Reference Document - OPW25 0085 (AS Published)

Executive Summary
This report details the results of a geo-environmental investigation undertaken for a proposed one into
nine lot residential subdivision at 44 Hoy Road, Lake Macdonald. The investigation comprised a site
walk-over by an experienced geotechnical engineer, together with excavating test pits at twelve
locations. This report summarises the results of the investigation and includes a slope stability
assessment along with recommendations regarding earthworks, retaining structures, footings, drainage
and pavements. Also included is a feasibility assessment for on-site wastewater treatment and disposal.
A concept layout plan indicates that the proposed subdivision will comprise nine new allotments with a
new central road, accessed via Hoy Road at the northern end of the site. The proposed lots are generally
about 1.5 ha in area, with one larger balance lot to the east, of 7.85 ha that includes a 110 m wide
environmental covenant that follows the natural drainage line. We understand that the site will remain
unsewered and therefore, on-site wastewater management will be required.
Subsurface conditions encountered in the test pits comprised loose, silty and clayey sand topsoil from
the ground surface to depths up to 0.3 m below ground level (BGL), followed by mostly medium
dense/stiff (or stronger), clayey sands or silty clays/clayey silts to depths between 0.8 m and 3.2 m BGL.
The soils were generally underlain by very low to low strength siltstone/sandstone to the test pit
termination depths.
No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over around
the proposed development areas. Groundwater seepage was encountered in five of the twelve test pits
during the investigation at depths of between 0.1 m and 0.9 m BGL. Groundwater may develop or be
encountered at higher levels following prolonged/heavy rainfall.
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 subdivision.
There is a risk that elevated soil moisture conditions could occur if the works are undertaken during a
period of wet weather and shallow groundwater could become perched in the sands above the
underlying less permeable clay. The upper sands could then become particularly unstable and very
difficult to work, proof roll or compact under elevated moisture contents. Allowance would then need to
be made for subgrade removal or improvement works during the earthworks phase.
Preliminary characteristic surface movements for the natural soil profile are generally estimated to be
equivalent to Class H1 in accordance with AS 2870-2011. Site classification will need to be reassessed at
the Building Works stage to consider the extent of any earthworks proposed. It should be noted that
filling must not exceed 1.5 m high unless assessed and approved by a geotechnical RPEQ before
construction.
All footings supporting the proposed residences and ancillary structures (including all retaining walls) on
slopes of 10° or less must penetrate through any fill and unsuitable natural soils to found at least 0.5 m
into stiff/medium dense (or stronger) residual soils, or future controlled fill. On slopes greater than 10°
footings must be founded at least 0.5 m into very low strength (or stronger) sedimentary rock. The
subsurface conditions encountered indicate that steel reinforced concrete bored piles or ‘bucket piers’
will generally be required unless excavations expose the required foundation materials at or near the
subgrade in which case conventional high level strip or pad footings would be appropriate.
Pavement design will be dependent upon the subgrade materials present after earthworks. Pavement
design could be carried out adopting a design CBR value of 3% for the natural clay and silt subgrades


Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                             ii