Monday, 12 September 2016

“NORTH SPUR DESIGN UNSAFE” CONCLUDES HYDRO ENGINEER - INSISTS PANEL OF EXPERTS CONDUCT REVIEW

Guest Post Written by James L. Gordon, P. Eng. (Ret'd)

Marine clay, commonly known as “quick clay” and as “sensitive clay” by geotechnical engineers is clay deposited through salt water where the particles pick up salt, which alters the properties of the clay. The clay becomes “sensitive” since it has a propensity to liquefy when disturbed or saturated. It has been avoided by dam engineers due to its sensitivity.

Wikipedia defines marine clays as – “Marine clay is a type of clay found in coastal regions around the world. In the northern, de-glaciated regions, it can sometimes be quick clay which is notorious for being involved in landslides. Clay particles can self-assemble into various configurations, each with totally different properties. When clay is deposited in the ocean the presence of excess ions in seawater causes a loose, open structure of the clay particles to form, a process known as flocculation. Once stranded and dried by ancient changing ocean levels, this open framework means that such clay is open to water infiltration. Construction in marine clays thus presents a geotechnical engineering challenge.”

This explains the controversy surrounding the North Spur dam at Muskrat Falls since it will be the first hydro dam built on a quick clay foundation.

However, I have come to the reluctant conclusion that the North Spur dam is not safe and there is no easy way to make the dam safe. This conclusion has been arrived at by applying some logic to the situation as outlined in the following.

Until recently, dam design has been based entirely on precedent, since there was no mathematical procedure available to determine the safety factor. 

Dam design began in 1857 with a paper by Professor Rankine titled “On the stability of loose earth”. Since then hundreds of scientists working at many universities have investigated the properties of soils and rocks in an effort to arrive at a methodology for calculating a dam safety factor. A breakthrough did not happen until about 1955, when the concept of a slip circle on the downstream face was conceived by A. W. Bishop and a methodology developed to determine the stability of the slip. It became known as the “limit equilibrium method”. However, it took some time before the methodology became generally accepted.

Jim Gordon, P. Eng (Retired)
During this time, in 1968, I attended a short dam design course at Berkeley, where a procedure for determining dam safety was developed by Professor Seed. The process was to build a model of the dam on a shaking table, and gently shake it if there was no earthquake, and more violently if there was. If the dam slopes remained intact, then the dam was safe. However, the geotechnical community did not follow this development since there were issues associated with calculating the reduced size of rocks, gravel, sand and clay used in the model.

It was the use of computers that helped solve the problem. Calculating the stability of a slip circle was a laborious and tedious process, since many slip circles had to be investigated to arrive at the circle with the lowest factor of safety. The computer solved this problem, and with the use of more sophisticated programs such as FLAC designed to calculate the stability of many slip circles, the minimum factor of safety of 1.5 could be easily determined.

The safety factor is simply the ratio of the failure force divided by actual force. 

Perhaps best explained is in the case of a pipe, where the bursting pressure is divided by the operating pressure to obtain the safety factor.

But how was a factor of safety of 1.5 arrived at. It was the spectacular failure of the Teton Dam in 1976, which energised the geotechnical community to determine an acceptable safety factor. The Canadian Dam Association was founded in 1986, with the prime purpose of developing safety standards. This was accomplished in 1995 after many years of work by a dedicated group of geotechnical engineers. A safety factor of 1.5 under normal loading was considered to be acceptable, and 1.3 under an unusual loading such as an earthquake.

But why 1.5, why not a higher factor of safety. At 1.5 the dam factor of safety is the lowest of any other component in a hydro development. For example, the pipe carrying the water from the intake to the powerhouse has a factor of safety of 3.0 to the bursting pressure - the allowable maximum working stress being 1/3 of the ultimate stress. The wire rope that lifts the generator has a factor of safety ranging between 4 and 8. For iron castings it is 8.

The relatively low factor of safety in a dam reflects the geotechnical community’s confidence in the calculation methodology and determination of soil/rock properties based on over 170 years of research work. However, all this research has been undertaken on non-marine clay and other materials, hence the same safety factors cannot be applied to dams founded on marine clays.

Research into the properties of marine clays has only recently been undertaken by a few scientists, with the sole objective of determining the safety of quick clay deposits – are they liable to liquefy, and hence only be suitable for farming, or are they stable, allowing the construction of permanent buildings such as housing. Absolutely no research has been undertaken on the safety of dams built on quick clay deposits.

