Attachment 12 - Geotechnical Investigation Report

                             PROPOSED RETIREMENT DEVELOPMENT
                             MYALL STREET, COOROY

Executive Summary
This report details the results of a geotechnical investigation undertaken for a proposed retirement
development at Myall Street, Cooroy. The investigation was performed by Tectonic for Ruby Developments
Pty Ltd. The investigation comprised drilling of boreholes at 13 locations, together with geotechnical
laboratory testing. This report summarises the results of the investigation and provides general geotechnical
recommendations regarding earthworks, footings, pavements, retaining walls and drainage.

According to preliminary layout plans (supplied subsequent to fieldwork being completed), the proposed
development will include around 175 standard residential lots spread across the entirety of the site area. A
two-storey club house with sporting facilities, including a swimming pool, a bowling green and a tennis court
at ground level is proposed central to the eastern boundary. The overall site would be accessed off Myall
Street, via a two lane driveway to the north-western corner, which appears to also link to a future club house
and car parking for the Cooroy Golf Club. Crossing of an existing creek which cuts across the north-western
corner of the site will be required to facilitate this access.
Large scale, bulk earthworks are anticipated, with the majority of the site being in cut (up to 6.5 m deep);
along with filling of around 3 m generally, with localised embankments up to 5 m height in the vicinity of bio-
basins proposed to the north and south of the overall site.
Boreholes encountered relatively consistent subsurface profiles, featuring topsoil to around 0.2 m below
ground level (BGL); overlying mostly stiff, high plasticity silty clays, becoming very stiff (or stiffer) below
about 1 m depth. Possible rock was encountered in one borehole (BH13) in the southern part of the site at
2.1 m BGL. We note that details regarding the scale of the proposed bulk earthworks were not available at
the time of planning and undertaking our assessment, and while weathered rock was not encountered
generally within the scope of the boreholes (generally to 3 m BGL), the presence of weathered rock cannot
be discounted within the depth of the proposed cut earthworks (up to about 6.5 m).

Standing groundwater was recorded in the lower lying ground in the north-western part of the site at 0.65 m
BGL; 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 be possible within the upper 1 m of the subsurface
profile, and from deeper horizons. Excavations for the proposed bio-basins to the north and south of the site
may experience the groundwater table/inflow within 1 m of the ground surface, resulting in more significant
dewatering.

Excavations within the natural soils should generally be achievable using conventional earthmoving plant, to
the maximum depth of the investigation boreholes. However, there is an outstanding risk that rock 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.

In cut areas where rock is not exposed at the foundation subgrade, an AS2870 site classification of Class H1
is recommended, and Class H2 in fill areas. Normally dimensioned, high-level footings should be appropriate
for all proposed structures, subject to Level 1 inspection and testing to produce ‘controlled’ fill. Footings
should found in to the following strata using recommended design allowable bearing capacities of 100 kPa
(Level 1 controlled fill); 200 kPa (natural very stiff clay); or 400 kPa (very low strength rock).

Design of pavements on natural clay subgrades or site won Level 1 controlled fill should be based on a CBR
value of no more than 3%.

Due to the difference in information supplied with the project brief, compared to more recent earthworks
proposals provided to us; the subsurface information gained during this investigation, is inadequate in
addressing ground conditions in several key areas for the development. These relate to excavatability,
rockhead levels; groundwater levels; lack of specific subsurface information at the bridge and clubhouse
sites; and global stability for large retaining walls. Therefore, a supplementary geotechnical investigation is
proposed, as detailed in this report.




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