Att 9 - Geotech and PASS report

                                                                                                Page 8 of 13




The estimates indicate total settlements per 20kPa of development load (or metre of new filling) of up
to 20mm. Most of this settlement is estimated to occur in the first six months after load is placed. With
alluvial soils, settlements can be irregular, resulting in differential platform movements. Potential
differential settlement will also depend on the variation in thickness of the underlying soft to firm clay.

It is also noted that in addition to the site settlements indicated above, new filling will also undergo
settlement due to self-weight. As a guide, well compacted clayey filling is expected to experience
settlements of about 0.5% to 1% of fill thickness.



8.6    Foundations

Provided that site preparation is carried out in accordance with the recommendations given in this
report, it is considered that strip footings up to 1 m wide and pad footings up to 2 m in width, founded
in the new controlled engineered filling could be designed using an allowable bearing pressure of
100 kPa. These bearing pressures are limited to ensure overstressing and excessive settlement of the
softer soils encountered at depth does not occur.

All footing excavations must be inspected and tested by an experienced geotechnical engineer from
DP to confirm bearing pressures prior to casting of concrete.

Where footings are located adjacent to underground services, the footings should extend to below
invert level for a distance of 1 m out from the trench. Beyond 1 m from the trench, the footings should
be taken to below an imaginary line drawn up at 45 degrees from the invert level. These requirements
do not override minimum structural footing base levels.

Slabs supported on high level foundations should be stiffened to suit the expected ground surface
movements.

Alternatively, where development loads exceed the above allowable bearing pressures and estimated
settlements, a suitable deep foundation will be required. Steel screw piles would be considered
suitable given the subsurface conditions encountered in the bores.

The ultimate geotechnical strength (Rd,ug) of screw piles can be calculated using the unfactored,
ultimate end bearing values given in Table 4. Due to installation disturbance, it is recommended that
the shaft friction on screw piles be ignored. The Rd,ug values given in Table 4 will need to be multiplied
by a suitable geotechnical strength reduction factor (Ø g) to obtain the design geotechnical strength
(Rd,g) of piles. In accordance with AS2159–2009 (Ref 6), the Øg value must be determined by the
designer, but based on the anticipated site, design and installation risk factors, a Ø g value of 0.45 is
considered likely for a low redundancy pile system.

If working stress methods are used in the pile design, the R d,ug values given in Table 4 will need to be
divided by a factor of safety of 2.5 to calculate the maximum single pile working load.




Geotechnical Investigation, Proposed Commercial Development                             90151.00.R.001.docx
11-19 Hilton Terrace, Tewantin                                                                    May 2017