HYDRO ENGINEER WRITES "OPEN LETTER" TO NALCOR CEO STAN MARSHALL

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

OPEN LETTER TO NALCOR CEO STAN MARSHALL 

Dear Mr. Marshall,

I write concerning safety issues with respect to the Muskrat Falls Project, and specifically concerning the North Spur.

You have probably been advised by your staff that, while the North Spur does indeed present significant technical challenges, they have been addressed by competent professionals, and so are no cause for concern.

In my professional opinion, such a conclusion is incorrect, and dangerously so.

Let me summarize the current situation at the North Spur, as I see it:

The North Spur Dam at Muskrat Falls will be the first dam ever built containing quick (or marine) clay in the dam body, and on a quick clay foundation.

To date, these clays have been avoided like the plague by dam designers, due to their propensity to liquefy if disturbed or fully saturated.

The dam design has been undertaken by SNC using the FLAC program, generally used on embankment dams.

The FLAC program has never before been used on a dam built on quick clay.

James L. Gordon, P.Eng. (Ret'd)

The design work has been reviewed by Dr. Serge Leroueil from Laval University. However, his review of a voluminous geotechnical document was confined to just over one page, and he admitted that “my knowledge on the dynamic behaviors of soils is rather limited”. He concluded that “the stabilization works increase the factor of safety from about 1.0 to 1.6 which is very significant”.  He did not express an opinion as to whether or not it is adequate, given the consequences of dam failure. 

Two other professors are known to have reviewed the North Spur, one was an earthquake specialist, but neither issued a report.

The North Spur dam design has also been reviewed by MWH, using the same FLAC program, with the same results.

One might argue that, since FLAC is the state of the art regarding soil stability, the concerns have thus been put to rest.  Unfortunately, this is not the case.

The scientific critique of the application of traditional methodologies to quick clays comes from an unlikely source.  Dr. Stig Bernander was, for many years, Chief Engineer for SKANSKA, an international Swedish contractor with over 55,000 employees. He and his team were asked by the Swedish government to determine the stability of several quick clay deposits being farmed in Sweden, due to the loss of life when several liquefied. Unfortunately, despite working on the problem for several years, they could not determine the safety with any precision, due to the lack of a reliable computation methodology.

When Dr. Bernander retired in 1991, he returned to the Lulea University of Technology to conduct research on quick clay stability.  In his doctoral research (Published in 2011 and updated in 2012), he developed a reliable computation methodology for determining quick clay stability, and successfully defended his 252 page thesis titled “Progressive Landslides in Long Natural Slopes”, before a geotechnical panel.

While I am not a geotechnical specialist, at times in my career I have worked with many geotechnical experts, including Dr. A. Casagrande from Harvard, and Dr. N. Morgenstern from U of A.  I have read Dr. Bernander’s thesis carefully, and I find its critique of applying traditional methodologies to quick clays to be persuasive. He demonstrates that the normal methods used to calculate the stability of embankment dams cannot be used where quick clay is involved, as they lead to inaccurate results. His methodology, which has accurately predicted instability in several cases, requires certain geotechnical data not normally obtained for dam analysis.

I called Dr. Bernander and asked him his opinion regarding the reliability of the FLAC analysis for quick clays. He said that he had compared results from FLAC with those of his own methodology, and found that they were not compatible, even after altering the FLAC program to account for a new variable.

Dr. Bernander has reviewed the reports and data made public so far concerning the North Spur, and finds it unpersuasive.  He visited the North Spur in October 2014. He has prepared a report, based on the data he has been able to consult, which has been filed with the Public Utilities Board in the context of the post-interconnection reliability review.  In this report, Dr. Bernander was unable to determine whether the proposed design was safe because there was insufficient geotechnical data and inadequate soil testing. One of his conclusions stated “The likely liquefaction of this kind makes the results of standard type soil investigations and the associated determination of the factors of safety in respect to slope stability, very unreliable. This applies in particular if calculations are based on the Plastic Equilibrium mode of analysis”, which underlies the FLAC program.

Dr. S. Bernander is currently working on a second report reviewing the SNC North Spur report issued this year. Unfortunately, he still cannot determine the safety factor due to the lack of the appropriate geotechnical soil tests.

The bottom line – the methodology used thus far to determine the safety of the North Spur Dam is not reliable for these types of soils. Until new analyses are performed by geotechnical engineers familiar with Dr. S. Bernander’s methodology, the possibility of catastrophic failure of the Muskrat Falls dam after impoundment cannot be ruled out.

Determining the North Spur dam safety factor is essential, in order to avoid both a financial disaster for NALCOR, and a fatal disaster engulfing the communities downstream of the Muskrat Falls Dam. Due to the deep deposit of quick clay below the North Spur, a failure would leave the Muskrat powerhouse high and dry, with no possibility of an economic repair.

Given the enormous human and economic cost of such an event, I strongly encourage you to give this issue your full attention.

The point I am trying to make is that the existing design is based on a discredited procedure. To bolster confidence in the design, it would be invaluable for someone familiar with Bernander's methodology review the work and determine the safety factor using the Bernander procedure. The cost, at something less than $50,000, would be insignificant compared to a failure.

For all I know, the design may be safe, but added reassurance would assuage the anxiety of the residents downstream, since they are convinced that the North Spur dam is not safe.

Sincerely,         
James L. Gordon P.Eng. (Retired)