220328 - Edwards Street - Slope Stability Assessment

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

The existing timber retaining wall to the north-eastern boundary should be removed and replaced with a new
structure preferably of more durable materials (e.g. concrete) behind the pool shells and pool terraces. While
only landscaping works appear to the proposed in the southern corner adjoining 166 Edwards Street, an
assessment should be made of the existing masonry blockwork/concrete wall and surcharge from features
above (e.g. the swimming pool) such that a non-worsening condition eventuates from the proposed
construction, and especially that no ‘undermining’ through cut earthworks occurs.

4.2       Earthworks
Care should be taken to ensure construction works do not cause excessive ground vibrations that could
result in damage to existing nearby structures. Should vibratory techniques be proposed (e.g. vibratory fill
compaction, sheet piling, etc.), ground vibrations should be monitored, and construction methods may need
to be altered should adverse impacts be anticipated or encountered. It would be prudent to conduct a
dilapidation survey of nearby structures before commencement of construction.

4.2.1         Site Preparation and Trafficability
Following removal of the existing structures (where required) and associated construction elements (e.g.
footings, services, etc.), any tree stumps and large roots should be ‘grubbed out’. Any organic rich topsoil
and uncontrolled fill should then be stripped and set aside. Any organic affected topsoil materials must not
be used in structural fill; however, may be placed across landscaping areas.
The natural subgrade following stripping will likely be in a loose condition. Where slab on ground
construction is proposed the ground surface should be compacted by several passes of a vibrating plate
compactor or small smooth drum roller (or similar), in conjunction with surface application of ample water.

After initial stripping the subgrade is expected to comprise a mixture of silty sands and relatively ‘clean’
beach ridge sands. Although relatively good trafficability should initially be encountered, these materials
could be subject to strength loss from repetitive vehicle loading (particularly from heavy rubber tyred plant)
and if they become excessively dry. Due to the potential for poor trafficability, placement of a minimum
150 mm thick granular working platform will likely be required, at least across access and laydown areas.

Groundwater seepage was not encountered during the Unearthed ‘soil test’ fieldwork; however, depending
on the prevailing weather conditions at the time of construction, groundwater seepage may develop at
relatively shallow depths.

The site should be graded to shed surface water runoff away from construction areas. Swale drains should
be constructed along the toes of any cuts, and above cuts where possible, with the drains graded to
discharge collected waters to a control point before release off-site. Appropriate erosion and sediment
control measures must be installed prior to commencement of earthworks at this site.

4.2.2          Filling
The existing sandy soils (excluding any topsoil) may be reused as structural fill, subject to appropriate
moisture adjustment. It should be noted that compaction of relatively ‘clean’ sands (expected to form most of
the semi-basement excavation spoil) as structural fill requires the addition of substantial volumes of water
(‘flooding’) combined with the use of vibratory compaction equipment (typically a smooth drum roller or
vibrating plate).

Should importation of fill be required, it is recommended that such fill should comply with the following:

          Minimum CBR of 10 %
          Maximum plasticity index of 15 %
          Shrink-swell index ≤ 1 %
          Maximum particle size of 100 mm

Any fill should be compacted at moisture contents within the range of ±2 % of optimum moisture content for
Standard Compaction. Select fill should have a maximum particle size of 100 mm for an uncompacted layer
thickness of 200 mm and shall be compacted by repeated rolling with a small compactor to achieve a dry
density ratio of at least 95 % of the Standard Maximum Dry Density for cohesive soils, or 70 % Dry Density
Index for any imported cohesionless soils.




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