Should earthquake loading be considered for open pit slope design in Australia?

This is an edited excerpt of a conference paper published and presented at AusIMM’s Open Pit Operators Conference 2022. AusIMM members can access the full paper via the Conference Proceedings in the AusIMM Digital Library.

Introduction

There is considerable debate in Australia regarding the need of seismic stability analyses for design of open pit slopes. The reasons for the debate are usually based on the following:

• limited reporting of pit slope instability or significant damage following strong earthquakes
• the common perception that earthquakes do not occur in Australia or earthquakes will not occur during the life of the mine
• management of risk and risk acceptance by assessing failure as having low likelihood and/or consequence.

This paper provides an overview of the reported impact of seismic events on mining operations, particularly the performance of open pits and the factors that may control performance under earthquake loading. The regional seismic hazard of Australia, the characteristics of the earthquake activity and behaviour of seismogenic faults are discussed to provide recommendations on the assessment and inclusion of earthquake loads in open pit mining.

Tailings dams have special considerations that are outside the scope of this paper and are not included in this discussion. There is specialised literature dealing specifically with aspects of seismic design of tailings dams.

Impact of seismic events on mining operations

Read and Stacey (2009) rightly state that there are few, if any, recorded instances in which earthquakes have resulted in significant instability of hard rock pit slopes. Reported damage after strong earthquakes indicate that in contrast to natural slopes, designed slopes such as pit slopes are less susceptible to induced earthquake effects.

Read and Stacey (2009) refer to evidence from a number of mines in highly active seismic zones in Papua New Guinea, Chile and Peru, which although exposed to significant earthquake activity have not reported large scale pit slope failure. There is limited documentation of earthquake induced failures of open pit slopes. Excluding tailings dams, reported impacts are typically:

• rockfalls
• pit slope failures of limited scale
• damage to mining infrastructure (roads, buildings and other equipment) due to failure of natural slopes.

The consequences of the above impacts to mining operations may include:

• loss of life or injury to personnel
• interruption to operations through a loss of road access, which prevents mobilisation of product and/or transit of personnel interrupting operations
• environmental damage through damage to infrastructure carrying concentrate or mining waste
• impact to operations due to loss of power.

An example of impacts associated with earthquakes can be demonstrated through the following brief case study. In Chile, a magnitude 8.8 earthquake on 27 February 2010 resulted in infrastructure damage and temporary closure of 20 per cent of the mining operations in the region (El Mundo, 08/03/2010). Then in 2015, a magnitude 8.3 earthquake resulted in infrastructure damage and temporary closure of 37 per cent of the mining operations in the region (Nueva Mineria y Energia, 08/03/2016). However, further information of the type of damage was not reported. More examples of impacts associated with earthquakes are available in our full paper.

Factors influencing open pit slope performance under seismic loading

There are numerous factors that may influence open pit slope performance under earthquake shaking. These factors include topography, geometry of the slope, water conditions, rock mass and intact rock properties. Table 1 summarises these factors, their impact and consequence for open pit slope stability.

Table 1. Factors affecting pit slope performance under earthquake load.

In highly seismic regions, pit slope design is commonly developed considering seismic loading.

Mining design in highly seismic zones

Minimum damage and/or uninterrupted operation reported after strong earthquakes in mine sites in highly seismically active regions such as Chile, Peru and Mexico are usually quoted as evidence of low risk of slope damage under earthquake shaking.

There is very limited available information and details of mine damage or slope failure during earthquakes. Although damage is seldomly reported, catastrophic failure has not been documented. However, and most importantly, it is usually overlooked that mining operations in highly active seismic regions such as Mexico, Peru and Chile are highly regulated and designed under strict seismic guidelines.

Open pit mines in countries such as Chile, Peru and Mexico are required to be designed with appropriate earthquake loads that depend on their site-specific seismic hazard. The pit slopes are designed considering earthquake loads from the national building codes or from mine site-specific seismic hazard assessments (ie Departamento de Seguridad Minera, 2010; Oldecop and Perucca, 2012; Chamorro, 2019). Additionally, these mining operations have detailed safety plans to follow during and after strong earthquake activity.

Consequently, the adequate performance of open pit mines in highly seismic regions support the inclusion of earthquake loads for design instead of justifying the contrary.

Earthquake activity in Australia

Australia is an intraplate region located away from plate tectonic boundaries where most of the world’s earthquake activity is concentrated. Consequently, Australia has much lower earthquake activity and seismic hazard than Mexico, Peru, Chile or other highly seismic regions such as PNG. Despite its intraplate location, earthquakes in Australia occur relatively frequently but their occurrence and characteristics are highly variable across the continent.

Figure 2 shows the location of the earthquake activity recorded in Australia from 1840 to 2018 (earthquakes extracted from the earthquake catalogue of Geoscience Australia). Most of the earthquake activity has been recorded with shallow depths (less than 15 km depth). However, the earthquake locations may involve significant error due to the sparsity of the Australian seismic network.

