RFI - 21327-001-Rev0 Plover St

                              PROPOSED RESIDENCE
                              34 PLOVER STREET, PEREGIAN BEACH

6.0      GEOTECHNICAL RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.

6.1       Development Limitations
It is anticipated earthworks will include cuts for the lower level of up to about 3 m deep in the western part of
the site, with filling expected to be required up to about 1 m in height to form finished surface levels. Should
proposed cuts/fills exceed these quantities, Tectonic must assess the impacts on slope stability before
construction.

6.2       Earthworks – General
6.2.1       Site Preparation and Trafficability
Following clearing of existing vegetation, any tree stumps and large roots should be ‘grubbed out’. Any
organic rich topsoil should then be stripped and set aside. Based on the boreholes, topsoil stripping depths
should generally range up to about 0.1 m deep; however, deeper areas of unsuitable materials may be
present across the site. Although not encountered in the boreholes any existing uncontrolled fill must be
stripped from construction areas. Therefore, it is possible that deeper pockets of topsoil or uncontrolled fill
may exist and deeper, localised grubbing out of tree roots may be required.
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 and infrastructure (e.g. sewer main
along the northern boundary). The impacts from ground vibration must be closely monitored and managed
during construction. 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).

6.2.2       Excavations
Excavations through the soils should be achievable using conventional earthmoving plant (e.g. small or
larger excavators with toothed buckets). Temporary unsurchaged cuts in the very loose to loose sand could
be formed at 1V:2H, and at 1V:1.5H in medium dense (or denser) sand, for batters up to 3 m in height. Flatter
batters or temporary retention may be required if groundwater seepage is encountered or if sloughing of dry
sand occurs at the batter surface. Deeper cuts, if required, should be specifically assessed by Tectonic.

Permanent cuts or fills on the dune slope greater than 1 m must be retained. Permanent cut and fill batters up
to 1 m high shall not exceed 1V:2.5H. Any permanent soil and fill batters will require erosion protection (e.g.
revegetation or surface protection). Higher unretained fill batters may be permissible along the eastern
boundary (dune toe) subject to an assessment by an RPEQ geotechnical engineer.




10 September 2021
Report No. 21327-001-Rev0                               9