21054-001-Rev0 Tristania Dr

Peter Williams                                                                                         21054-001-Rev0
                                                                                                      25 February 2021

installation, as piling will not encounter drilling refusal at the recommended founding depth, and piles could
extend to much greater depth than necessary.

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

4.4     Retaining Walls
Retaining structures must be founded as per the recommendations in Section 4.3 above. 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.

We suggest the parameters outlined in Table 3 below 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.
Table 1: 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 Sands                   1.8          30                0.33                     0.5                3.0


 Medium Dense (or
                               1.9          34                0.28                    0.44                3.5
 Denser) Sands


 Future Controlled 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. It is assumed that the pool floor will include a
hydrostatic pressure relief valve.

All excess stormwater collected around the residence (including overflow from rainwater tanks and pool)
should be directed by pipes or lined channels to discharge into the existing stormwater infrastructure along
Tristania Drive.

Subsurface disposal of stormwater must not be undertaken.




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