Appendix B - Geotech and Acid Sulfate report - MCU19-0030

Gympie Terrace Noosaville Pty Ltd



6.0      GEOTECHNICAL COMMENTS & RECOMMENDATIONS
  6.1 Site Preparation and Trafficability
Site preparation in construction areas should include the following:

     Removal of trees and other vegetation including root systems
     ‘Proof roll’ the stripped surface by several passes of a minimum 10 tonne roller to identify loose or
         soft areas. Proof rolling in areas of cut (i.e. basement excavation) may be deferred until after
         excavation.
An experienced geotechnical engineer from Core should carry out an inspection of the subgrade 'proof roll'.
Loose or soft areas identified by proof rolling may require treatment by either of the following methods:

     Over-excavation and replacement with suitable fill compacted in layers; or
     Re-compaction of the loose or soft material provided that it does not contain substantial organic
         matter or deleterious substances (e.g. construction waste after demolition of existing structures).
Due to the potential for poor trafficability in soil across the site during periods of wet weather, it would be
prudent to allow for the placement of a nominal 250 mm thick (or thicker depending on traffic loadings)
granular layer to facilitate movement of construction machinery.

  6.2 Excavatability & Stability
Excavation for the basement to approximately 2 m BGL (~RL -0.3 AHD) should encounter mainly loose to
medium dense (or denser) sands.
Such excavation should be achievable using conventional earthmoving equipment (e.g. 12 to 20 tonne
tracked excavators).
A standing groundwater level was recorded at 2 m BGL (RL -0.23 m) in the standpipe installed in borehole
BH1. Groundwater inflow is expected to be encountered during the basement excavation, particularly if
conducted during/following a period of heavy or prolonged rainfall, and therefore dewatering will likely be
required (refer Section 6.2.1).
Based on conceptual sketches provided, the use of temporary batter slopes during basement excavation are
not anticipated due to space constraints. However, if temporary batter slopes are to be used; the loose to
medium dense sand above the groundwater level will be expected to temporarily stand unsupported at
slopes of 1V:2H (without loads behind the batter crest). A minimum distance of at least 2.5 m (horizontal)
between the crest of such batter slopes and any adjacent services or structural elements is recommended.
Excavations in wet sands will not stand unsupported and will require the installation of temporary support
(refer Section 6.2.1).
Excavations must consider the presence of adjacent structures and services. A dilapidation survey of
adjacent structures is recommended prior to basement excavation, and consideration must be given to
construction vibration impacts.
Care should be taken to confirm underground service locations, as instability of trench backfill can impact the
temporary support of the basement excavation.

    6.2.1 Temporary Support & Dewatering
As groundwater will likely be encountered, temporary shoring of the basement excavation using engineer-
designed anchored (or internally propped) sheet-pile walls will be required during basement construction.
Additional measures such as grouting the joints in sheet piles should reduce groundwater inflows.
Based on our experience on similar basement excavation projects, steel sheet piles will need to be propped
or anchored to limit bending moments and deflections and to provide control of soil movements behind the
wall. These movements will be both lateral and vertical, occurring at the crest of and behind the excavated



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