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NOOSA COUNCIL
REFERENCE DOCUMENT
OPW25/0085 28 JULY 2025
Geo-Environmental Investigation
for Proposed Subdivision
44 Hoy Road, Lake Macdonald
7 Proposed
Geo-Environmental Investigation for February 2025 | Ref. 24386-001-Rev0
Subdivision
7 February 2025 | 24386-001-Rev0
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Full text of pages 2 to 41 (of 63)
Page 2
Document Review
Doc No. Revision Prepared By Reviewed By Date Issued
001 0 Jarrad McVey & Mark David Pollock 7 February 2025
Thomson
Doc No. Revision Method of Delivery Issued To
001 0 Electronic Peter Stone ([email protected])
Prepared By
Reviewed By
Jarrad McVey BEng(Civil) BBus MIEAust David Pollock RPEQ 14121 CPEng BE(Civil) MIEAust
Geotechnical Engineer Principal Geotechnical Engineer
Mark Thomson B.Sc (Environmental) CPESC 7057
Senior Environmental Scientist
Geo-Environmental Investigation for Proposed Subdivision
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Executive Summary This report details the results of a geo-environmental investigation undertaken for a proposed one into nine lot residential subdivision at 44 Hoy Road, Lake Macdonald. The investigation comprised a site walk-over by an experienced geotechnical engineer, together with excavating test pits at twelve locations. This report summarises the results of the investigation and includes a slope stability assessment along with recommendations regarding earthworks, retaining structures, footings, drainage and pavements. Also included is a feasibility assessment for on-site wastewater treatment and disposal. A concept layout plan indicates that the proposed subdivision will comprise nine new allotments with a new central road, accessed via Hoy Road at the northern end of the site. The proposed lots are generally about 1.5 ha in area, with one larger balance lot to the east, of 7.85 ha that includes a 110 m wide environmental covenant that follows the natural drainage line. We understand that the site will remain unsewered and therefore, on-site wastewater management will be required. Subsurface conditions encountered in the test pits comprised loose, silty and clayey sand topsoil from the ground surface to depths up to 0.3 m below ground level (BGL), followed by mostly medium dense/stiff (or stronger), clayey sands or silty clays/clayey silts to depths between 0.8 m and 3.2 m BGL. The soils were generally underlain by very low to low strength siltstone/sandstone to the test pit termination depths. No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over around the proposed development areas. Groundwater seepage was encountered in five of the twelve test pits during the investigation at depths of between 0.1 m and 0.9 m BGL. Groundwater may develop or be encountered at higher levels following prolonged/heavy rainfall. Based upon the existing site conditions, together with our qualitative assessment and quantitative analysis, and subject to the implementation of recommendations given in this report, it is our assessment that there would be a Low Risk of global slope instability affecting the proposed subdivision. There is a risk that elevated soil moisture conditions could occur if the works are undertaken during a period of wet weather and shallow groundwater could become perched in the sands above the underlying less permeable clay. The upper sands could then become particularly unstable and very difficult to work, proof roll or compact under elevated moisture contents. Allowance would then need to be made for subgrade removal or improvement works during the earthworks phase. Preliminary characteristic surface movements for the natural soil profile are generally estimated to be equivalent to Class H1 in accordance with AS 2870-2011. Site classification will need to be reassessed at the Building Works stage to consider the extent of any earthworks proposed. It should be noted that filling must not exceed 1.5 m high unless assessed and approved by a geotechnical RPEQ before construction. All footings supporting the proposed residences and ancillary structures (including all retaining walls) on slopes of 10° or less must penetrate through any fill and unsuitable natural soils to found at least 0.5 m into stiff/medium dense (or stronger) residual soils, or future controlled fill. On slopes greater than 10° footings must be founded at least 0.5 m into very low strength (or stronger) sedimentary rock. The subsurface conditions encountered indicate that steel reinforced concrete bored piles or ‘bucket piers’ will generally be required unless excavations expose the required foundation materials at or near the subgrade in which case conventional high level strip or pad footings would be appropriate. Pavement design will be dependent upon the subgrade materials present after earthworks. Pavement design could be carried out adopting a design CBR value of 3% for the natural clay and silt subgrades Geo-Environmental Investigation for Proposed Subdivision 7 February 2025 | 24386-001-Rev0 ii
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and design CBR value of 5 % for the natural clayey sands. A presumptive CBR value of 10 % could be adopted for design in areas of deeper cut where very low strength (or stronger) weathered rock is exposed consistently over the full width of the road. Considering the site and soil conditions, it is recommended that on-site effluent treatment and land application for each new lot should comprise a household sewage treatment plant producing a minimum of Secondary standard effluent and connected to shallow subsurface or surface irrigation system. Based on site and soil evaluation and including conservative slope reduction and design irrigation rate, a minimum land application area of 350 m2 will be required for the new residences. There is sufficient room on the proposed lots to accommodate this. Geo-Environmental Investigation for Proposed Subdivision 7 February 2025 | 24386-001-Rev0 iii
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Contents
1. Introduction ............................................................................................................................................................ 1
2. Proposed Development ....................................................................................................................................... 1
3. Aims & Method of investigation .......................................................................................................................3
3.1 Fieldwork ...................................................................................................................................................................... 3
3.2 Laboratory Testing ................................................................................................................................................... 3
4. Results of Investigation ...................................................................................................................................... 4
4.1 Site Description ......................................................................................................................................................... 4
4.2 Topography................................................................................................................................................................. 7
4.3 Surface Drainage ....................................................................................................................................................... 7
4.4 Vegetation ................................................................................................................................................................... 8
4.5 Buildings and Other Structures ........................................................................................................................... 8
4.6 Geology ........................................................................................................................................................................ 8
4.7 Subsurface Conditions ............................................................................................................................................ 9
4.8 Groundwater............................................................................................................................................................. 10
4.9 Geotechnical Laboratory Test Results ............................................................................................................. 11
5. Slope Stability Assessment ............................................................................................................................... 12
5.1 Existing Conditions................................................................................................................................................. 12
5.2 Slope Stability Assessment.................................................................................................................................. 13
5.2.1 Qualitative Assessment ....................................................................................................................... 13
5.2.2 Numerical Modelling ........................................................................................................................... 14
6. Geotechnical Recommendations ..................................................................................................................... 16
6.1 Site Layout and Geotechnical Constraints..................................................................................................... 16
6.2 Earthworks ................................................................................................................................................................. 16
6.2.1 Site Preparation and Trafficability................................................................................................... 16
6.2.2 Excavations .............................................................................................................................................. 17
6.2.3 Batter Slopes ........................................................................................................................................... 18
6.2.4 Filling ......................................................................................................................................................... 18
6.3 Building Foundations ............................................................................................................................................ 19
6.3.1 Site Classification .................................................................................................................................. 19
6.3.2 Footing Design....................................................................................................................................... 19
6.4 Retaining Structures .............................................................................................................................................. 20
6.5 Drainage ..................................................................................................................................................................... 21
6.6 Pavement Design Parameters ............................................................................................................................ 22
6.7 Geotechnical Certification of Operational Works ....................................................................................... 22
7. Wastewater Treatment and Disposal ............................................................................................................ 23
7.1 Wastewater Flow Design Allowance ................................................................................................................ 23
7.2 Terrain-Soil Assessment ....................................................................................................................................... 23
7.3 Recommendations ................................................................................................................................................. 24
7.3.1 System Type ............................................................................................................................................ 24
7.3.2 System Positioning ............................................................................................................................... 24
7.3.3 System Construction ............................................................................................................................ 26
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7.3.4 Maintenance and Inspection ............................................................................................................ 26
7.3.5 Water Resource Catchment Overlay .............................................................................................. 27
8. Limitations ............................................................................................................................................................ 27
Tables
Table 1: Summary of Subsurface Profiles ................................................................................................................................ 9
Table 2: Summary of Quality of Materials Testing............................................................................................................. 11
Table 3: Stability Risk Levels ....................................................................................................................................................... 13
Table 4: Details of Qualitative Risk Assessment for Property (AGS 2007) ................................................................ 14
Table 5: Retaining Wall Design Parameters.......................................................................................................................... 21
Text Figures
Text Figure 1: Conceptual subdivision layout, courtesy of IPS........................................................................................ 1
Text Figure 2: Conceptual earthworks plan for the subdivision, courtesy of Walsh ............................................... 2
Text Figure 3: 2025 aerial image of the site and surrounds (courtesy of Nearmap imagery) ............................. 4
Text Figure 4: North-easterly view across proposed Lots 8, natural drainage line behind .................................. 5
Text Figure 5: Oblique drone image of the site looking south-west along the central ridgeline ..................... 5
Text Figure 6: Looking east across the natural drainge line, drone image................................................................. 6
Text Figure 7: Drone image looking north, towards Hoy Road ...................................................................................... 6
Text Figure 8: Image showing ground surface contours (courtesy of QGlobe)........................................................ 7
Text Figure 9: Extract from Gympie Surface Geology Map .............................................................................................. 8
Text Figure 10: Typical subsurface profiles TP1 - sandstone (LHS) and TP7 - siltstone (RHS) .......................... 10
Text Figure 11: Extract from SCC landslide hazard overlay mapping ......................................................................... 12
Text Figure 12: Slope/W analysis output for proposed residential construction – short term
conditions .............................................................................................................................................................................. 15
Text Figure 13: Footing depth required to minimize risk of undermining and surcharging ............................. 20
Text Figure 14: Location of site relative to Mary River ..................................................................................................... 27
Figures
Figure 1: Site Plan
Appendices
Appendix A Test Pit Logs & Explanatory Notes
Appendix B Geotechnical Laboratory Test Certificates
Appendix C AGS Guidelines – Qualitative Terminology for Use in Assessing Risk to Property
Appendix D Limitations
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1. Introduction
This report details the results of a geo-environmental investigation undertaken for a proposed
subdivision at 44 Hoy Road, Lake Macdonald. The investigation was carried out by Tectonic for Indigo
Group, generally in accordance with proposal P24681-Rev0, dated 14 November 2024.
