Reference Documents - MCU23 0070

Cambooya Pty Ltd


     Loose grading to very dense dune sand (aeolian) with layers of weakly cemented indurated sand (i.e.,
      ‘coffee rock’; refer below for description of coffee rock formation).
Groundwater was not encountered in any of the boreholes.

 Coffee rock on the Sunshine Coast: what, where and why?
 Coffee rock is the colloquial term for cemented (indurated) sand, that often has the physical characteristics of
 weathered rock.

 It comprises ancient alluvial / aeolian sands of the Pleistocene age (2 million to 10,000 years ago). Coffee
 rock is typically dark brown in colour, and when disturbed breaks up into dark brown silty sand (hence the
 colloquial name ‘coffee rock’).
 Cementation in these sands is thought to have occurred as a result of the interaction of organic matter and
 mineral-rich sand, e.g., flocculation at the interface of a fluctuating groundwater level.
 Coffee rock can be weakly or strongly cemented and is often discontinuous and of variable strength and
 thickness. It is generally of low permeability, and often associated with high ‘perched’ groundwater.
 On the Sunshine Coast, coffee rock is common at many locations along the coastline, generally buried under
 younger alluvial soils (e.g., dune or beach sand). Coffee rock is often exposed and re-buried in the intertidal
 zone by cycles of natural sand movement.
 Previous investigations have found the coffee rock to be underlain by dense, non-cemented sand in most
 areas, or soft alluvial clay in some instances.
 Coffee rock presents geotechnical constraints and opportunities to infrastructure development and
 maintenance, e.g., excavatability, groundwater management, and foundations. It is considered a key element
 in the stability of the region’s coastline, likely providing protection from erosion and storm events.


6.0     GEOTECHNICAL RECOMMENDATIONS

6.1   Earthworks

6.1.1 Site Preparation & Filling
Prior to construction at-grade, the following works are recommended:

     Strip the surface of existing vegetation and root affected soils and loose disturbed material likely to be
      present following the demolition of the existing building. Localised deeper stripping/excavation may
      also be required where larger tree root systems need to be removed and/or beneath the existing
      building if any debris or remnant building items remain. Any organic rich topsoil must not be used as
      fill in proposed building areas, however it may be used for non-structural purposes (i.e., landscaping).

     ‘Proof roll’ the stripped surface by several passes of a minimum 10 tonne smooth drum roller to
      identify loose areas.

     A suitably qualified and experienced geotechnical engineer should carry out inspections of the
      subgrade to help identify any areas of unsuitable material or weak subgrade that may be present
      following stripping. Loose and/or over dry sand will require moisture adjustment and compaction.

     Loose areas identified by proof rolling should be treated by over excavation and re-compaction of the
      natural aeolian sand (providing these are free from organic matter).
Contractors carrying out compaction of existing subgrade will need to be aware of potential damage to
adjacent buildings due to vibration effects. Compaction may need to be carried out without vibrations. We
also recommend undertaking a condition assessment (dilapidation survey) of adjacent and neighbouring
structures prior to commencement of earthworks. It would also be prudent to confirm the foundation systems
of the adjacent properties, to ensure that these foundations are not undermined during any excavation
works.



J001873-001-R-Rev1                                                                                              Page 5