geotech report · 132003.220660.5

6.9.2 Engineered Fill
For treatment of any poor subgrade areas and raising of site surface levels, engineered fill should be used.
This would require removal of the thin road base layer placed to facilitate proof rolling. Consequently, when
reinstating poor subgrade areas, it would not be necessary to place the thin road base layer before placement
of the engineered fill to treat such poor subgrade areas.

Engineered fill should be free from organic materials, other contaminants and deleterious substances and
have a maximum particle size not exceeding 40mm. We expect the excavated sands may be used as
engineered fill. Engineered fill comprising sands should be placed in layers not exceeding 100mm loose
thickness and compacted with the above-mentioned rollers to achieve a minimum density index (ID) of 70%.
However, the ID may be reduced to 65% in landscaped areas, where the designer considers that settlements
are not critical. We note that sandy soils are difficult to compact, particularly silty sand which is usually very
moisture sensitive. Care must be taken with the compaction with no vibrations being used.

Backfill to conventional retaining walls should also comprise engineered fill comprising the excavated sands.
Alternatively, well graded imported granular materials such as demolition rubble would be suitable for this
purpose provided it is also free of contaminants or deleterious substances and has a maximum particle size
not exceeding 40mm. Imported well graded granular fill should be compacted to at least 98% of Standard
Maximum Dry Density (SMDD) and within 2% of their Standard Optimum Moisture Content (SOMC). The
compaction requirement may be reduced to 95% of SMDD in landscaped areas, where settlement can be
tolerated. Such fill should be compacted in horizontal layers as described above. Care will be required to
ensure excessive compaction stresses are not transferred to the retaining walls.

Density tests should be carried out at the frequencies outlined in AS3798 (Table 8.1) for the volume of fill
involved. At least Level 2 testing of earthworks should be carried out in accordance with AS3798. Any areas
of insufficient compaction will require reworking.

As an alternative, single sized granular material (or ‘no fines’ gravel) may be used as backfill to the landscape
retaining walls and this would also act as the drainage behind the wall and would only require nominal
compaction (with no compaction testing). The drainage material should be wrapped in a non-woven
geotextile fabric (e.g. Bidim A34) to act as a filter against subsoil erosion. Further, retaining wall backfill
should be provided with a cap of at least 0.3m thickness of compacted crushed sandstone at surface level to
reduce the likelihood of surface water surcharging the retaining wall.



6.10 Sewer Pipeline
A Unitywater sewer pipeline orientated approximately north-south crosses the eastern end of the site below
the existing pool and will extend below a portion of the walkway connecting the addition to the lift. Our
recommendation is for new footings to comprise piles founded in at least loose relative density sands in
accordance with the advice provided in Section 6.7 above. We recommend that the invert level of the sewer
pipeline be confirmed in order that the proposed founding level of the piers is below the pipe invert and/or



32032URrpt                                          18