Guest Post by James L. Gordon, P. Eng. (Ret'd)
I have mentioned several times that a review board is essential for the North Spur.
Perhaps I should summarise the reasons particularly since the Owner’s engineering consultancy - Hatch - has made the same recommendation.
It is acknowledged that the North Spur natural dam is the first time a dam containing marine clay and founded on a deep deposit of marine clay has been used in a hydro dam. All major “firsts” always have a review board to add assurance to the design. I have worked on three dam “firsts” and all had review boards, and all benefited from their advice.
The first was at Duncan in BC, which is founded on a deep deposit of unconsolidated liquefiable silt. It holds the world record for settlement as predicted, at now over 6m. We had a 4-man review board which made significant changes to the project layout, adding to security.
The second was at Bighorn in Alberta, where the deepest (to date) cut-off through gravel and boulders was part of the dam. There we had a 2-man review board, including Dr. A. Casagrande from Harvard. He made a major change in the dam design which, on hindsight, avoided a possible dam failure.
The third was at Jebba in Nigeria, where the dam is founded on Aeolian unconsolidated sand blown in from the Sahara Desert. Again we had a 2-man review board including Dr. R. B. Peck, Professor of Geotechnical Engineering from the University of Illinois, who suggested a consolidation method which added significantly to the dam safety.
|James L. Gordon, P. Eng. (Ret'd)|
Every utility I have worked with has welcomed a review board, with the exception of NALCOR, who adamantly assert that such a board is not necessary, since the design has been thoroughly reviewed by professors and other geotechnical engineers. Such is not the case, since any thorough review would have discovered the glaring inconsistency in the liquidity-strength relationship, as uncovered by Mr. Maurice Adams, in the test data for the North Spur soils.
So who has undertaken reviews of the North Spur?
NALCOR has affirmed that two renowned professors have reviewed the design.
The first was Dr. Serge Leroueil from Laval University who stated at the start of his just over one-page review dated 24 August 2014, that “my knowledge on the dynamic behaviour of soils and its analysis is rather limited. Moreover, I received the main report but not the appendices”. His conclusions – “the stabilization works increase the factor of safety from about 1.0 to about 1.6 which is very significant …….in the section on Limitations and sensitivity, I have been surprised to read that the granular material can be very sensitive to their saturation. This is true but I am not sure that this aspect has been considered in the analysis” Certainly not an endorsement of the design.
The other was Dr. I. M. Idriss, Professor Emeritus of Geotechnical Engineering from the University of California, Davis Campus. He made some comments on the earthquake magnitude, but never issued a report.
Other companies which may have undertaken a review are –
The Independent Engineer, MWH. However, their mandate only includes reporting on the construction progress for the Federal Government as part of their loan guarantee.
The Owner’s Engineer, Hatch has reviewed the design. I was under the impression over a year ago, that they had undertaken a detailed review, and one of their senior geotechnical engineers assured me that I should have no worries about the dam safety. I was relieved, since he mentioned the three geotechnical engineers who had looked at the design, and I had previously worked with all three.
This resulted in my flip to pronouncing the North Spur dam as being safe. However, on further investigation, I found that their review was superficial, and I flopped over to again advising that the dam needed a thorough review by a board of geotechnical engineers with experience in soft sensitive clays. Now, Hatch do not claim to have undertaken part in the design and only provided a computer program to model the seepage and effect on the internal piezo-metric pressures when the reservoir is filled.
There have been several reports produced by Hatch. Most are on the effect of ice, the dam safety monitoring and the pump-well system. But there is a report on the design titled “Cold eye review of the design and technical specifications for the North Spur. Jan. 2014.” which is worth a more detailed comment.
The review is based on incomplete data - “The review of the North Spur design relied on the Engineering Report - SLI document No. 505573-XXXX – XXER- rev # which is dated November 2012 (Ref 1). It is understood that this report is in a preliminary form and does not contain much of the analysis and design that has been undertaken more recently”.
The liquidity-strength anomaly was not mentioned, but there is a pertinent comment on the liquidity –“The Liquidity Index (of the upper clay strata) averages 1.5 with a range of 0.7 to 3. Values in excess of 1 are an indication of the potential for both liquefaction and flow type failures”. And “The Lower Clay sits above the Lower Aquifer generally between El. 10 to -50 m. This layer consists of clay of low to medium plasticity with a liquidity index that would classify the clay as slightly sensitive”. Sensitive clays are prone to liquefaction, but there is no further comment on the dangers posed by the clay sensitivity.
However, there is a comment that – “The sensitivity of the upper marine clay has been reported to be in the range of 2 to 28 with an average of 11 as obtained from cone tests carried out in 1979. This does not agree with the results from the 1978 Acres report which indicate sensitivities one magnitude higher”. This discrepancy was not pursued further.
Even Hatch recommends a review board with their comment in the conclusions – “Further analysis on the sensitive marine clays with regards to potential loss in strength when subjected to seismic loading is required. This should be coupled with engaging two eminent consultants with specific expertise on sensitive marine clays”.
Another concern is the steepness of the North Spur dam slopes when compared with other dams founded on sensitive non-marine clay with similar strength characteristics. The main example is the Gardiner Dam in Saskatchewan, which has a height equal to the height of the North Spur dam, and is founded on similar sensitive clay. The slopes at Gardiner are far flatter, and for the North Spur dam to have comparable slopes, the base width would need to be more than doubled.
The bottom line – North Spur design reviews to date have been superficial, and even the only worthwhile review by Hatch, based on incomplete data, recommends a review board.
Jim Gordon. P. Eng. (Retired).