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19009-004-Rev0 RIP Advice · OPW22/0166
From the archive 220906 - OPW220166_RFI.zip, inside OPW22/0166 - Response to Request for Information. File date in the archive: 2022-09-01 12:24.
1 September 2022 Project No. 19009-004-Rev0 CBD Settlers Cove Pty Ltd Attention: Mr Shannon Gillard Email: [email protected] Cc: [email protected] GEOTECHNICAL ASSESSMENT OF PROPOSED RAPID INFILTRATION PITS 16 (LOT 9) SERENITY CLOSE, NOOSA HEADS Dear Shannon, 1.0 INTRODUCTION At the request of CBD Settlers Cove Pty Ltd, an experienced geotechnical engineer from Tectonic (A. Davey) has undertaken a geotechnical assessment of proposed rapid infiltration pits (RIPs) to deal with stormwater at the above site. This report has been prepared to respond to Items 1 and 2 of a Noosa Council information request (Ref. letter OPW22/0166, dated 25 July 2022), specifically: 1. Provide a Geotechnical Engineer’s report that demonstrates the suitability of the soils to support the use of the proposed Rapid Infiltration Pits. The report must be prepared by a suitably qualified person (RPEQ – Registered Professional Engineer of Queensland). 2. Provide additional detail demonstrating that the location of the proposed Rapid Infiltration Pits follow ‘best practice’ methods and will not create adverse effects on the stability of the existing retaining wall, proposed pool and overall conditions of the site. This detail must be prepared by a suitably qualified person (RPEQ – Registered Professional Engineer of Queensland) and must align with the recommendations made by a Geotechnical report. As part of this assessment, one borehole (denoted BH9) was drilled in the location of the proposed RIPs. The location of the borehole is shown in Figure 1 which is attached along with the borehole report and explanatory notes. The following documents were also reviewed: Alta Stormwater Management Plan for the site (Ref. a6107/I9/SWMP/Rev a, dated August 2021). Sketches received by email dated 23 August 2022 from Alta regarding the approximate extents of proposed sheet piling to enable construction of the RIPs. Advice received by email dated 30 August 2022 from ASP regarding details of proposed sheet piling. 2.0 OBSERVATIONS As shown in Text Figures 1 and 2 overleaf, five RIPs are proposed in the southern corner of the site, off-set in places less than 1 m from the back of an existing boulder retaining wall that runs along the boundary. Each RIP will have a chamber 1800 mm in diameter and 2400 mm deep, with an open base founded at about RL 5.85 m on a 300 mm thick layer or rock spalls sitting on a natural sand subgrade. This would be slightly below the toe level of the adjacent boulder wall, which is indicated to be at about RL 6.2 m. Stormwater would feed into the RIPs via 300 mm and 375 mm diameter pipes connected to surface catchments within the site and overflow from a nearby 80 kL rainwater harvesting tank, respectively. When the capacity of the RIPs is exceeded (>Q100), water would flow up out of grated covers and be directed by a swale to a 3 m wide, cemented grouted rock overflow weir discharging to the south-east. Tectonic Geotechnical Pty Ltd PO Box 899, Buderim, QLD 4556 Office: 07 5478 9016 Email: [email protected] www.tectonicgeo.com.au A.B.N. 83 165 727 828
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CBD Settlers Cove Pty Ltd 19009-004-Rev0
1 September 2022
Text Figure 1: Sketch plan showing proposed locations of RIPs and extent of sheet piles (supplied by Alta)
Text Figure 2: Sketch section showing proposed RIP details and sheet pile position (supplied by Alta)
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CBD Settlers Cove Pty Ltd 19009-004-Rev0
1 September 2022
Appendix D of the Alta SWMP indicates that the maximum infiltration rate across the five RIPs will be
265 L/s, or 53 L/s per pit, with the pits full of water.
As indicated by borehole BH9, the RIP bases at about 3 m to 3.5 m below existing ground level will be
founded in medium dense to dense, fine to medium grained sand/silty sand. The medium dense to dense
sand extends to borehole termination at 9 m depth. Groundwater seepage was not encountered during
drilling, with the sands being moist and becoming moist to wet at about 8 m depth (approx. RL 1 m). Similar
foundations and deeper subsurface conditions are expected under the adjacent boulder wall.