NALCOR’s engineers have developed a design for the North Spur dam using the FLAC program which has been shown to give incorrect results both by Dr. Bernander and Dr. Locat when used on marine clays.

In Dr. Bernander’s thesis he states (Page XXII) “Landslide hazards in long natural slopes of soft sensitive clays may – on a strict structure-mechanical basis – only be reliably dealt with in terms of progressive failure analysis. There exist, for instance, no fixed relationships between safety factors based on the conventional limit equilibrium concept and those defining risk of progressive failure formation. In consequence, the safety criteria have to be redefined for landslides in soft sensitive clays”. In other words - conventional safety factors are not applicable to sensitive clays.

And from Page XX in the Bernander thesis - Considering deformations and strain-softening in the assessment of slope stability normally results in a higher computed risk of slope failure than that emerging from the conventional ideal-plastic approach, depending in particular on the nature and the location of the applied additional load. In other words the FLAC approach currently used in conventional dam safety analysis is not correct.

In the abstract for the lecture titled “Spreads in Eastern Canadian Sensitive Clays” by Dr. Locat recently  presented at Memorial University, Dr. Locat states “Based on witnesses, spreads (landslides) generally occur rapidly, without any apparent warning sign, and cover large areas (> 1 ha). In addition, conventional stability analyses give too large safety factors when applied to this landslide type. Spreads are therefore serious threats to population and infrastructures on sensitive clays and the need for tools enabling their prediction and mitigation is quite necessary”. Again, regular stability analysis in not applicable to marine clays.

So, if the FLAC stability analysis cannot be used, can the old method of precedent be invoked, neglecting for the moment that there is no precedent for a dam founded on marine clays, and assuming the North Spur is founded on a soft non-marine clay.

Why soft clay? Over a month ago, I had the opportunity to discuss drilling on the North Spur undertaken in 2013 with the mechanic operating a vibrating drill rig. What he told me is not at all reassuring. He mentioned that on several occasions, the casing would slowly descend under its own weight. On other occasions it would drop suddenly by about 4 ft. On one memorable occasion, when the casing was left protruding some 20ft above the earth at the end of the shift, on returning the next morning, it had disappeared and was found some 20ft below ground. It had descended 40ft under its own weight overnight. All this indicates a soft to very soft foundation. Samples obtained from the drilling were placed in core boxes, now stored on site. The logs are also stored in NALCOR site files.


                            Figure 1 – North Spur modified downstream slope.

SNC and MWH have developed a design for the downstream slope of the North Spur with a 1:8 slope from El. 25m to water level at about El. 2m, which requires a horizontal slope length of about 184m. Above El 25.0m to berm at El. 40.0m, the slope is at 1:3, requiring a horizontal slope length of 45.0m. Above the berm at El. 40.0m and on to top of Spur, the slope is 1:2.5. With top of spur at about El. 64m, the horizontal distance from berm to top will be about 60m. Allowing for three berm thickness at 12m each, the total horizontal distance from the shore up to the crest is then about 326m. Source – Poster presentation by “Lower Churchill Project Geotechnical delivery team” October 28-30, 2013. All as illustrated in Figure 1.

At a total horizontal distance from shore to crest of 326m, the downstream work will cover over half of the Spur’s thickness of 570m at the narrowest section. Current work on the downstream face is shown in Figure 2. Normally, the upstream face of a dam has a flatter slope than the downstream face due to the lower friction from water lubrication of the particles. With over half of the Spur thickness taken up by the downstream slope, there is insufficient room for the flatter upstream slope.


Figure 2 – re-shaping work on the downstream face. August 2016.

A Google Earth view of the North Spur is shown in Figure 3, marked up to show the location of landslides. It is interesting to note that the large 1978 landslide extended upstream to the middle of the Spur, not a very reassuring development, since it could happen again.
Figure 3 - Source – North Spur Stabilization Works Progressive Failure Study Figure 1-1.
 Aerial photo of the North Spur.  SNC-Lavalin 21 Dec. 2015.

One of the simplest measures of the dam stability using the precedent analysis, is the ratio of base thickness to dam height. For a dam founded on rock, this ratio can be as low as 3.5, but the ratio increases rapidly as the foundation material becomes softer, and more so as the dam height increases. For the North Spur, with crest at El. 64.0m, and a base thickness of 570m, the ratio is 570/64 = 8.9. The question of a precedent then becomes – what is the ratio for a dam of similar height founded on a soft clay foundation.

