Geotehnical Investigation Report · OPW22/0140

                                PROPOSED UNIT DEVELOPMENT
                                37 BOTTLEBRUSH AVENUE, NOOSA HEADS

Retaining structures greater than 1 m high, or those required to support surcharge loads or sloping backfill,
will require engineer design and certification of construction. Retaining wall design should be carried out in
accordance with AS4678-2002 Earth retaining structures.
Design for temporary and permanent retaining structures could be carried out using a pressure distribution
applicable to the wall type (e.g. trapezoidal for propped or anchored walls) and based on the parameters
listed in Table 4. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.
Table 4: Retaining Structure Design Parameters
                                                                     Lateral earth pressure coefficients
                              Unit
                                         Friction
 Retained material           weight
                                         angle (°)            Ka                        Ko
                             (t/m3)                                                                        Kp
                                                       (Cantilever wall)        (Non-yielding wall)


 Very Loose/ Loose
                               1.8           28               0.36                     0.53                2.77
 Sands


 Medium Dense (or
                               1.9           34               0.28                     0.44                3.54
 Denser) Sands


 Future Controlled Fill*        *            *                   *                       *                  *

Note:    * Depends on fill material type and level of compaction.

Retaining structures must be founded as per the recommendations in Section 6.3.2 above with the piled
footings, if required, also designed to resist lateral loads imposed by the retaining wall. Detailed design must
include an assessment of stability. A solution using screw piles to support retaining walls, possibly including
raked piles, may be feasible although bending and shear capacities would need to be carefully checked in
design from a soil-structure interaction viewpoint. Additional lateral capacity may be achieved by
incorporating a deeper pile cap that also acts as a shear key attracting passive resistance in the sands in front
of the wall.

6.5       Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment or
retaining walls (e.g. grassed or landscaped swales or diversion mounds). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building walls, footings, batters and retaining
walls.

Although unlikely, if groundwater seepage is encountered during excavations, Tectonic must be notified to
assess the impact on stability.

Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.
All excess stormwater collected around the residence (including overflow from rainwater tanks etc.) must be
directed by pipes or lined channels to discharge into the existing stormwater infrastructure along Bottlebrush
Avenue or Berrima Row. Subsurface disposal of stormwater must not be undertaken.




13 May 2022
Report No. 22151-001-Rev0                                   14