A field test carried out by an Alta engineer (see Appendix C of the Alta SWMP) indicates that the infiltration
rate under 2700 mm of head (when the RIPs are full) would be approximately 40 mm/sec. Anecdotal
evidence indicates that existing RIPs in earlier stages of the development have never been seen to
overflow, except for one occasion where the basal geosynthetic filter fabric became clogged with fines.
Design drawings (Ref. Alta Dwg. No. A6107/22/L9/EW02, Rev A) indicate that the proposed pool adjacent
to the RIPs will be supported by screw piles.
To enable the RIP construction adjacent to the existing boulder wall, sheet piles will be installed within the
approximate 1 m wide space between the back of the retaining wall and the proposed RIPs. ASP has
indicated that the sheet piles will be 4 mm thick and will be installed to depths of 6 m to 8 m below existing
ground level (behind the wall crest). The sheets will be interlocking with a clutch mechanism so that there
would be minimal opportunity for fine grained sand and/or water to penetrate through the joints.
Based on AS2159-2009 Piling - Design and installation, the sandy soil conditions at this site (allowing for
regular saturation by ‘groundwater’) would be classified as non-aggressive. From Table 6.5.3 of AS2159,
corrosion allowance for steel piles would therefore be <0.01 mm/year. However, if we conservatively
assumed a Mild exposure and corrosion at 0.02 mm/year, for a design life of 50 years, the sheets could lose
1 mm from both sides (2 mm from total thickness) so there should be at least 2 mm still in place after 50
years. Therefore, although the sheet piles are proposed to maintain temporary stability of the existing
boulder wall, the would remain in place sufficiently to provide protection to the wall from infiltrated water for
the long-term.
3.0 ASSESSMENT & ADVICE
As demonstrated by previous field testing at Settlers Cover by others, and visual observations of the long-
term performance of existing RIPs within Settlers Cove by Alta, the local sands have sufficiently high
permeability to enable the proposed rapid infiltration of stormwater. Such sands were identified to at least
9 m depth in a borehole at the proposed location of the RIPs at the subject site. It is noted that groundwater
could affect the performance of the RIPs, however the adjacent borehole indicates that groundwater is at
about RL 1 m, almost 5 m lower than the underside of the proposed RIPs. It is therefore concluded that the
soils are suitable to meet the design intent of the RIPs at this site.
It is acknowledged that the proposed RIP operation could potentially have the following adverse effects on
the existing boulder retaining wall:
a) Infiltration causing densification of foundation sands and settlement of the wall foundations.
b) Saturation of the subsurface reducing the angle of friction in the sands beneath the wall and
impacting global slope stability.
c) Uncontrolled overflow from the RIPs (>Q100 events) causing scour erosion of the wall backfill
materials.
With regard to potential impact a), the existing wall foundations are in a medium dense to dense condition,
thus there is negligible potential for the foundation sands to densify due to water infiltration. From our
extensive experience with densification of the natural sands at Settlers Cove, it is necessary to impart
significant vibration (typically by an excavator mounted vibrating plate) in combination with ample water to
effect sand densification (from loose to medium dense, or better). The foundations would not be subject to
such concentrated vibration during operation.
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CBD Settlers Cove Pty Ltd 19009-004-Rev0
1 September 2022
It is noted that the proposed swimming pool will be supported by piles adjacent to the RIPs so any potential
adverse impact on the pool foundations/stability from the RIP operation would be mitigated such that it
would be negligible risk.
Noosa Council contour mapping indicates that the ground surface below the toe of the boulder wall dips at
less than 10° to the south for a distance of about 15 m, then becomes relatively flat for some distance
across a broad natural terrace at about RL 5 m. This area is also densely vegetated. Sand dunes are
typically marginally stable where the face slope is about 30°. Given the gentle to relatively flat gradients
extending away from the toe of the retaining wall, it is assessed that the proposed RIP operation would
present a Low Risk of causing global instability to the boulder wall and adjacent area.
Regarding potential impact c), we note anecdotal advice which indicates that the RIPs are unlikely to
overflow. The design calls for a grouted rock spillway fed by an open channel with sides battered at 1V:2H
from the RIP overflow grates. There would be increased potential for overflow to develop if the basal
geosynthetic filter fabric in the RIPs becomes blocked with fines. Migration of RIP overflow waters into the
retaining wall backfill could also occur, however this will be prevented in the long-term by the proposed
sheet piles.
