Excecutive Summary

SRK Consulting UK (SRK) has been engaged by Rio Tinto (the Client) to undertake the site-specific seismic hazard assessment of the Simandou Mine area. The Simandou site located in a very low seismicity area in the mountain range of southern Guinea’s Nzérékoré Region.

A Probabilistic Seismic Hazard Assessment (PSHA) was performed at the site. PGA and spectral acceleration hazard values were computed for return periods up to 10,000 years for rock site conditions. Hazard values in terms of PGA and spectral ordinates were reported for median values and +50% percentiles and site class AB (firm rock). The maximum design earthquake (median values) in firm rock assumed a target annual exceedance probability(AEP) of 1:10,000 years and resulted in \(PGA\approx\) 0.076 \(\mathrm{g}\)

In addition, a scenario-based (deterministic) model was implemented to assess the impact of the closest neotectonic faults with no information available about its productivity. A fault located 38 km from the project site, with possible hypocentres between 5 and 35 km and \(M_w\approx\) 6.8 was identified as the controlling scenario for the local seismic hazard. The maximum credible earthquake in rock obtained from the controlling scenario resulted in \(PGA\approx\) 0.18 \(\mathrm{g}\) (84th percentile).

Site-amplification factors were reported for all spectral ordinates and site classes B,C,D and E, which represents the most common geotechnical scenarios expected at the site. spectral ordinates were reported for median values and \(84^{th}\) to \(99^{th}\) quantiles. Table 1 presents mean PGA values expected at Simandou site from both probabilistic and deterministic approaches.

For preliminary design purposes, it has been assumed for all foundation scenarios, shear-wave velocities for the top 30 m \((V_{s.30})\) ranging between 400 and 800 m/s, corresponding to class “C” sites according to the Uniform Building Code, ASCE 7-16 and NEHRP. This class is representative of dense soils to soft rocks.

From the probabilistic approach, Peak-ground accelearation (PGA) values associated with a 1:10,000-year AEP event in class-C site conditions, reported \(PGA\approx\) 0.146 \(\mathrm{g}\) for the 84% percentile.

From a deterministic approach, PGA values associated obtained from a scenario-based analysis, reported \(PGA\approx\) 0.248 \(\mathrm{g}\) (84%) assuming class-C site conditions. Until further neotectonic evidence confirms that the sources selected as possible control scenarios are not active faults, the maximum credible earthquake will be conservatively assumed as the maximum design earthquake for the Simandou site.

Finally, two seismic coefficients \(k_h\) and \(k_{max}\) for pseudo-static analysis were estimated for the Simandou site.

For the design of natural soil slopes and low compacted embankments (\(T_n\)=0.03-0.06 s) during the construction and operation stages (O), the seismic coefficient with a target performance of 1”(25 mm) results \(k_h \approx\) 0.34 relative to a \(PGA=\) 0.058 \(\mathrm{g}\), equivalent to a pseudo-static coefficient \(k_{max} \approx\) 0.0197 \(\mathrm{g}\). The pseudo-spectral acceleration for the “shifted” period of the slope is \(Sa(T_d)\approx\) 0.096 \(\mathrm{g}\). This scenario assumes a target design earthquake of 1:2,500 years AEP and it is consistent with the overall methodology of ASCE 7-16.

For the design of WRD slopes (\(T_n\)=0.3-0.6 s) during closure and post-closure stages (C), the seismic coefficient with a target performance of 10” (25 cm) results \(k_h \approx\) 0.053 relative to a \(PGA=\) 0.146 \(\mathrm{g}\), equivalent to a pseudo-static coefficient \(k_{max} \approx\) 0.0077 \(\mathrm{g}\). The pseudo-spectral acceleration for the “shifted” period of the slope is \(Sa(T_d)\approx\) 0.12 \(\mathrm{g}\). This scenario assumes a target design earthquake of 1:10,000 years AEP.

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