Geotech Report - techtonic · RAL22/0021

                             PROPOSED RESIDENTIAL DEVELOPMENT
                             39 PICTURE POINT CRESCENT, NOOSA HEADS

The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation of
the proposed retention and subsurface drainage system. It will be necessary to conduct numerical analysis
to optimise the retention system. Detailed design will also need to account for relevant factors of safety to
maintain the temporary and long-term stability of the retention system.

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 trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type 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
                             1.7         28              0.36                 0.53                2.77
 Sands


 Medium Dense Sands          1.8         34              0.28                 0.44                3.54

 Dense (or Denser)
                             1.9         36              0.26                 0.41                3.85
 Sands


Retaining structures must be founded as per the recommendations in Section 6.3.2 above with the footings
also designed to account for lateral loads imposed by the retaining wall. Detailed design must include an
assessment of internal and global 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 from 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 the nearest legal point of discharge.
Where possible, excess stormwater collected from the roof and other elevated hard surfaces should be
piped to discharge into the kerb along Picture Point Crescent. Given the relative elevations involved, the use
of a ‘wet’ or ‘charged’ drainage system may be required. Where this can only be designed to take some
flows; smaller volumes of excess stormwater will need to be directed to the dune slope to the north


10 February 2023
Report No. 22511-001-Rev0                              16