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- Bernander, Stig, et al.
(författare)
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Dam Bank Stability in loosely layered silty sands and lean silty sandy clays : Comments on the risk of failure in the North Spur at Muskrat Falls in the Churchill River Valley, Labrador, Newfoundland
- 2018
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The differences in landslide analysis between the classic limit equilibrium method (LEM) and a progressive failure procedure is outlined. In LEM the soils are presumed to be fully plastic, whereas in the progressive failure approach the joint effect of strain-softening material properties and deformations in the soil mass are considered.The risk of failure in the North Spur ridge due to the dam impoundment at Muskrat Falls in the Churchill River Valley (Labrador/Newfoundland) is investigated. An important issue in this context is e.g. that sloping failure surfaces near the cut-off wall (COW) are bound to be much more critical than the horizontal failure planes, which have hitherto been considered according to Nalcor/SNC-Lavalin Engineering Reports.Results from progressive failure analyses have now been obtained, applying plausible deformation-softening material properties to the soils in the ridge. These results, which are presented at the end of this report, render unsatisfactory safety factors – i.e. lower than 0.5, thus indicating potential risks of failure when the water surface is raised to the proposed levels.Three reports and a summing up are appended, where Dr Bernander strongly emphasizes the need of stability evaluations based on proper progressive failure analysis – i.e. using soil properties based on tests that are not carried out under fully drained conditions.Measures to reducing the detrimental effects of high in-situ porosity are also proposed.
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- Bernander, Stig, et al.
(författare)
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Riverbank stability in loose layered silty clays : Comments on the North Spur Dam at Muskrat Falls in Churchill River, Labrador, Newfoundland
- 2017
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Rapport (refereegranskat)abstract
- The differences are outlined in landslide analysis between the classic limit equilibrium method with assumed plastic properties of the soil and a progressive analysis applying softening material properties.The risk for failure is studied in the dam at the North Spur riverbank ridge at Muskrat Falls in Churchill River in Labrador, Newfoundland, Canada. A sloping failure surface is much more critical than the horizontal surfaces which have hitherto been studied. Results from new analyses have now been obtained applying softening material properties probable for the ridge. The results indicate safety factors lower than 0.5, i.e. there is a high risk that the ridge will fail if the water level is raised to the proposed level.Three reports are appended where Stig Bernander argues in detail for the need for a proper progressive failure analysis based on measured material properties. He also proposes how such properties may be obtained and gives an example of a way to stabilize the ridge if the soil properties show a softening behaviour. Finally examples of progressive failure analyses are included using probable material properties.
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- Bernander, Stig, et al.
(författare)
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Stability of the North Spur at Muskrat Falls : Comments and Discussion of a Paper by Stig Bernander and Lennart Elfgren
- 2021
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In Part A, the geotechnical background is presented to a stability problem regarding the North Spur dam wall at Muskrat Falls in Churchill River in Newfoundland, Canada. This land was formed in the regression of the sea during and after the last ice age with deposits of multiple layers of silty sands and silty sandy clays that formed the valleys and plains that are now above sea level. Some of these layers, deposited thousands of years ago in post-glacial times, are vulnerable to liquefaction when they are disturbed. These conditions have in the past repeatedly caused slides along the banks of the Churchill river. In the report, a specific type of possible progressive failure –the most dangerous one in respect of the safety of the North Spur – is discussed. This type of landslide development may be caused by the rising water pressure, when - or after - the dam is impounded. As will be explained, such a slide could force part of the North Spur ridge to slide along a failure surface sloping East-wards into the deep river whirlpool downstream of Muskrat Falls. In the following, a brief overview is provided of the geotechnical background behind our concerns, also discussing methods of mitigating the risk of the kind of slopefailure in question. Hence, we propose measures such as compacting the soil by piling or by methods of grouting and drainage. We also suggest the need for an expert Advisory Panel to look further into the long-term safety of the North Spur.In Part B, the paper in Part A is discussed and the risk for a failure is denied.In Part C, finally, the authors of Part A, give their answer to the criticism and uphold their view: There may be a serieois risk for a dam breach caused by weak soil layers in the dam. This risk should be properly investigated and mitigated.
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