The site investigation comprised a walk-over by an experienced geotechnical engineer from Tectonic,
excavations of twelve test pits, together with in situ soil strength and geotechnical laboratory testing.
This report summarises the results of the investigation and includes a slope stability assessment, together
with recommendations regarding geotechnical matters for the development pertaining to earthworks,
retaining structures, footings, drainage and pavements. Quantitative and qualitative slope stability
assessments are included, as well as an assessment of feasibility for on-site wastewater treatment and
disposal.
2. Proposed Development
A concept plan provided to Tectonic (Ref. Innovative Planning Solutions [IPS], Dwg No. GA2022-085-CS-
004, Rev A, dated 20 November 2024) indicates that the proposed subdivision will comprise nine new
allotments with a new central road. The new road will be accessed via Hoy Road at the northern end of
the site. The proposed lots are generally about 1.5 ha in area, with one larger balance lot to the east, of
7.85 ha (Proposed Lot 9). This lot includes a 110 m wide environmental covenant that follows the natural
drainage line. Text Figure 1 illustrates the proposed subdivision layout.
Text Figure 1: Conceptual subdivision layout, courtesy of IPS
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Concept earthworks drawings provided (Ref. Walsh Consulting Engineers [Walsh], Dwg No.
C6535/24/SK02, dated December 2024) indicate that mostly cut earthworks are proposed for the new
central road and driveways. Only minor filling is required on the fringes of the road alignment. No
depths/heights were given on the drawing, however considering gradients observed on site it is
anticipated that cut up to 2 m depth and filling less than 1 m height will be required for the road and
driveway construction. Text Figure 2 shows the conceptual earthworks plans for the subdivision.
Text Figure 2: Conceptual earthworks plan for the subdivision, courtesy of Walsh
Details regarding earthworks for future residential construction were unknown at the time of reporting.
We understand that the site will remain unsewered and therefore, on-site wastewater management will
be required.
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3. Aims & Method of investigation
The aim of this investigation is to provide guidance regarding general geotechnical matters, such as
earthworks requirements for building works, and to address slope stability issues and on-site wastewater
management. It is understood that this report will be submitted to Noosa Council (NC) in support of a
Development Application for Reconfiguration of a Lot. The investigation and report have therefore been
designed to address the requirements of the relevant NC codes and policies. It should be noted that this
slope stability assessment only covers geotechnical matters to address relevant parts of the NC Landslide
Hazard Overlay Code and does not consider any other requirements of the Code (e.g. site aesthetics,
scenic amenity etc.).
3.1 Fieldwork
An experienced geotechnical engineer from Tectonic undertook a site walk-over on 19 December 2024
to visually assess any indicators of slope instability. The walk-over included assessment of soil and rock
outcrops, surface drainage conditions, topography, evidence of erosion, and vegetation
coverage/condition.
This was incorporated with a subsurface investigation, which comprised the excavation of twelve test pits
(designated TP1 to TP12). The test pits were undertaken using a 5.5 tonne excavator with a toothed
bucket and ripper attachments to depths between 2.3 m and 3.2 m below ground level (BGL), across the
likely new building areas and adjacent to the new road alignment.
In situ soil strength was assessed by conducting dynamic cone penetrometer (DCP) testing from the
ground surface adjacent to the test pits when coarse grained materials were encountered. Pocket
penetrometer (PP) testing was carried out on fine grain samples recovered from the test pits and within
test pit sidewalls, where safe to enter. Disturbed samples were recovered from select test pits.
The test pits were backfilled progressively on completion, with excavation spoil left slightly mounded at
the surface. Co-ordinates were obtained using a handheld GPS unit with a recorded accuracy of ±3 m.
Test pit reports, along with explanatory notes, are presented in Appendix A. Approximate test locations
are shown in Figure 1, attached at the end of this report.
3.2 Laboratory Testing
Select soil samples were dispatched to a NATA accredited laboratory for geotechnical testing.
Laboratory testing was undertaken in accordance with Australian Standard AS1289 Methods of Testing
Soils for Engineering Purposes and comprised:
Two Atterberg Limits tests and particle size distribution (PSD) analyses for soil classification
purposes; and
One (1 No.) single point, four day soaked CBR test for pavement design parameters.
Laboratory test results are discussed in Section 4.6. The laboratory test certificates are attached in
Appendix B.
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4. Results of Investigation
4.1 Site Description
The site is described as Lot 3 on RP171041, covers an area of about 20.84 ha, with access to Hoy Road
along the northern boundary. At the time of the investigation several agricultural sheds were situated in
the north-western part of the site. Surrounding properties generally comprised smaller sized rural
residential allotments. A 2025 aerial image of the site and surrounds is provided in Text Figure 3 below.
Existing sheds
Lake MacDonald
Subject site
Text Figure 3: 2025 aerial image of the site and surrounds (courtesy of Nearmap imagery)
Site conditions at the time of our investigation are shown in Text Figure 4 to Text Figure 87 below and
overleaf.
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Test pit TP7
Text Figure 4: North-easterly view across proposed Lots 8, natural drainage line behind
Approx. site boundary
Central ridgeline
Agricultural sheds
Text Figure 5: Oblique drone image of the site looking south-west along the central ridgeline
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Lake MacDonald
Approx. site boundary
Hoy Road
Natural drainage line
Central ridgeline
Text Figure 6: Looking east across the natural drainge line, drone image
Hoy Road
Approx. site boundary
Natural drainage line
Central ridgeline
Text Figure 7: Drone image looking north, towards Hoy Road
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4.2 Topography
During the site walk-over, topographic features were noted, and ground slopes were recorded using a
hand-held clinometer. As illustrated in the contour plan in Text Figure 8 below, topography comprises a
meandering central ridgeline which runs over a gentle saddle generally in a north-south direction . A
drainage line was present on the eastern side of the ridge flank, that was dammed at two locations within
the property. Two smaller drainage lines ran off the ridge to the east and west in the southern part. To
the east of the main drainage line (Proposed Lot 9) ground slopes fell westward towards the main
drainage line.
Gradients atop the crest of the ridgelines were gently sloping at about 6° to 10°, steepening to between
about 10° and 17° down the flanks across the anticipated building areas. Steeper gradients up to about
20° were recorded in some areas of the site, generally on the immediate flanks of the drainage lines
where future residential development is not anticipated.
