Geotechnical Investigation Report

J001369-001-R-Rev0                                                                                      August 2021


      ▪    Fill should be compacted in layers, typically about 250 mm thick (loose thickness). However, layer
           thickness will be dependent upon the type/size of compaction plant, use of vibration, material type
           and condition.
      ▪    Fill shall be compacted by repeated rolling to achieve a dry density ratio of at least 95% of the
           Maximum Standard Dry Density for cohesive soils, or 70% Dry Density Index for cohesionless soils.
      ▪    Imported fill (if required) should be cohesive in nature and have a plasticity index of < 15%, a
           maximum particle size of 100 mm, and a soaked CBR of > 10%.
      ▪    Slopes and existing drainage channels should be benched to assist with fill placement and
           compaction.
Further details for control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on
Earthworks for Commercial and Residential Developments.
If filling greater than 0.5 m thickness is proposed, Core must assess the impacts on slope stability before
construction. Fill greater than 1 m thickness (if required) must be retained (refer Section 7.6).

7.3       Excavations
Due to space constraints, basement excavation will require positive support by engineer-designed retaining
structures. Possible retention systems include sheet piles (cantilevered, anchored or propped). Design and
construction of temporary support must consider adjacent structures (e.g. loads or undermining). It is
recommended that information on adjacent footings is obtained through site investigation or certified ‘as built’
plans. Vibration impacts of sheet piling must be considered. Temporary support must be designed by a
suitably experienced engineer (RPEQ) and installed by a specialist piling/shoring contractor.
Temporary excavations of less than 1 m height into the loose to medium dense sand (e.g. trenches) could be
battered at 1V:2H. Any cuts greater than 1 m must be retained (refer Section 7.6).
Based on the materials encountered in the boreholes, excavation for the basement is expected to encounter
loose to dense sands. Excavations in the anticipated soil materials should be achievable using conventional
earthmoving equipment.

7.4       Foundations
      7.4.1    Site Classification
Site classification derived in accordance with Australian Standard AS2870-2011 Residential slabs and
footings can provide an indication of the likely magnitude of reactive (shrink and swell) movements
associated with normal seasonal moisture variations.
Based on the results of the investigation the site is classified as ‘Class ‘P’ due to the presence of
uncontrolled fill.
However, the natural soils on site may be classed as equivalent ‘Class ‘S’ in accordance with AS 2870-2011.
The estimated characteristic ground surface movement due to seasonal moisture variation (‘ys’ value) for this
site is less than 20 mm.
It should be noted that the predicted order of movement is based on the existing soil profile (i.e. no cutting or
filling), and moisture content changes within soils due to normal seasonal moisture variations only (i.e. does
not consider soil settlement).

      7.4.2    Footing Design
The proposed building could be supported on high-level footings founded in the dense natural sands. A
design allowable bearing pressure of 200 kPa may be adopted for the dense sand.
Footing excavations should be inspected by an engineer from Core to confirm the foundation material meets
the design pressure requirements.
To achieve the recommended allowable bearing pressures, founding materials must be dry and cleaned of
any loose, disturbed or saturated materials prior to pouring concrete. Allowance should be made for
compaction of disturbed sand in the base of footing excavations.



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