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.
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| 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).
