Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                             PROPOSED RESIDENCE
                             33 ROSS CRESCENT, SUNSHINE BEACH

6.1      Earthworks
Bulk excavations in the order of up to 3.5 m depth/height are anticipated for the terraced construction.
Isolated deeper excavations may be required for underground services and footings. The pool is shown to
be mostly above ground on the drawings, with filling required to create the building terraces (supported by an
RPEQ designed retaining walls). Filling elsewhere is anticipated to be for landscaping only, should be less
than 0.5 m high.

The advice give herein should be treated as preliminary to aid in project planning, and Tectonic must review
detailed design drawings for the residence prior to construction.

6.1.1      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. Similarly, all foundations and remaining structural
elements from the existing residence should be removed and ‘grubbed out.’ Based on the boreholes and site
observations, stripping depths to remove existing unsuitable upper soils should be relatively shallow
however, deeper areas of unsuitable materials may be present across the site, particularly related to the
existing structure (e.g. retaining wall backfill, service trenches, footing excavations). Topsoil materials must
not be used in structural fill; however, may be placed across landscaping areas.

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. It would be prudent to conduct a dilapidation
survey of neighbouring structures prior to commencement of construction; and furthermore it is
recommended that a suitably qualified and experienced civil/structural RPEQ is engaged to provide a
vibration management plan for the proposed construction works.

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).

6.1.2      Excavations
Excavations through the soils should be achievable using conventional earthmoving plant (e.g. small or
larger excavators with toothed buckets). Temporary, unsurcharged 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 3.5 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, may require temporary shoring,
unless assessed and approved by Tectonic during construction.

Permanent cuts or fills 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). Similarly, improvements to the vegetation (i.e. with site appropriate natural species) on
the natural high dune face downslope of the residence would aid in restricting the development initial surficial
failures.

Based on the preliminary design details provided, the above temporary batter slopes may be achievable due
to the favourable proposed positioning of the new structure. However, if spatial or other constraints (e.g.
undermining of sewer and stormwater services under Ross Crescent) do not allow this, temporary shoring


18 June 2024
Report No. 24096-001-Rev0                              14