Concave shaped ground was observed near the heads of minor drainage lines on either flank of the
central ridgeline. The natural slopes across the anticipated development areas were relatively linear or
convex shaped, with no obvious signs of previous global instability.
Existing dams
Subject Site
Prevailing natural drainage line
Minor drainage lines
Text Figure 8: Image showing ground surface contours (courtesy of QGlobe)
4.3 Surface Drainage
Surface water flows would follow ground surface topography sheeting away from the ridgeline and
flanks and accumulating in the natural drainage lines. Natural surface drainage across the majority of the
elevated areas and minor drainage lines was generally considered to be good, owing to the moderate
natural gradients and clayey residual soils which promote runoff. Some boggy conditions were noted
around the main drainage line and on the flatter crest near the existing agricultural sheds were drainage
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was poorer. However, it should be noted that the investigation was completed shortly after very heavy
rainfall.
4.4 Vegetation
Vegetation across the site at the time of the investigation comprised a mixture of long grasses across the
open undulating hills and small groves or linear features of large mature trees. These appeared to be
relatively straight, with no obvious signs of distorted growth which could be attributed to slope
instability. Although difficult to determine due to the long grasses at the time of the investigation,
possible shallow soil creep may be present on the steeper slopes at the site.
4.5 Buildings and Other Structures
Three agricultural sheds were present in the north-western corner of the site, they are expected to be
retained on Future Lots 1 and 2. Based on an external assessment, they generally appeared to be in fair
structural condition, considering age, with no obvious signs of distress (e.g. tilting, cracking,
misalignment etc.) which would indicate global slope instability.
4.6 Geology
Published geology mapping 1 (Ref. Text Figure 9) indicates that the site is underlain by the middle Triassic
age sedimentary strata of the Kin Kin Beds formation generally comprising mudstone siltstone and
sandstone.
Indicative site location
Text Figure 9: Extract from Gympie Surface Geology Map
1
State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, Sheet 9445, 2018
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4.7 Subsurface Conditions
Subsurface conditions encountered in the test pits are summarised as follows:
Topsoil – comprising either loose, silty sand or firm and stiff clayey silt from the ground surface to
depths between 0.17 m to 0.3 m BGL; then
Residual Soils – comprising either initially loose, becoming medium dense to dense with depth,
clayey sands or silty clays/clayey silts of high plasticity/liquid limit and initially of stiff consistency,
quickly becoming very stiff and hard to depths between 0.8 m and 3.2 m BGL (test pit target depth in
TP2 & TP4); underlain by
Weathered Sedimentary Rock – being either highly weathered sandstone (Test Pit TP1) or siltstone
(TP3 and TP5 to TP12), initially being very low strength, then generally grading to low strength (or
stronger) to target depths of 2.3 m to 3 m BGL.
Table 1 summarises the subsurface profile in each test pit. Text Figure 10 overleaf provides and
illustration of the typical subsurface profiles encountered within the sandstone and siltstone profiles.
Detailed descriptions of the subsurface conditions encountered in the test pits are provided in the
reports attached in Appendix A, along with definition sheets which describe terms and abbreviations
used on the reports.
Table 1: Summary of Subsurface Profiles
Residual Soil
Residual Soil Weathered
Test Pit No. Topsoil Medium Dense /
Loose or Firm Sedimentary Rock
Stiff (or stronger)
TP1 0.0 – 0.2 0.2 - 0.6 0.6 – 1.1 1.1 – 2.3 *
TP2 0.0 – 0.3 0.3 – 0.6 0.6 – 3.0 -
TP3 0.0 – 0.3 NE 0.3 – 1.6 1.6 – 3.0
TP4 0.0 – 0.2 0.2 – 0.4 0.4 – 3.0 -
TP5 0.0 – 0.2 NE 0.2 – 1.1 1.1 – 3.0
TP6 0.0 – 0.2 NE 0.2 – 1.1 1.1 – 2.7*
TP7 0.0 – 0.2 NE 0.2 – 1.2 1.2 – 2.6 *
TP8 0.0 – 0.17 NE 0.17 - 0.8 0.8 – 2.5 *
TP9 0.0 – 0.2 NE 0.2 – 2.0 2.0 – 3.0
TP10 0.0 – 0.2 NE 0.2 – 1.7 1.7 – 2.7*
TP11 0.0 – 0.2 NE 0.2 – 0.9 0.9 – 3.0
TP12 0.0 – 0.2 NE 0.2 – 1.4 1.4 – 3.0
Notes: 1. Depths noted in m BGL at time of investigation (13 & 19 December 2024)
2. NE denotes not encountered
3. * denotes test pit terminated due to very slow digging, toothed bucket or ripper refusal
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Text Figure 10: Typical subsurface profiles TP1 - sandstone (LHS) and TP7 - siltstone (RHS) 4.8 Groundwater It is noted that the investigation was completed during a period or prolonged heavy rainfall, however no signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over. Groundwater seepage was encountered in five of the twelve test pits during the investigation at depths of between 0.1 m and 0.9 m BGL. It is inferred that the groundwater seepage encountered during the test pitting was surface waters (from the recent rainfall) perched in the sandy upper soils, that were underlain by less permeable clays/silts than an established groundwater table. It should be noted that groundwater conditions are expected to be reflective of prevailing weather conditions and seepage may develop or at higher levels, particularly in the sands following periods of prolonged or heavy rainfall. Geo-Environmental Investigation for Proposed Subdivision 7 February 2025 | 24386-001-Rev0 10
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4.9 Geotechnical Laboratory Test Results
Table 2 below summarises the results of laboratory classification testing conducted on two soil samples
recovered during the investigation. Full test results (laboratory certificates) are presented in Appendix B.
Table 2: Summary of Quality of Materials Testing
Atterberg Limits
Test Pit Linear Percent
Depth (m) Liquid Limit Plastic Plasticity
No. Shrinkage Fines (%)
(%) Limit (%) Index (%)
(%)
TP1 0.5 – 0.7 30 17 13 7.0 28
TP8 0.4 – 0.7 61 33 28 8.0 83
Notes: % fines denotes percent passing 0.075 sieve;
Laboratory testing indicates that the residual sample (sandstone origin) tested from Test Pit TP1 was a
clayey sand with low plasticity fines. The residual sample (siltstone origin) tested from TP8 was a clayey
silt with a high liquid limit. As such the clayey silts/silty clays encountered across the majority of the
property are estimated to have a moderate to high potential soil reactivity to normal moisture content
changes.
CBR testing was completed on the clayey silt sample from TP8 located within the proposed road
alignment, and returned a CBR value of 6% which is typical of those often achieved in the laboratory for
silty soils. However, the materials are expected to be susceptible to strength loss particularly if they
become wet, with much lower CBR values likely for materials even just slightly moist of optimum
moisture content (OMC) for compaction. The field moisture content of the sample tested was 4.8 % wet
of OMC. Testing indicates that subgrade moisture conditioning (drying back) will likely be required
during construction. It is noted that the samples had negligible swell recorded.
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5. Slope Stability Assessment
5.1 Existing Conditions
NC Landslide Hazard overlay mapping indicates that the areas of the site, mainly around the natural
drainage lines are mapped as having to Moderate to High landslide susceptibility. Other areas generally
in the western and northern parts of the site are mapped as having Very Low/Low (no shading)
susceptibility. An extract of this overlay is given below in Text Figure 11.
Subject site
Text Figure 11: Extract from SCC landslide hazard overlay mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility
only and is superseded by this site-specific assessment. The site did not exhibit any obvious indicators of
global slope instability, with residual soil and weathered sedimentary rock encountered at shallow depths
in the test pits.
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5.2 Slope Stability Assessment
5.2.1 Qualitative Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 3 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix C.
Table 3: Stability Risk Levels
Risk Level Example Implications(1)
VH Very High Unacceptable without treatment. Extensive detailed investigation and research, planning
Risk and implementation of treatment options essential to reduce risk to Low; may be too
expensive and not practical. Work likely to cost more than value of property.
H High Risk Unacceptable without treatment. Detailed investigation, planning and implementation of
treatment options required to reduce risk to Low. Work would cost a substantial sum in
relation to the value of the property.
