Appendix F - Geo-Environmental Investigation Report · RAL24/0003

                                      PROPOSED SUBDIVISION
                                      41 TALLOWWOOD PLACE, BLACK MOUNTAIN

Executive Summary
This report details the results of a geo-environmental investigation undertaken for a proposed 1 into 2 lot
residential subdivision at 41 Tallowwood Place, Black Mountain. The investigation comprised a site walk-
over by an experienced geotechnical engineer, together with excavating test pits at four locations. This
report summarises the results of the investigation and includes a slope stability assessment along with
recommendations regarding earthworks, retaining structures, site classification, footings, and drainage.
Also included is a feasibility assessment for on-site wastewater treatment and disposal.

A concept layout plan indicates that the proposed subdivision will comprise the reconfiguration of the existing
allotment into two new allotments. The easternmost allotment will retain an existing residence and driveway,
with a new driveway and residence proposed on the allotment to the west.
Subsurface conditions encountered in the test pits across the proposed new building envelope (i.e. TP1, TP3
and TP4) are summarised as follows:
            Topsoil/Fill – comprising friable silty clay of medium plasticity, with varying organic, gravel and
             cobble content to depths between 0.3 m and 0.6 m below ground level (BGL); then
            Residual Soils – very stiff to hard, silty clay of high plasticity and clayey silt of high liquid limit to
             depths between 1 m and 1.2 m BGL; underlain by
            Extremely Weathered (XW) Materials/Weathered Phyllite – hard, silty gravelly clay of medium
             plasticity and very low strength phyllite, rapidly grading to low strength (or stronger), with toothed
             bucket refusal or very slow digging achieved at depths between 1.7 m and 2.3 m BGL.

Test pit TP2 was excavated on the proposed driveway alignment, downslope of a rock fill embankment
(inferred to be work to remediate a landslide) and encountered a friable topsoil layer to 0.3 m BGL, then a
stiff to very stiff, silty gravelly clay of high plasticity, comprising colluvium 1/landslide debris, to 3 m BGL.
Based on our site observations this area appeared to have been impacted by previous landslide movement
and thus, the proposed driveway alignment will need to be repositioned. It is envisaged that the new
driveway alignment could trace an existing track to the south of the proposed new building area.

No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over around the
proposed development area. Increased soil moisture conditions were observed within the colluvium
materials in test pit TP2, excavated in the previous landslide.

Based upon the existing site conditions, together with our qualitative assessment and quantitative analysis,
and subject to the implementation of recommendations given in this report, it is our assessment that there
would be a Low Risk of global slope instability affecting the proposed new residential development.

Based on the results of the investigation, the likely new building area would be equivalent to Class H1 in
accordance with AS 2870-2011 Residential Slabs and Footings. Site classification will need to be
reassessed at building stage to consider the extent of any earthworks proposed.
All footings supporting the proposed new residence and ancillary structures (including all retaining walls) must
penetrate through any fill and natural soils to found at least 0.3 m into very low strength (or stronger) phyllite.
High-level footings (typically in deeper cut areas) and steel reinforced concrete bored piles (or similar)
founded at least 0.3 m into very low strength (or stronger) weathered phyllite may be designed using an
allowable bearing pressure 400 kPa (footings) and 600 kPa (piles), respectively.
It is recommended that on-site effluent treatment and land application for the new allotment should comprise
a household sewage treatment plant producing a minimum of Secondary standard effluent and connected to
a wastewater irrigation system. The soil profile across the likely effluent disposal area typically comprised
Light Clay (Soil Category 5). Including a slope reduction on the design irrigation rate, a minimum land
application area of 560 m2 will be required for the proposed new residence. There is sufficient room on the
proposed lot to accommodate this.




1
    A mixture of soil and/or rock (of various sizes) deposited over geological time due to movement induced by gravitational force


22 May 2023
Report No. 23099-001-Rev0                                             ii