MCU14/0106 Appendix E - Engineering Report

Noosa Civic
Engineering Report




4                    Geotechnical and Acid Sulfate Soil Issues
A detailed sub-surface geotechnical investigation was completed by Golder Associates prior to the
development of the existing Noosa Civic site in August 2004 (Report on Geotechnical Investigation – Noosa
Business Centre – Stage 1, Golder Associates Pty Ltd, August 2004). 21 test pits and boreholes with depths
varying from 2.50 to 5.00 m were excavated across that part of the site west of the OS7 area. The results for
the testing showed a consistent profile across the area of investigation, with silty and clayey sand at the
surface generally overlying silty and sandy clays grading at depth into low strength sandstones. A copy of
the previous report in included herein as Appendix A.
GA15, GA11, GA17 and GA18 are located generally close to the western boundary of the proposed
development. The logs for each of these test pits show consistent stratigraphy with sands overlying sandy
clays. No evidence of sandstone was found in these eastern pits which were excavated to depths of 2.50 to
3.00 m. Test pits GA11, GA17 and GA18 were terminated at levels of 3.50, 2.00 and 1.50 m AHD
respectively.
At this stage, no test pits have been dug on the development area, since to do so will require significant
removal of vegetation and other land disturbance. However, surface investigation provides no sign that sub-
surface conditions will vary from that found on the western part of the Noosa Civic site. Golder Associates
concluded that all materials should be excavatable using conventional earthmoving equipment, and that the
sand and clays soils won by excavation would be suitable for reuse as structural fill. It is noted that the
extent of fill required for the development is significantly less than on the western part of the Noosa Civic site,
primarily because of the presence of the undercroft carpark. A significant volume of material excavated from
the site will need to be disposed of as spoil external to the project.
It is proposed that a level of geotechnical investigation to confirm the above findings will be undertaken prior
to the commencement of construction. This will include excavation of test pits across the site east of OS7.
Golder Associates also undertook an acid sulfate soils investigation for the existing Noosa Civic site (Section
7 of the Geotechnical Investigation Report). According to acid sulfate soil mapping prepared by the
Department of Natural Resources, the site is defined as “land >5 m AHD with a low to negligible probability
of ASS occurrence”. Both field and laboratory tests were undertaken on soil samples collected from test pits
GA17 and GA18, albeit at levels below 5.0 m AHD. It was determined that there was negligible probability
for the occurrence of Potential Acids Sulfate Soils (PASS) on the site, which was expected since the tested
layers were above the water table level, i.e. soils on the site are likely to have already been oxidized by
exposure to the atmosphere. Correspondingly, the soils displayed slight actual acidity. Surface soils in
these tests (at 5.0 m AHD) required a minimal liming rate of about 5 kg of lime per tonne of soil. The
proposed floor level of the undercroft carpark is 5.5 m AHD. Allowing for excavation for concrete slab
construction purposes, the minimum level of excavation generally over the site will therefore be
approximately 5.2 m AHD. There will be a small basin adjacent to the undercroft carpark associated with a
stormwater detention and bio-retention basin that will include excavation below 5.0 m AHD. The State
Planning Policy for Queensland specifies that acid sulfate investigations should be undertaken where
disturbance takes place below 5.0 m AHD, and the volume of excavation is at least 100 m 3. In this case, the
volume of excavation below 5.0 m AHD is likely to exceed 100 m 3, and an acid sulfate soils investigation will
therefore be required.
It is noted that the previous testing was undertaken at a time when no distinction was drawn between sulfidic
and organic acidity. The source of the actual soil acidity in this case is not known, but may in fact be organic
since no sulfur content was detected during testing.
Council’s Planning Scheme (Natural Hazards Overlays Code) also requires acid sulfate soils assessment
when earthworks are undertaken below 5.0 m AHD. Given the small area of disturbance in the proposed
detention basin (see Figure 2) the requirement for acid sulfate soils investigations will be relatively minor. In
addition, it is unlikely that extensive treatment of acid sulfate soils will be required because of the low acid
generating capacities of the existing soils. These investigation works will be undertaken prior to
commencement of any earthworks on site after relevant site clearing has taken place.


QIC Noosa Civic Pty Ltd                              Version 1                                       November 2014
engineering report_final.docx                                                                              Page vi