Response to and Comments on “Geotechnical Peer Review of Dr. S. Bernander’s Reports and Analysis of the North Spur”

The concerns regarding the stability of the North Spur can be summarized in three points: (1) None of the most critical inclined failure surfaces have been studied by Muskrat Falls Corporation. These failure surfaces may be initiated on the upstream side of the dam containment. Here the effects of the deformations, caused by the pressure of the rising water level, have to be resisted by the metastable soil layers in the North Spur. A local failure may occur progressing downwards towards the downstream side of the Spur. A catastrophic dam breach would follow. The GPRP further categorically overlooks the fact that horizontal failure planes cannot possibly represent the highest risk of instability irrespective of whether the analysis is based on the Limit Equilibrium Mode (LEM) or on the Progressive Failure Mode. (2) The stress/strain deformation properties of the porous soils in the North Spur have not been made available. Only strength properties, related to fully drained conditions, have been given. How stresses relate to simultaneous deformations under undrained (or partially undrained) conditions have not been defined in any way. Such relationships are crucially essential for any up-to-date analysis of slope stability. (3) A high risk of North Spur instability has been found related to impoundment. A series of investigatory calculations have been made, based on deformation properties from similar landslides and on a wide variety of assumed input data for possible critical failure surfaces. The results of these analyses indicated a safety factor far below 1. The peer review does not address the above three points. It gives a good view of the general conditions but also contains misconceptions, erroneous considerations and refutable comments indicating that the earlier reports by Bernander have not been fully understood by the panel members. As no up-to-date analysis of the stability of the North Spur has been provided, our conclusion is that an independent group of experts, appointed by government, should be entrusted with this important task