Geo-environmental Investigation · MCU25/0020

Executive Summary
This report presents the findings of a geotechnical and acid sulfate soils (ASS) investigation for a
proposed residential development at 11 Ravenwood Drive, Noosa Heads. The investigation, conducted
by Tectonic for Sunshine Beach Noosa Developments Pty Ltd, involved drilling boreholes at two
locations, in situ soil strength testing, and laboratory testing for ASS. This report outlines the
investigation results and provides recommendations on earthworks, footings, retention systems,
groundwater management, and the extent and severity of ASS.
The proposed development consists of a two-storey house over a single level basement and includes an
arc-shaped pool in the north-eastern corner, near the existing canal revetment wall. Excavations for the
basement will range between 3 m and 4 m deep, with deeper excavations for a car lift pit. Contiguous
piles are planned around the perimeter of the basement.
Subsurface conditions were found to consist of loose to medium dense, silty sand fill, containing gravel
and deleterious materials (e.g. cobbles, building debris etc.) to depths of 2.3 m to 3 m below ground
level (BGL). Below this, mostly medium dense to very dense sands were present, overlying stiff clays at
depth. A firm sandy/silty clay layer was encountered between 4.8 m and 5.5 m BGL in both boreholes,
with some loose sands above. Groundwater seepage was encountered at depths of 1.8 m to 2 m BGL,
with a standing groundwater level of 1.8 m BGL recorded at the completion of the investigation.
Groundwater levels are expected to fluctuate with weather and tidal variations.
After excavation to the design basement level, the subgrade is expected to comprise loose to medium
dense sand, possible with some firm clays in deeper excavation areas. A geotechnical RPEQ must inspect
the foundation subgrade to determine whether any over-excavation is required to form a stable working
platform and foundation for the building. An internal dewatering system, such as sand spears or wells,
will be required to manage seepage and prevent ‘boiling’ of the subgrade. It should be possible to
manage the groundwater extraction system such that the impact on the surrounding, external
groundwater table is negligible. The design of basement walls (temporary and permanent) and floor
must account for hydrostatic pressures.
Settlement analysis has been carried out using Settle 3D and considered a flexible footing system
founded on loose to medium dense sand at around RL – 1.4 m AHD. Our analysis indicates that elastic
and consolidation settlements would be in the order of 30 mm to 45 mm. It is anticipated that a well
stiffened raft slab footing system could be designed to support the proposed building provided that the
estimated total and differential settlements can be accommodated. Should the anticipated settlements
preclude the use of a high-level raft footing solution, a piled footing system would be required.
Considering the likely order of column loads to be supported, steel screw piles (or similar) would need to
be socketed into the dense to very dense sands at about 8 m BGL (existing ground level).
Laboratory test results indicate that the soils above 4 m BGL have minimal acid generating capacity given
that the recorded Acid Neutralising Capacity exceeded the net acidity, presumably due to previous lime
treatment. As such, these soils do not require specific ASS management and n ASS management plan is
not required. Should significant excavations (i.e. >50 m3) extend below 4 m BGL, Tectonic should be
notified to assess the implications associated with ASS.
Groundwater extracted would be expected to require quality adjustment prior to off-site release to
stormwater. Effective treatment of groundwater to achieve suitable release criteria requires reasonable
spatial and temporal consideration. An experienced dewatering sub-contractor should be consulted to
ensure appropriate outcomes can be achieved.



Geo-environmental Investigation Proposed Residence
11 March 2025 | 25040-001-Rev0                                                                             ii