Geotechnical Stability Report · OPW24/0017

GMW Projects Pty Ltd                                                                             24008-001-Rev0
                                                                                                 31 January 2024

4.2.3       Excavations
Excavations through the fill and natural soils encountered in the Global boreholes should be achievable
using conventional earthmoving plant (e.g. small excavators). Non-surcharged temporary cuts in the very
loose to loose sands could be formed at 1V:2H, and in medium dense sands at 1V:1.5H, for batters up to
1.5 m in height. Although unlikely, if groundwater seepage is encountered, flatter batters may be required.
Deeper cuts, if required, should be specifically assessed by Tectonic. Permanent fill batters up to 1.5 m high
(vertical height) may be battered at no steeper than 1V:3H, or otherwise retained. 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. Excavations in close proximity to existing structures/infrastructure will need to be specifically
designed to prevent undermining. 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 to a control point(s).

4.3     Footings
A suitably qualified and experienced structural RPEQ must be consulted to ensure the existing footings and
structural elements of the residence and ancillary structures (e.g. retaining walls) have sufficient capacity to
support additional loads imposed by the proposed extensions. It is recommended that footing exposures
and/or building records searches be undertaken prior to construction. Alternatively, the proposed extensions
should be designed to be structurally separate from the existing structures.

No design loadings were available at the time of reporting. Footings for the proposed extensions, carport
and any ancillary structures (e.g. 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 inferred to be present at depths between about 1.1 m and 1.5 m BGL (existing ground levels) in
the Global borehole logs. Thus, piled footings will be required unless medium dense sands are exposed at or
near the foundation level. Piles founded at least 0.5 m into medium dense (or denser) natural sand may be
designed using an allowable end bearing pressure of 300 kPa.

High-level footings may be possible in areas of future cut, where founded at least 0.3 m into natural medium
dense (or denser) sand, may be designed using an allowable bearing pressure of 100 kPa. Excavations for
footings may loosen the base so it is recommended that a vibrating plate with ample water should be used to
densify footing bases prior to the placement of reinforcing steel.

Given the sandy soil profile and depths of footings required, steel screw piles are an appropriate footing type
for the proposed development. 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.




                                                       10/13