Appendix D - Geotechnical Report

Executive Summary
This report presents the results of a geotechnical investigation and slope stability assessment undertaken
by Tectonic for Threedom Nominees Pty Ltd.
Proposed layout plans indicate that an existing single rural allotment (Lot 3 on SP258991) will be
subdivided in to five future allotments. No significant earthworks are anticipated for the subdivisional
works, with all lots having direct frontage to Black Mountain Range Road.
The investigation comprised a site walk-over by an experienced engineering geologist and excavation of
seven test pits to depths between 1.2 m and 3.1 m below ground level (BGL), together with in situ
strength testing and targeted laboratory testing. No groundwater was encountered in the test pits at the
time of investigation; however, seepage may develop following prolonged or heavy rainfall.
Published mapping indicates the site is underlain by the middle Triassic age Kin Kin Beds (meta-
sedimentary strata). Test pits generally encountered a profile in line with this, featuring a thin topsoil
layer (0.1 m to 0.2 m BGL) underlain by very stiff residual soils and extremely weathered (XW) materials to
about 0.8 m to 1.3 m BGL, then highly to distinctly weathered, very low to low strength phyllite to the
termination depths. Quartz veins were encountered at two test pit locations.
The site walk-over did not identify indicators of existing slope instability within the proposed
development areas (e.g. tension cracking, hummocky ground or the like) and the proposed building
areas were generally positioned on favourable rounded spur crests or broad flank slopes. A Low Risk of
instability for the proposed subdivision would be achievable, with an acceptable minimum factor of
safety against global instability in excess of 1.5.
From a geotechnical and slope stability perspective, the proposed subdivision layout is considered
acceptable. Key recommendations for future development (per-lot, at Building Works stage) include:
minimising cut and fill; limiting unretained fill to 1 m height where natural slopes are 10° or greater (or
otherwise providing engineered retention); and undertaking site-specific geotechnical assessment for
each dwelling location within the nominated building envelopes.
Footings supporting structures (including retaining walls) are to penetrate through fill and any unsuitable
natural soils (if encountered) and be founded a minimum of 0.5 m into stiff (or stiffer) residual soils.
Conventional footings may be suitable in some areas; however, bored piles are anticipated where fill
pads are constructed or buildings are located on steeper ground. A preliminary site classification of
Class M (according to AS 2870-2011) would apply for the existing natural subsurface profiles, extending
to Class H1 where up to 1 m of fill is placed.
Surface and subsurface drainage must be designed to divert stormwater away from building platforms,
batters and retaining structures, with collected flows discharged in lined drains/pipes to stable outlets at
least 5 m downslope of structures. Subsurface disposal of stormwater is not recommended.
As reticulated sewer is not available, on-site wastewater treatment and disposal will be required for each
lot. The land application area required for each lot varies due to slope reduction factors (per
AS1547:2012). For feasibility these have been estimated as the following:
▪   Lots 1 & 2:          470 m²
▪   Lots 3 & 4:          840 m²
▪   Lot 5:               420 m²
Prior to construction, a site specific ‘soil test,’ slope stability assessment and effluent disposal design
should be prepared for future building works on each of the proposed allotments.

Geotechnical Investigation
10 June 2026 | 26073-001-Rev0                                                                                  ii