Based upon the above, to achieve a Low Risk of adverse impact to the existing/proposed structures and
general area from the operation of the RIPs, we recommend the following:
Sheet piles should extend at least 3 m beyond the ends of the RIP zone to provide effective
stabilisation during temporary works, and to increase the distance that infiltrated water needs to
travel to outflow under the retaining wall.
The basal geosynthetic filter fabric in the RIPs should be regularly checked and replaced if it is found
to have become blocked with fines. We understand that Alta has developed a management plan to
deal with maintenance of the RIPs.
5.0 CLOSURE
Your attention is drawn to the document Limitations, which is attached to this letter report.
Please contact the undersigned should you wish to discuss any of the above matters.
Yours faithfully
TECTONIC GEOTECHNICAL PTY LTD
Ashley Davey BEng (Geological) RPEQ 8159
Principal Geotechnical Engineer
Attachments: Figure 1 – Site Plan
Report for Borehole BH9 & Explanatory Notes
Limitations
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Page 5
Tectonic Geotechnical Pty Ltd
PO Box 899
Buderim Qld 4556
Ph: 07 5478 9016
PROJECT:
DCP6 BH6
STAGE 5 GEOTECHNICAL
INVESTIGATIONS
BH7
BH3 TITLE:
BH11
SITE PLAN
CLIENT:
BH1
CBD SETTLERS COVE PTY LTD
BH9 BH8 DRAWN: DATE:
BH4
MCC 16/08/2022
BH2 CHECKED: DATE:
BH10 BH12 ACD 16/08/2022
BH5 SCALE:
AS SHOWN (APPROX.)
LEGEND:
Subject Site APPROX. PREVIOUS
BOREHOLE LOCATIONS (2021)
APPROX. PREVIOUS
STANDSLONE DCP LOCATION
(2021)
APPROX. ADDITIONAL
BOREHOLE LOCATIONS (2022)
Notes:
Base plan adopted from Nearmap imagery dated 10 September 2021 &
ALTA Projects Drawing No. A6107/21/L901 Rev A, dated August 2021 0m 20 40m PROJECT NO.: FIGURE: REV:
19009-004 1 0
Page 6
Borehole No.
BH9
Page 1 of 1
Engineering Log - Borehole Project No.: 19009
Client: CBD Settlers Cove Pty Ltd Commenced: 11/8/2022
Project Name: Proposed Rapid Infiltration Pit Completed: 11/8/2022
Hole Location: Settlers Cove, Serenity Close, Noosa Heads Logged By: LM
Hole Position: See Site Plan Checked By: MCC
Drill Model and Mounting: 4WD Auger Inclination: -90° RL Surface: No survey
Hole Diameter: 100 mm Bearing: 360° Datum: AHD Operator: Tectonic
Drilling Information Material Description and Observations
Relative Density
Classification
Pocket DCP TEST
Penetration
Consistency
Graphic Log
Samples Material Description (AS 1289.6.3.2-1997)
Penetrometer
Recovery
Condition
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
Remarks secondary and minor components, colour
Water
RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
VL HW
SM Silty SAND, fine grained, dark grey
SM becoming grey
L
Augered to 1.9m
2
MD
Augered to 2.7m
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 19009-003.GPJ <<DrawingFile>> 16/8/2022 12:54 10.02.00.04 Datgel Lab and In Situ Tool - DGD | Lib: Tectonic 3.00.2 2021-03-30 Prj: Tectonic 3.00.2 2021-03-30
SP SAND, fine to medium grained, pale grey
M
Not Encountered
4 Augered to 4.1m
AD/T
Augered to 7.6m
6
MD to
D
8
Augered to 8.6m
W to
M
DS
8.4-8.8m
Hole Terminated at 9.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
AD/T-Auger Drilling TC Bit VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
AD/V-Auger Drilling V Bit E - Easy DS - Disturbed Sample M - Moist S - Soft
RT - Rotary Tri-cone Bit Inflow BDS - Bulk Disturbed Sample W - Wet F - Firm
RD - Rotary Drilling F - Firm SPT - Standard Penetration Test VSt - Very Stiff
Partial Loss
NMLC -Rock Core H - Hard H - Hard
VH - Very hard Complete Loss
HA - Hand Auger Plastic Limit VL - Very Loose
Graphic Log/Core Loss L - Loose
Classification Symbols and < PL MD - Medium Dense
Core recovered (hatching Soil Descriptions
indicates material) = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Core loss Fr - Friable
Classification System
Page 7
TERMS USED ON LOGS
DRILLING/EXCAVATION METHOD
AD* Auger Drilling RA Rotary Air BH Tractor Mounted Backhoe
ADH Hollow Auger RD Rotary Blade or Drag bit EX Tracked Hydraulic Excavator
HA Hand Auger RT Rotary Tri-cone bit HMLC Core – 63mm
*T TC-Bit HQ Core – 63mm
*V V-Bit NMLC Core – 47mm
NQ Core – 52mm
R Ripper
RH Rock Hammer
WATER
GROUNDWATER NOT The observation of groundwater, whether present or not, was not possible due to drilling water,
OBSERVED surface seepage or cave in of the borehole/test pit.