M Moderate May be tolerated in certain circumstances (subject to regulators’ approval) but requires
Risk investigation, planning and implementation of treatment options to reduce risk to Low.
Treatment options to reduce to Low risk should be implemented as soon as practicable.
L Low Risk Usually acceptable to regulators. Where treatment has been required to reduce the risk
to this level, ongoing maintenance required.
VL Very Low Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and
may depend on the nature of the property at risk; these are only given as a general guide.
Considering the existing site conditions, and subject to the implementation of recommendations given in
Section 6, it is assessed that there would be a Low (L) Risk of global slope instability affecting the
proposed subdivision. Regulators (NC) normally require that a Very Low or Low Risk of landslide
affecting property must be demonstrated to enable development approval. Summarised in Table 4
overleaf is our qualitative assessment of landslide risk for the site. A summary of qualitative terminology
for use in assessing risk to property is attached in Appendix C (taken from AGS 2007).
It should be noted that this risk assessment excludes geotechnical risk associated with the existing
structures and areas of the site outside of the proposed development areas (environmental covenant
area around the natural drainage line).
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Table 4: Details of Qualitative Risk Assessment for Property (AGS 2007)
Hazard Likelihood Consequence Assessed Risk Comments
Landslide through Unlikely Minor Low Given the gentle to moderate gradients
future fill and natural across the development areas and
soils above the relatively shallow depth to stiff/medium
foundation depth dense (or stronger) residual soils it is
considered Unlikely that a landslide
would develop, subject to
implementation of the
recommendations in Section 6. Given a
potential Minor impact from such
shallow instability the resultant risk
would be Low as per AGS 2007.
Deep seated failure Rare Major Low The likelihood of a deep failure through
within weathered the weathered sedimentary rock below
sedimentary rock the footing depths is assessed as Rare
below the foundation due to the strength of these materials,
depth moderately sloping ground and no
signs of previous deep-seated
instability. Although the consequence
of such a failure could be Major, the
resultant risk is Low as per AGS 2007.
The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 6 have been designed to mitigate those impacts, so that a Low
Risk of instability is achieved and can be maintained for the proposed development.
5.2.2 Numerical Modelling
In order to estimate the factor of safety (FOS) against slope instability for the proposed development,
numerical analysis has been conducted using Slope/W, including consideration of adverse short-term
groundwater conditions. The modelled conditions for future construction comprised a 30 kPa surcharge,
to represent filling up to 1.5 m high. As all footings for the future structures on slopes greater than 10°
will be founded into weathered rock, minimal building loads are expected to surcharge the soil profile.
The subsurface profile for this scenario was modelled on the conditions encountered in the test pits
across the likely building areas, comprising stiff (or stiffer) residual clayey silt/silty clay overlying
weathered siltstone. The model assumes any existing topsoil and any firm/loose (or weaker) materials
will be stripped from the development areas prior to filling.
Groundwater was encountered within five of the twelve test pits and the investigation was completed
just after a period of prolonged heavy rainfall at the site. As such, groundwater conditions have been
modelled with a piezometric surface at about 0.5 m BGL to assess the FOS during an adverse temporary
(short-term) case.
Results of the numerical slope stability analysis are illustrated in Text Figure 12 overleaf. The key points
from our analysis are summarised as follows:
Unit weight and shear strength parameters adopted for the soil and weathered rock strata were
estimated based on our interpretation of the geotechnical investigation data and were typically
within the mid range of values recommended for similar materials in Appendix D of Australian
Standard AS4678-2002 Earth Retaining Structures.
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Analysis was undertaken for specified slip circle centroids (over a grid) and radii to assess global
slope instability. The adopted ground surface profile was based on the steepest slope measured
during the field investigation across the likely building areas, that being about 17°. The analysis
returned an acceptable minimum FOS of at least 1.5 for both short-term and long-term conditions.
This result indicates that the industry standard for short term conditions (FOS≥1.3, e.g. for shallow
groundwater) and the minimum long term conditions FOS against slope instability for hillside
development (≥1.5) would be achievable for the proposed development.
Text Figure 12: Slope/W analysis output for proposed residential construction – short term conditions
Our analysis indicates that an acceptable FOS against slope instability could be achieved for the new
residential development and that there would be a Low Risk of slope instability, subject to
implementation of the recommendations given in Section 6.
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6. Geotechnical Recommendations
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached in Appendix C.
6.1 Site Layout and Geotechnical Constraints
From the conceptual earthworks plans it is anticipated that cut earthworks up to 2 m depth and filling up
to 1 m high will be required for construction of the new road and driveways. Earthworks depths/heights
must not exceed these values unless assessed by a geotechnical RPEQ before construction.
From a geotechnical and slope stability perspective there would not be significant limitations over the
majority of lots in terms of building envelopes. However, to avoid potentially adverse zones, building
areas would be suited to the highest points of the sites and away from the natural drainage lines.
To achieve a Low Risk of instability, the following is recommended:
For future building works, unretained fill heights on slopes between 10° (18%) and 20° (35%) shall be
limited to no more than 1.5 m for future residential construction, unless assessed and approved by a
suitably qualified and experienced geotechnical RPEQ.
For future building works, no fill should be placed on slopes greater than 20°, unless assessed and
approved by a suitably qualified and experienced geotechnical RPEQ.
All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
and certified, which must include an assessment of global and internal stability.
Design and construction of footings shall be undertaken in accordance with Section 6.3.2.
Formal drainage should be constructed to ensure all stormwater is directed away from the proposed
structures and effluent disposal areas.
Site specific site classification and slope stability assessments must be carried out for each new
allotment, where required by NC/Private Certifier, which appropriately consider the proposed
residential construction plans prior to building works commencing.
Final Operational Works earthworks design drawings should be assessed by Tectonic prior to
construction.
6.2 Earthworks
6.2.1 Site Preparation and Trafficability
In construction areas, following clearing of vegetation, tree stumps and large roots must be ‘grubbed
out’ and any organic rich topsoil and/or any unsuitable materials (e.g. firm/loose or water softened
materials) must be stripped and removed. Topsoil materials must not be used in structural fill; however,
may be placed to shallow depths (<0.5 m) across landscaping or effluent disposal areas (if required).
It is recommended that, following removal of unsuitable materials, all building/pavement subgrades
should be proof rolled using a minimum 4 tonne roller in the presence of a civil/geotechnical RPEQ to
identify any ‘soft spots’. Materials in any such soft spots should be excavated and replaced with select fill
material, constructed as recommended in Section 6.2.4.
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Based on the results of the investigation, it is likely that excavation depths up to about 0.2 m to 0.6 m will
be required to remove the existing topsoil and loose clayey sands across the proposed pavement areas;
however, deeper pockets may be present.
After stripping and excavation to design levels, the subgrade is expected to comprise mostly medium
dense/stiff (or stronger) residual soils or very low strength (or stronger) sedimentary rock (areas of
deeper excavations). These materials could be subject to strength loss from repetitive vehicle loading
(particularly from rubber tyred plant) or if they become overly wet. Due to the potential for poor
trafficability during and after wet periods, placement of a minimum 150 mm thick granular working
platform may be required, particularly along access ways and laydown areas.
There is a risk that elevated soil moisture conditions could occur if the works are undertaken during a
period of wet weather and shallow groundwater could become perched in the sands above the
underlying less permeable clay. The upper sands could then become particularly unstable and very
difficult to work, proof roll or compact under elevated moisture contents. Draining water from these
materials can be very difficult and subgrade removal might be required if the materials become wet.
Although the CBR strength is moderate at close to OMC, it can be cost prohibitive to re-work, dry out
and re-compact such materials. Measures to divert water away from work areas and using lighter
earthmoving equipment should be considered to minimise the potential for development of unstable
subgrades. It would also be prudent to plan for earthworks to be conducted during the typically ‘drier’
months of the year.
Bridging layers may be needed if weak subgrade conditison are and encountered and this could
comprise:
Placement of a non-woven geosynthetic filter fabric (e.g. Bidim A39, or equivalent) across t subgrade
to prevent ‘pumping’ of ‘fines’ (silt/clay materials) into the rock fill.
