geotech report · 132003.220660.5

6.7   Footing Design
As noted in Section 6.2 above, the very loose sands are not considered to be a suitable foundation material
due to the low allowable bearing capacity and potential for high elastic settlement. In addition, the fill
encountered at the site has been assessed to be poorly compacted. The poorly compacted fill and very loose
sands are also not considered suitable for supporting building loads as they are susceptible to additional
settlements due to vibrations and infiltration of surface water run-off. In this regard we note that the
structural drawings indicate that “all footings to be founded 300 into medium dense sands or natural stiff
clays, with a minimum allowable bearing capacity of 100kPa to be confirmed by geotechnical testing”. Based
on the previous and current investigation results, this comment would be referring to natural sands of
medium dense relative density.

Based on the current and previous investigation results, natural sands of at least loose relative density were
encountered at depths between 1.4m and 1.6m depth. Consequently, steel screw piles or bored piles (with
an allowance for sacrificial liners) formed using an auger attachment to an excavator or hand auger
techniques will be required. Lined hand auger piles or steel screw piles (say 0.35m diameter) founded at a
depth of least 2m or 4 pile diameters (whichever is deeper) into loose (or denser) natural sands may be
designed using an allowable bearing pressure of 200kPa. This will require the proposed structure to be fully
suspended and supported on a ‘pier and beam’ footing system where any piles located close to the existing
sewer line can span over the pipe line, provided the piles extend below the sewer pipe invert level and/or a
ZoI line advised by Unitywater. This is so that the sewer line is not surcharged by the piles.

For screw piles founded in soil, the assessment of the pile geotechnical capacity must be based on the
recommendations above and not empirical correlations based on installation torque. Further, no allowance
is to be made for an increase in capacity due to a multi helix pile or skin friction on the shaft. If the structural
engineer determines the proposed additions can be supported by screw piles, we strongly recommend the
pile design be reviewed by ourselves prior to the piling contract being let so that conformance with our
recommendations can be confirmed.

During construction, a geotechnical engineer must assess the depth to loose or denser sands by carrying out
additional DCP tests at specific pile locations.

For the retaining walls supporting the rear yard fill we also recommend that a ‘pier and beam’ footing be
adopted with the piles comprising lined hand auger piles as described above.

Excavations for footing beams that extend through sands should be supported with formwork, as vertical
cuts will be potentially unstable.




32032URrpt                                          16