Att 9 - Geotech and PASS report

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Preference should be given to adopting thin soil layers and using small hand-controlled compaction
equipment during backfilling against retaining walls. This is in order to limit the stress applied to the
walls during construction. Should heavy compaction be required, then wall stresses will be well in
excess of Ko and temporary propping should be used.

Clayey backfill should not be placed too dry of optimum moisture content, as this can lead to increased
future swelling with changes to moisture content or inundation from water creating additional load on
the back of the wall.

It is recommended that all retaining walls be drained for full height in order to minimise hydrostatic
pressure build-up behind the wall. Additional guidelines on wall drainage are provided in Appendix G
of AS 4678-2002.



8.8     Pavement and Slab-On-Ground

Subgrade conditions are expected to typically comprise either new engineered filling or natural sands.
The design of pavements and on-ground floor slabs will depend on the quality of the new imported
filling which is to be placed in order to raise site levels.

Soaked CBR testing will need to be carried out at the time of construction to confirm CBR values of
the imported filling material.

The results of the laboratory testing indicate a soaked CBR value of 35% for the natural silty sand
encountered onsite. The high laboratory CBR value in the silty sand material is a result of the lateral
confinement of the material in the sample mould. Such confinement is not likely to be achieved in the
field, therefore it is suggested that a maximum value of 10% be used for these sands based on
experience with similar soils.

For composite subgrades (ie. where imported filling is less than 1m) the Japan Road Association
method of assessing a weighted subgrade strength should be used:

                              0.33                  0.33 3
       CBRW = (DF x CBRF             + (1-DF) x CBRS    )

       where:         CBRW = weighted subgrade CBR (%)
                      DF = depth of filling (m)
                      CBRF = CBR of filling material (%)
                      CBRS = CBR of natural subgrade (%)

On-ground building floor slabs (ie. floating slabs) should be detailed so they can accommodate
potential ground movements, including settlements and shrink-swell movements. Differential
settlements could be expected to vary across the site, depending on the floor loads, as well as the
thickness and consistency of the near surface soils. Dowelling of floor slabs will minimise differential
movements between adjacent slabs.

The satisfactory on-going performance of pavements is dependent on the subgrade not being allowed
to become excessively wet. To improve pavement life, it is essential that sufficient subsoil drainage be
installed in areas where there is potential for water to enter the subgrade, (ie. adjacent to unsealed
areas, table drains, landscaped areas etc).


Geotechnical Investigation, Proposed Commercial Development                            90151.00.R.001.docx
11-19 Hilton Terrace, Tewantin                                                                   May 2017