The Block Model derived from the updated Resource estimate was used for the Savannah FS. Modelling was undertaken by Panoramic staff using DatamineTM Studio 5D Planner and Enhanced Production Scheduler (EPS) software. Design parameters are shown in Table 1.
Table 1 – Savannah North mine design parameters
Historically, Savannah paste filled stopes that have been undercut by stoping below have experienced excessive paste dilution (crown failures) impacting metallurgical performance. Therefore, the Savannah North stope shapes have been designed so that the paste filled stopes are not undercut. Typical stope and development designs for wide (>15m) and narrow (<15m) zones are shown in Figure 1.
Figure 1 – Stope shapes for wide (>15m) ore zones (A) and narrow (<15m) ore zones (B)
A range of different sequencing and mine design options were considered for Savannah North, including accessing the deposit at either the 1380-1360 level or top-down, and “centre out” versus “east to west” stope retreat sequences. Amongst the models, the “centre out” option with the 1380-1360 level access was superior, and was selected as the preferred option.
Once the preferred design for Savannah North was selected, the impact of varying a number of rate parameters was tested, including paste curing time, paste fill rate, bogging rate, production drilling rate and jumbo development rate. An overall constraint of 85,000t ore per month was applied, to reflect current mill capacity. Rate parameters adopted for the Savannah FS are shown in Table 2.
Table 2 – Savannah North mining rate parameters
Scenarios for the relative timing of the start-ups for Savannah and Savannah North were considered, including recommencing mining at Savannah concurrently with commencing access development to Savannah North, and delaying recommencement of mining at Savannah until access development to Savannah North is complete. The concurrent development option results in a slightly lower milling profile in the initial quarters, however has the clear advantage of being cash-generating almost immediately. Accordingly this option was selected as the most appropriate schedule for the Savannah FS.
Beck Engineering Pty Ltd was engaged to undertake a geotechnical study to forecast mine scale stability and deformation for Savannah North. The method of analysis used was Discontinuum Finite Element modelling using geological structures on a mine scale. This method has previously been used by Beck Engineering (August 2015) to accurately model rock damage and seismic activity at Savannah. Two mining sequences were modelled; “centre out” and “east to west” stope retreat sequences. There was no definitive geotechnical difference between mining sequences as both maintain an inclined continuous mining front and avoid retreating to pillars.
The Upper Zone is expected to have generally good mining conditions. Some local areas of potentially significant rock mass damage are forecast at fault intersections or areas of convex hanging wall geometry. At 1300m below surface there is an increase in footwall and hanging wall damage, due to increasing stress with depth. These geotechnical risks will be mitigated by detailed stope design taking into account geological structures, hangingwall shape, hangingwall ground support and consideration for leaving rib pillars, if necessary.
The Lower Zone has more challenging mining conditions due to the magnitude of the stress forecast with depth and the current understanding of the rock mass strength. To account for this, stoping dilution was increased from 10 to 20% and stope mining recovery reduced from 90 to 80%.
The “centre out” stoping sequence was confirmed as the preferred option as it maximises the stoping areas per level, reduces pressure on decline development and allows more time to assess geotechnical risks and mitigation strategies before developing the Lower Zone.
Ozvent Consulting Pty Ltd was engaged to undertake a ventilation study of the preferred combined Savannah and Savannah North mine design and schedule. The scope of the study included:
- Primary airflow requirements based on the estimated mining fleet;
- Review previous ventilation methods for initial access/production and permanent primary ventilation for Savannah North;
- Review optimum airway sizes;
- Model LOM primary fan duties;
- Review heat loads and model LOM cooling requirements based on climate data; and
- Provide budget capital costs and specifications for primary fans and cooling plant.
Historically, Savannah has utilised a series primary ventilation circuit with chilled air delivered to lower working headings via fresh air raises (FARs). This style of primary ventilation circuit will not be suitable for Savannah North due to heat loads from the additional mining fleet and the depth of mining. The recommended airflow strategy for the combined mining schedule involves establishing air-change locations along the decline where hot air is replaced with fresh chilled airflow from FARs.
A new Savannah North surface FAR and the conversion of Savannah FAR2 to a return air raise (RAR), with a new primary ventilation fan are required to achieve sufficient air-change locations along the decline.
A combination of parallel and series ventilation has been assumed with production bogging activities exhausted on the level. Airflow estimates are based on the minimum regulatory airflow requirements with allowance for leakage and air compression with depth.
The maximum airflow rate of 530m3/s is required by Year 5. Establishment of the Savannah North FAR and conversion of Savannah FAR2 to an RAR is required for full production from Savannah North (450m3/s). A number of options were reviewed with raiseboring contractors for the Savannah North surface FAR. The preferred option is a 900m long, 5m diameter shaft to the 1570 exploration drive with a separate 100m extension to the initial stoping level. The maximum modelled exhaust rate through Savannah’s existing exhaust fan is 367m3/s. As such the construction of the Savannah North surface FAR and new surface fan to convert FAR2 to an RAR will be prioritised upon the decision to re-start operations.
Exhaust airflow transfer from Savannah North to Savannah is via an exhaust drive. This exhaust drive will be mined parallel to and concurrently with the Savannah North decline.
To determine the Savannah North cooling requirements, a detailed analysis of site climate data was completed. VentsimTM was used to model heat loads based on the proposed mining fleet. Cooling requirements, based on site weather station data and heat load modelling to achieve target decline temperatures, are initially 6.0MW and increase up to 11MW at full LOM depth.