Dynamic stability analysis of tailings dams

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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 there is 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[1]. 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[2]

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). [3] [4]

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. [3]

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. [3] 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.[5]

Seismic Source Characterization

Seismic source characterization involves the identification and documentation of relevant possible earthquake sources in a region. [5] 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. [6]


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 [6]:

  • Identification of seismotectonic provinces related to the project
  • Geologic history of the area
  • Description of the regional geologic formations, rock types, and soil deposits to evaluate seismic energy transmission, effects, and duration of ground motion
  • Location of major regional geological structural features, including folds and joint families
  • Interpretation of the regional tectonic mechanisms and associated types of faulting
  • Location and description of shear zones and faults
  • Estimation of the relative degrees of fault activity for each of the faults of concern to the study area


Once the regional study has been completed a local/site specific study can be conducted. This is necessary to determine the potential for surface rupture and to evaluate local site response effects on earthquake ground motions. The information for a site specific study can be collected through reviewing published geological reports, field observations, and detailed site specific investigations that may include geological mapping, trenching, shear wave velocity measurements, drilling, material sampling, and laboratory testing. The local study typically covers [6]:

  • Geotechnical character, depositional history, orientation, lateral extent, and thickness of soil units beneath the site and on adjacent slopes
  • The character, lateral extent and thickness of rock units
  • The structural geology of the site to include rock unit attitudes, faults, and joint systems, folding, and intrusive bodies
  • The age and activity level of faults in the dam and reservoir area
  • The geohydrology of the site area to include water table conditions, soil and rock transmissivity coefficients, and recharge areas
  • The existing and potential ground failure and subsidence, including rock and soil stability, dispersive soil conditions, and soil units exhibiting characteristics for potential liquefaction


The seismic hazard study can provide support to determine the ground motions at the project site and the attenuation of the area. Attenuation studies are difficult due to inhomogeneous ground conditions and therefore usually estimated by studying historical, empirical and/or theoretical data. Information that is required in attenuation studies are [6]:

  • Instances where ground motion data are available, to develop site-specific attenuation relationships or scattering functions
  • Isoseismal maps constructed from historical earthquake data provide an indirect measure of ground motion attenuation

Development of Hazard Curves

Seismic hazard curves are curves that relate the probability at which a specific ground motion will be exceeded at the site of interest. Hazard curves typically have “annual probability of exceedance” or its reciprocal “return period” on the vertical axis on a logarithmic scale and peak ground acceleration on the horizontal axis[5].

A hazard curve is developed for each seismic source, these curves are combined to develop a cumulative hazard curve for the project site. The seismic hazard curves are calculated using ground motion attenuation relationships between peak ground acceleration or spectral acceleration and the distance between the seismic source and site, and the magnitude of the earthquake associated with the source. Figure 1 shows a common curve showing peak ground acceleration for multiple materials[6].

(thumbnail)
Figure 1: Example of a hazard curve plot[7]

Development of Uniform Hazard Spectra

Peak ground acceleration is the traditional parameter used to describe intensity, however it does not always correlate well with sustained damage. Usual practice is to complete a sensitivity analysis from a typical earthquake as the guide to what values to take for other frequencies relative to the standard 40 Hz. This methodology does not take into account the specific site conditions and at some frequency values the intensity will be either over estimated or under estimated. [8] For a given exceedance probability the values are taken from the hazard curves for each spectral acceleration and an equal hazard response spectrum is created. The Hazard Spectra curves are created for a specified annual exceedance probability of interest as shows in Figure 2. [5]

(thumbnail)
Figure 2: Example of a uniform hazard spectra for Vancouver, Montreal, Toronto, and Winnipeg at 2%/50year probability on firm ground conditions.[9]

Development of Acceleration Time-Histories/Accelerograms

The development of time-histories (accelerograms) can be completed through two different methods: selecting a suite of recorded motions and synthetically developing or modifying one or more motions. Accelerograms should be developed to be consistent with the design response spectrum and have an appropriate strong motion duration for the particular design earthquake. Caution must be exercised when developing accelerograms for near-source earthquake ground motions (source-site distance is less than 10 km), strong intermediate-to long-period pulses should be included as these are often observed in the near field and usually responsible for significant damage. [6]

