Compiled Reference Document - MCU25/0020 (As Published)

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 two 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 from about 7 m deep generally up to 8 m deep for a car lift pit. Shoring is 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 medium dense to
dense sand/indurated sand. 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 medium dense sand at around 7 m BGL. Our analysis indicates that elastic and consolidation
settlements would range up to the order of 30 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, or other
considerations, preclude the use of a high-level raft footing solution, a piled footing system would be
required.
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. However, the soils excavated
below 4 m BGL, up to 7 m to 8 m BGL, would likely include ASS for which an ASS management plan
would be required and is included herein.
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
4 June 2025 | 25040-001-Rev1                                                                               ii