Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                              PROPOSED RESIDENCE
                              33 ROSS CRESCENT, SUNSHINE BEACH

6.2.2       Footing Design
No design loadings were available at the time of reporting. Footings for the proposed residence and any
ancillary structures (e.g. retaining walls, 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) natural
sands. High-level footings may be possible in areas of deeper excavation where medium dense to dense (or
denser) sands are exposed at or near the subgrade. This would be subject to an assessment by a suitably
qualified and experienced geotechnical RPEQ (including verification testing). High-level footings, so founded,
could be designed using a maximum allowable bearing pressure of 100 kPa.

Elsewhere (excluding the shear pile wall), based on the results of the investigation, pile founding depths of
about 1.5 m to 4 m BGL (existing ground levels) are anticipated. Deeper pile depths may be necessary
subject to an assessment of building loads at the detailed design stage.
Excluding the shear pile wall, steel screw piles would be an appropriate footing option for the residence. Piles
may be designed using a maximum allowable end bearing pressure of 600 kPa in medium dense sands, or
at least 1 MPa in the deeper dense sands. Due to the presence of very loose to loose upper sands, together
with the restricted site conditions and close proximity of adjacent houses, open bored piers or driven piles
are not recommended for this site. A possible alternative to screw piles could be grout-injected, CFA piles,
provided that construction water loss is carefully monitored and controlled. Steel reinforced concrete bored
piles may be possible; however, the use of sacrificial liners (e.g. Formatube) would be required.

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.

Ultimately, the advice of a specialist piling contractor should be sought regarding pile design and
construction. However, where steel screw piles are adopted site specific load testing and the results of this
geotechnical investigation should be considered by the structural engineers in validating any generic torque
to capacity ratios provided as part of the proposed design. An assessment of potential vibration effects on
nearby structures and monitoring requirements should also be included.

Footing design should also account for potential soil reactivity to be equivalent to Class M. It should be noted
that the placement of reactive clay may result in a more significant equivalent site classification, such as Class
H1 (or higher).

All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
retaining walls, or in ground structures that are not designed to support building loads. Footings should
found at least below a plane extending 1 m horizontally from the base of trenches/excavations/swimming
pool, then rising up at 1V:1H, as illustrated in Text Figure 11 overleaf. Where consideration is given to the
use of high level footings to the eastern side of the residence, structural designers should allow for the
deepest founding depth of either:
       taking footings below the zone of active earth pressure (within this zone reduced foundation capacity
        and adverse interaction with the seaward retention system are anticipated); or
       the arrangement should comply with Text Figure 11 overleaf, with the relevant plane taken from the
        equivalent ground level at the front of the exposed piled wall following the most onerous erosion
        case in the stability modelling (Ref. Section 5.2.1).




18 June 2024
Report No. 24096-001-Rev0                               16