Dynamic stability analysis of tailings dams

Introduction

The regulation of dams and dam safety falls under the responsibility of the provincial/territorial government. Canada is unique from other countries in the fact that no federal agency or over-arching program which guides the development of requirements for the safe management of dams. In the 1980’s the Canadian Dam Association was formed to provide dam owners, operators, consultants, suppliers and government agency with a national forum to discuss issues of dam safety. It is evident that in high or moderate seismicity areas the design of tailings dams must take potential seismic activity into consideration. Many failures have occurred due to a lack of concern with respect to seismic hazard. Most failures are strongly related to inertial and/or weakening instabilities of the dam slopes

Classification of Dams

Due to the size of these structures and the impacts that a failure a dam can have, dam safety regulations and recognized practices are in place for the design, construction, operation, and maintenance. As some structures carry more risk than others, the required standard of care can vary based on dam classification. Dams are classified based upon estimates of potential consequences association with dam failure. This classification provides guidance on the standard of care expected of dam owners and designers as well as distinguish lower risk dams from higher risk dams (Table 1).

Standard practice for safety evaluations of dams in industry today consist of a standards-based approach. This is a deterministic concept and because it is computationally straightforward, provides the reassurance of a well-known method that provides easy to understand measures (Safety Factors). The probability of a dam failure cannot be quantified using deterministic approaches as it does not consider the large amount of uncertainty regarding the load intensities and the ability of a dam to resist given loads. Risks associated with the dam are managed through the use of classification schemes that reflect the potential consequences. Table 2 outlines the frequency based target levels for earthquakes, for use in load resistance performance analysis.

Assessing the Seismic Risk

Before designing a tailings dam, the required seismic design parameters (Earthquake Design Ground Motion {EDGM}) should be calculated though the completion of a seismic risk assessment. Products from a seismic hazard analysis typically consist of a seismic source characterization, development of hazard curves, development of uniform hazard spectra, and development of acceleration time-histories.

Seismic Source Characterization

Seismic source characterization involves the identification and documentation of relevant possible earthquake sources in a region. Typically this is completed through literature reviews focusing on tectonics, seismology, and geology of the site area. The review may also include analyses of remote sensing imagery, geophysical data and site investigations to evaluate documented or suspected active structures. Study of the regional geology should encompass an approximate area with at minimum a 100 km radius around the site with consideration to extend the study in order to encompass any major fault or structure that could affect the ground motions at the site. Typical topics covered in a regional study include :