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Appendix D - Geotechnical Report
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Geotechnical Investigation
Proposed Subdivision, 220 Black Mountain Range Road,
Black Mountain
Geotechnical Investigation 10 June 2026 | Ref. 26073-001-Rev0
10 June 2026 | 26073-001-Rev0
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Page 2
Document Review
Doc No. Revision Prepared By Reviewed By Date Issued
001 Draft Robert Gibb Ashley Davey 1 April 2026
001 0 Robert Gibb Ashley Davey 10 June 2026
Doc No. Revision Method of Delivery Issued To
001 0 Electronic Jack Boman – Threedom Nominees
([email protected])
Reviewed By
Prepared By
Robert Gibb BSc (Hons) Ashley Davey BEng (Geological) RPEQ 8159
Senior Engineering Geologist Director/Principal Geotechnical Engineer
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Executive Summary This report presents the results of a geotechnical investigation and slope stability assessment undertaken by Tectonic for Threedom Nominees Pty Ltd. Proposed layout plans indicate that an existing single rural allotment (Lot 3 on SP258991) will be subdivided in to five future allotments. No significant earthworks are anticipated for the subdivisional works, with all lots having direct frontage to Black Mountain Range Road. The investigation comprised a site walk-over by an experienced engineering geologist and excavation of seven test pits to depths between 1.2 m and 3.1 m below ground level (BGL), together with in situ strength testing and targeted laboratory testing. No groundwater was encountered in the test pits at the time of investigation; however, seepage may develop following prolonged or heavy rainfall. Published mapping indicates the site is underlain by the middle Triassic age Kin Kin Beds (meta- sedimentary strata). Test pits generally encountered a profile in line with this, featuring a thin topsoil layer (0.1 m to 0.2 m BGL) underlain by very stiff residual soils and extremely weathered (XW) materials to about 0.8 m to 1.3 m BGL, then highly to distinctly weathered, very low to low strength phyllite to the termination depths. Quartz veins were encountered at two test pit locations. The site walk-over did not identify indicators of existing slope instability within the proposed development areas (e.g. tension cracking, hummocky ground or the like) and the proposed building areas were generally positioned on favourable rounded spur crests or broad flank slopes. A Low Risk of instability for the proposed subdivision would be achievable, with an acceptable minimum factor of safety against global instability in excess of 1.5. From a geotechnical and slope stability perspective, the proposed subdivision layout is considered acceptable. Key recommendations for future development (per-lot, at Building Works stage) include: minimising cut and fill; limiting unretained fill to 1 m height where natural slopes are 10° or greater (or otherwise providing engineered retention); and undertaking site-specific geotechnical assessment for each dwelling location within the nominated building envelopes. Footings supporting structures (including retaining walls) are to penetrate through fill and any unsuitable natural soils (if encountered) and be founded a minimum of 0.5 m into stiff (or stiffer) residual soils. Conventional footings may be suitable in some areas; however, bored piles are anticipated where fill pads are constructed or buildings are located on steeper ground. A preliminary site classification of Class M (according to AS 2870-2011) would apply for the existing natural subsurface profiles, extending to Class H1 where up to 1 m of fill is placed. Surface and subsurface drainage must be designed to divert stormwater away from building platforms, batters and retaining structures, with collected flows discharged in lined drains/pipes to stable outlets at least 5 m downslope of structures. Subsurface disposal of stormwater is not recommended. As reticulated sewer is not available, on-site wastewater treatment and disposal will be required for each lot. The land application area required for each lot varies due to slope reduction factors (per AS1547:2012). For feasibility these have been estimated as the following: ▪ Lots 1 & 2: 470 m² ▪ Lots 3 & 4: 840 m² ▪ Lot 5: 420 m² Prior to construction, a site specific ‘soil test,’ slope stability assessment and effluent disposal design should be prepared for future building works on each of the proposed allotments. Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 ii
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Table of Contents
Table of Contents ............................................................................................................................................................iii
1. Introduction ............................................................................................................................................................ 1
2. Proposed Development ....................................................................................................................................... 1
3. Aims & Method of Investigation .......................................................................................................................3
3.1 Fieldwork ...................................................................................................................................................................... 3
4. Laboratory Testing ................................................................................................................................................3
5. Results of Investigation ...................................................................................................................................... 4
5.1 Site Description ......................................................................................................................................................... 4
5.1.1 Proposed Lot 1 ......................................................................................................................................... 5
5.1.2 Proposed Lot 2 ......................................................................................................................................... 6
5.1.3 Proposed Lot 3 ......................................................................................................................................... 8
5.1.4 Proposed Lot 4 ......................................................................................................................................... 9
5.1.5 Proposed Lot 5 ....................................................................................................................................... 10
5.2 Geology ...................................................................................................................................................................... 10
5.3 Subsurface Conditions .......................................................................................................................................... 11
5.4 Groundwater............................................................................................................................................................. 12
