Attachment 12 - Geotechnical Investigation Report

                             PROPOSED RETIREMENT DEVELOPMENT
                             MYALL STREET, COOROY

6.4       Pavement Design Parameters
Pavement design will be dependent upon the subgrade materials present after earthworks.

Following earthworks, the pavement subgrade may comprise silty clays of mostly high plasticity, or rock.
CBR testing indicates that the existing natural clays have CBR values in the order of 2.5 % to 4 %. Where
future fill provides at least 500 mm deep coverage across the natural clays or in areas of cut exposing rock,
pavement design may be based upon the CBR value of these materials. Where rock is exposed following
bulk earthworks, a regulating layer of 150 mm minimum thickness is recommended below the main
pavement make-up.

Design of pavements on natural clay subgrades or site won controlled fill may be based on CBR value of no
more than 3 %. Proposed pavement subgrades must be ‘proof rolled’ and visually assessed by an
experienced RPEQ engineer to ensure no heaving or unacceptable subgrade deflection occurs. Should
heaving or deflection occur, excavation of ‘soft spots’ may be required, followed by reinstatement with
compacted select granular fill.

The recommended design CBR value is applicable to properly drained subgrades only, and must be
confirmed by subgrade inspection and testing, if necessary, by an experienced RPEQ engineer following
earthworks. ‘Properly drained subgrades’ require open graded drains, site grades that shed water and
prevent ponding, and subsurface drains adjacent to pavements.

6.5       Groundwater and Drainage
Groundwater was not encountered in the majority of the investigation boreholes, although noting that most
holes only extended to 3 m BGL during this investigation. A standing groundwater level was recorded in
borehole BH5 in the lower lying ground to the north of the site at 0.65 m BGL, following drilling; with
seepages noted on the flanks of the higher ground at between 0.5 m and 0.75 m BGL.

Across the cut areas therefore, minor groundwater seepages may therefore be possible within the upper 1 m
of the subsurface profile, and could also be present at the soil/rock interface, and deeper. If it is encountered
during construction, such seepage should be manageable using graded open surface drainage and/or a
submersible sump-pump(s). Excavations for the proposed bio-basin to the north of the site are shown in the
drawings to extend to around 2 m below the existing ground level. This is in the vicinity of borehole BH5
noted above, and therefore the groundwater table may be expected within 1 m of the ground surface. The
result of this would be that more significant dewatering may be required during construction.

Adequate site drainage must be installed to ensure that surface water runoff is directed away from
pavements, walls and footings. Subsurface disposal/infiltration of stormwater is not recommended due to
the potential to adversely affect the performance of footings and pavements, and the relatively impermeable
nature of the natural clays on site. Swale drains must be constructed upslope of any batters and retaining
walls.
Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).

All stormwater should be piped or channeled to discharge to the future inter-allotment drainage network, or
at least 10 m downslope of any structure (including retaining walls) or fill embankments to the creek in the
north-west of the site.




17 May 2019
Report No. 19071-001-Rev0                              13