Compiled Reference Documents - OPW25/0053 (As Published)

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Table 1: Results of Plasticity and Particle Size Distribution Testing

Bore      Depth        Material                           Plasticity                        Particle Size Distribution
No.       (m)                                                                               (%)
                                                          LL      PL      PI     LS         Gravel   Sand        Silt/Clay
                                                          (%)     (%)     (%)    (%)
4         0.9-1.2      Silty Sandy CLAY                   35      18      17     8.5        0        49          51
5         1.6-1.9      Silty Sandy CLAY                   43      27      16     8.5        2        50          48
Where LL = Liquid Limit, PL = Plastic Limit, PI = Plasticity Index, LS = Linear Shrinkage



5. Comments

The design of the proposed retaining wall could be undertaken using a triangular pressure
distribution and the earth pressure parameters given in Table 2. Flexible walls are those which
are free to rotate or tilt (such as gravity and cantilevered walls) and should be designed using an
active (Ka) earth pressure coefficient. Rigid walls are those which are laterally restrained against
rotation or tilt (ie. single anchored/propped walls) and should be designed using the at-rest (Ko)
earth pressure coefficient.

Table 2: Earth Pressure Coefficients (non-sloping crest backfill)
 Material                                            Unit           Friction         Active       At Rest      Passive
                                                     Weight         Angle            Ka           Ko           Kp
                                                     (kN/m3)        (degrees)
 New controlled fill*, stiff (or stronger)           19             26               0.40         0.55         2.5
 natural clays
 Loose to medium dense sand                          18             30               0.33         0.50         3.0
Notes: * Depends on fill material type and level of compaction. Assumes well compacted clayey material.


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).

Allowance for surcharge loads and sloping crest should also 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 2.

Preference should be given to adopting thin soil layers and using small hand-controlled
compaction equipment during backfill against retaining walls. This is in order to limit the stress
applied to the walls during construction.

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 those indicated in Table 2 or inundation from water creating additional load on the back
of the wall.




Proposed Retaining Wall                                                                                  222438.01.R.001.Rev0
10 Surfside Court, Sunshine Beach                                                                               August 2024