Geotechnical Report

                               PROPOSED UNIT DEVELOPMENT
                               31 BOTTLEBRUSH AVENUE, NOOSA HEADS

6.4        Retaining Structures
Retaining structures must be founded as per the recommendations in Section 4.2 above. 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.

Retaining structures must be founded as per the recommendations in Section 4.2 above with the piled
footings, if required, also designed to resist lateral loads imposed by the retaining wall. 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.

We suggest the parameters outlined in Table 4 may be adopted for retaining wall design: The values in
Table 4 assume that the walls are fully drained and no allowance is included for hydrostatic pressures,
construction loading, up-slope footing loadings, the effects of sloping ground and backfill compaction
stresses.
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.

6.5        Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment (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 retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).

All collected surface water, roof water, and overflow from any tanks/pools should be directed by pipes or
surface channels (with erosion protection) to the kerbside drainage alongside Bottlebruch Avenue. Due to
potential for stormwater infiltration, any permeable surface channels must be located a minimum of 5 m from
any future structures or batters.



7.0        SUMMARY & CONCLUSIONS
Based on the findings of our assessment it is assessed that there would be a Low Risk of slope instability
affecting the proposed development subject to implementation of the recommendations given in this report.



17 December 2020
Report No. 20370-001-Rev0                                  13