June 2022 G-01483.84-017-R-Rev0 Because the 47 Site investigations including test pits and cored boreholes have been completed in the footprint of the SFA as shown in Figure 5. Laboratory testing has also been carried out on samples from the test pits and boreholes to characterise the encountered materials. The ground conditions can be summarised as follows: Boreholes (WFBH015/WFBH016) – both of the boreholes are located upslope of the infrastructure area. ▪ Very soft SILT (topsoil) from the ground surface to approximately 0.20 m bgl ▪ Soft to stiff silty CLAY from 0.20 m bgl to approximately 11.6 m bgl ▪ A very soft lenses of CLAY at approximately 9.5 m bgl ▪ Medium dense silty GRAVEL from 11.6 m bgl to approximately 12.7 m bgl ▪ Very stiff silty CLAY from 12.7 m bgl to approximately 15.45 m bgl ▪ Piezometric levels were measured at about 2 m below surface in WFBH015 and 5 m below surface in WFBH016 Test pits (WFTP020-WFTP023) – these test pits are located downslope of the infrastructure area: ▪ Very soft SILT from ground surface to approximately 0.20 m bgl ▪ Firm to stiff silty CLAY from 0.20 m bgl to approximately 4.1 m bgl ▪ Groundwater was encountered in WFTP023 at about 3 m below surface Test pits (WFTP030-WFTP033) – These test pits are based upslope of the infrastructure: area ▪ Very soft CLAY from ground surface to approximately 0.20 m bgl ▪ Firm to very stiff sandy CLAY from 0.20 m bgl to approximately 1.5 m bgl ▪ Very stiff CLAY from 1.5 m bgl to approximately 4.0 m bgl ▪ Groundwater was not encountered in WFTP030 to WFTP033 The geotechnical investigations indicate that the site is underlain by silts and clays interpreted to be volcanic ash or colluvium, gravelly alluvium and underlying highly weathered, very weak tuff. The thickness of these various layers varies across the site. Consideration will need to be given to stability of the cut slopes and suitability of the excavated material and subgrade materials for construction of the proposed fill platform. Widespread shallow slope instability is evident on the hill slopes around the site. It is therefore possible that some form of slope retention, such as mass gravity retaining walls may be required for part of the cut slopes. The in situ volcanic and alluvial soils are expected to generally form a suitable subgrade, though sheeting (nominally 0.5 m thickness) with a suitable granular aggregate will be required to maintain a reliable running surface. This granular material is unlikely to be available from tunnel spoil early in the project and will likely need to be imported. However, some suitable (non-acid generating) material may be available elsewhere on site or may be selectively won from tunnel spoil comprising strong volcanic rock. Fills placed as part of the mine industrial area will require careful evaluation because some of the volcanic soils exhibit geomechanical properties that may lead to difficultly in effective compaction. Further evaluation
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