4. 220148 - Slope Stability Report (2) · 132006.2554.01

Seachange Builders                                                                               22134-001-Rev0
                                                                                                    26 April 2022

4.3   Footings
No design loadings were available at the time of reporting. New footings for the proposed renovations (e.g.
lift base, pool, etc.) must penetrate through any fill materials and unsuitable natural soils (i.e. very loose to
loose sands) to found at least 1 m into medium dense (or denser) sands which were mentioned at about 2 m
BGL (existing ground levels) in the CSI borehole/DCP tests. A minimum founding depth of 3 m BGL
(existing ground levels) is recommended.
Given the sandy soil profile and depth of footings required, steel screw piles could be adopted. 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 suitable 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. Due to the extent of the residence, access is restricted, so specialised tight access
pilling equipment will likely be required.

Please note that a suitably qualified and experienced structural RPEQ must be consulted to ensure that
existing footings have sufficient capacity to support any additional loads imposed by the proposed
renovations.

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.

It is therefore recommended that site specific static load testing be conducted from screw piles, if adopted, at
a minimum. Where tension or compression capacity developed in the footings is critical for the future
structure (e.g. resistance to uplift forces from high winds), then cyclic load tests (including measurement of
deflections under tension and compression cycles) should also be undertaken prior to construction.
Kentledge for static testing, or simulated cyclic loads, should prove the piles to a minimum of two times the
expected working load within tolerable deflections.

All footings should be founded such that they are not adversely affected by or affect any adjacent
excavations, trenches, retaining walls, or in ground structures that are not designed to support building
loads. Footings should be founded at least below a plane extending 1 m horizontally from the base of
trenches/excavations/retaining walls/swimming pool, then rising up at 1V:1H, as illustrated in
Text Figure 9.




                            Text Figure 9: Footing depth required to prevent undermining




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