4. 220148 - Slope Stability Report (2) · 132006.2554.01

Seachange Builders                                                                                    22134-001-Rev0
                                                                                                         26 April 2022

If any soil conditions encountered during construction are found to differ from those noted in the geotechnical
investigation, Tectonic and CSI should be notified immediately, and an inspection carried out to determine if
changes to footing design are required.

Should bored piles be adopted, it is essential that the piling contractor have a ‘mud bucket’ or similar to
enable clean out of the pile bases. It is recommended that bored pile sockets be inspected by a suitably
qualified and experienced engineer prior to placement of steel reinforcement and pouring of concrete to
confirm allowable bearing pressures and cleanliness.

4.4     Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.3 above with the piled
footings also designed to resist lateral loads imposed by the retaining wall. 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. Detailed design must include an assessment of global stability.

The parameters outlined in Table 3 may be adopted for retaining wall design. The values in Table 3 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. Any temporary
shoring required to support excavations during construction may also be designed using the parameters in
Table 3.
Table 3: Retaining Structure Design Parameters

                                                                     Lateral earth pressure coefficients
                               Unit       Friction
 Retained material           weight       angle Ø
                                                                Ka                      Ko
                             (kN/m3)         (°)                                                             Kp
                                                         (Cantilever wall)      (Non-yielding wall)


 Very Loose/ Loose
                               17            30                 0.33                    0.50                3.00
 Sands


 Medium Dense (or
                               18            34                 0.28                    0.44                3.54
 Denser) Sands


 Future Fill                    *             *                  *                       *                    *

Note:    *      depends on fill material type and level of compaction

4.5     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 and footings.

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, pool etc.)
must be directed by pipes or lined channels to discharge into the existing stormwater infrastructure.

Subsurface disposal of stormwater, such as by an infiltration pit, must not be undertaken.




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