220328 - Edwards Street - Slope Stability Assessment

Bev Jenner Const.                                                                                         22073-001-Rev0
C/- Westera Partners                                                                                       25 March 2022

4.4     Temporary and Permanent Retention
Excavations are expected to range up to around 3 m BGL (existing ground levels) and, given that the
planned extent of the excavations extends close to some of the site boundaries, some form of temporary
shoring may be required for part of the excavations.
Based upon our experience with similar sites, we anticipate that temporary shoring may be achieved by use
of sheet piles, subject to vibratory assessment and parameters regarding adjacent properties. Shoring of
deeper cuts would likely require lateral restraint, and this is conventionally achieved by helical steel anchors
(similar to screw piles). However, the proximity to neighbouring properties would require permissions to be
obtained for subterranean access. Therefore, where lateral support by cantilever only is not feasible, it is
suggested that internal struts/propping be considered to provide lateral restraint for the shoring system.

Sheet piles may require pre-drilling to prevent adverse vibrations during sheet pile installation. Other shoring
methods that could be considered include bored contiguous or secant piles. Non-contiguous or soldier pile
walls are not feasible for the proposed development due to the likelihood that loose sands would ‘flow’
through gaps between the piles.

The advice of a specialist shoring contractor must be sought regarding feasibility, design and installation of
the proposed retention 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 3. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.

Table 3: Retaining Structure Design Parameters
                                                                    Lateral earth pressure coefficients
                              Unit
                                         Friction
 Retained material           weight
                                         angle (°)            Ka                       Ko
                             (t/m3)                                                                        Kp
                                                       (Cantilever wall)       (Non-yielding wall)


 Loose Upper Sands
                               1.8           30               0.33                     0.5                 3.0
 (<2 m BGL)


 Medium Dense Lower
                               1.9           33               0.29                    0.46                 3.39
 Sands (>2 m BGL)


 Future Controlled Fill*        *            *                  *                       *                   *

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

As a minimum, retaining structures must be founded as per the recommendations in Section 4.4 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, together with required bearing capacity and founding
depth, by the retaining wall design engineer.




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