Guest Post by James L. Gordon P.Eng.(Retired)
Way back in 2014, my interest in the North Spur started when I read a short book by Cabot Martin titled “Muskrat Madness”. It concentrated on the questionable stability of a natural side dam called the
North Spur which contained layers of sandy silt and sensitive clay. I emailed Cabot, a member of the 2041 committee, on August 27th commenting on his book, and have
commented several times on the Uncle Gnarley blog on the North Spur safety.
NALCOR describes the Spur as -
The north
spur forms a natural earthfill dam,
with
a crest elevation of about
60 m, and about one km long, which connects the
rock knoll to the north
bank of the valley……. The crest width varies
from
about 1,000 m at its north
end to about 70m at its south
end where it has been narrowed by erosion and landslide activity in the past. The head across the spur is presently 16 m from river
level upstream to downstream. The impounding of the reservoir to El. 39 m will increase the
hydraulic head across the spur to 36 m and
stabilization measures are then necessary to ensure its long-term stability under both normal and extreme water
levels. The soils forming the spur consist of a complex interbedded sequence
of relatively low
permeability silty
sands and sands, and sensitive marine
clays.
Unfortunately, there is a history of landslides in the valley, with several occurring on the upstream
and
downstream banks of the Spur. A very large landslide
occurred at Edwards Island, some 65km
upstream of Goose
Bay about the end of February 2010. It involved about 2,000,000m3 of
material. Source – AMEC Geotechnical investigation: Edwards Island Landslide,
August 2011. There are many other
landslides both upstream and downstream of Muskrat Rapids.
Due to the questionable stability of the Spur, Dr. Stig Bernander was requested by Grand Riverkeepers Labrador in Happy Valley-Goose
Bay to undertake an analysis of the stability, all pro bono, with
travel expenses paid by various concerned citizens. He issued three extensive reports. A remarkable achievement, since he has just
turned
90!
 |
| James L. Gordon P. Eng. (Retired) |
Dr. Bernander’s report
is far too technical to
explain. It is
filled
with geotechnical engineering terms and
equations, to such an extent that only someone with a doctorate in geotechnical engineering and extensive experience
with sensitive clays will be able to follow the reasoning. His conclusions are as follows
–
1. The SNC Lavalin elastic-plastic methodology used to determine stability is incorrect.
2. Insufficient analysis of failure planes.
3. The
use of the Limit
Equilibrium Method
(LEM) of analysis is
not justified.
4. The
applicability of the “elastic-plastic” (see comment #1)
methodology is not
proven.
5.
The safety factors determined using
the SNC-Lavalin methodology are not correct.
6. Finger
drains are not effective.
7. The cutoff wall may be detrimental to the stability.
His conclusions were so alarming that I
authored several articles requesting the formation
of a Review Board,
under a government mandate, to look into his concerns
and
determine the Spur
stability. Only an expert Review Board which is independent
of Nalcor and has full access to data and third-party expertise, working under
transparently set terms of reference, can provide definitive conclusions in a
situation like this.
The NL government never acted upon this
recommendation. Instead, NALCOR convened
a Geotechnical Peer Review Panel (GPRP) at the end of 2017. The members included –
· Prof. Bipul C. Hawlader. Geotechnical
Professor at
Memorial University, St. John’s.
· Prof. Serge Leroueil. Retired
professor at Université Laval,
Québec City,
Canada.
· Dr. Jean-Sébastien L'Heureux.
Technical Lead, Norwegian Geotechnical Institute,
Norway
· Prof. Ariane Locat.
Professor
at
Université Laval, Québec City, Canada.
They issued a report titled Geotechnical Peer Review of Dr. S. Bernander's Reports and Analysis
of the North Spur, dated 2nd February, 2018. The 30 page report concluded that:
1.
… most of the landslides along the Churchill River valley and at the North Spur are either rotational slides or retrogressive flowslides, and
that
they are comparable to many of the
landslides observed in sensitive clays elsewhere in Eastern
Canada and Norway.
2.
… The clayey soils found
at the North Spur are comparable to those found in Eastern Canada and Norway…..
3.
The methodology applied using
the LEM by SLI (SNC-Lavalin Inc.) to evaluate the stability of the North Spur for an initial landslide
corresponds to the current
state of practice.
4. In view of the analyses performed by SLI, the GPRP finds that the approach used is conceptually acceptable to take
into account the initiation
of progressive failure.
5.
The GPRP considers that SLI used State-of-the-Art methodology to assess the resistance of
the North Spur to earthquakes.