Placement of a minimum 300 mm thick rock mattress. We suggest the use of 75 mm to 150 mm,
angular, durable rock fill.
Fully wrapping the rock fill with non-woven geosynthetic filter fabric (Bidim A39, or equivalent), prior
to placement of select controlled fill.
Suitably sized plant should be used for earthworks, and heavy vehicles should avoid trafficking directly
on the subgrades. Care must be taken with the use of vibratory equipment to ensure that there are no
adverse impacts on surrounding structures, and that vibration does not cause groundwater to ‘pump’
into and weaken the subgrade. The impacts from ground vibration must be closely monitored and
managed during construction.
The site should be graded to shed surface water runoff away from all construction areas. Swale drains
should be constructed along the toes of any cuts, and above cuts where possible, with the drains graded
to discharge collected waters to a control point before release off-site. Appropriate erosion and
sediment control measures must be installed prior to commencement of earthworks at this site.
Specific design would need to be carried out to determine working platform requirements for any heavy
plant (e.g. mobile cranes and piling rigs), and a thicker rock fill platform may be required than suggested
above.
6.2.2 Excavations
Excavations in the natural soils and very low to low strength sedimentary rock to the test pit termination
depths should be achievable using conventional earthmoving plant, such as medium to large (8 to 20
tonne) excavators with toothed buckets and/or ripper attachments.
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As indicated by toothed bucket very slow digging terminations at depths of about 2.3 m and 2.7 m BGL, low to medium strength (or stronger) sedimentary rock could be expected in deeper excavations. Excavations into any low to medium strength (or stronger) sedimentary rock would likely require the use of ripper and/or hydraulic rock breaking attachments with large equipment (e.g. 20 to 40 tonne excavator(s)), particularly in any confined excavations (e.g. service trenching). If high strength (or stronger) rock requires excavation, other specialised excavation methods (e.g. pre-splitting the rock using expansive grout etc.) may be required. Should vibratory excavation techniques be adopted (e.g. rock hammering), ground vibrations should be monitored, and excavation methods may need to be altered should adverse impacts be anticipated or encountered on neighbouring structures. Should groundwater seepage be encountered during construction, such seepage should be readily removable using gravity drainage at a low point in the excavation. 6.2.3 Batter Slopes Unsurcharged cut batters to a maximum of 2 m deep may be temporarily formed at 1V:1H in stiff/medium dense (or stronger) residual soils and very low strength sedimentary rock, provided groundwater is not encountered. Steeper temporary batters may be permissible in low strength (or stronger) rock subject to an assessment during construction by a geotechnical RPEQ. Where space limitations occur or the above temporary batter slopes are not achievable, installation of temporary retention would be required. Permanent batters in medium dense/stiff (or stronger) natural soils and very low strength sedimentary rock up to 2 m deep may be formed at 1V:2H. Steeper and higher batters may be permissible in low strength (or stronger) weathered rock subject to an assessment during construction. Permanent batters up to 1.5 m high in future controlled fill on slopes <20° may be formed no steeper than 1V:2.5H. Fissured clays were noted in the test pits and so it is recommended that a geotechnical RPEQ inspect all batters at the time of construction. Permanent batter slopes will require some form of erosion protection such as revegetation by landscaping (e.g. topsoil, Jute matting and planting). 6.2.4 Filling Future Building Works must consider the limitations on earthworks given in Section 6.1 above. The site soils, where free of deleterious materials (e.g. organics, oversize etc.), could be reused as structural fill, or as non-structural fill (e.g. in landscaping areas, batters, etc.). The weathered rock could be used as structural fill provided the materials can be broken down to suitable particle sizes (<100 mm). Should importation of fill be required, it is recommended that such fill should comply with the following: Minimum CBR of 10% Maximum plasticity index of 15% Maximum particle size of 100 mm All structural fill should be compacted at moisture contents within the range of ±2% of OMC for Standard Compaction. Fill should be placed in 250 mm thick (loose thickness) layers and be compacted by repeated rolling to achieve 75% of the Density Index for cohesionless fills (e.g. sands) or a dry density Geo-Environmental Investigation for Proposed Subdivision 7 February 2025 | 24386-001-Rev0 18
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ratio of at least 95% of the maximum Dry Density Ratio (DDR) for cohesive soils, or 100% DDR based
upon the Standard compaction method within 0.3 m of pavement subgrades.
Confirmatory compaction testing must be carried out at regular intervals. A minimum of two tests per
vertical lift is recommended where structural fill heights are to exceed 300 mm. Further details for
control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for
Commercial and Residential Developments. It is recommended that Level 1 geotechnical inspection and
testing be conducted for all Operational Works filling, and for future building pad construction where fill
heights exceed 0.5 m.
Due to the sloping nature of the site, all fill must be ‘benched’ into the natural slope, over-filled by 0.5 m
(horizontally) and then trimmed back to the well compacted material.
6.3 Building Foundations
6.3.1 Site Classification
Based on the results of the investigation, site classifications for the proposed allotments are estimated to
be typically Class H1 in accordance with AS 2870-2011 Residential Slabs and Footings. It should be noted
that the predicted orders of movement are based on the existing natural soil profiles (i.e. no additional
cutting or filling), and moisture content changes within soils due to normal seasonal moisture variations
only. In areas of deeper clayey sands or shallower weathered rock, site classifications may reduce to
Class M.
Reactive soil movements and site classification should be assessed by the footing design engineer at
Building Works stage to consider the extent of earthworks proposed on each lot, together with any
retained trees (trees located near footings can influence the potential ground movement, and result in
more severe site classifications). A site classification ‘soil test’ would be required prior to building works
to confirm the site classification and footing design parameters.
It should be noted that the placement of highly reactive clay fill may have an adverse impact on the site
classifications.
6.3.2 Footing Design
Footings supporting any structures (including retaining walls) are to penetrate through any uncontrolled
fill or unsuitable natural soils to be founded as follows:
On slopes of 10° (18%), or less – at least 0.5 m into stiff/medium dense (or stronger) residual soils, or
future controlled fill (Level 1).
On slopes of more than 10° – at least 0.5 m into very low strength (or stronger) rock, which is
typically anticipated to be about 1 m to 2 m BGL (existing ground levels) on the steeper slopes.
Due to the depths of foundations recommended, conventional high-level footings (e.g. slab on ground)
should be suitable for some areas, whereas bored piles (or similar) would likely be required for sloping
building sites. High-level footings on slopes greater than 10° could be possible in areas of future cut
where the target foundation stratum is exposed at the subgrade level.
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High level footings founded as described above may be designed using allowable bearing pressures of:
100 kPa in the stiff/medium dense (or stronger) residual soils and future controlled fill.
200 kPa in the very stiff to hard residual soils.
400 kPa in the very low strength (or stronger) weathered rock.
Piles founded as described above may be designed using allowable end bearing capacities of:
150 kPa in the stiff/medium dense (or stronger) residual soils.
300 kPa in very stiff to hard residual soils.
600 kPa in very low strength (or stronger) weathered rock.
The above footing recommendations are considered to be minimum requirements from a slope stability
viewpoint. Future residential footing/slab design must also consider site specific ‘soil testing’.
All footings should found such that they are not adversely affected by any adjacent excavations, trenches,
or retaining walls that are not designed to support loads. Footings adjacent to any fill batter crests
should be deepened to found below the toe of the batter. To minimise the potential for any adverse
interaction effects, footings should found at least below a plane extending 1 m horizontally from the
base of trenches/ excavations/retaining walls, then rising up at 1V:1H. This requirement is illustrated in
Text Figure 13.
Text Figure 13: Footing depth required to minimize risk of undermining and surcharging
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over
the full base area. For bored piles, it is essential that the piling contractor have a ‘mud bucket’ or similar
device to enable clean out of the pile bases. A suitably qualified and experienced structural/geotechnical
RPEQ should inspect the footing excavations to confirm appropriate foundation materials and cleanliness
prior to pouring concrete.
If any soil conditions encountered during construction are found to differ from those noted in the
geotechnical investigation, Tectonic should be notified immediately, and an inspection carried out to
determine if changes to footing design are required.