GROUNDWATER NOT The borehole/test pit was dry soon after excavation. However, groundwater could be present in
ENCOUNTERED less permeable strata. Inflow may have been observed had the borehole/test pit been left open
for a longer period.
SAMPLING AND TESTING
SPT Standard Penetration Test to AS1289.6.3.1-2004
5,4,10 N=14 5,4,10 = Blows per 150mm. N = Blows per 300mm penetration following 150mm seating
30/65mm Where practical refusal occurs, the blows and penetration for that interval are reported
RW Penetration occurred under the rod weight only
HW Penetration occurred under the hammer and rod weight only
HB Hammer double bouncing on anvil
DS Disturbed sample
BDS Bulk disturbed sample
SV Field shear vane test expressed as uncorrected shear strength (sv = peak value, sr = residual value)
PP Pocket penetrometer test expressed as instrument reading in kPa
U50 Thin walled tube sample - number indicates nominal sample diameter in millimetres
DCP Dynamic cone penetration test
CPT Electronic cone penetration test
CPTu Electronic cone penetration test with pore pressure (u) measurement
Rev1
February 2021
Page 8
METHOD OF SOIL DESCRIPTION
USED ON LOGS
CLASSIFICATION AND INFERRED STRATIGRAPHY
Soil is classified and described in borehole and test pit logs using the preferred method given in AS1726 – 2017.
The material properties are assessed in the field by visual/tactile methods.
Particle Size Plasticity Properties
Major Sub Division Particle Size
Division
BOULDERS > 200 mm
COBBLES 63 to 200 mm
Coarse 19 to 63 mm
GRAVEL Medium 6.7 to 19 mm
Fine 2.36 to 6.7 mm
Coarse 0.6 to 2.36 mm
SAND Medium 0.21 to 0.6 mm
Fine 0.075 to 0.21
SILT 0.002 to 0.075
CLAY < 0.002 mm
NOTE: The U line is an approximate upper bound for most materials.
Data which plot above the U line may represent unusual/problem soil
behavior, or unreliable data and should be considered carefully.
MOISTURE CONDITION AS1726 - 2017
Symbol Term Description
D Dry Sands and gravels are free flowing. Clays & Silts may be brittle or friable and powdery.
M Moist Soils are darker than in the dry condition & may feel cool. Sands and gravels tend to cohere.
W Wet Soils exude free water. Sands and gravels tend to cohere.
CONSISTENCY AND DENSITY AS1726 - 2017
Symbol Term Undrained Shear Symbol Term Density Index % SPT “N” #
Strength
VS Very Soft 0 to 12 kPa VL Very Loose Less than 15 0 to 4
S Soft 12 to 25 kPa L Loose 15 to 35 4 to 10
F Firm 25 to 50 kPa MD Medium Dense 35 to 65 10 to 30
St Stiff 50 to 100 kPa D Dense 65 to 85 30 to 50
VSt Very Stiff 100 to 200 kPa VD Very Dense Above 85 Above 50
H Hard Above 200 kPa
In the absence of test results, consistency and density may be assessed from correlations with the observed behaviour of
the material.
# SPT correlations are not stated in AS1726 – 2017, and may be subject to corrections for overburden pressure and
equipment type.
Rev1
October 2017
Page 9
LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)
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