OPW052025 Report - Part 2

Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed driveway and
future residence at 146 Middle Creek Road, Federal. The investigation was performed by Tectonic for
Oceantobush Pastoral Pty Ltd. The geotechnical investigation comprised site mapping by an experienced
engineering geologist, excavation of test pits, and laboratory testing. We understand that the preferred
site for the residence would be on a topographical high point within the site towards the north-western
corner. The driveway would connect from the existing access point off Middle Creek Road to the east and
extend along a strongly defined ridgeline running through the site.
The crest of the ridgeline was generally rounded, sloping at no more than 8° to either side, estimated to
be between about 10 m and 20 m wide along the majority of the alignment, before dropping to steeper
slopes, ranging from about 15° to 20°. Beyond this, steeper slopes were present extending down towards
the inverts of larger gullies and drainage lines. The site of the proposed house consisted of a relatively
wide, gently sloping knob crest; of at least 20 m width at around 5°. Around 10 m from either side of the
proposed construction ground slopes increased to about 10° to 11°, increasing to 15° downslope.
Test pits TP1 to TP3 were excavated progressively from lower ground to the east/north-east to higher
ground in the south-west. Test pits TP1 and TP2 encountered residual soils and extremely weathered
(XW) materials to depths of between 0.5 m and 1.3 m below ground level (BGL), underlain thereafter by
very low strength to low strength phyllite. Test pit TP3 was positioned below the final rise to the
proposed residence, where slopewash was logged to to 0.25 m BGL, then residual soil and XW material
of stiff to very stiff, silty clay with varying gravel content to 2.5 m BGL, with phyllite below. Test pits TP4
and TP5 were positioned to the edges of the proposed building area, with a relatively similar subsurface
profile logged; of topsoil up to 0.2 m depth, followed by high plasticity, residual silty clay to between 0.4
m and 0.7 m BGL; underlain by highly fractured, medium strength basalt.
Diversion drainage will be required around the future residence, and along the driveway alignment such
that fill embankments are not allowed to become saturated, and sensitive natural drainage features
adjacent are not agitated. Additionally, to minimise the downslope effects of surface waters, channel bars
could be installed at key breaks in slope along the alignment, to intercept flows incrementally.
In accordance with AS2870-2011, the existing subsurface profile could have a characteristic surface
movement due to normal seasonal moisture variation (ys) of up to 75 mm and therefore would be
equivalent to Class H2. All footings for the proposed buildings and any ancillary structures (e.g. retaining
walls etc.) must penetrate through any future fill and found at least 0.5 m below ground level (existing)
into stiff to very stiff residual soils. Footings founded as recommended above in stiff (or stiffer) clays/silts
could be designed using a maximum allowable bearing pressure of 100 kPa (high level/pads), or up to
150 kPa maximum allowable end bearing pressure for steel reinforced, concrete bored piles (e.g. if this
was preferred for post footings).
Following earthworks subgrades are expected to comprise mostly silty clays with varying gravel content
and gravelly clays. Laboratory testing for the high plasticity silty clay soils with no visual gravel content
returned a CBR values of 1.5 % with a relatively high swell value of 5 %. Where the reactive silty clays with
no visual gravel content are present in the subgrade, and a hard surfacing (e.g. concrete) is to be used, a
nominal over-excavation and replacement to 300 mm depth with ‘roadbase’ (or similar granular
materials) would be required to achieve a design CBR value of 3 %. Alternatively, gravel surfacing could
be used with an expectation of some future maintenance. Where the clays contain a significant
proportion of gravel or are gravelly clays (i.e. >30 % gravel) a design CBR of 3 % could be adopted,
subject to the subgrade passing a ‘proof roll.’


Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                 ii