The base thickness is the actual thickness of the dam at the contact with the foundation from upstream to downstream, as shown in Figure 4. For example, a 10m high dam on bedrock could have an upstream slope of 2:1 for a horizontal length of 20m. The downstream slope could be 1.5:1 for a downstream horizontal length of 15m. Neglecting the crest thickness, the base thickness is then 20+15 = 35m, and the thickness/height ratio is 35/10 = 3.5. For dams on softer materials, such as deep deposits of clay overlying bedrock, the side slopes are much flatter, resulting in higher thickness/height ratios. Also, as the height of the dam increases, the weight of the dam increases, requiring the slopes to become even flatter, again increasing the thickness/height ratio. 

 At Muskrat, the problem is even more complex due to the layers of quick clay within the body of the natural dam. Theoretically, this will require some further flattening of the slopes, but the effect has been neglected in the                    precedent analysis. 


            Figure 4. Thickness to height ratio.

For Muskrat, fortunately there is a precedent in the Gardiner Dam on the South Saskatchewan River in Saskatchewan. It is also 64m high and founded on soft clay. There the base thickness is 1,500m, for a base-height ratio of 1,500/64 = 23.4, or 2.6 times the North Spur ratio. Thus precedent indicates that the North Spur dam cannot be stable when founded on soft clay.

Figure 5 - Gardiner Dam. 64m high, base thickness 1,500m. Thickness/height ratio = 23.4

A photograph of the Gardiner Dam is shown in Figure 5, where the flat downstream slope is clearly evident. In fact, the slope is so flat, that it is rented to a local farmer as a hay field!

This analysis can be criticised as being incorrect, since the North Spur dam crest could be cut down to El 45.0m, requiring a much shorter base thickness, since the thickness ratio is also a function of the height for a dam on the same soft foundation. Fortunately, there is precedent for this in the Rafferty Dam, also in Saskatchewan, which  has a height of only 20m, and a base thickness of 278m, for a thickness/height ratio = 13.9. If it is assumed that the thickness/height ratio is a linear function of the height for dams on the same type of foundation, a 45m high dam on a soft clay foundation would require a thickness/height ratio = 19.3, for a base thickness of 869m. This is considerably wider than the Spur thickness, hence there is insufficient room for a 45m high dam with sufficiently flat side slopes to be stable.
This analysis has indicated that –            

1.         Dam stability analysis using conventional liquid equilibrium methods    cannot be applied to dams on marine clays.
2.        Safety factors developed for dams on non-marine clays cannot be
applied to dams on marine clays.
3.        There has been no research into the stability of dams founded on
marine clays.
        4.     The North Spur foundation consists of soft to very soft marine clay.
        5.     There is no precedent for a dam founded on a soft clay foundation with      
                the steep slopes shown for the North Spur, where the thickness to          
                height ratio is only 8.9.
        6.     Based on precedent, the thickness to height ratio for a 45m high dam        
                on the North Spur has to be at least 19.3 for a base thickness of about       
                870m.
        7.     Based on precedent, the North Spur with a 570m thickness, has is              
                insufficient thickness to construct a 45m high dam with safe side       
                slopes.

Conclusion – the dam design developed for the North Spur is just not acceptable, and I hope that this analysis will be proved to be incorrect by a panel of international experts which should be convened immediately to resolve this issue.

- Jim Gordon, P. Eng. (Retired)

_________________________________________________________________
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.

16 comments:

  1. PEGNL, where is the duty of care? This engineer feels you should be made an optional organization. Your official status makes it appear to the public that engineering is regulated in this province -- and that implies protecting public safety and not just collecting fees.

    Note to other Canadian Engineers: Is the engineering association in your province willing to intervene in Newfoundland on behalf of the engineering profession in Canada?

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  2. When a engineer as respected and as experienced as Gordon speaks publically with such concern in a design, there is no way that the PUB or the PEGNL can ignore it.

    This is a one of a kind construction. In addition to the complexities above, this dam has the irregular boundary condition of the rock Knoll (Spirit Mountain). How does the analysis take this boundary condition into account.

    How are the ice loads (very dynamic) factor into the calculations? Does the ice affect the pore pressure for example, along with the actual loadings. Does the odd shape of the resevoir increase the ice loads from a normal dam?

    Finally who has ownership of the entire system? Hatch, SNC, Nalcor and a series of other consultants have had their fingers in this. Who is the engineer of record?

    PEGNL need to make a public statement about this. Considering Nalcor's abysmal record to date on this project, the risk are too high.

