Attachment 12 - Geotechnical Investigation Report

                             PROPOSED RETIREMENT DEVELOPMENT
                             MYALL STREET, COOROY

To gain information on the rock material prior to the construction stage, a supplementary site investigation is
recommended (Ref. Section 6.6 below). This would be particularly relevant for the proposed club house and
adjacent pool, situated in an area of between 3 m and 6 m cut below the existing ground level.
Temporary (short term) unsupported cut batters may be constructed at a maximum slope of 1V:1H in the
natural soils and 2V:1H in low strength (or stronger) rock, provided that groundwater is not encountered.
Flatter batters may be required if groundwater seepage is encountered, or if earthworks are undertaken
during or immediately following a period of heavy rainfall.

Permanent cut/fill batters must be formed no steeper than 1V:2H. Steeper batters may be permissible in
weathered rock subject to inspection and confirmation by an RPEQ geotechnical engineer. Where such
battering is not possible, retention will be required.

Permanent soil or fill batters will require erosion protection such as revegetation by landscaping (e.g. topsoil,
jute matting and planting).

Any excavations required adjacent to existing structures (e.g. where construction is staged), will need to be
specifically designed to ensure that the structures are not adversely affected ('undermined').

6.1.4        Filling
Generally, the existing natural soils where free of deleterious materials (e.g. organics etc.), and weathered
rock (if broken down to a suitable particle size), are expected to be suitable for re-use as structural fill,
subject to appropriate moisture adjustment. Dependent upon weathering and defects, it may not be possible
to break down rock to a suitable particle size for placement in controlled fill (further site investigation is
recommended to mitigate this outstanding issue). Laboratory testing indicates that the natural residual soils
are very highly plastic, and thus may be difficult to handle and place, particularly if they are moist of optimum
moisture content (OMC). The site won high plasticity clays may be re-used as structural fill, subject to
appropriate moisture conditioning, however the impacts on site classification and pavement subgrades
should be considered.

Should importation of fill be required, it is recommended that such fill should comply with the following:

       Minimum CBR of 10%
       Maximum plasticity index of 15%
       Shrink-swell index ≤ 1%
       Maximum particle size of 100 mm
All imported and suitable site-won fill should be compacted at moisture contents within the range of ±2% of
OMC for compaction. Fill particles (e.g. weathered rock) greater than 150 mm must be excluded. Fill should
be placed in maximum 250 mm thick (loose thickness) layers and be compacted by repeated rolling to
achieve a dry density ratio of at least 95 % of the maximum Dry Density Ratio (Standard Compaction
method), or 70 % of the Dry Density Index for cohesionless fills (e.g. sands or gravels).

Confirmatory compaction testing must be carried out at regular intervals. Details for inspection and testing of
fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and Residential
Developments. We recommend placement of fill to Level 1 standards, as residential lots are present across
the majority of the proposed fill areas (based on the preliminary earthworks information, an extract of which
is presented in Text Figure 1); as the fill is to support building/slab loads within residential allotment
boundaries.




17 May 2019
Report No. 19071-001-Rev0                               9