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.