Compiled Reference Documents - OPW25/0053 (As Published)

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Resisting forces to sliding and rotation are provided by the shear resistance at the footing/soil
interface and the passive soil resistance in front of the footing. Passive resistance (Kp) at the toe
of the wall should be ignored in the zone where future disturbance (eg. services trenches) could
occur.

It is recommended that factors of safety of 2 against overturning and sliding stability and 1.5 for
global stability, be adopted in the design of all retaining walls.

For limit state design methods, the ultimate parameters provided above in Table 2 will need to
be factored in accordance with AS 4678 (2002). 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 soils.

High level wall strip footings founding into stiff or stronger silty sandy clay could be designed for
an allowable bearing pressure of 100 kPa. All footing excavations should be inspected and tested
by an experienced geotechnical engineer from Douglas to confirm bearing pressures prior to
casting of concrete.

Alternatively, bored piers founding into stiff or stronger silty sandy clay would also be considered
suitable.

The ultimate geotechnical strength (Rd,ug) of bored piers can be calculated using the ultimate,
unfactored shaft adhesion and end bearing values given in Table 3. The Rd,ug values given will need
to be multiplied by a suitable geotechnical strength reduction factor (Ø g) to obtain the design
geotechnical strength (Rd,g) of piles. In accordance with AS 2159 (2009) the Øg value must be
determined by the designer, but based on the anticipated site, design and installation risk factors,
a Øg value of 0.45 is considered likely for a low redundancy pile system.

If working stress methods are used in the pile design, the Rd,ug values given in Table 3 will need to
be divided by a factor of safety of 2.5 to calculate the maximum single pile working load.

Table 3: Ultimate (Unfactored) Bored Pier Design Parameters
 Material                                                  Ultimate Unfactored     Ultimate Unfactored
                                                           Shaft Adhesion* (kPa)   End Bearing* (kPa)
 Stiff (or stronger) natural clays                         30                      300
Notes: * Geotechnical strength reduction factor or factor of safety required


It is recommended that pile excavations be inspected by an experienced geotechnical engineer
from Douglas during drilling to ensure the design parameters adopted are suitable for the
ground conditions and to ensure that there is no soft or loose material remaining at the base of
the excavations.

Numerical global stability analysis for the proposed retaining wall was also carried out using the
commercially available software Slope/W.

The assumed material properties used in the analysis were derived from the interpretation of the
field and laboratory test results, general soil parameters given in Table D4 of AS 4678 and
Douglas’ experience with similar conditions. An assumed pore water pressure coefficient (Ru) of
0.1 (20% saturation) was also adopted in the analysis.




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