geotech

                                PROPOSED UNIT DEVELOPMENT
                                9 BOTTLEBRUSH AVENUE, NOOSA HEADS

6.4       Temporary and Permanent Retention
Excavations are expected to be around 4 m BGL (existing ground levels) and, given that the planned
basement footprint extends close to some of the site boundaries, some form of temporary shoring will likely
be required to support the basement excavation.

Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles. Such shoring techniques would likely require lateral restraint, and this is conventionally
achieved by anchoring. However, the proximity to neighbouring properties would require a number of
permissions to be obtained for subterranean access. Therefore, it is suggested that internal struts/propping
be considered to provide lateral restraint for the shoring system.

Anchored or propped sheet pile would likely require pre-drilling to prevent adverse vibrations during sheet
pile installation. Temporary lateral support can be provided by installation of helical steel anchors (similar to
screw piles) or internal props/struts connected to walers. As a permanent basement wall is constructed
inside the temporary structure, the sheet pile wall becomes sacrificial.

Other shoring methods that could be considered include contiguous or non-contiguous piles and secant
piles. 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
                               1.8           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.

Given the relatively poor condition of the timber retaining wall along the northern boundary, it would be
prudent to replace this wall with a more durable structure (e.g. a concrete sleeper wall) during the proposed

13 July 2021
Report No. 21169-001-Rev0                                   13