Attachment 12 - Geotechnical Investigation Report

                                  PROPOSED RETIREMENT DEVELOPMENT
                                  MYALL STREET, COOROY

6.2       Building Foundations
6.2.1        Indicative Site Classification
According to AS2870-2011 Residential Slabs and Footings, we have determined the site classification
resulting from the proposed range of earthworks (up to about 5 m of filling; up to 6.5 m of cutting).

In cut areas, where rock is not exposed, the natural clay soils are expected to have a classification of
Class H1, with an estimated characteristic surface movement (ys) equivalent to 40 mm to 50 mm. Due to the
highly plastic, and highly reactive nature of the potential site-won clay material, in fill areas ys ≈ 50 mm to
60 mm has been estimated, and consequently a classification of Class H2 is recommended as per AS2870.

To reduce the differential ground surface movements across the site earthworks could be designed and
constructed to use a fill ‘capping layer’ material, with a low reactivity to moisture content variation. Presented
in Table 3 are the required thicknesses of the inert fill capping layer required to achieve reduced ground
surface movements.
Table 3: Effect of inert engineered fill in reducing characteristic ground surface movement
                                                             Estimated characteristic ground surface
   Thickness of inert engineered fill layer (m)
                                                                        movement (mm)
                            0.5                                    40 to 50 (Class H1 equivalent)
                        ≥0.75                                      30 to 50 (Class M equivalent)


The above would also apply where an inert fill capping layer was constructed in cut earthworks areas where
highly reactive natural clays are exposed at subgrade level, resulting in over-excavation and replacement.

Clean granular fill is not recommended for use in construction of the inert engineered fill platform. Any fill
should contain adequate fines so that water does not permeate through the inert fill platform and infiltrate
into the underlying highly reactive clays.

The imported low reactivity fill material would need to be relatively inert to moisture variation having a shrink-
swell index (Iss) value of less than 1 %. The appropriate select fill material is expected to have a plasticity
index of less than 15 %, and a fines content of no greater than 30 %. Confirmatory laboratory testing is
recommended to be undertaken on the potential fill material to assess the suitability for use on site. Site won
weathered rock, if available from deeper excavations, may be suitable for re-use as inert fill capping.

Movement sensitive structures would need to be positioned a minimum of 3 m from the edge of the fill
replacement areas to limit effects from potential shrink-swell behaviour of the adjacent natural ground.




17 May 2019
Report No. 19071-001-Rev0                               10