Appendix B - Geotechnical Assessment · OPW23/0179

 Project No. PG-10375                                                                               Page | 9
 June 2023
 Ref: PG-10375, 2023-06-09, LTR VER 1
 Michael Cullen – Geotechnical Investigation - Proposed Residence, 128 Gympie Terrace, Noosaville




Assuming an excavation depth of 3m for the basement, it is expected indurated sands
will be exposed across the basement floor. Provided sands above the indurated sands
are removed, an allowable bearing pressure of 300kPa could be adopted in the
indurated sands. If any sands remain across the base of the excavation, provided they
are compacted to a minimum Density Index of 75%, an allowable bearing capacity
of 200kPa could be adopted for the foundation design.

Where sands are exposed at the foundation level, this compaction could be
achieved through the over-excavation of the founding soils, compaction of the
exposed material and the replacement of the excavation using “clean” sand.

It should be noted some difficulties have been experienced in compacting sands and
particularly silty sands immediately above the indurated stratum, due to the difficulties
in suitably dewatering.

It is strongly recommended that if a high-level footing raft type system is to be
adopted, sufficient “clean” sand from higher in the excavation profile be stockpiled
on-site, to allow for the replacement of the silty sands at the founding level, or the
sands removed to expose the indurated sand.

It should be further noted that immediately below the indurated sand layer, weaker
soil layers may be present that were not identified as part of the geotechnical
investigation. It is strongly recommended that following excavation to the basement
floor level that additional testing be carried out under the supervision of Pacific
Geotech to further assess the nature of the soils below the indurated sands.

To enable the required ground improvement works to be undertaken, the following
earthworks operation is recommended:

   i.      Install the temporary retention system and dewatering system.

  ii.      Dewater the site to at least 0.6m below the excavation level or to the top of
           the indurated sand.

  iii.     Excavate the site to the design basement level and test the base of the
           excavation to determine the extent of over-excavation required (if any).

 iv.       Following determination of the required extent of over-excavation, excavate
           the site to the required level and compact the exposed sands using a large
           (300kg plus) vibrating plate compactor or a small smooth drum roller. It should
           be noted that the silty sands encountered at the test locations could be difficult
           to compact and manage, if not appropriately dewatered. It may be
           necessary to remove the silty sands and replace them with clean sands from
           higher in the excavation profile or extend the foundation into the underlying
           stiff clays.