Geotechnical Report Dec 2013 Revision 1

                              55 LAKE MACDONALD DRIVE, COOROY


The presence of deeper deposits of alluvial materials within the drainage gullies should not be discounted.
However, our assessment of the site topography and the subsurface conditions in the test pits and boreholes
indicates that there is a low risk of deep alluvial deposits accumulating within the gullies.

6.1.4        Filling
The proposed earthworks include filling of about 0.6m depth across the proposed Lots 16, 18, 19, and 28 to 31
in Stage 3, and Lots 13 to 15 in Stage 4.

Future fill located within residential areas, pavement areas, or within the gully, should be compacted at
moisture contents within the range of ±2% of optimum moisture content for Standard Compaction.
Confirmatory compaction testing must be carried out at regular intervals. A minimum of two tests per vertical
lift is recommended where structural fill heights are to exceed 300 mm. Further details for control and testing
of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial and
Residential Developments. Fill should be placed in 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 (DDR) for
residential areas and gully fill, and 98% DDR for pavement fill (all Standard Compaction method).

Fill must be ‘benched’ into the natural slope, over-filled by 0.5 m (horizontally) and then trimmed back to the
well compacted material.

6.2       Pipelines
The invert level for the sewer and water pipelines are typically expected to be about 1m to 2m BGL. The
anticipated materials at that depth are expected to comprise mostly stiff (or stiffer) high plasticity sandy and
silty clay, and medium dense clayey sand.

The bases for the manholes are expected to be about 2.5m depth BGL.

The founding strata for the pipeline and manholes is expected to have an allowable bearing capacity of at
least 100kPa.

As noted in Section 6.1.2, some groundwater seepage was encountered in the test pit at TP10 in Stage 3
thorough sandy layers within the sandy clay stratum. Seepage should be readily removable from pipeline
excavations by pumping from a sump.

6.3       Pavement Design Parameters
Pavement design will be dependent upon the subgrade materials present after earthworks. Following
anticipated topsoil stripping (and where no filling is proposed), the exposed subgrade will comprise about
100mm of loose to medium dense silty sand over/or clayey sand.

Laboratory testing indicated CBR values of 17% and 19% for the clayey sand (with low plasticity clay fines).
Presumptive CBR values for clayey sand are 5% to 10% (Table 11 of Austroads Guide to Pavement
Technology, Vers 2.1- 2013). Therefore, we recommend adoption of a design CBR of 10% for the clayey
sand.

The silty/sandy clay subgrade is expected to have a CBR value less than 5%, and possibly less than 3% for
silty clay, in which case, additional subgrade treatment may be required, such as over-excavation and
replacement with granular fill (e.g. roadbase)

The above values are applicable to properly drained subgrades only, and should be confirmed by testing
following earthworks. Properly drained subgrades’ require open graded drains, site grades that shed water
and prevent ponding, and subsurface drains adjacent to pavements.

Subgrade inspection and CBR testing should be carried out following bulk earthworks to confirm design CBR
values.

If subgrade materials are allowed to ‘dry out’, significant softening (and resulting subgrade strengths less than
those nominated above) could occur on ‘wetting up’.




2 December 2013
Report N0. 13008-001-Rev1                               12