6.4 Retaining Structures
Design for retaining structures could be carried out using a pressure distribution applicable to the wall
type and based on the parameters listed in Table 5. The values in Table 5 assume that the walls are fully
drained, and no allowance is included for hydrostatic pressures, construction loading, up-slope footing
loadings, the effects of sloping ground and backfill compaction stresses.
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Table 5: Retaining Wall Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle Ka Ko Kp
(kN/m3) (Degrees) (Cantilever wall) (Non-yielding
wall)
Loose/Firm Residual
17 23 0.44 0.61 2.28
Soils
Medium Dense/Stiff
18 25 0.41 0.58 2.46
Residual Soils
Very Low Strength
(or Stronger) 21 32 0.31 0.47 3.25
Sedimentary Rock
Future Fill * * * * *
Note: * depends on fill material type and level of compaction.
At a minimum, retaining structures must be founded as per the recommendations in Section 6.3.2 above
with the footings also designed to account for lateral loads imposed by the retaining wall. Detailed
design must include an assessment of internal and global stability, together with required bearing
capacity and founding depth, by the retaining wall design engineer.
6.5 Drainage
Construction activities may be affected by groundwater, as seepage may occur particularly through the
upper sand which would worsen if works are undertaken during or following a prolonged/heavy rainfall
event. Control of seepage during construction activities may be achieved by grading the base of the
excavations towards a suitable discharge point. Where groundwater seepage is encountered in subgrade
areas it will be necessary to install subsurface trench underdrains to promote drainage across road and
building areas.
During construction temporary open swales and diversion drainage should be installed to divert seepage
and/or runoff away from pavement and building areas, to prevent saturation of the subgrade.
Specific drainage recommendations are as follows:
Construct diversion drainage along the upslope edge of pavement and building areas (e.g. turf,
concrete or rock lined drain or similar) to discharge any captured stormwater from the development
area downslope of any structure, embankment or effluent disposal area.
Grade any building platforms (minimum 1V:100H) to achieve positive drainage into edge drains.
To prevent the migration of stormwater into the proposed effluent disposal area, install diversion
drainage (e.g. landscaped swale) along the upslope side of the proposed disposal area.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in
filter ‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and
site-specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to
be accounted for in the design of the retaining structure. Any pools must include a hydrostatic pressure
relief valve (or similar), which must daylight at the ground surface.
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All excess stormwater collected around the proposed development area, including the effluent disposal area, must be directed by pipes or surface channels (e.g. rock lined ‘dry creek’ beds) to discharge at least 10 m away from and downslope of any structure or embankment, and not towards the effluent disposal areas. Discharge should be directed towards the natural drainage lines, where possible. A surface dissipation device such as rock weir or rock lined contour drain must be installed at the stormwater outlet(s). Subsurface disposal of stormwater must not be undertaken (including infiltration pits). 6.6 Pavement Design Parameters Laboratory testing of the clayey silt from test pit TP8 indicated a CBR value of 6 %. Pavement design will be dependent upon the subgrade materials present after earthworks. Following preparatory earthworks, we anticipate the pavement subgrades in the centre and southern end of the site will likely comprise mostly stiff (or stiffer), high plasticity silty clays or high liquid limit clayey silts; or weathered siltstone in areas of deeper cuts. Pavement subgrades at the northern end of the site are likely to comprise clayey sands/sandy clays or weathered sandstone in deeper cuts. Due to the high silt/clay content, the natural soil subgrades would be susceptible to poor engineering qualities if they become significantly wet of OMC. Earthworks planning would be prudent to avoid saturation of the natural subgrades. Provided removal of any unsuitable materials is undertaken as outlined within this report, preliminary pavement design could be carried out adopting a design CBR value of 3% for the natural clay and silt subgrades and design CBR value of 5 % for the natural clayey sands. Should significant ‘wetting’ of the natural subgrade occur, over-excavation of the silts and replacement with select, granular materials may be required during construction to achieve a design CBR value. Where future fill provides at least 500 mm deep coverage across the natural soils, pavement design may be based upon the CBR value of those materials. In sections of the alignment where very low strength (or stronger) weathered rock is exposed consistently over the full width of the road, a presumptive CBR value of 10 % could be adopted for design. All pavement subgrades must be ‘proof rolled’ and visually assessed by an experienced RPEQ engineer to ensure no heaving or unacceptable subgrade deflection occurs. Should heaving or deflection occur, excavation of ‘soft spots’ may be required, followed by reinstatement with compacted select granular fill. Subgrades should be compacted to 100% DDR (standard) at optimum moisture content with confirmatory testing carried out prior to pavement construction. The recommended design CBR values are applicable to properly drained subgrades only and must be confirmed by subgrade inspection and testing (if required), following earthworks. ‘Properly drained subgrades’ require open graded drains, site grades that shed water and prevent ponding, and subsurface drains adjacent to pavements. 6.7 Geotechnical Certification of Operational Works NC generally requires subdivision Operational Works on hillside sites to be certified by a geotechnical RPEQ prior to Plan Sealing. Should Tectonic be required to certify the Operational Works, inspections must be completed by Tectonic at various stages during construction (e.g. following site stripping, post completion etc.) to ensure the recommendations provided within this report have been adhered too. Tectonic should be contacted prior to commencement of Operational Works to confirm specific inspection requirements. Geo-Environmental Investigation for Proposed Subdivision 7 February 2025 | 24386-001-Rev0 22
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7. Wastewater Treatment and Disposal
An assessment of the type and configuration of wastewater system that may be suitable for a proposed
development requires:
Assumptions to be made regarding the typical building/visitor configuration and probable
wastewater flow; and
Evaluation of the terrain-soil capacity to accept treated wastewater; and
Consideration of NC wastewater treatment and disposal system construction and siting
requirements.
7.1 Wastewater Flow Design Allowance
For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy
of seven people. This assumption is consistent with current NC development standards and has been
used to demonstrate feasibility of on-site effluent disposal. Should the proposed future residences
exceed this size, it is the owner’s responsibility to demonstrate feasibility of the design, meeting all
relevant regulations.
For each occupant, a typical wastewater flow allowance of 120 L/person/day has been taken from
Appendix H of AS1547:2012. This represents the flow allowance for households with standard water
reduction fixtures (including duel flush toilets, shower flow restrictors, aerator faucets and water
conserving automatic washing machines) serviced by rainwater tank supply. We therefore estimate a
typical daily household wastewater flow rate of 840 L/day (7 x 120 L/person/day).
It should be noted that wastewater flow rates may be further reduced by the adoption of full water
reduction facilities (e.g. reduced flush 6/3 L toilets, shower flow restrictors, aerator faucets, front-load
washing machines and flow/pressure control valves on all water-use outlets, etc.). Conversely, the use of
extra wastewater producing fixtures (e.g. spa baths) has the potential to increase the household
wastewater flow rate.
Final wastewater flow rates must be calculated at the time of detailed design of the wastewater system,
at building stage once the house design is known.
7.2 Terrain-Soil Assessment
The terrain-soil combination(s) encountered at a site are assessed so that an Expected Available Area
(EAA) can be delineated for suitable methods of wastewater treatment and land application.
It is anticipated that residential building areas will be suitably positioned near the crest, or across
elevated consistent slopes, within each respective lot. The terrain across the likely effluent disposal areas
associated with probable residential dwellings may vary, however it is expected that typically Linear
Planar areas available, with low to moderate potential for diverging or converging terrain. Such slopes
have less effective natural drainage as distance from the crest increases, with no spreading or
acceleration of run-off as it moves down slope.
It has been assessed from the results of the investigation that the soil profile across the likely building
and effluent disposal areas are consistently silty and sandy clay soils across most of the site, with some
sandier soils encountered on proposed lots 1 and 2. Based on this understanding a conservative
characterisation has been applied to all sites equivalent to Light Clay (Soil Category 5). This
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categorisation has been derived via the method described in Table M1 of AS1547:2012 and accounts for
the following factors:
Variable silty/clayey sands and sandy silt topsoil (Loams, Soil Category 3) were found to depths of
0.4 m to 1.1 m across proposed Lots 1 and 2.
The underlying subsoils generally comprise silty/ and sandy clay soil (Soil Category 5 light clay).
