Attachment 12 - Geotechnical Investigation Report

                             PROPOSED RETIREMENT DEVELOPMENT
                             MYALL STREET, COOROY

6.6      Supplementary Site Investigation
As has been mentioned above, due to the limited information supplied at the scoping stage with the project
brief; the subsurface information gained during this investigation, while giving a good general understanding
of the site is inadequate in addressing ground conditions in several key areas for the project. These relate to:

       Rockhead levels across the site, which have implications for excavatability for bulk earthworks and
        footings, and the pool adjacent to the clubhouse.
       Groundwater levels, which are now more pertinent due to deeper than anticipated cuts; and
        proposed bio-basin invert levels.
       Lack of site-specific information at the proposed bridge and club house sites.
       Global stability of large retaining structures, especially in close proximity to the creek banks to the
        north-west.

Therefore, prior to advancing the project to the detailed/final design stage, we recommend that a
supplementary geotechnical investigation program is completed comprising the following:

       Drilling of three boreholes to 7 m depth using a specialised rotary drilling rig capable of solid flight
        augering, cased wash boring, and rock coring. Rock coring would be proposed when (and if) low
        strength (or stronger) rock is encountered. These three boreholes would be positioned east to west
        across the crest of the rounded knoll to the central-south part of the site, and at the proposed
        clubhouse and pool location. One borehole would have a standpipe piezometer installed to gain an
        understanding of groundwater levels for dewatering purposes in the areas of maximum proposed
        cutting. In-situ testing comprising standard penetration testing (SPT) at selected depths would be
        scheduled during the drilling. Subsequent laboratory testing could be completed on the recovered
        rock core (if encountered) to determine excavatability characteristics.
       Two boreholes (one at each abutment location) at either side of the proposed bridge structure.
        These would be drilled using a specialised rig as specified above into rock (probable foundation for
        bridge piles) to an estimated depth of 15 m BGL. One borehole would have a standpipe piezometer
        installed to understand groundwater level differences compared to the creek level, and those close
        to the proposed bio- basin excavation.
       Detailed field mapping and visual appraisal of the existing creek bank geometry and stability;
        together with conceptual advice regarding bank stabilisation measures (if required).
       In addition a quantitative slope stability assessment would be proposed using the computer program
        Slope/W to confirm that the proposed retaining walls will have a factor of safety against global slope
        instability of at least 1.5 (including how this affects the in-situ stability of the existing creek bank).


7.0      LIMITATIONS
Your attention is drawn to the document “Limitations”, which is included in Appendix C of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of this
report should be, and to present you with recommendations on how to minimise the risks associated with the
services provided by Tectonic for this project.




17 May 2019
Report No. 19071-001-Rev0                               14