5.5 Geotechnical Laboratory Test Results ............................................................................................................. 12
6. Slope Stability Assessment ............................................................................................................................... 13
6.1 Existing Conditions................................................................................................................................................. 13
6.2 Slope Stability Assessment.................................................................................................................................. 14
6.2.1 Qualitative Assessment ....................................................................................................................... 14
6.2.2 Numerical Modelling ........................................................................................................................... 15
7. Geotechnical Recommendations ..................................................................................................................... 17
7.1 Development Limitations..................................................................................................................................... 17
7.2 Earthworks ................................................................................................................................................................. 17
7.3 Building Foundations ............................................................................................................................................ 18
7.3.1 Preliminary Site Classification........................................................................................................... 18
7.3.2 Footing Design....................................................................................................................................... 19
7.4 Retaining Structures .............................................................................................................................................. 19
7.5 Drainage ..................................................................................................................................................................... 20
7.6 Geotechnical Certification of Operational Works ....................................................................................... 20
8. Wastewater Treatment and Disposal ............................................................................................................. 21
8.1 Wastewater Flow Design Allowance ................................................................................................................ 21
8.2 Terrain-Soil Assessment ....................................................................................................................................... 21
8.3 Existing Wastewater Infrastructure .................................................................................................................. 22
8.4 Recommendations ................................................................................................................................................. 22
8.4.1 System Type ............................................................................................................................................ 22
8.4.2 System Positioning ............................................................................................................................... 23
8.4.3 System Construction ............................................................................................................................ 24
8.4.4 Maintenance and Inspection ............................................................................................................ 25
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9. Limitations ............................................................................................................................................................ 25
Tables
Table 1: Summary of Soil Classification & Shrink-Swell Index Testing ...................................................................... 12
Table 2: Stability Risk Levels ....................................................................................................................................................... 14
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007) ................................................................ 15
Table 4: Retaining Wall Design Parameters.......................................................................................................................... 20
Table 5: Required EAAs ................................................................................................................................................................ 23
Table 6: Setbacks for subsurface land application area for on-site sewage treatment plant ........................... 23
Table 7: Setbacks for surface irrigated land application area for on-site sewage treatment plant ................ 23
Table 8: Setback distances for on-site sewerage facilities (Protection of surface and groundwater) ............ 24
Text Figures
Text Figure 1: Extract of sketch layout plans (courtesy MWA) ........................................................................................ 2
Text Figure 2: 2025 aerial image of the site and close surrounds (courtesy NC Intramaps) ............................... 4
Text Figure 3: Oblique aerial view looking south across site ........................................................................................... 5
Text Figure 4: Site layout (courtesy MWA) and aerial view of Lot 1, looking south ............................................... 6
Text Figure 5: Site layout (courtesy MWA), aerial view (centre) and looking north along spur (RHS)............. 7
Text Figure 6: View of western shed gable ............................................................................................................................. 7
Text Figure 7: Site layout (courtesy MWA) existing residence (centre) and general view looking
north over spur (RHS) ......................................................................................................................................................... 8
Text Figure 8: View of concrete water tanks to east of house (LHS) & spearated apron slab (RHS)................ 9
Text Figure 9: Site layout (courtesy MWA) and oblique aerial view looking south................................................. 9
Text Figure 10: Site layout (courtesy MWA) and looking north along spur within proposed buiding
area (RHS) .............................................................................................................................................................................. 10
Text Figure 11: Extract of Gympie Surface Geology Map................................................................................................ 11
Text Figure 12: Extract from NC landslide hazard overlay mapping ........................................................................... 13
Text Figure 13: Slope/W Analysis output (critical plane indicated) ............................................................................. 16
Text Figure 14: Footing depth required to minimise risk of undermining and surcharging ............................. 19
