Geotehnical Investigation Report · OPW22/0140

                             PROPOSED UNIT DEVELOPMENT
                             37 BOTTLEBRUSH AVENUE, NOOSA HEADS

3.0      METHOD OF INVESTIGATION
As part of our assessment, a desk-top study was carried out comprising a review of published geological
maps and Noosa Shire Council (NSC) planning overlays and site information.

An experienced geotechnical engineer from Tectonic undertook a site walk-over on 21 April 2022 to
visually assess any indicators of past slope instability. The walk-over included assessment of soil
exposures, surface drainage conditions, topography, evidence of erosion or past instability, and vegetation
coverage/condition.
This was incorporated with a subsurface investigation comprising the drilling of three boreholes
(designated BH1 to BH3) in accessible areas across the site. Borehole BH1 was drilled within the north-
eastern driveway area using Tectonic’s 4WD mounted solid flight auger rig to 6 m below ground level
(BGL). Due to access restrictions, boreholes BH2 and BH3 were drilled along the southern outdoor area to
3 m BGL using hand augering techniques.

In-situ testing, using the dynamic cone penetrometer (DCP) method, was carried out from the ground
surface and within each borehole, extended downhole below drilling termination to depths of 4.5 m and
5.2 m in boreholes BH2 and BH3 respectively. Boreholes were backfilled on completion using spoil arising
during drilling, with excess material slightly mounded at surface.

Laboratory testing was not warranted due to the sandy nature of the subsurface conditions encountered.
Borehole locations are shown in Figure 1 attached at the end of this report, with borehole and DCP logs
presented in Appendix A, together with explanatory notes. Borehole coordinates were recorded using a
handheld global positioning system (GPS).

The results of the desk-top study and walk-over survey are included in Section 4 below.



4.0      RESULTS OF INVESTIGATION
4.1      Site Description
The site is formally described as Lot 133 RP 88934, covering an area of 491 m2, with frontages to
Bottlebrush Avenue and Berrima Row to the south-west and south-east, respectively. At the time of the
investigation the site contained a two-storey, residential unit building, with a concrete driveway on the
eastern side of the property connecting to Berrima Row. The grounds contained various small garden
spaces, with an outdoor area having artificial grass and rubber mat surfacing on the southern side of the
building. Neighbouring properties consisted of residential allotments to the north and west.

Inferred cut and fill earthworks had taken place across the site and adjacent areas. The site had been
levelled with cutting estimated up to about 2.2 m deep along the southern driveway boundary and filling
visually estimated to be up to about 1.5 m high to the south. Cut and fill embankments were retained by the
existing building as well as concrete block and wooden walls ranging in heights up to 2.2 m.

An existing brick retaining wall located on the western frontage of the allotment appeared to be leaning
slightly or tilted. A wooden retaining wall located downslope of the allotment also appeared slightly bowed
and poorly maintained. Generally, the existing building and other retaining walls, together with structures on
neighbouring properties. appeared to be in good condition, exhibiting no obvious signs of structural distress
(e.g. cracking, misalignment, etc.). Minor cracking was observed in the concrete driveway slab. This and the
above mentioned retaining wall tilting are likely related to consolidation and lateral/creep movement of the
upper loose sands as opposed to slope instability.
A 2021 aerial image of the site and surrounds is shown in Text Figure 2 overleaf.




13 May 2022
Report No. 22151-001-Rev0                             2