RFI - slope stability reporting

RECOR Pty Ltd                                                                                     22393-001-Rev0
                                                                                                  26 October 2022

and infrastructure, including on neighbouring properties, will need to be specifically designed to prevent
undermining. Based upon our experience with similar sites, Unitywater requirements for sewer/water
infrastructure may restrict the use of sheet piles (if required).

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   Site Classification
Site classification for footing design is determined in accordance with AS 2870-2011 Residential Slabs and
Footings, and depend on the depth of cut / fill undertaken at a site, variations in the natural soil/rock depths,
and the reactivity of the in situ soil / rock and any imported fill materials.

Based on the results of the investigation, the site is classified as Class P in accordance with AS 2870-2011
Residential Slabs and Footings, due to the presence of poor bearing capacity soils at shallow depths, and
trees. If not for these factors, the site would have the characteristics of Class S, with the estimated
characteristic surface movements (‘ys’) ranging up to about 20 mm. However, due to the potential variability
in subgrade density a site classification of Class M is recommended in relation to assessment of flexibility
requirements for ground slabs, footings and plumbing design.

The use of reactive fill materials (clays) can adversely affect the characteristic ground surface movement and
must be considered during design if fill material other than sand is proposed to be used on the site.

4.4   Footings
No design loadings were available at the time of reporting. Footings for the proposed duplex buildings and
ancillary structures (e.g. pool, retaining walls etc.) must penetrate through any fill materials and unsuitable
natural soils (i.e. very loose to loose sands) to found at least 0.5 m into medium dense (or denser) sands
which were encountered at depths between 1 m and 1.2 m BGL (existing ground levels) in the boreholes.
Thus, piled footings will be required unless excavations expose medium dense sands at or near foundation
subgrade level.

Piles so founded may be designed using an allowable end bearing pressure of 300 kPa. High level footings,
where achievable in deeper cut areas, may be designed using an allowable bearing pressure of 100 kPa
provided that they are founded in the natural medium dense (or denser) sands.

Given the sandy soil profile and depths of footings required, steel screw piles are an appropriate footing type
for the proposed development generally. However, in areas of cut exposing medium dense (or denser)
sands at the subgrade level high-level footings may be adopted. It should be noted that excavations for
concrete strip footings (or similar) may loosen the base. 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 the presence of loose sands, together with the close proximity to surrounding infrastructure, open
bored piers or driven piles are not considered to be appropriate for this site. Steel reinforced concrete bored
piles could be feasible; however, the use of sacrificial liners (e.g. ‘formatube’) would be required to prevent
the caving of upper loose sands.

The advice of a specialist piling contractor must be sought regarding feasibility, design and installation of the
piling system for this site, including an assessment of potential vibration effects on nearby structures, and
monitoring requirements.

If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling may
not encounter sufficient drilling resistance (from torque measurement) in sands at the suggested founding
depth, and piles could extend to much greater depth than necessary.




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