6. The GPRP does not expect that the cut-off walls will "create a gigantic force", as calculated by Dury and Bernander, which could trigger a downward progressive failure. Actually, the existing piezometer data show that water pressure within the Spur is already at a level similar
to the level of the reservoir
after impoundment.
7.
The GPRP agrees that the finger drains
are necessary to maintain appropriate drainage on the slopes on the downstream
face of the North Spur
and to reduce infiltrations.
It is unfortunate
and unusual that the GPRP did not consult Dr. Bernander, or give him any
opportunity to respond to their concerns. Also, it is unfortunate that none of the staff within the NALCOR organization have the experience to
discuss and question the GPRP findings.
Nevertheless, in view of the recent revelation that NALCOR edited reports by
the “Independent Engineer” this inexperience may not have prevented NALCOR
staff from “marking-up” or “vetting” the report of the GPRP or setting
parameters/mandates that the GPRP had to follow which would only give the
answer sought by NALCOR.
I still question whether the North Spur
is safe, based
on the lack of data and absence
of geotechnical analysis in
the GPRP report to support their conclusions. The report was prepared in three days, totally
insufficient to assess the vast amount of geotechnical data on the Spur. This
can be compared with the time required to investigate the Mount Polley dam
failure, which occurred on August 4th, 2015, and the failure report
was issued 5 months later on 30th January 2015. It is available here.
In particular, I prefer to have seen –
1. The results of stress-strain tests on the soils. This is where Dr. Berdander and the
consultant SNC-Lavalin depart in their expectation of the
soil strength. SNC-Lavalin
have assumed a linear relationship with a softening factor, whereas Dr. Bernander assumes a loss of strength at relatively low levels
of strain, as shown in the following diagram extracted from his report.
Such tests must
have been undertaken. But results
have not been divulged.
There
is a reference to the strength reduction in the report as follows - Recent research in Norway (refs. (29) and
(30)) recommends the use of the LEM approach in practice for the analysis of
slope stability in sensitive and quick clays, with the addition of a correction
factor, called Fsoftening, to account for the reduction of the clay
shear strength after the peak shear strength has been reached, and to partially
account for strain compatibility on the failure surface. (Report page 15)
The
report then states - As an example of the
effect of the mitigation measures for a slope on the Eastern side of the North
Spur, the critical factor of safety was increased from 1.0 to 1.6. This is a 60%
increase in the stability of the slopes, and within the reduction associated
with Fsoftening, if one should use the results of some of the most
recent researches.
However,
there is no discussion on how the Fsoftening,
factor was determined for the soils in the Spur. It has been assumed from
other studies. Since the safety of the Spur is so critical, stress-strain test
results should be available for the North Spur soils.
2. Dr. Bernander has based his analysis on the lowest strength soils, whereas the GPRP has used the average soil strength,
stating – Atterberg limits indicate that
the clay has low to medium plasticity. Plasticity charts based on soil testing
in 1979 and 2013 investigation are presented in Figure 6 for the Upper Clay and
Lower Clay. For the Upper Clay, the plasticity index ranges between 3 and 22,
with an average of 11. Only a few values are below 7 and most of them seem to
be associated with a mixture of silty layers and clayey layers. (Report, page
10)
There has been no justification for this.
What is the
saying – the strength of a chain is equal to the strength of its weakest link.
The soils in the Spur have
been deposited in layers, with
each layer
having a different strength and
characteristic. If there is a layer of weak soil, then the Spur will fail by
sliding on that layer as occurred at Mount Polley in BC. The GPRP has not demonstrated that there is
no such layer. The extent of soft and low-strength materials is unknown at this time. I
would have preferred to see a three-dimensional computer
model of the Spur showing the layer strength.
Mount Polley dam
in BC. The downstream
slope failed on August
14th, 2014.
Report extract - (The) Independent Expert Engineering Investigation and Review Panel concluded that the
dominant contribution to the failure resides in the design. The design did not take
into
account the complexity of the sub-glacial
and pre-glacial geological environment associated with the Perimeter Embankment foundation.
As a result, foundation
investigations and associated site
characterization failed
to identify a continuous
(soft) layer in the vicinity of the breach and to recognize that it was susceptible to undrained
failure.
3. The GPRP concludes that the cut-off wall is a necessary component
of the Spur safety
enhancement. My experience with such walls is that they contain defects in the form of permeable windows caused by
the permeable material falling off the vertical sides of
the wall during the back-filling process. Impermeability can be seriously
affected, and
will not be known until
the
reservoir is filled.