On average 100 earthquakes of magnitude 3.0 or more occur in Australia every year. Earthquakes above magnitude 5.0 occur in average one to two years and earthquakes of magnitude 6.0 or more occur every ten years, approximately (Leonard, 2008; Geoscience Australia).

Figure 2. Earthquake activity in Australia. (a - top) Distribution of earthquake magnitudes. (b - bottom) Depth of earthquake activity. Earthquake records from the earthquake catalogue of Geoscience Australia.

Earthquakes can occur anywhere within Australia but there are four regions where current earthquakes are more likely: north-west of Australia, south-west of Australia, Flinders Ranges and south-east Australia (Figure 2).

Australian earthquake records show the seismicity has been steady in the last 100 years in the south- east corner of Australia, the Flinders Ranges and north-west Australia. In the south-west of Western Australia, the seismicity has dramatically increased since 1940 (Leonard, 2008; Estrada, 2013).

South-east Australia and the Flinders Ranges have the highest recorded earthquake activity. However, the largest earthquakes recorded in the last 50 years have occurred elsewhere (Table 2). Note that the Newcastle earthquake with magnitude 5.6 on 28 December 1989 which resulted in the deaths of 13 people is not included in Table 2.

Table 2. Largest recent earthquakes in Australia.

Fault behaviour in Australia

It is well documented that earthquakes occur in geological faults. However, in Australia the association between earthquake and hosting faults is not always evident due to:

• limited knowledge on the character of seismogenic faults
• errors on earthquake location, which prevent the association between earthquake and causative fault
• lack of evidence of fault rupture in the surface after a strong event.

Despite the difficulties associating earthquakes with hosting faults, in the last  50 years, 11 earthquakes have been associated with surface rupturing faults in Australia (Clark, McPherson and Collins, 2011; King, Quigley and Clark, 2019).

Analysis of the earthquake associated surface rupturing faults in Australia has provided insights into the seismic behaviour of the faults. More than 360 geomorphic features with similar characteristics to the surface rupturing faults associated with recent large earthquakes have been recognised across Australia (Figure 4). These features are known or suspected to be associated with earthquakes mostly greater than magnitude 6.0 and are considered likely to have been the source of potentially damaging earthquakes in the recent geological past (Clark, McPherson and Collins, 2011). In addition, these features could again be associated with earthquakes in the future.

Figure 3. Location of recent historic surface rupturing earthquakes in Australia (stars), neotectonic features (red lines), onshore earthquakes with magnitude greater than 4.0 (grey dots), seismic zones, and crustal provinces (modified from King, Quigley and Clark, 2019).

Seismic hazard considerations for open pit mines

The seismic hazard of a mine is relative to its location. Although earthquakes can occur suddenly anywhere within Australia, they are more likely to occur in the north-west of Australia, south-west of Australia, Flinders Ranges and South-east Australia.

Considerations for assessing the seismic hazard of a mine include but are not limited to the following:

• If a fault classified as active or neotectonic is located within a few kilometres for the mine, it may control the seismic hazard. This is especially relevant for south-east Australia and the Flinders Ranges zone where palaeoseismological studies (ie Clark, McPherson and Collins, 2011) suggest that more frequent earthquakes are hosted by these faults than in any other onshore Australian region. The probability of activity of the fault, its rate of activity and magnitude distribution must be considered in the seismic hazard assessment.
• Given the highly episodic fault behaviour in the central and western parts of Australia, the seismic hazard in these regions may be better assessed by probabilistic seismic hazard analysis based on randomly occurring earthquakes that are modelled by distributed earthquake sources (King, Quigley and Clark, 2019). A probabilistic analysis of the seismic hazard aims to quantify the uncertainties associated with future earthquake occurrence such as location, size, shaking level at a site (Baker, 2013). Distributed earthquake source model earthquakes as randomly occurring within a specific area.
• Special consideration should be given to mining operations located along the north-west coast where offshore earthquakes and associated phenomena such as tsunami may result in significant detrimental effects for mining infrastructure.

Recommendations

The assessment of the specific seismic hazard of the mine is necessary to select appropriate earthquake loads for design. However, given the relatively low seismic hazard in Australia, the inclusion of earthquake loads for open pit slopes may not always be required.

There are instances where assessing slope stability with appropriate earthquake loads is recommended, including:

• All slopes where a failure has a high or catastrophic consequence.
• Slopes, excavated or natural, that interact with mine infrastructure (including roads), where failure or damage may result in high financial or social consequences, significant interruption of production and/or injury.
• Slopes with complex or adverse geology or structural features with potential instability issues.

