20220109 - Geotech · OPW23/0182

                             PROPOSED RESIDENCE
                             21 MAHER TERRACE, SUNSHINE 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       Additional Investigation
Based on the results of the investigation and accessibility issues at the time of the investigation, a deeper
investigation is required to confirm the required footing depths for the detailed design phase. This
investigation would be undertaken following the demolition of the existing residence and would comprise the
drilling of one borehole using a 4WD mounted solid flight auger rig to depths between 6 m and 8 m BGL, or
shallower refusal. Tectonic can undertake such an investigation for an additional fee, following detailed
design drawings and demolition of the existing residence, if requested.

6.2       Development Limitations
It is anticipated earthworks will include cuts for the basement level of up to about 4 m deep in in the rear with
only minor filling expected to be required up to about 1 m in height to form finished surface levels. Should
proposed cuts/fills exceed this quantity, Tectonic must assess the impacts on slope stability before
construction.

6.3       Earthworks – General
6.3.1       Site Preparation and Trafficability
Following removal of the existing buildings and associated construction elements (e.g. footings, services,
etc.), any tree stumps and large roots should be ‘grubbed out’. Any organic rich topsoil and uncontrolled fill
should then be stripped and set aside. Based on the boreholes, topsoil stripping depths should generally
range between 0.1 m and 0.2 m BGL, with uncontrolled fill encountered to 0.8 m BGL (existing ground
levels) in borehole BH1. 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 natural material such as any saturated, loose or soft materials encountered should also be
removed during subgrade preparation for bulk earthworks.

After stripping and excavations to design levels, the subgrade is generally expected to comprise stiff to very
stiff residual clays, medium dense (or denser) sands and possibly weathered sandstone. Although relatively
good trafficability should be encountered during ‘dry’ weather conditions, the clayey soils could be subject to
strength loss from repetitive vehicle loading (particularly from rubber tyred plant) or if they become wet.
Placement of imported rock (about 150 mm thick) may be required to provide stable access along high traffic
areas, or on steeper gradients (e.g. access ramps), particularly following periods of heavy or prolonged
rainfall.

Following bulk excavation, the subgrade must be inspected by a suitably qualified and experienced RPEQ
geotechnical engineer (‘Geotechnical Engineer’) to determine whether any over-excavation is required to
achieve a stable subgrade for construction activities, and a suitable subgrade for the selected foundation
system (Ref. Section 6.4.2).
Groundwater seepage was not encountered during the investigation; however, depending on the prevailing
weather conditions at the time of construction groundwater seepage may be encountered at shallower
depths.

The site should be graded to shed surface water runoff away from construction areas. Swale drains should
be constructed along the toes of any cuts, and above cuts where possible, with the drains graded to
discharge collected waters to a control point before release off-site. Appropriate erosion and sediment
control measures must be installed prior to commencement of earthworks at this site.




16 July 2021
Report No. 21236-001-Rev0                              11