Guest Post by James L. Gordon, P.Eng. (Ret'd)
THE MUSKRAT NORTH SPUR
WILL FAIL!
Mr. Robin Dury, an engineering student at the Luleå
University of Technology, Department of Civil, Environmental and Natural
Resources Engineering, INSA Lyon, has undertaken a geotechnical analysis of the
North Spur as part of his studies towards a Master’s degree in Geotechnical
Engineering. He concludes that –
For assumed material properties and geometries of failure,
the critical load-carrying capacity is below 1000 kN/m whereas a rise of the
water level with 21 m (to El. 39m) will give an increased load of Nq = 2420 kN/m. This is more than
twice of what the ridge may stand with the assumed properties.
The work with the thesis has
been conducted under the guidance of Emeritus Professor Lennart Elfgren,
Structural Engineering, and Professor Jan Laue, Soil Mechanics and Foundation
Engineering, Luleå University of Technology, LTU. I am grateful to them for
their help and commitment. I also wish to express my gratitude to Dr. Stig
Bernander for the time and effort he took in sharing his knowledge on
progressive landslides with me.
Mr. Dury went to the
engineering school INSA de Lyon to study civil engineering. After 4 years spent
at INSA Lyon where he studied mostly geotechnics and structural analysis, he went
in 2016 to Luleå Tekniska Universitet in order to do his last year of Master
Degree before graduation. Then he wrote this thesis at the Department of Civil,
Environmental and Natural Resources Engineering.
The mathematical analysis in
the thesis is far too complicated to describe, as indicated by the following
extract for two typical equations –
Hence the thesis will be
described in non-technical terms as far as possible. All quotations from the
thesis are in bold italics. The summary states -
The so called Muskrat Falls Project consists
in the ongoing construction of a hydroelectric power plant in Churchill River
Valley, Labrador, Canada. The site hosting the project includes a land ridge
which is supposed to be used as a natural dam and thus be submitted to
important water pressures. Yet, previous landslides in the area have shown that
a stability analysis is worth to be carried out in order to ensure the safety
of the facility.
Until now, investigations have only been
carried out using the traditional limit equilibrium method and related
elastic-plastic theory. For the sake of simplicity, this approach does not take
into account deformations outside and inside the sliding body. However, because
of the soil features in Churchill River Valley and particularly its
‘deformation softening’ behavior, there is increasing evidence that the
conventional analysis is not relevant in this situation. Further, when
analyzing the total stability of the ridge, only a horizontal failure surface
has been used and not an inclined one, which is very optimistic and rather
unrealistic.
The difference between the LEM and the Dury
analysis, is that the LEM methodology only assesses the stability of the dam
slopes, determining a safety factor against slips or a slope failure. On the
other hand, the Dury analysis looks at the entire Spur from upstream water
level to downstream water level, assessing the stability to resist the
horizontal force imposed on the Spur by the full reservoir water. The two
analysis are not comparable.
In order to provide a more reliable study, a
progressive failure analysis has been performed according to the finite
difference method of Dr. Stig Bernander. The development of a spreadsheet
adapted to this particular problem has allowed getting quickly and easily
numerical results for several cases of study and assumptions. For assumed
material properties and geometries of failure, the critical load-carrying
capacity is below 1000 kN/m whereas a rise of the water level with 22 m (to El. 39.0m) will
give an increased load of Nq = 0,5 gw Hd 2 =
0,5∙10∙22 2 = 2420 kN/m. This is more than twice of what the ridge may stand
with the assumed properties.
The investigation has led to the conclusion that the situation will
be risky for many combinations of soil properties if the water level is raised
as high as initially planned. The investigation also shows that more material
tests are necessary and that stabilization work may be needed to eliminate the
risk for a landslide.
In other words, the geotechnical analysis
undertaken by SNC cannot be used on the soils in the North Spur, and a more
detailed analysis is required as advocated by Dr. Bernander as applicable to
sensitive clays. This is due to the reduction in strength when the sensitive
soil is subjected to significant deformation under load, not included in the
current method of analysis.
In sensitive soils, the failure mode has two
stages –
Stage I: After an elastic phase with shear
strength up to the linear limit, a plastic phase begins and the peak value c is
reached. This last event corresponds to the beginning of the formation of the
slip surface.
Stage II: A decline in strength occurs until
only the residual strength remains and the slope finally collapses.
Nalcor has performed its own stability
analysis by using the traditional limit equilibrium method (LEM). The main
issue is that this procedure is not justifiable for soils having such a high
porosity. In fact, high porous materials have a ‘deformation softening’
behavior far from the perfect elastic plastic behavior assumed with the LEM.
Thus, the analysis and safety factors calculated by Nalcor cannot be reliable.
The computer model developed for the analysis
broke the North Spur section into a mesh of triangles where the strength and
deformation of each triangle was calculated to determine the safety factor
against failure. This resulted in -
Safety Factor - The safety factor related to
local failure in this case is defined as:
Fs = Ncrit/Nq
= 981.7/2420 = 0.38
The
safety factor would have to be quadrupled to achieve a safety factor above 1.5
in order to avoid a failure, as recommended in the Canadian Dam Safety
guidelines.
In view of this result it is
now absolutely essential to convene a North Spur Review Board to determine the natural
dam safety factor and the remedial measures required to ensure stability.
Jim
Gordon, PEng. (Retired)
____________________________________________________________________
Editor's Note: Partly in connection with the conclusion of the Thesis referred to in Jim Gordon's Piece, and as a follow up to earlier demands made of government regarding the safety and stability of the North Spur, the Labrador Land Protectors and the Grand River Keepers sent a new "Open Letter" to Premier Dwight Ball on July 12, 2017. The letter was aalso ccompanied by attachments which readers may want access. A Link to the Thesis is found here. An abstract of the Thesis is found here. A copy of the "Open Letter" to Premier Dwight Ball and "Structure Document" explaining both the rationale and an acceptable Panel formation is also provided.