Figures
Figure 1: Site Plan
Appendices
Appendix A Test Pit Logs & Explanatory Notes
Appendix B Laboratory Test Certificates
Appendix C AGS Guidelines – Qualitative Terminology for Use in Assessing Risk to Property & Some
Guidelines for Hillside Construction
Appendix D Limitations
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1. Introduction This report details the results of a geotechnical investigation undertaken for a proposed five lot subdivision at 220 Black Mountain Range Road, Black Mountain. The investigation was carried out by Tectonic for Threedom Nominees Pty Ltd, generally in accordance with our proposal P25610-Rev0, dated 9 December 2025. The site investigation comprised a walk-over by an experienced engineering geologist and geotechnical engineer from Tectonic; and the excavation of eight test pits, together with in situ soil strength and 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, and drainage. Quantitative and qualitative slope stability assessments are included. 2. Proposed Development Proposed development layout plans by Max Watterson & Associates (MWA, Ref. 25-6775AG, dated 20 April 2026) were provided to Tectonic. These indicate that it is proposed to subdivide an existing single rural allotment in to five future allotments. Proposed Lot 3 towards the centre of the development would retain an existing residence, with new building envelopes to be established on Lots 1 & 2 and 4 & 5, mostly focussed around the crests of spurs or broad flank slopes. The proposed lots would range in size from about 2.931 ha to 7.699 ha. All lots have direct frontage to Black Mountain Range Road, so it is expected that separate accesses will be feasible for each of the lots. Therefore, to effect the subdivision, no significant earthworks should be required, although concrete crossovers established outside of the site boundaries are expected. For future building works, lots focussed towards the crest of rounded spurs may have more flexibility in terms of earthworks and building construction, with those on slightly steeper flanking spurs (e.g. Lots 2 and 4) limited to about 1 m depth/height of cut/fill, to create localised building pads and drainage diversion etc. Mains water and sewerage connections are not available, and therefore collection of rainwater in tanks and on-site disposal of effluent will be required. Text Figure 1 on the following page provides an extract of the layout drawing provided. Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 1
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Text Figure 1: Extract of sketch layout plans (courtesy MWA) Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 2
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3. Aims & Method of Investigation The main aims of the investigation were to address potential geotechnical constraints for the proposed subdivision, along with an assessment of slope stability. 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. However, it should be noted that this report 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 engineering geologist from Tectonic undertook a site walk-over on 3 March 2026 to visually assess indicators of past slope instability. The walk-over included assessment of soil and rock outcrops, surface drainage, topography, evidence of erosion, vegetation, and condition of structures. This was incorporated with a subsurface investigation, which comprised the excavation of eight test pits (designated TP1, TP2, TP3A, TP3B to TP7) supervised by a geotechnical engineer from Tectonic. The test pits were undertaken using a 5.5 tonne excavator with a toothed bucket and ripper attachments to depths between 1.2 m and 3.1 m below ground level (BGL), across the proposed development areas. In situ soil strength was assessed using dynamic cone penetrometer (DCP) testing from surface adjacent to each test pit (other than TP7), and using a pocket penetrometer (PP) on undisturbed U50 tube samples, relatively undisturbed clayey block samples and 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. The test pits 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. 4. Laboratory Testing An undisturbed U50 tube sample and a small disturbed sample 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 one Shrink-Swell Index analysis and one Atterberg Limits and complimentary Particle Size Distribution (PSD) test. Laboratory test results are discussed in Section 5.8. The full laboratory test certificates are attached in Appendix B. Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 3
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5. Results of Investigation
5.1 Site Description
The site is described as Lot 3 on SP258991, and totals some 22.83 ha. At the time of our investigation, an
existing residence was located to the centre/south of the lot, with a shed adjacent to the south-west. The
site generally consisted of a number of rounded spurs and flank slopes, interspersed with established
drainage lines. The site is bounded by Black Mountain Range Road to the west and south, the Bruce
Highway road corridor to the north; and moderately sized rural-residential properties to the east.
The site had been most recently utilised for pastoral activities, with grassed paddocks being cleared of
most significant vegetation. Small to medium sized trees tended to be sporadic across the land area,
although were concentrated in copses around gully inverts and along the eastern boundary.
A recent aerial image of the site is provided in Text Figure 2 below.
Overall lot boundary
Existing residence
Text Figure 2: 2025 aerial image of the site and close surrounds (courtesy NC Intramaps)
General site conditions at the time of our investigation are shown in Text Figure 3 on the following page.
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Broad flank slope on
proposed Lot 2
Eastern edge of site
along spur on Existing residence,
proposed Lot 5 (proposed Lot 3)
Spur on proposed
Lot 1
Bruce Highway
Dams well down slope of
proposed building areas
Text Figure 3: Oblique aerial view looking south across site
During the site walk-over, topographic features were noted, and ground slopes were recorded using a
hand-held clinometer.
On a wider scale, as mentioned above, the allotment generally consisted of a number of northerly
dipping spurs, with well defined drainage lines between. Drainage lines to the centre, east and west had
been dammed, with the dam reservoirs holding water at the time of our assessment. It is noted that
Black Mountain Range Road follows the crest of a ridgeline, which wraps around the west and south of
the site, and forms a regional drainage divide. Therefore, the potential catchment for surface water is
limited to internal surfaces only within each lot. No signs of significant ongoing soil erosion were
observed in the proposed development areas during our walk-over.
Detailed assessment was carried out in the vicinity of the building areas nominated in the project brief,
which were generally closer to the southern/south-western frontages to Black Mountain Range Road. It is
expected that the lower parts of the site to the north, including established dams, would largely be
maintained and would not be affected as a result of the subdivisional works.
Specific comments relating to each proposed lot, are provided below.