    The cost of a public review is minor, compared to the consequence of getting it wrong. The reason this project is in the mess it is in is largely due to NAlcor bypassing the normal controls of a democracy.

    PEGNL - Where are you? Will you continue to play your reactive role in the engineering profession?

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  3. But if they built it right the first time, those billions in cost overruns just wouldn't have come. Does anyone else catch that reeking stench of organized crime here?? ... Can the RCMP accompany the panel of Professional Engineers? Or can they come first to ensure the Engineers arrive at all??

    Organized Crime is defined in the Criminal Code of Canada as a group of three or more people whose purpose is the commission of one or more serious offences that would "likely result in the direct or indirect receipt of a material benefit, including a financial benefit, by the group.

    Just sayin'

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  4. A year or more ago there was commentary on Uncle Gnarley as to this North Spur design. With my limited knowledge of civil engineering, and experience as a student engineer on building dams on the Churchill project in the 1960s, I weighted in on the discussion. I had seen the aerial views of past slides in this areas, and aware of the large crater some 150 ft deep in the river just downstream of the North Spur, and aware of the danger of the marine clay issue. I felt a very low slope downstream would be necessary, but this would entail reaching the crater, and requiring an enormous amount of fill. I crudely estimated that a fix could run from .5 to .75 billion. A comment by another stated that the fix was all included at a cost of a mere 20 million or so. I believe original estimated from the 1960s indicated a fix cold be as much as 10 percent of the over all project cost, but got much reduced in what appears to be unsafe design. So I am not surprised by Mr Gordon`s very long base measurement to provide a degree of safety.
    And the facts of the drilling pipes sinking into this structure under it`s own weigh is just alarming. I theorised previously that a large mechanical vibrator operating on this Spur, tuned to the right frequency, could trigger resonance, whereby a small force could cause a large movement, that could trigger a slide of the Spur. Nalcor must be aware of this possibility, as I believe their tenders have some restrictions on equipment operating there.
    Let`s see what the response will be from Stan Marshall and other having authority and accountability. We know Gil Bennett has previously indicated no concern as to the design and safety. Thank you Mr Gordon for your interest and concern.
    Winston Adams

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    1. I'm thinking that just the combination of additional factors to those stated above; like a steady stream of rolling trucks, existing hydrology unknowns, and unforseen impacts from nearby infrastructure could create a slip of a 1.5 safety factor designed dam on quick clay, it doesn't seem to be enough even to a labourer like me.
      Jeez and the other factors mentioned...what a world.
      Signed
      Mr. Ditch Digger

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  5. I have just received the following - "MWH have never at any stage been involved in the design of the North Spur. We act as Lender’s Engineer to the Federal Government and have never at any point been actively involved in the design of any of the project components. I think you should correct this as I believe some of this ends up in the Newfoundland Press."

    I was under the impression that MWH had reviewed the SNC design. Apparently i was wrong and regret any confusion this may have produced.

    Jim Gordon

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  6. On the VOCM broadcast today, the host Patrick Daly asked me if I had heard of the Waba dam in Ontario founded on marine clay. I mentioned that I had not heard of it. The internet provides the following information on the dam -

    Waba dam
    The Waba dam is a relatively low dam in Eastern Ontario, Canada built of clayey materials and founded on a deep deposit of marine clay (Law et al., 2000; Law et al. 2005). The dam has wide berms on both the
    upstream and downstream sides to achieve the required margins of safety against instability under static conditions because of the soft weak foundation. The dam is in an area of moderate seismicity and performance of the dam in the event of an earthquake has become an issue for the owners and operators.
    The generation of excess pore-pressures and the associated possible liquefaction are not an issue at this site, due to the clay foundation and embankment. However, possible plastic yielding of the foundation soil during earthquake shaking and the resulting permanent deformation is a concern.
    Figure 1 shows a cross-section of the dam. The embankment is only 11 m high with wide side berms 6 m high. The depth of the foundation clay is 66 m and the depth of the reservoir is only 8 m.

    The dam height is only 11m, and the base thickness is 200m, for a thickness/height ratio of 18.2

    Jim Gordon

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  7. It is with profound fear that I call upon The Professional Engineers of NL to actively take action to investigate this deplorable situation presented by Mr.Gorden in his latest comments. It appears that an accident is waiting to happen and the Client will then investigate the problem. Much cost and lives will have been by then all at the blunder of Engineers!!
    What a pity.

