220328 - Edwards Street - Slope Stability Assessment

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

Confirmatory compaction testing should be carried out at regular intervals. A minimum of two tests per
vertical lift is recommended where structural fill heights are to exceed 300 mm. Further details for control
and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for Commercial
and Residential Developments.

Fill embankment edges must be over-filled by 0.5 m (horizontally) and then trimmed back to well compacted
material.

4.2.3      Excavations
Excavations through the existing soils should be achievable using conventional earthmoving plant (e.g. small
excavators) with toothed bucket attachments. It is understood that a cut of up to about 3 m depth would be
required to site the garages proposed beneath the dual occupancy buildings. We would recommend that non
surcharged temporary batters for the excavation do not exceed 1V:1.5H in the loose sands (to about 2 m
depth at the location of the cut); and up to 1V:1H in the medium dense materials below. This assumes that
groundwater, or other seepage, is not encountered and allows for a maximum batter height of 3 m. Although
unlikely, if groundwater seepage is encountered, flatter batters may be required. Deeper cuts, if required,
should be specifically assessed by Tectonic. Permanent cut and fill batters (e.g. for landscaped areas) up to
1 m high (vertical height) may be battered at no steeper than 1V:3H in loose sands or 1V:2H in medium dense
sands. All remaining permanent cut/fill batters must be supported by engineer designed retaining walls. Any
permanent soil and fill batters will require erosion protection (e.g. revegetation or surface protection).

If the above temporary batter slopes cannot be achieved, due to spatial restraints, temporary shoring will be
required, as per the recommendations in Section 4.4 below. Excavations in close proximity to existing
footings will need to be specifically designed to prevent undermining. It is recommended that footing
exposures or confirmation of foundations via certified building plans be undertaken to assess the possibility
of undermining where close interaction is anticipated (possibly the pool on 166 Edwards Street). If
undermining is a potential issue the existing structures may need to be underpinned or the excavation may
require temporary shoring.

The advice of a suitably qualified and experienced structural RPEQ must be sought regarding any potential
undermining and design and construction of any underpins/temporary shoring (if required).

The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).

Due to the highly permeable nature of the sands at this site, stormwater is expected to rapidly infiltrate the
ground, making un-sealed surface drainage ineffective for the control of flows. Any sealed areas should be
graded to ensure free drainage of stormwater away from construction areas, and to a control point(s).

4.3     Footings
No design loadings were available at the time of reporting. Footings for the proposed dual occupancy
buildings and ancillary structures (e.g. retaining walls etc.) must penetrate through any fill materials and
unsuitable natural soils (i.e. loose sands) to found at least 0.5 m into medium dense (or denser) sands which
were encountered/inferred at depths below about 2 m BGL (existing ground levels). With the scope of the
basement excavation, it is likely that a mixture of normally dimensioned, conventional high-level footings and
relatively short piles would be utilised for the structure.

Footings founded as described above could be designed with a maximum allowable bearing pressure of
100 kPa (high level), or 300 kPa (maximum end bearing pressure for piles). Settlement under such loads is
expected to be in the order of up to 20 mm or 1 % of footing width.

It should be noted that excavation for high level footings may loosen the foundation subgrade. Therefore,
the use of a vibrating plate would be required to densify any high-level footing bases prior to the placement
of reinforcing steel.

Due to close proximity to the surrounding properties, driven piles are not considered to be suitable for this
site, and the presence of loose sands in the near subsurface would preclude the use of open bored piles
unless the construction methodology can address sloughing (sidewall collapse) of sands by use of liners or




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