Natural gradients exist which increase likelihood of runoff.
Vertical drainage beyond about 1 m depth may be affected by the presence of weathered rock in
some areas.
Where the natural ground slope exceeds 6° (10%), wastewater irrigation areas must incorporate a slope
reduction rate, as described in Table M2 of AS1547:2012, or shall be effectively terraced as necessary to
reduce slope to < 10% negating the effect of natural slopes. As it is not currently known whether
terracing of slopes will occur at the building stage, a slope reduction factor of 20% has therefore been
applied to all sites to account for slopes of up to 20%.
7.3 Recommendations
7.3.1 System Type
It is recommended that wastewater from the future residences be directed to a suitably sized (max. 10
EP) aerated Sewage Treatment Plant (STP). The STPs must be licensed to produce at least Secondary
standard effluent, with Chief Executive approval from the Department of Housing and Public Works.
However, to maximise available space and to reduce setback requirements, it may be necessary/desired
to use an Advanced Secondary treatment system.
The STPs should be connected to either shallow subsurface drip irrigation system or surface irrigation. It
is noted that surface irrigation is not supported for areas with greater than 10% slope.
Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 3 mm/day for Category 5 soil.
Therefore, a minimum irrigation area of about 280 m² (840 L/day / 3 mm/day) would be required.
However, an additional 20% slope reduction rate for application areas with slopes between 10-20% shall
be applied in accordance with Table M2 of AS1547:2012. Therefore, a minimum irrigation area of about
350 m² will be required. This may be reduced at final design stage depending on the final location of the
building and land application area and associated slope configuration.
The need for a reserve area is generally applied to septic tank systems, producing low quality (Primary
Standard) effluent, due to increased potential for fowling and failure of the trenched land application
area to occur. As the use of an aerated wastewater treatment system is proposed, which can deliver high
quality (Secondary Standard or better) effluent, the risk of fouling and system failure is negligible.
Shallow subsurface drip or surface irrigation can also be easily repaired in-place as required, therefore
the addition of a separate reserve area is not warranted.
7.3.2 System Positioning
Positioning of the land application systems must be in accordance with the horizontal and vertical
‘setback’ distances recommended in Tables T2, T3 and T5 of the Queensland Plumbing and Wastewater
Code 2019, as summarised in Tables 6, 7 and 8 below and over page.
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Table 6: Setbacks for subsurface land application area for on-site sewage treatment plant
Horizontal separation distance
Site Feature
Upslope Downslope Level
Property boundaries, pedestrian paths, walkways, 2m 4m 2m
recreation areas, retaining wall, and footings for
buildings and other structures.
Inground swimming pools. 6m
Inground potable water tank. 6m
(Secondary standard effluent or better)
Note: Distances measured from the edge of trench/bed excavation or subsurface irrigation distribution
pipework to the nearest point of the feature.
Table 7: Setbacks for surface irrigated land application area for on-site sewage treatment plant
Site Feature Horizontal separation distance
Property boundaries, pedestrian paths and walkways 2m
Water edge of a swimming pool 6m
Dwellings, recreation areas 10 m
Note: Distances measured from the edge of the irrigated wetted area to any point of the feature.
Table 8: Setback distances for on-site sewerage facilities (Protection of surface and groundwater)
Separation Distance
Site Feature Advanced Secondary Secondary Effluent
Effluent
Surface Waters: 10 m 30 m
Top of bank of permanent or intermittent water
course, lake, bay or estuary;
Top water level of a surface water source used for
agriculture, aquaculture or stock purposes;
Open stormwater drainage channel or drain;
Bore or a dam.
Unsaturated soil depth to a permanent water table 0.3 m 0.6 m
(vertically)
Note: Distances measured from the edge of the irrigated wetted area to any point of the feature.
Investigation test pits did encounter isolated seepage in some sandy layers however generally indicate
adequate unsaturated soil depths are available across the respective anticipated building and application
areas.
The conceptual wastewater layout is shown in Figure 1 Site Plan, where it can be seen that a suitably
sized wastewater treatment and disposal areas can be positioned within each proposed allotment. It is
noted that this plan demonstrates conceptual layout only for the purpose of demonstrating feasibility.
Individual site layouts shall be decided at building stage.
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7.3.3 System Construction
Construction of wastewater land application areas must be carried out in accordance with Appendix M of
AS1547:2012 and SCC requirements. In order to prevent any adverse impacts on the water quality from
effluent disposal, the following shall be implemented:
For shallow subsurface drip irrigation, systems must be installed at 100 mm to 150 mm depth into
150 mm to 250 mm of good quality topsoil, in grassed of other suitably vegetated areas. Any sandy
topsoil material won from site works may be used to supplement the soil within the respective
disposal fields or alternatively may be imported to the site.
Where stiff or hard clays are encountered, receiving soils should be prepared by undertaking deep
ripping / shallow cultivation combined with the addition of organic matter (10 kg/m² raw mulch) and
gypsum (1 kg/m²) to ‘break up’ any relatively hard clay subsoils and improve overall drainage
characteristics.
Shallow subsurface irrigation lines will run parallel to slope contours. To evenly distribute
wastewater throughout the irrigation area, a pressure equalisation system (i.e. pressure
compensating drip emitters) should be installed.
Surface diversion drainage shall be installed on the up-slope sides of the land application area(s) in
order to divert surface water and shallow subsurface seepage flows away. Such drainage should
penetrate to a minimum depth of 0.3 m BGL, to capture and divert any seepage which may develop
between the topsoil and underlying soils.
Plant and maintain grasses, reeds and/or small shrubs within the land application areas. Preference
should be given to native plant species endemic to the local area, which are suitable for mostly wet
soils and are tolerant of nutrient loads (e.g. native riparian vegetation)
Erect signage at the boundaries of the area in at least two places in accordance with Council
requirements to warn people regarding the presence of wastewater pipes (e.g. “Sewage Effluent
Pipework Installed Below – DO NOT DIG” or “Recycled Water – Avoid Contact - DO NOT DRINK”).
Detailed design of the effluent disposal systems must be carried out by a qualified wastewater designer
to accompany the plumbing application at the Building Works stage, once the building designs are
finalised.
7.3.4 Maintenance and Inspection
Operation and maintenance of an STP is the responsibility of the property owner. A licensed service
agent must be appointed to undertake routine servicing works in line with regulatory authority approval
conditions. To comply with NC requirements, the following management and inspection shall be
implemented once the effluent disposal systems are established:
Continuous monitoring by the property owner for any signs of system failure such as septic odour,
seepage, soil and/or plant discoloration.
Should any signs of system failure be observed by the property owner, including the in-built STP
failure alarm, the manufacturer shall be contacted immediately to arrange for the necessary repairs.
Yearly inspection, testing and repair (if necessary) to the STP and irrigation area by a Licensed
Inspector (normally a service provided by the STP manufacturer).
Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0 26
Page 33
7.3.5 Water Resource Catchment Overlay
The site is located within a mapped water resource buffer for Lake MacDonald (Ref. Text Figure 15
overpage), however the site is located outside of the Lake MacDonald catchment. All site runoff, drains
to internal pathways, creeks and streams that drain away from the site to the northwest, entering Six Mile
Creek and the Noosa River catchment, which are not drinking water sources.
While proposed development does technically trigger NC’s water resource catchments overlay code, the
proposed site development should be considered accepted development, as there is no potential for
effluent to impact the drinking water catchment.
Water Resource
Catchment
Lake
MacDonald
Subject Site
Water Resource
Buffer
Text Figure 14: Location of site relative to Mary River
8. Limitations
Your attention is drawn to the document Limitations, which is included in Appendix D of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of
this report should be, and to present you with recommendations on how to minimise the risks associated
with the services provided by Tectonic for this project.
Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0 27
Page 34
LEGEND PROJECT APPROX. TESTPIT LOCATIONS GEO-ENVIRONMENTAL ASSESSMENT PROPOSEDS9 LOT SUBDIVISION - 44 HOY ROAD, LAKE MACDONALD s DRAWING TITLE NOTES SITE PLAN BASEMAP COURTESY OF NEARMAP, DATED 24 DECEMBER 2024 Pi DOC NO. FIGURE NO. REVISION [SCALE 0 CONCEPTUAL EFFLUENT DISPOSAL AREA (MIN 350M
Page 35
Appendix A
Test Pit Logs & Explanatory Notes
Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0 28
Page 36
Test Pit No.