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  8. The residents of Mud Lake should apply for a court injunction to halt this project until NALCOR can prove that this is SAFE. Given the concerns raised by Eng. Gordon and others this injunction needs to happen as soon as possible. Human safety is the primary concern of our governments, our justice system and the governing body of the professional engineers. All economic and political concerns are now sidelined until we can ensure that everything possible is being done to protect life and property.

    John D Pippy, B.Eng (Civil), MBA

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  9. I am an engineer, albeit not a dam engineer. From my university days I remember a dam having to be designed for both global response (sliding, overturning) and local response (slope failures). The initial infomation released from Nalcor was all about the local failure (slope stability). There was never any calculations to look at the sliding resistance of the mass. What is most interesting from Mr. Gordon's assessment is that he has reduced this into a simple rule of thumb, or a simple benchmark. Now some may argue that in the day of complex computer modelling this is not relevant. I disagree. A simple check like this will show quickly if we are in the remit of "historical benchmarks" or if we are in "new territory". It is a simple analysis that can not be discounted.

    Remember that the slope stability theory is derived for an enscarpment, and not a dam. The second free edge may alter the theory?

    I dont know the answers to these questions, but a look through the work provided on the internet (upto 6-8 months ago) raised many questions.

    Gordon seems to have invested time in this. I see no reason not to commence this review board, in the public domain. Why not?

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  10. This is a relatively complicated topic which Mr Gordon attempts to explain in lay person’s language. I am a technical person but having some trouble following all the logic. I follow that he uses the precedent methodology because he doesn’t believe the FLAC analysis is appropriate to arrive at a suitable safety factor but am unclear about the following:

    1. The Gardiner Dam has a base to height ratio that is 2.6 times greater than MF. It is unclear to me how that sets a precedent of safety. Was Gardiner designed to have such a minimum ratio or is it a situation of the river and dam geography that produced the ratio. Either which way it would come back to whatever calculations were done for Gardiner to determine the minimum ratio.
    2. If the dam can be cut to 45 M wouldn’t the base to height ratio be 570/45 = 12.67 which is within 10% of the 13.9 for the Rafferty Dam. It is not clear to me how the calculation jumps to 19.3 nor is it clear how the Rafferty Dam is the precedent setting baseline.
    3. What about the sump pumps that are supposed to be part of the North Spur design. Would they change the analysis?
    4. What about the liner along the dam / water edge. Is there any benefit from it that would lower the possibility of a slide?

    Perhaps it is just me who is unclear but these are questions that I have.

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  11. Regarding your question #2, Mr Gordon assumes that the base/height ratio of such dams varies inversely with the height. He has found two points to define the graph, the Gardiner and Rafferty dams. MF falls between them. A base/height ratio 10% steeper than a much lower dam seems like pure craziness if his assumptions are correct - and I expect they are.

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  12. Can someone put the paragraph below (sinking well casing) into perspective? To me it is shocking that the core of an earth dam could be this soft. Also, if you pump water out, won't it be replenished from surface water and lose its salinity over time? I seem to remember that salt stiffens up quick clay.

    "On one memorable occasion, when the casing was left protruding some 20ft above the earth at the end of the shift, on returning the next morning, it had disappeared and was found some 20ft below ground. It had descended 40ft under its own weight overnight. All this indicates a soft to very soft foundation."

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    Replies
    1. I agree that this seems to be a shocking incident, and now coming to light after 3 years!. Yes pumping out water is part of the design, and they have been doing that this past 20 years or more least the Spur give way. And with time and fresh water moving through, the stability of the marine clay gets worse as times goes on. I`m an electrical engineer, but appreciate the concerns raised, originally by Cabot Martin, who has engaged the interest of professionals like Mr Gordon. The Telegram has not taken much interest it seems.
      Winston Adams

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  13. Thank you Mr. Gordon for making explicit in both technical and plain language the spur geotechnical issues.

    Has PEGNL received a request from a member or the public for an investigation of the spur plan? The profession is at risk of disgrace if the spur fails. They should be challenged to act on the disturbing technical issues Mr. Morgan raises.

    Are any groups willing to file an injunction on behalf of Mud Lake Residents?

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  14. An experienced and qualified Senior Engineer has put his best technical advice publicly to the Governing agency for the Muskrat "Boondoggle ".

    Having been a practicing Engineer myself, I have in the past observed that the Powers and Authority have a deaf ear and proceed down a different, and sometimes dangerous path.

    Where are the Powers of dissent on Muskrat before it is too late to prevent some probable catastrophic event, such as Mount Polley and other earth dam and tailings pond failure?

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