Information response - GEOTECH REPORT

Report: 15960, May 11 Page 2

Numerous studies into the causes of landslides within southeast Queensland comment upon an
equilibrium existing between weathering of the underlying rock and removal of the soil mantle. Where
landslides are known to occur it can be concluded that some change to this equilibrium has occurred.
The key factor in maintenance of this equilibrium is water; whether that is infiltration reducing soils
shear strength and/or an increase in pore (groundwater) pressure.

Typically the removal of natural forest will reduce transpiration by trees and hence during high rainfall
events allow increased pore (groundwater) pressures. Additionally with tree removal there is a
diminished binding of the upper soils by root systems.

1.6 Our initial review included:
# Maroochy Shire Council “Landslip Hazard Mapping”- which identifies the allotment to
include both hazard and non-hazard areas;
Review of regional geotechnical mapping;
“Landslide Risk Management Concepts and Guidelines”, March 2000 by Australian
Geomechanics Society;
4 Review referenced soil investigation and effluent disposal reports.

2. PHYSIOGRAPHY

2.1 Land Description
Residential allotment regionally located over dissected topography to the west of Cooroy township. The

area is represented by narrow ridges, spurs and deep intervening gullies. The allotment is(Qng in area
and locally is accessed by Sie “cannon Pgs off Black Mountain Road.

2.2 Vegetation

“ Medium/long pasture grasses and numerous srhali/medium treéon the lower flank, Some recent tree
enn .

removal was evident, Refer to site photographs.

 

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2.3 Crossfall

Locally the allotment includes a narrow crown with flanks grading to the east. The proposed residential
area incorporates the crown and near flank. Contours supplied indicate a relative fall of 1.5 metres over the
house area, with typical grades of 1:5 (V:H). Area to the southeast of the building area is locally flatter
while the northeastern flank steepens to 1:3.

2.4 Drainage
The building area is well drained with the flanks of planar shape.

2.5 Regional Geology - Regional geology describes this allotment to be upon the Kin Kin Beds.
Typical rock types include phyllite, shale, arenite and argillite. All these rock types contain fine grained
sediments and can be micaceous. Locally quartz bands will be encountered. Rock is bedded and visually
evident in surrounding road embankments.

 

Office at: 1/8 Denna St, Maroochydore, Q, 4558.
Phone: 5443 6622, Fax: 5479 1899, Email: [email protected]