Also, the
GPRP rejects Dr. Bernander’s description of a “gigantic force” applied to the
wall, pointing out, correctly, that the location and height of the wall has
been misinterpreted by Dr. Bernander. The GPRP report states –
In
their analyses, Dury and Bernander assumed “a gigantic external force (locally
on the (cut-off
wall) COW)”, assuming the water pressure
resulting from impoundment on only one side of the COW ……., in addition to
using incorrect geometry and incorrect location for the COW. Actually, the many
piezometers installed in the North Spur show that the water pressure in the
Spur will be acting on both sides of the COW (red triangle on Figure 13). If
the calculation is performed for a COW at the actual location, the force on the
wall will be much less than the force calculated by Dr. Bernander. (Page 21)
This statement is patently not correct
and indicates a misunderstanding of the hydraulic forces exerted by the
reservoir waters on the Spur. There has to be an impervious barrier to the headpond
water within the Spur to avoid excessive seepage. It makes is no difference to
the hydraulic forces if the barrier is a cut-off wall constructed from the dam
crest down to the impervious clay layer below the spur, as assumed by Dr.
Bernander, or if the barrier is a short cut-off wall topped by an impervious
blanket, as built at the upstream face of the Spur. Moreover, the impervious
barrier can be located anywhere within the upstream half of the natural dam
formed by the Spur.
To reinforce their theory about the
forces acting on the cut-off wall, the GPRP has added a counterforce on the
downstream face of the cut-off wall (Report Figure 13) equal to the pressure on
the upstream face. If this was the case, then the cut-off wall and blanket
would not be required.
What Dr. Bernander is describing is the
concentration of the forces at the watertight barrier resulting in high
compressive forces on the soil immediately downstream of the watertight
barrier. Without the watertight barrier, the dam would be built with a
homogeneous glacial till, as at Bay d’Espoir and Cat Arm, and the hydraulic
forces would be dissipated throughout the dam, instead of being concentrated at
the watertight barrier.
The effect of the force concentration
has not been determined. It would have to be undertaken by a fine-mesh finite
element analysis.
Teton
Dam failure due to excessive seepage on
first filling. June 5th,
1976. This was the unexpected event which started the dam safety campaigns in
North America.
4. The
GPRP has assumed that the safety factors applicable to the Spur stability are those
recommended by the Canadian Dam Association.
As first mentioned by
Phil
Helwig, these safety factors were developed for dams constructed with known homogeneous materials such as rock, gravel, and silt, all
within a determined size, placed and
compacted under strict
specification requirements,
and tested throughout construction.
On the other hand, the North Spur contains
a mixture of sand, silt and clay, resting
naturally, and not compacted, nor tested as in an engineered dam. The safety factor for
such a natural dam should be
higher. The
GPRP
has not demonstrated that the CDA factors can be applied to the Spur.
Also,
I would refer the GPRP to a paper (reference provided by Phil Helwig) authored
by J. Michael Duncan, titled “Factors of safety and reliability in Geotechnical
engineering” which demonstrates that using a simple factor of safety is
insufficient, and that a reliability analysis is also required. (ASCE Geotechnical
Journal, Oct. 1999. *1) A reliability analysis has not been undertaken by the
GPRP.
So what can be concluded from all this? – is the North spur absolutely safe? The reservoir spillway has been designed to pass a 1/10,000
flood, and the Spur is able to
withstand a 1/10,000 earthquake. But is the risk of a
slope failure at the North Spur also assessed at 1/10,000? If not, then how risky is
it? Nobody knows, since a reliability analysis has not been
undertaken.
Recommendations:
The government of
NL should appoint an eminent panel of geophysical experts, completely
independent of Nalcor, to assess the scientific evidence and undertake all
necessary additional research, and undertake a comprehensive review of the
safety and stability of the North Spur. The Panel’s work should include:
1. Giving Dr. Bernander and his associate Dr. Elfgren the
opportunity to respond to the GPRP report, and include their comments therein.
2. Assessing the results of stress-strain tests on the
various soils within the Spur, to demonstrate that the linear relationship and
softening factors have been correctly determined and applied to the
geotechnical calculations.
3. Producing a three-dimensional computer model of the
strength of the various soil layers within the spur, to demonstrate that there
are no extensive layers having a strength below average strength, which could
form a sliding plane.
4. Undertake a reliability analysis of the safety of the
Spur.
5. Seeking precedents for the use of CDA dam safety factors in
slope safety calculations in a natural dam.
Given the recent revelations that Nalcor staff have been
“marking-up” reports by the “Independent” Engineer, it is imperative that the
chosen Panel is verified to be Independent of Nalcor. Only then will the residents downstream of
the Spur — and those who are responsible for paying for the dam — feel that Government
has done its utmost to ensure their safety and the safety of the Spur.
Jim Gordon. Hydropower
consultant, retired.