Attachment 12 - Geotechnical Investigation Report

                              PROPOSED RETIREMENT DEVELOPMENT
                              MYALL STREET, COOROY

6.0      ENGINEERING ASSESSMENT
6.1      Earthworks
It is anticipated that bulk earthworks for the site would include an extensive cut area across the majority of
elevated area, of up to 6.5 m depth. Filling of the lower lying ground the north and south is expected to
generally be in the order of 1 m to 4 m height, although in areas adjacent to the proposed bio-basins, it is
noted that retained embankments may range from 3 m to 5 m.

6.1.1        Site Preparation
Any existing vegetation and topsoil should be stripped away from the construction area. We anticipate
stripping depths on average of about 0.2 m BGL to remove existing topsoil, although deeper pockets may
exist. Historical aerial images indicate that some mature trees were present sporadically across the site until
at least early 2016. Therefore, any large roots or remaining stumps should also be ‘grubbed out’ as part of
the preparatory works. Organic rich topsoil materials and water softened upper soils (which may occur to at
least 0.3 m to 0.4 m BGL [Ref. Section 3.0 / Figure 1]) should not be used as structural fill or be placed under
pavement areas. Site-won material from the bulk earthworks cutting below these depths comprising high
plasticity, residual clay soils would be suitable for re-use as structural fill on site, subject to the suitable
moisture conditioning and handling (Ref. Section 6.1.4 below).
Following stripping, and excavation to the design levels, the subgrade may be expected to comprise mostly
high plasticity, residual silty clays; however in areas of the site currently at higher elevations, it is possible
that rock would be exposed.

Subgrade areas should be ‘proof rolled’ using a fully laden water cart or minimum 10 tonne roller in order to
identify any ‘soft spots’. Treatment of ‘soft spots’ may involve excavation and replacement with select fill or
tyning, appropriate moisture adjustment and compaction. Proof rolling must be carried out in the presence of
a suitably qualified and experienced RPEQ engineer.

The site should be graded to shed surface water runoff away from the construction area. It should be noted
that during wet weather, the natural soils present will tend to cause turbid stormwater run-off due to their
clayey nature. It is recommended that subgrades should be ‘sealed off’ by several passes of a smooth drum
roller at the end of each working day to minimise erosion potential during rainfall events. This would be
particularly pertinent given the presence of the creek in the north-west corner of the site.

6.1.2        Trafficability
Following stripping, the natural clay (and possible rock) subgrades should generally be trafficable for most
construction plant, however would tend to lose trafficability when affected by rainfall and from repetitive
wheel loadings, particularly from heavy rubber tyred plant. Placement of a granular select fill (e.g. minimum
CBR 15%) nominally 150 mm thick may be required on heavily trafficked areas (e.g. site entry/driveway and
laydown areas) to prevent softening of the clay subgrade and loss of trafficability.
Specific design would need to be carried out to determine working platform requirements for any heavy plant
(e.g. cranes or piling rigs), and a rock fill platform may be required.

6.1.3        Excavations
Excavations within the natural soils should generally be achievable using conventional earthmoving plant
(e.g. 20 to 30 tonne excavator(s) with toothed bucket attachments), to at least the maximum depth of the
investigation boreholes (generally 3 m depth).

The presence of very low strength (or stronger rock) was indicated in BH13, to the south-west of the site at
2.1 m BGL. The extent or nature of the weathered rock material has not been confirmed by the investigation.
Therefore, there is an outstanding risk that such material may be encountered across significant portions of
the proposed cut areas. Depending on the weathering profile, strength and jointing characteristics of the
rock, localised ripping (with excavators); larger scale heavy ripping (with dozers); or indeed pneumatic
breaking may be required (particularly in confined excavations (e.g. service trenches).




17 May 2019
Report No. 19071-001-Rev0                                8