Meyer Judgment Sealed

 

tec sMic PROPOSED RESIDENTIAL DEVELOPMENT
beste Rye Aether 39 PICTURE POINT CRESCENT, NOOSA HEADS

 

6.3. Building Foundations

6.3.1 Indicative Site Classification

Site classifications for footing design are determined in accordance with AS 2870-2011 Residential Slabs and
Footings, and depend on the depth of cut undertaken at a site, variations in the natural soil depths, and the
reactivity of the in situ materials 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 (‘y;’) 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, footing and plumbing design.

6.3.2 Footing Design

No design loadings were available at the time of reporting. Footings for the proposed development 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 and medium dense sands). Footings should be taken to found at least 7 m in to
the underlying dense sands. This would result in relatively long piles ranging up to the order of 10 m BGL as
shown indicatively in Text Figure 9.

Concrete piles so founded may be designed using an ultimate end bearing pressure of 3 MPa, and ultimate
side adhesion (in dense sand) of 60 kPa. Settlement under such loads is expected to be in the order of up to
20 mm or 1% of footing width. In addition, piles on the downslope edge of the buildings (at least) will likely be
required to have resistance to normal shear forces.

A review of the design plans should be undertaken, once available, such that the footing design for the
structures can be optimised.

Given the sandy soil profile and depths of footings required, steel screw piles could be considered a suitable
foundation type, where more onerous shear resistance is not required. The use of driven piles would be
precluded with the proximity to neighbouring residences, and the presence of sewer infrastructure within the
site. Steel reinforced, concrete bored piles could be considered, however installation may not be
straightforward, given the presence of very loose to loose sands in the upper soil profile. The use of a
continuous flight auger, grout-injected pile system would be required for bored piles. 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.

Depending on the detailed design of the footing system, once access becomes available (following building
demolition) it is recommended that additional geotechnical investigation is undertaken to confirm parameters
for piling. This should comprise two boreholes drilled to at least 2 m beyond the design pile toe depths (say
about 12 m BGL) with standard penetration testing at regular, 1.5 m intervals.

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.

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

 

10 February 2023
Report No. 22511-001-Rev0 14

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