TP1
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 13/12/2024
Project Name: Proposed Subdivision Completed: 13/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: JM
Hole Position: 492621.8 m E 7080333.0 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 114.90 m
Excavation Dimensions: Datum: AHD Operator: Bebrok
Excavation Information Material Description and Observations
Relative Density
Classification DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty SAND (TOPSOIL), fine to medium
grained, dark brown, with rootlets
SC Clayey SAND, fine to coarse grained, brown,
low plasticity, trace gravel (RESIDUAL) M L
114.4
0.5
DS_0.5-0.7m
MD
D to
M
Not Encountered
113.9
1.0 D
15/80
EX
SANDSTONE, highly weathered, fine to
coarse grained, pale yellow, grey red, very low
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
to low strength, (ROCK)
113.4
1.5
DS_1.5-1.8m
112.9
2.0
R
Hole Terminated at 2.30 m
Bucket Slow Progress on low strength (or
stronger) sandstone
112.4
2.5
111.9
3.0
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
Page 37
Test Pit No.
TP2
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 19/12/2024
Project Name: Proposed Subdivision Completed: 19/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: JM
Hole Position: 492729.9 m E 7080418.6 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 103.60 m
Excavation Dimensions: Datum: AHD Operator: Ryano's Civil
Excavation Information Material Description and Observations
Relative Density
Classification DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty SAND (TOPSOIL), fine to medium
grained, dark brown, with rootlets
W
EX
L
SC Clayey SAND, fine to medium grained, pale
brown, pale orange grey, low plasticity clay
fines M
103.1
0.5
CI Silty Sandy CLAY, medium plasticity, orange
Strong [email protected]
brown red, fine to medium grained sand,
(RESIDUAL) 150
w<
St
PL
102.6
1.0
SC Clayey SAND, fine to coarse grained, orange
pale grey brown, low plasticity clay fines
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
102.1
1.5
MD
R
101.6
2.0
M
becoming pale grey orange red, with sandy
clay seams
12/80
101.1
2.5
D
100.6
3.0
Hole Terminated at 3.20 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
Page 38
Test Pit No.
TP3
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 19/12/2024
Project Name: Proposed Subdivision Completed: 19/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: BC
Hole Position: 492912.3 m E 7080299.8 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 111.10 m
Excavation Dimensions: Datum: AHD Operator: Ryano's Civil
Excavation Information Material Description and Observations
Relative Density
Classification DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Clayey SILT (TOPSOIL), low liquid limit,
brown, with organics, trace fine to medium w<
grained sand F
PL
CH Silty CLAY, high plasticity, orange brown, 110
trace sand (RESIDUAL)
110.6
0.5 w=
St
Slight [email protected]
PL
DS_0.8-1.2m becoming orange brown mottled pale grey
110.1
280
1.0
w<
VSt
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
tending to extremely weathered material, PL
220
becoming red brown mottled pale grey, with
fine to coarse sized gravel
109.6
1.5
EX
SILTSTONE, highly weathered, fine grained,
red brown mottled pale grey, very low
strength, with extremely weathered clay
seams up to 100mm thick
109.1
2.0
108.6
2.5
108.1
3.0
Hole Terminated at 3.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
Page 39
Test Pit No.
TP4
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 19/12/2024
Project Name: Proposed Subdivision Completed: 19/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: JM
Hole Position: 492693.0 m E 7080278.8 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 116.50 m
Excavation Dimensions: Datum: AHD Operator: Ryano's Civil
Excavation Information Material Description and Observations
Relative Density
Classification DCP TEST
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS Blows per 100 mm
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty SAND (TOPSOIL), fine to medium
grained, dark brown, with rootlets W
Slight [email protected]
L
SC Silty Clayey SAND, fine to medium grained,
grey brown, low plasticity clay fines M
(NATURAL)
CI-CH Silty CLAY, medium tohigh plasticity, orange St
116.0
300
0.5 brown, trace sand, trace fine sized gravels,
(RESIDUAL)
becoming red grey
350
VSt
115.5
1.0
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
115.0
1.5
EX
w< 550
becoming high plasticity, pale grey mottled PL
red, possible fissured structure
114.5
2.0
H
114.0
2.5
600
DS_2.7-2.9m
113.5
3.0
Hole Terminated at 3.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
Page 40
Test Pit No.
TP5
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 19/12/2024
Project Name: Proposed Subdivision Completed: 19/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: JM
Hole Position: 492767.4 m E 7080220.7 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 117.70 m
Excavation Dimensions: Datum: AHD Operator: Ryano's Civil
Excavation Information Material Description and Observations
Relative Density
Classification
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Silty CLAY (TOPSOIL), medium plasticity,
brown, with rootlets w=
F
PL
110
DS_0.2-0.4m CH Silty CLAY, high plasticity, pale brown, pale
orange, trace sand, tending to clayey silt, St
(RESIDUAL)
becoming red orange brown
117.2
300
0.5
VSt
w<
PL
becoming pale grey red, with fine to coarse
sized siltstone gravels
500
H
116.7
1.0
SILTSTONE, highly weathered, pale red grey,
fine grained, very low to low strength, with thin
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
extremely weathered seams
Not Encountered
116.2
1.5
EX
115.7
2.0
becoming distinctly weathered, low to medium
115.2
2.5 strength
114.7
3.0
Hole Terminated at 3.00 m
Target depth
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
Page 41
Test Pit No.
TP6
Page 1 of 1
Engineering Log - Test Pit Project No.: 24386
Client: Indigo Group Commenced: 19/12/2024
Project Name: Proposed Subdivision Completed: 19/12/2024
Hole Location: 44 Hoy Road, Lake Macdonald Logged By: JM
Hole Position: 492701.2 m E 7080167.2 m N MGA2020 Zone 56 Checked By: MT
Equipment Type and Model: 5.5t Excavator RL Surface: 119.30 m
Excavation Dimensions: Datum: AHD Operator: Ryano's Civil
Excavation Information Material Description and Observations
Relative Density
Classification
Penetration
Pocket
Consistency
Graphic Log
Samples Material Description
Penetrometer
Condition
Recovery
Tests soil type: plasticity or particle characteristics,
Moisture
UCS
Symbol
Method
secondary and minor components, colour
Water
Remarks RL Depth (kPa)
(m) (m) 0 5 10 15 20 25
100
200
300
400
500
Clayey SILT (TOPSOIL), low liquid limit,
brown, with roots & rootlets w=
F
PL
CH Silty CLAY, high plasticity, pale orange brown, 110
trace sand, tending to clayey silt (RESIDUAL) St
becoming orange brown, trace siltstone
118.8
0.5 gravels
380
w<
PL
becoming pale grey red brown, with gravels VSt
118.3
1.0
Not Encountered
SILTSTONE, highly weathered, fine grained,
pale grey red, very low to low strength, with
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 24386.GPJ <<DrawingFile>> 5/2/2025 10:49 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
thin extremely weathered seams
EX
117.8
1.5
117.3
2.0
116.8
2.5
becoming low to medium strength
R
Hole Terminated at 2.70 m
Bucket Slow Progress on low strength (or
stronger) siltstone
116.3
3.0
Method Penetration Water Samples and Tests Moisture Condition Consistency/Relative Density
N - Natural Exposure VE - Very Easy Level (Date) U## - Undisturbed Sample (## mm) D - Dry VS - Very Soft
X - Existing Excavation E - Easy DS - Disturbed Sample M - Moist S - Soft
BH - Backhoe Bucket Inflow BDS- Bulk Disturbed Sample W - Wet F - Firm
R - Ripper F - Firm VSt - Very Stiff
EX - Excavator H - Hard H - Hard
RH - Rock Hammer VH - Very hard VL - Very Loose
Plastic Limit
L - Loose
Classification Symbols < PL MD - Medium Dense
and Soil Descriptions = PL D - Dense
Based on Unified Soil < PL VD - Very Dense
Classification System Fr - Friable
The text continues on page 42.