21054-001-Rev0 Tristania Dr

Peter Williams                                                                                     21054-001-Rev0
                                                                                                  25 February 2021

4.3   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 into medium dense (or denser) sands which were
mentioned in the CSI borehole and DCP reports at depths between 1 m and 3 m BGL. As a minimum it is
recommended that footings found at least 2.5 m BGL (existing ground levels), or deeper in areas where
loose sands extend below this depth (e.g. CSI borehole BH1).
Given the sandy soil profile and depths of footings required, steel screw piles are considered to be a suitable
foundation type, generally. However, in areas of deeper cuts exposing medium dense (or denser) sands at
the subgrade level, conventional slab on ground or high-level strip or pad footings may 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.

Screw piles socketed at least 0.5 m into medium dense (or denser) natural sands and at least 2.5 m BGL
(existing ground levels) may be designed using an allowable end bearing pressure of 300 kPa. In areas of
deeper excavations exposing medium dense (or denser) natural sands at the subgrade level, provided the
natural subgrade is compacted as outlined in Section 4.2, slab edge beams, internal beams and any isolated
strip or pad footings must found at least 0.5 m into medium dense (or denser) natural sands and may be
designed using an allowable bearing pressure of 100 kPa.

Footing design should also account for potential soil reactivity, advised by CSI to be equivalent to Class S. It
should be noted that placement of reactive clay may result in a more severe equivalent site classification.

All footings should found such that they are not adversely affected by any adjacent excavations, batter
slopes, 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/batter
slopes/excavations/retaining walls, then rising up at 1V:1H, as illustrated in Text Figure 7 below.




                  Text Figure 7: Footing depth required to prevent undermining or surcharge loading

Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. It is recommended that footings be inspected by a suitably qualified and experienced
engineer prior to placement of steel reinforcement and pouring of concrete to confirm allowable bearing
pressures and cleanliness of excavations. It will be important to ensure close supervision of screw pile




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