
‘cut-off wall’ or by seepage friction.
Conclusively we maintain that, the GPRP panel did not either address – or even refer to Bernander’s report of October 23, 2017 titled:” Summing up of North Spur
stability issues”, This document deals with, refutes and explains many of the main
issues that had been brought up by the Geotechnical Peer Review Panel (GPRP, 2018).
The consultant SLI has continued to disregard modern R&D approaches in its construction report (Ceballos 2019), and in its post-construction assessment, see Rattue (2018) and Bouchard (2019).
We have also expressed our concerns in letters to the Hon. Ms. Siobhan Coady, Minister at the Department of Natural Resources in the Government of Newfoundland and Labrador, Canada, (Bernander & Elfgren, 2019a). Serious criticism regarding the assessment of the North Spur stability has also been made public by Mr. James L Gordon, hydropower consultant (Gordon, 2019), through the Uncle Gnarley Blogspot (http://unclegnarley.blogspot.com/) and in the hearings of the Commission of Inquiry Respecting the Muskrat Falls Project, (https://www.muskratfallsinquiry.ca/).
We are of course aware that you cannot teach new tricks to old dogs and that a paradigm shift takes time. Yet, it constitutes a failure to mankind when disasters such as the Brumadinho dam failure (Brazil Jan. 2019) are needed before new R&D-
methodology is taken for granted.
Possible mitigation measures
In this section, options to avoid possible downhill progressive disasters of Type (b) are summarized.
Our analysis suggests that the properties of metastable soils and sensitive clays in the soil layers of the North Spur structure represent a significant risk of a Type (b) forward downhill progressive landslide failure. We suggest that certain measures should be put into effect in order to lessen the hazards of post-flooding dam breakage
.Methods for investigating the soils, and establishing valid geotechnical Safety Factors, which have already been proposed by Bernander (2016b) include testing, compaction, grouting and drainage, see Bernander (2017), Bernander &
Elfgren (2018a-c, 2019a, b).
Yet, given the currently prevailing conditions at Muskrat Falls, the most effective and practically possible mitigation measure appears to be drainage or careful (long-term) compaction of critical water-saturated metastable soil layers, as well as seams of highly sensitive clays. This would ensure that the shear resistance in sensitive layers would increase - roughly speaking in the way friction will increase in a motor if the oil were removed.
Conclusion
The crucial and main point of our criticism is that the analyses presented in the SLI reports have been based on the Limited Equilibrium Mode of analysis (LEM), which in terms of modern Research & Development is totally irrelevant for the types of soil making up the North Spur. No dependable factors of safety (SF) based on undrained soil conditions have been established simply because of the lack of tests defining valid stress/deformation relationships. Furthermore, the decisive effect on slope stability of inclining failure surfaces has in some assessments been totally disregarded.
Yet, even if the water level, related to the intended impoundment, were to be attained without failure, the effects of drastically raised flood levels due to up-river landslides in the future must be correctly investigated. The effects of seismic tremor must also be determined. In both of these cases, the Limited Equilibrium Mode of analysis, and corresponding stress/strain relationships, may not be applied when calculating valid factors of safety. The world history of structures on foundations of metastable soils is full of disastrous events.
Since no up-to-date analyses of the stability of the North Spur has been provided,
neither by Nalcor/SNC-Lavalin nor by the Geotechnical Peer Review Panel, our
conclusion is that an independent group of experts, appointed by government, should
be entrusted with this important task.
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About the Authors
Stig Bernander was born on February 5, 1928, in Mnene, Rhodesia, Africa. He attended primary and secondary schools in Mnene and Bulawayo, Rhodesia, and in Gothenburg, Sweden. He studied civil engineering at Chalmers University of Technology and obtained a M.Sc. in 1951. Having worked for the Swedish Board of Roads in Stockholm 1951-53, he moved to Skanska Contracting Co, which in those days was named Skånska
Cementgjuteriet AB. In 1972 he became Head of their Design Department in Gothenburg. He retired in 1991 and started a consulting company of his own, Congeo AB.
Stig Bernander has designed or been engaged in major civil engineering works such as bridges, dams, harbors, tunnels, dry docks, off-shore structures, buildings, underground storages and water supply structures in Sweden, Denmark, Norway, Poland, Monaco, Egypt, Saudi Arabia, India, Sri Lanka and Zimbabwe.
In the years 1980 – 98, Stig Bernander served as an Adjunct Professor at the Division of Structural Engineering at Luleå University of Technology, working primarily with crack prevention and modeling of temperature stresses in hardening concrete - taking various boundary conditions into account.
After the large landslide in Tuve (Gothenburg, 1977), Stig Bernander began developing a finite difference model for slope stability analysis taking the deformation-softening of soft sensitive clays into consideration. In the model, the mean down-slope deformation in each element caused by normal forces is maintained compatible with the deformation generated by shear stresses.
He developed software for the model and presented it at international soil mechanics conferences during the 1980-ies. In 2000 he summarized his findings in a Licentiate thesis.
An easy-to-use spread-sheet has also been developed.
In 2011 he further conveyed his experiences of slide modelling in a PhD thesis focusing on the nature of triggering agents and the different phases that a slope may undergo before its stability becomes truly critical.
Lennart Elfgren was born on July 9, 1942, in Gothenburg, Sweden. He studied civil
engineering at Chalmers University of Technology and obtained a M.Sc. in 1965 and a PhD in 1971 with a thesis on “Reinforced concrete beams loaded in combined torsion, bending and shear. A study of the ultimate load-carrying capacity”
After a post doc stay at University of California at Berkeley 1972-73 working with curved box- girder bridges he was appointed to a position as Associate Professor in Structural
Engineering at the recently started Luleå University of Technology. In 1981-83 he worked as a Consulting Engineer with Jacobson & Widmark in Gothenburg and in 1982-83 as part time Researcher in the Swedish Research and Testing Institute in Borås.
He returned to Luleå as Full Professor in 1983 and has served as Department Head and Dean of the Faculty of Engineering Sciences. He has studied anchorage of sheet piling in soft clays, anchorage to concrete, fatigue, fracture mechanics and strain-softening materials and, in the last 20 years, assessment and strengthening of existing structures including numerical modelling and full scale testing to failure of bridges. He has been the main supervisor for 14 PhDs and an associate supervisor for another 15. He has published more than 300 papers and reports, see https://ltu.diva-portal.org/.
He is a Member of the Royal Swedish Academy of Engineering Sciences (IVA), an Honorary Member of the International Association for Bridge and Structural Engineering (IABSE), a Fellow of the American Concrete Institute (ACI) and of the Swedish Concrete Association, and a long-time Member of the International Federation of Concrete (fib).
