Geotech Report · OPW26/0094

Project No. PG-10899                                                                                                 Page | 8
November 2023
Ref: PG-10899, 2023-10-16, GR VER 1
DD Sunnyside CD Sunnyside Pty Ltd – Geotechnical Investigation - Proposed Residence, 30 Mossman Court, Noosa Heads




11.0 BUILDING FOUNDATIONS

Given the encountered subsurface profile, a high level footing system would
generally be suitable for the support of the proposed structure, with appropriate
ground improvement works and due consideration of the potential hydrostatic uplift
forces.

Assuming an excavation depth of 3m for the basement, provided the founding soils
are compacted to a minimum Density Index of 75% for a minimum depth of 750mm
below the footings, an allowable bearing capacity of 200kPa could be adopted for
the foundation design.

This classification could possibly be achieved through insitu compaction of the
exposed sands but may require the over-excavation of the founding soils (up to
400mm), compaction of the exposed material and the replacement of the
excavation using “clean” sand.

It should be noted that silty sands were encountered in the sand profile and filling
should be preferably disposed of. Some difficulties in treating these sands should be
expected.

It is strongly recommended that if a high level footing 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.

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 0.9m below the final excavation level.

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

  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 some 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.

  v.       Replace the over-excavation with “clean” sand and compact in layers to the
           design founding level. The founding soils should be compacted to a
           minimum Density Index of 75% (in accordance with AS 1289 5.6.1).