Conclusions

The lack of significant damage after strong earthquakes in mining operations in highly seismic areas such as Chile, Peru and Papua New Guinea is usually used to justify the avoidance of considering earthquake loads for design. It is frequently overlooked that pit slopes that have withstood strong earthquakes in these regions are often designed under strict seismic guidelines.

The seismic hazard in Australia is lower in comparison with regions located at or close to plate tectonic boundaries. However, strong earthquakes with magnitudes 6.0 or greater are not uncommon. Earthquakes can occur suddenly anywhere within the Australian continent, but there are four regions with more likelihood of earthquakes: north-west of Australia, south-west of Australia, Flinders Range Zone and south-east Australia.

Factors that may affect pit slope performance during earthquakes include topography, slope geometry, rock contrasts, water management, and static factors of safety used for pit designs.

The earthquake occurrence and fault behaviour across Australia have implications for the regional seismic hazard and site-specific seismic hazard of open pit mining operations. Depending on the mine location, its seismic hazard may be controlled by a fault previously recognised as active or neotectonic or its seismic hazard may be more appropriately modelled by distributed earthquake sources.

There are particular cases where seismic design and appropriate selection of seismic loads are relevant for the design of open pit slopes. These cases include but are not limited to:

• All slopes where a failure has a high or catastrophic consequence.
• Slopes (excavated or natural) that interact with mine infrastructure (including roads), where failure or damage may result in high financial or social consequences, and significant interruption of production and/or injury.

The discussion and conclusions presented in this paper are not applicable to tailings dams.

References

Anggraeni, D. 2010. Modelling the Impact of Topography on Seismic Amplification at Regional Scale. Thesis. International Institute for Geo-Information Science and Earth Observation Enschede, The Netherlands. 46 pp.

Ashford, S.A., Sitar, N., Lysmer, J., & Deng, N. 1997. Topographic effects on the seismic response of steep slopes. Bulletin of the Seismological Society of America, 87(3):701-709.

Azhari, A. and Ozbay, U. 2017. Investigations The Effect Of Earthquakes On Open Pit Mine Slopes. International Journal of Rock Mechanics and Mining Sciences, 100, 218–228.

Azhari, A., 2016, Evaluating the effects of earthquakes on open pit mine slopes, PhD Thesis, Colorado School of Mines. Baker, J. 2013. Probabilistic Seismic Hazard Analysis. White Paper Version 2.0.1, 79 pp.

Chamorro, M. 2019. Evaluacion de la Estabililidad Pseudostatica Para Botaderos Apoyados Sobre Laderas Coluviales Considerando un Evento Sismico Severo, Ubicado en la Mina Chepica, Region del Maule. Thesis. Universidad de Talca.

Clark, D., McPherson, A. and Collins, C. 2011. Australia’s seismogenic neotectonic record. Geoscience Australia, Record 2011/11. GeoCat #70288.

Departamento de seguridad minera, 2010, Guia methodologica de seguridad para presentacion de projectos mineros a tajo abierto. Gobierno de Chile, Servicio Nacional de Geologia y Minería.

El Mundo. 2010. Available online: {https://www.elmundo.es/america/2010/03/08/noticias/1268082114.html}. Estrada, B. 2013. Insights into the seismic hazard of Western Australia. Australian Geomechanics. Vol 48, No 2. Geoscience Australia. Available Online {https://earthquakes.ga.gov.au/}.

King, T., Quigley, M., Clark, D. 2019. Surface-Rupturing Historical Earthquakes in Australia and Their Environmental Effects: New Insights from Re-Analysis of Observation Data. Geoscience, 9, 408.

Leonard, M., 2008. One hundred years of earthquake recording in Australia. Bulletin of the Seismological Society of America, 98(3): 1458-1470.

Nueva Mineria. 2016. Available online: [ http://www.nuevamineria.com/revista/faenas-mineras-afectadas-por-terremoto- se-recuperan-tras-quedar-con-danos-el-37-de-ellas/].

Oldecop, L and Perucca, L. 2012 Applicability of Evaluation Methods of Seismic Hazard to a Mining Project (Based on neotectonics,  Instrumental  and  Historical  data)  Central  Andes,  San   Juan   Argentina.   Bol.   Soc.   Geol. Mex vol.64 no.2 México ago. 2012.

Outlet Minero. 2017. Available online: [https://outletminero.org/tag/mineria-chile/].

Read, J., and Stacey, P., (Eds) 2009. Guidelines for Open Pit Slope Design. CSIRO Publishing. Collingwood Australia. 496 pp.

Schnebele, E., Jaiswal, K., Luco, N and Nassar, N. 2019. Natural hazards in mineral commodity supply: quantifying risk of earthquake disruption to South America copper supply. Resources Policy, 63 101430.

Toh, J.C.W. 2010. Seismic Analysis for Open Mines, Australian Geomechanics Society, Sydney Chapter Symposium, 13 October 2013 Sydney Australia.