Reference Document - RAL25/0020 & OPW25/0194 (As Published)

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Allowance should be made for hydrostatic pressure build-up behind the retaining 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).

Also, allowance for surcharge loads and sloping crest should be made as appropriate. The effect
of surcharge should be included by multiplying the vertical pressure developed by the surcharge
by the appropriate lateral earth pressure coefficient as given in Table 7.

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 at more than 2% dry of optimum moisture content, as this
can lead to increased future swelling with changes to moisture content and pressures well in
excess of Ko, or inundation from water creating additional load on the back of the wall.

It is recommended that factors of safety of 2 against overturning and sliding stability be adopted
in the design of all retaining walls.

For limit state design methods, the ultimate parameters provided above in Table 7 will need to
be factored in accordance with AS 4678. Guidance on the selection of material strength partial
factors is provided in Section 5.2 of AS 4678 and is dependent upon the nature and state of the
insitu soil.

6.8    Pavements

It is anticipated that the subgrade conditions are likely to comprise sandy silt and sandy clays.
The results of laboratory testing indicate a soaked CBR value of 4.5% for the sandy silt. Based on
DCP testing, insitu CBR values at current moisture conditions were typically in the order 3% to
5%.

Provided site and subgrade preparation is carried out in accordance with this report and allowing
for minor variations in subgrade type and strength, it is recommended that a design CBR value
of 3% or a modulus of subgrade reaction of 25 kPa/mm be adopted.

Rigid pavements should be designed using a soil modulus of 20 MPa for short term loads and
15 MPa for long term loads.

Modulus of subgrade reaction values given above are for standard wheel load applications only.
For loaded areas of different proportion or different load intensity to standard wheel loads,
Douglas should be contacted for further advice.

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




Proposed Subdivision                                                             237894.00.R.001.Rev0
12-14 Gympie Street, Tewantin                                                           October 2025