Meyer Judgment Sealed

 

tecténic PROPOSED RESIDENTIAL DEVELOPMENT
geotechnical & environmental a 39 PICTURE POINT CRESCENT, NOOSA HEADS

 

In an adverse case, where ongoing near surface instability occurs on the dune face below the proposed
buildings, and retrogresses; it is possible that the piles may become exposed above the ground surface
temporarily. Thus, some form of lateral capacity must be allowed for in the pile design to address this.

A grout ‘curtain’ must be installed along the along the front (northern) edge of the buildings (at the slope
crest) to a depth of no less than 6 m. The plane extending at 28° from the toe has also been used to inform
the need for protection to the side boundaries. It is possible that sand would be removed from neighbouring
properties with instability on the dune slope, which has the potential to affect the subject site where the
building envelopes do not extend as far north/downslope. The grout curtain should also extend from the
northern edge of the building envelopes, and south along each respective site boundary for about 10 m,
tapering from 6 m deep at the northern end.

It would be prudent to consider the following for the final design and construction of the development
regarding the dune slope below the site.

= One of the primary triggers for large-scale instability of sand dunes is when the toe of the dune is
undermined. This may be caused by excavations or natural erosion. The toe of the dune must be
protected from any such disturbance.

= — In order to minimise future erosion (and initial surficial slipping) it is recommended that the faces of all
dune and batter slopes are protected using with topsoil/mulching of the surface and revegetation, in
combination with a cellular confinement system, such as GeoWeb (or similar), in critical areas such as
stormwater outlet zones. Any erosion or instability will need to be appropriately remediated as soon
as possible after it is identified. If ongoing instability occurs, the slope may need to be retained to
prevent further movement.

6.2 Earthworks
6.2.1 Site Preparation and Trafficability

Following clearing of existing vegetation, any tree stumps, large roots and remnant foundation elements from
the existing structure should be ‘grubbed out’. Any organic rich topsoil should then be stripped and set
aside. Based on the boreholes, topsoil stripping depths are expected to be minimal with pockets of
uncontrolled fill being around 0.7 m depth locally; deeper areas of unsuitable materials may be present on
site.

Topsoil materials must not be used in structural fill; however, may be placed across landscaping areas.
Unsuitable saturated or loose materials encountered should also be removed during subgrade preparation.

It should be noted that compaction of such relatively ‘clean’ sands as structural fill requires the addition of
substantial volumes of water (‘flooding’) combined with the use of vibratory compaction equipment (typically
a vibrating plate). Care must be taken with the use of vibratory equipment (e.g. compactors or sheet piling)
to ensure that there are no adverse impacts on surrounding structures. The impacts from ground vibration
must be closely monitored and managed during construction, to be detailed in a construction management
plan. It would be prudent to conduct a dilapidation survey of neighbouring structures prior to commencement
of construction.

The exposed natural subgrade following excavations will likely be in a loose condition and must be
compacted by several passes of a vibrating plate or minimum 4 tonne vibrating smooth drum roller, in
conjunction with surface application of ample water, to achieve the required compaction.

Due to the potential for development of poor trafficability during construction, placement of a minimum
150 mm thick granular working platform may be required across laydown areas. Specific platform design
would be required prior to mobilisation and trafficking of any heavy mobile plant on the site (e.g. mobile
cranes, concrete pumps or piling rigs).

Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).

 

10 February 2023
Report No. 22511-001-Rev0 12

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