Selecting Recorded Motions

When selecting recorded motions, it is necessary to select multiple time histories such that, valleys of individual spectra that fall below the design response spectrum are compensated by peaks of other spectra. The advantage of selecting recorded motions is that each accelerogram is a historical event and hypothetically the most representative of what the structure may be subjected to. There are some disadvantages as well and include: multiple dynamic analyses are needed for the accelerograms selected; and there will typically be some exceedances of the smooth design spectrum by individual peaks. Although a good fit may be achieved for one horizontal component, simple scaling factors applied to the other components may cause the spectral fit to not be as good. [6]

Synthetically Developing or Modifying Motions

In order to create and develop synthetic accelerograms multiple computer programs have been created with the use of many techniques. These are used to create synthetic accelerograms or modify recorded accelerograms so that the response spectrum closely matches the design spectrum. Recent advances in the creation of synthetic accelerograms have used frequency-domain techniques with an amplitude spectrum based upon band limited white noise and a simple, idealized source spectrum, or kinematic models that produce three components of motion using complex source and propagation characteristics. There are both advantages and disadvantages to synthetically developing accelerograms. Advantages include: the natural appearance and strong motion duration can be maintained in the accelerograms. The disadvantage is that these motions are not “real” as real motions do not generally exhibit smooth spectra. [6]

Seismic Potential Failure Modes

Earthquake shaking may result in many different types of damage to a dam. Failure may result due to:

  • Disruption of the dam by major fault movement in the foundation (not typically seen in Canada) [3]
  • Loss of freeboard due to differential tectonic ground movement
  • Slope failures induced by ground motions
  • Loss of freeboard due to slope failures or soil compaction
  • Sliding of dam on weak foundation materials
  • Piping failure through cracks induced by ground motions
  • Overtopping of dam due to slides or rockfalls into the reservoir [10]
  • Liquefaction and loss of shear strength due to increased pore pressures in the embankment or its foundation.[5]


References

  1. P. Campbell, P.Eng., MCIP, M. Dolbec, ing, M.B.A., G. Ford, G. I. Haack, P.Eng, N. Heisler, W. Jolley, R. Kamel, Ph.D., P.Eng., S. Kaczmarek, L. Marcoux, C. McLean, MASc, MBA, P.Eng., T. Moulding, M.Sc, M. Passey, A. Roy ing, M.Sc., X. Su, Ph.D., P.Eng, J. Theakston, P.Eng., and K. M. Wog, M.Eng., P.Eng., "Regulation of Dams and Tailings Dams in Canada," in CDA 2010 Annual Conference - Congres annuel 2010 de l'ACB, Niagara Falls, 2010.
  2. P. N. Psarropoulos and Y. Tsompanakis, "Stability of Tailings Dams Under Static and Seismic Loading," Canadian Geotechnical Journal, vol. 45, no. 5, pp. 663-675, 05 2008.
  3. 3.0 3.1 3.2 3.3 3.4 Canadian Dam Association - Association Canadienne des Barrages, Dam Safety Guidelines 2007 (2013 Edition), Toronto, Ontario: Canadian Dam Association, 2013.
  4. 4.0 4.1 Government of British Columbia: Ministry of Forests, Lands and Natural Resource Operations, "Downstream Consequence of Failure Classification Interpretation Guideline for Dam Safety Officers," Ministry of Forests, Lands and Natural Resource Operations, Victoria, 2016.
  5. 5.0 5.1 5.2 5.3 5.4 U.S. Department of the Interior, Bureau of Reclamation and U.S. Army Corps of Engineers, Best Practices in Dam and Levee Safety Risk Analysis, vol. 4, Washington, District of Columbia: U.S. Department of the Interior, Bureau of Reclamation and U.S. Army Corps of Engineers, 2015.
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Federal Emergency Management Agency, "Federal Guidelines for Dam Safety," Federal Emergency Management Agency, Washington, 2005.
  7. United States Geological Survey, "Hazard Curve Application," USGS, 11 January 2017. [Online]. Available: https://geohazards.usgs.gov/hazardtool/application.php. [Accessed 25 02 2017]
  8. British Geological Survey, "Spectral Hazard," Natural Environment Research Council, 2017. [Online]. Available: http://www.earthquakes.bgs.ac.uk/hazard/haz_guide/spectral.html. [Accessed 25 02 2017]
  9. Natural Resources Canada, "Sample Uniform Hazard Spectra for Four Canadian Cities," Natural Resources Canada, 10 02 2016. [Online]. Available: http://www.earthquakescanada.nrcan.gc.ca/hazard-alea/zoning-zonage/UHSsample-en.php. [Accessed 25 02 2017].
  10. H. B. Seed, "Earthquake-Resistant Design of Earth Dams," in First International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics , St. Louis, 1981.
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