Bjørn Kalsnes

Tailings Dam Stability

Open image in new window Tailings are waste materials from mining operations, which need to be disposed of and safely stored. They are commonly transported as slurry in pipes to the storage facility and surface impoundments through spigots. Different types of tailings dam construction and disposal method include tailings dams designed as water retention dams, and dams built using the upstream method, downstream method or centreline method. Several examples of recent tailings dam failures involved dams constructed by the upstream method, where the new embankments are founded on tailings, causing the dam to become progressively more dangerous as its height increases. From back-calculation of historical failures, two distinct failure mechanisms seem to be dominant. The first mechanism is related to the development of progressive failure in a weak soil layer in the dam foundation. The second dominant failure mechanism is related to static or dynamic liquefaction of loose tailings material at a critical state. Static or dynamic liquefaction of loose tailings may occur at a critical condition, where a rapid (undrained) small increase in the shear strain results in a large increase in pore pressure, reduced effective stresses and a dramatic reduction of shear strength. Typical for these types of failures is that they occur rapidly with no warning. There is often no sign of increased displacement rates or pore pressure increase in the days prior to dam failure. Failure is often initiated by a local instability at a critical location, where redistribution of stresses due to reduced shear strength upon further deformations rapidly develops into a global failure mechanism without any additional load actions.

Plenary: Progress of Living with Landslide Risk in Europe

The need to protect people and property with a changing pattern of landslide hazard and risk caused by climate change and changes in demography, and the reality for societies in Europe to live with the risk associated with natural hazards, were the motives for the research project “SafeLand” on landslide risk in Europe.

Risk Assessment for Quick Clay Slides – The Norwegian Practice

The approach for the assessment of the risk associated with quick clay slides in Norway is a qualitative/semi-quantitative procedure developed as part of work for The Norwegian Water Resources and Energy Directorate. Slide areas are classified according to “engineering scores” based on an evaluation of the topography, geology and local conditions (to qualify hazard) and an evaluation of the elements at risk, persons, properties and infrastructure exposed (to qualify consequence). The risk score to classify the mapped areas into risk zones is obtained from the relationship R S = H WS × C WS , where R S is the risk score, H WS is the weighted hazard score and C WS is the weighted consequence score. The risk matrix is divided in five risk classes. Guidelines for the implementation of the risk matrix are administered by NVE. In practice, the approach is used to make decisions on required mitigation measures to reduce the risk. The approach is simple and makes room for engineering experience and judgment. For detailed regional planning, slope stability calculations need to be made. Methods for quick clay slide stability calculations taking into account the brittle behaviour of the material are under development. This chapter provides an illustration of this development work.

A Web-Based Landslide Risk Mitigation Portal

The mitigation of landslide risk to human-valued physical and non-physical assets is a fundamental component in the disaster risk management cycle Open image in new window . The reduction of risk can be pursued through the selection, planning and implementation of suitable mitigation measures and/or actions. The selection of the most appropriate mitigation measures is a complex process which depends on both the characteristics of the expected landslide event and the potential impacts on the physical, economic, environmental, cultural and societal human-valued assets. Each risk mitigation effort is thus markedly case- and site-specific. A web-based portal is in course of development within the Norwegian research project Klima 2050. The project is aimed at reducing the risks associated with climate changes and enhanced precipitation and flood water exposure within the built environment. The portal implements the analytic hierarchy process (AHP) for the purpose of selecting the most appropriate landslide risk mitigation measures based on user inputs and dynamic expert scoring of an extensive set of candidate mitigation measures. This paper outlines the conceptual standpoints and the present and foreseeable future structure of the portal.

Case History: Failure of a Clay Slope Involving Time Effects

This paper discusses the sudden failure of a clay slope, ca. 3 years after excavation of a ca. 20 m high cut. The landslide had a volume of ca. 15,000 m3 and involved very sensitive clay. A warehouse at the foot of the slope was partly filled with soil, and later collapsed when hit by after-slides from the landslide scarp. Almost miraculously, no humans were present in the building at the time of the landslide. The failure is considered a direct result of the excavation of a cut in the previously stable slope, increasing the slope gradient considerably. The reverse consolidation process initiated by unloading offers an explanation for the time delay of a drained failure. Gradual reduction of effective stress has also resulted in gradual reduction of undrained shear strength due to soil swelling. Another factor possibly contributing to failure is the increased shear stress at the foot of the slope resulting from increased slope gradient. Having verified that very sensitive clay is present close to the foot of the slope, creeping to failure at high levels of mobilization may be a relevant mechanism, possibly contributing to the slope failure.