Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                                PROPOSED RESIDENCE
                                33 ROSS CRESCENT, SUNSHINE BEACH

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 and based on the parameters listed in Table 6. These values do not include
allowance for hydrostatic pressures, backslopes (assumed to be horizontal), nor transient (e.g. construction)
or adjacent footing loadings.
Table 6: Retaining Structure Design Parameters
                                                                     Lateral earth pressure coefficients
                               Unit
                                         Friction
 Retained material           weight
                                         angle (°)            Ka                        Ko
                             (kN/m3)                                                                       Kp
                                                       (Cantilever wall)        (Non-yielding wall)
 Very Loose/ Loose
                                17           27               0.38                     0.55                2.66
 Sands

 Medium Dense Sands             18           34               0.28                     0.44                3.54

 Dense (or Denser)
                                19           36               0.26                     0.41                3.85
 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 6.2.2 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.

6.4       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.
It is assumed that the pool floor will include a hydrostatic pressure relief valve. The outlet pipe from this
valve must be taken to ‘daylight’ at the ground surface.
Collected roof water should be directed by pipes to the council stormwater system along Ross Crescent
(e.g. through the use of a ‘wet’ or ‘charged’ drainage system) or; where not physically possible, surface
water should be directed to infiltrate into the ground around the edges of hard surfaces, with infiltration
areas at least 10 m upslope of the rear boundary. Hydraulic design should be such that under no
circumstances are uncontrolled stormwater flows permitted over the dune face; where channelised erosion
and scour features could be allowed to develop.
Design of the stormwater system should be undertaken by a suitably qualified and experienced RPEQ.




18 June 2024
Report No. 24096-001-Rev0                                   18