#### Executive Summary

SRK Consulting (Australasia) Pty Ltd (SRK) has been engaged by Gruyere Management Pty Ltd (the Client) to undertake the site-specific seismic hazard assessment of the Gruyere Gold Mine (GGM) area. The mine is located approximately 200 km northeast of Laverton in the Eastern Goldfields region of Western Australia.

GGM is situated within the eastern most known greenstone belts of the Archaean Yilgarn Craton, the Yamarna and Dorothy Hills Greenstone Belts. The Gruyere TSF is located on an area predominantly underlain by basalt that is mantled by highly to moderately weathered rock. The TSF site extends across 230 ha and two geotechnical shallow foundation typical profiles can be distinguished. The mean average shear-wave velocity of the top 30 m ranges $$V_{s30}≈620-655\,\mathrm{m/s}$$, corresponding to class C sites according to the Uniform Building Code, ASCE 7-16 and NEHRP.

According to the dam failure classification performed by SRK,1 ANCOLD2 rates it as “High C,” DMP3 as “High, Category 1” and GISTM4 as “High.” The design criteria adopted for Active Care corresponds to an AEP of 1:2475 years and for Passive Care to the MCE or an AEP of 1:10,000 years.

A Probabilistic Seismic Hazard Assessment (PSHA) is performed for the site. Peak ground acceleration (PGA) and spectral acceleration hazard values are computed for return periods up to 10,000 years for rock foundation conditions. Hazard values in terms of PGA and spectral ordinates on a site class AB (firm rock) are reported for mean values (Table 1) and an 84th percentile (Table 2).

A disaggregation analysis is performed on the results of the PSHA for fundamental periods $$T_n$$ ranging between $$T_n=0$$ (PGA) and $$T_n=4.0\,\mathrm{s}$$. For rigid and PGA-controlled structures ($$T_n<<0.5\,\mathrm{s}$$), the hazard disaggregation reported bins with $$M_w \sim 4.75$$ as main contributors. For flexible structures $$(T_n>0.5\,\mathrm{s})$$ reported events with $$M_w \sim 5.80$$. For all AEPs and spectral ordinates, nearby sources (20 - 60 km) have a greater contribution on the site’s seismic hazard.

In addition, a scenario-based (deterministic) model is implemented to assess the impact of the closest neotectonic faults without information about its productivity. A fault located beneath the project site ($$R_{EPI}=$$ 5 km), with possible hypocentres between 5 and 15 km and $$M_w\approx$$ 5 is identified as the controlling scenario for the local seismic hazard. The mean and 84th percentile PGA are included in Table 1 and Table 2.

Site-amplification factors are reported for all spectral ordinates and site classes B, C and D, which represent the general geotechnical scenarios expected at a site. Spectral ordinates are reported for mean values and 84th to 99th percentiles. Table 1 and Table 2 are completed with the PGA values expected at GGM from both probabilistic and deterministic approaches.

For the site particular conditions (class C, representative of dense soils to soft rocks) the mean and 84th percentile site amplification factors of the PGA for a 10,000 year event resulted in $$AF\approx$$ 1.194 and $$AF\approx$$ 1.78. The peak-ground acceleration (PGA) values associated with a 1:10,000-year AEP event in class C site conditions, reported $$PGA\approx$$ 0.23 $$\mathrm{g}$$ (mean) and $$PGA\approx$$ 0.359 $$\mathrm{g}$$ (84% percentile). From a deterministic approach, PGA values obtained from a scenario-based analysis, reported $$PGA\approx$$ 0.27 $$\mathrm{g}$$ (mean) and $$PGA\approx$$ 0.424 $$\mathrm{g}$$ (84%). This result is based on the available neotectonic evidence.

 Design earthquakes (PGA) [g] - (mean) NEHRP TR=500 TR=1000 TR=2500 TR=5000 TR=10000 MCE AB 0.021 0.038 0.076 0.124 0.194 0.2303 B 0.022 0.039 0.083 0.139 0.222 0.2653 C 0.028 0.048 0.095 0.151 0.230 0.2704 D 0.035 0.060 0.116 0.183 0.276 0.3234
 Design earthquakes (PGA) [g] - (+84%) NEHRP TR=500 TR=1000 TR=2500 TR=5000 TR=10000 MCE AB 0.028 0.051 0.104 0.171 0.268 0.3218 B 0.033 0.061 0.128 0.215 0.344 0.4132 C 0.043 0.075 0.148 0.235 0.359 0.4238 D 0.055 0.093 0.181 0.286 0.433 0.5087

The pseudo-static coefficient $$k_{max}$$ is estimated for the GGM for two service scenarios. For a service life equal to the mine operation, a design earthquake with an AEP of 1:2475 years is adopted. For critical slopes that must remain stable during closure and post-closure stages, a ground-motion with 1:10,000 years AEP is adopted as the target service level. These levels of service for closure and post-closure are consistent with the prescriptions of the ANCOLD 2019 and the GISTM design guidelines. The fundamental period for the projected geometry of the tailings storage is $$T_{n,1}≈0.423\pm 0.031\,\mathrm{s}$$. The pseudo-static coefficient for different target displacements are presented in Table 3 for the ground motions under certain exceedance probabilities, and in Table 4 for the scenario-based ground motion.

 AEP-based pseudo-static coefficients [g] NEHRP Ts Da TR=500 TR=1000 TR=2500 TR=5000 TR=10000 C 0.423 0.25 0.0429 0.0701 0.1247 0.1857 0.2728 C 0.423 1.00 0.0213 0.0358 0.0658 0.0999 0.1494 C 0.423 2.50 0.0123 0.0214 0.0405 0.0627 0.0955 C 0.423 5.00 0.0076 0.0136 0.0268 0.0423 0.0655 C 0.423 10.00 0.0042 0.0080 0.0166 0.0271 0.0430
 Scenario-based pseudo-static coefficients [g] Ts Repi Mw SID Da Kmax 0.423 5 5 C 0.25 0.1921 0.423 5 5 C 1.00 0.0994 0.423 5 5 C 2.50 0.0600 0.423 5 5 C 5.00 0.0388 0.423 5 5 C 10.00 0.0233

Alejandro Verri Kozlowski, P.Eng.
Corporate Consultant (Geotechnical Earthquake Engineering)
SRK Consulting (Argentina)

1. “Gruyere TSF,” 2021.↩︎

2. Guidelines Consequence on the Consequence Categories for Dams (ANCOLD, 2012); Guidelines for Design of Dams and Appurtenant Structures for Earthquake (ANCOLD, 2019).↩︎

3. Code of Practice: Tailings Storage Facilities in Western Australia (DMP, 2013).↩︎

4. Global Industry Standard on Tailings Management (GISTM, 2020).↩︎