Attachment F - Geotechnical Report

                              PROPOSED COMMERCIAL & RESIDENTIAL DEVELOPMENT
                              30-32 DOONELLA ST, TEWANTIN

6.0      ENGINEERING ASSESSMENT
6.1      Earthworks
Excavations up to approximately 3 m BGL are proposed for the basement, extending to within about 2 m to
5 m of the property boundaries. No significant filling is proposed.

6.1.1        Site Preparation
Following demolition and clearing of vegetation, stumps, tree roots and disused services should be grubbed
out. Any topsoil should be stripped and stockpiled away from the construction area. We anticipate stripping
depths up to about 300 mm, although deeper pockets of topsoil may exist. Organic rich topsoil materials
should not be used as structural fill, however may be used in landscaping areas.
Following stripping and excavation to the design levels, the subgrade is expected to comprise mostly very
stiff natural clays. Subgrade areas should be ‘proof rolled’ using a fully laden water cart 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 excavation subgrade should be graded to shed surface water runoff away from the construction area. It
would be prudent to install diversion drains at regular intervals around the site to control stormwater runoff
and to prevent flows over cut batters.

It should be noted that during wet weather, the natural soils will tend to cause turbid stormwater run-off due
to the high clay component. 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.

6.1.2        Trafficability
Following stripping, the natural clay 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) to prevent
softening of the clay subgrade and loss of trafficability, which may be incorporated into the permanent works.

Specific design will need to be carried out for working platform requirements for any heavy plant (e.g. cranes
or piling rigs).

6.1.3        Excavations
Excavations within the existing fill and natural soils to the design levels (about 3 m BGL) and for
services/footings (i.e. up to about 2 m deeper) should generally be achievable using conventional
earthmoving plant (e.g. excavators with toothed bucket attachments or small dozers with blades).

6.1.4        Batter Slopes
Temporary cuts in the natural clay materials on site should be formed no steeper than 1V:1H. This may be
achieved by ‘stepped’ vertical batters and horizontal benches not more than 1 m high/wide. Flatter
temporary batters may be required if groundwater seepage is encountered, or if earthworks are undertaken
during a period of heavy rainfall.

Subject to detailed assessment of proposed excavation depths and existing site conditions (e.g. proximity of
existing structures such as buildings and services), it is anticipated that temporary shoring should not be
required. If necessary, it is anticipated that sheet piles (cantilevered, propped or anchored) would be
suitable for the encountered ground conditions to provide temporary lateral restraint.

Permanent cuts in natural stiff (or stiffer) materials and controlled fill batters may be formed at 1V:2H for
batters up to 3 m in height. We understand that the basement cuts will be supported using engineer
designed retaining walls.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations e.g. limits on personnel entry into trenches.


22 June 2018
Report No. 18151-001-Rev0                                7