5.1.1 Proposed Lot 1
Proposed Lot 1 was located to the western side of the development area, and was dominated by a
rounded spur, which dipped down to the north/north-east. The crestline could be traced through the
western part of proposed Lot 1 from south to north through the entirety of the proposed lot. The crest of
the spur was relatively wide and gently rounded, with inclinations ranging from about 6° to 10°. The
rounded crest was estimated to be at least 30 m wide in the proposed building area. The upper flanks
sloped at about 14° to 16°, developing to 18° to 20° midslope; and up to 29° in a short steep reach closer
to the invert of the drainage lines downslope.
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Downslope to the north-eastern corner of proposed Lot 1, an established dam was present, which was
holding water at the time of our assessment. The drainage line upstream was noted to be wet underfoot
at the base, but with no discernible flow. Some exposed soils were observed on the lower flanks of the
spur to the south, described as pale brown and pale grey silty clay, however in general the ground cover
was maintained and no signs of significant channelised erosion was observed.
Text Figure 4 provides some context to the above.
Approx. spur
crest on Lot 1
Text Figure 4: Site layout (courtesy MWA) and aerial view of Lot 1, looking south
5.1.2 Proposed Lot 2
Proposed Lot 2 also featured a well defined spur running through the allotment, approximately from
south to north. However, the crest of the spur extends on to Lot 3 to the east. This means that at the
frontage, a drainage line head and upper gully ran oblique across the south of the proposed block,
potentially limiting access to a building location on the spur further down in to the lot. We understand
that ultimately a building envelope may be established on the drainage line flanks in the south-western
corner or considered on the spur crest further downslope to the north.
To the south-west of the block, the drainage line flanks were moderately steep, increasing from about 8°
at the road frontage to 13° downslope towards the drainage line invert. While the slope shape was
convex, the overall shape was approximately linear. This type of ground would naturally tend to promote
sheet flow of surface water, rather than cause concentrated/channelised run-off.
To the south-east of the proposed block a ‘pinch-point’ would potentially existing around the head of
the drainage line. An established track and shed were present, which could be utilised for access to the
spur further downslope, however on the downslope side there was sloping ground of up to 18°.
The spur downslope included a relatively wide crest area (~30 m) which dipped at 8° to 9°, and the flanks
which extended to between 15° and 18°. The rounded crest of the spur would help to shed water in a
divergent manner, with positive gradients available down and away from proposed building areas to the
north, east and west.
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Approx. spur
crest on Lot 2
General alternative
building area
Potential ‘pinch point’
around gully head
Text Figure 5: Site layout (courtesy MWA), aerial view (centre) and looking north along spur (RHS)
An existing shed was located to the south-eastern corner of proposed Lot 2, which was a single storey,
brick walled structure, with a clad roof and a concrete ground slab. The western gable wall was cracked,
with the crack being open and with an aperture of about 2 mm to 3 mm. A power pole directly adjacent
was however observed to be straight. A view of the shed is shown below in Text Figure 6.
Crack in end wall
Text Figure 6: View of western shed gable
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5.1.3 Proposed Lot 3
Proposed Lot 3 contained an existing residence to the south off Black Mountain Range Road. We
understand that the building would be maintained in situ as part of the development, and no additional
building envelopes are planned elsewhere on the proposed lot.
The topography on proposed Lot 3 was dominated by a rounded spur, which generally dipped down to
the north at about 10°, and the flanks were relatively steep at about 25°. The block would extend via a
‘dog leg’ to the north-east, to the frontage with the Bruce Highway corridor, with a small dam also
contained in this part of the site.
The existing residence was of one storey height with masonry brick walls and a tiled roof. The walls were
mostly in good alignment, however some cracking was present to the north-east and north-west (in the
north facing wall). The cracks were persistent from ground to window sill level, with an aperture of about
1 mm to 3 mm. An exterior concrete apron around the perimeter had dropped about 20 mm to 30 mm,
and separated ≤40 mm from the building walls. This was most prevalent to the north, east and west of
the building. Partially in-ground concrete water tanks were surrounded by concrete surfacing off the
eastern gable. The tanks had been sutured in a number of locations, and the adjacent surfacing was
cracked in several places. Additionally, the surfacing appeared to have slid laterally along the ground
surface to the north-east.
The wastewater pipe from the bathroom in the north-eastern corner of the house appeared to connect
to a broken clay pipe, which in turn extended under the house through the slab. It was not possible to
determine the position of the septic trenches for the property at the time of our walk-over.
Downslope of the house to the north-east, there was a fenced flat pad (presumed to be a former tennis
court). This was estimated to have involved historic earthworks of about 1.4 m depth of cut, and up to
1.2 m height of filling downslope. The cut was subvertical, and exposed about 0.6 m of residual soil (silty
clay with gravel) then grading to extremely weathered (XW) material of hard, silty clay and dense, clayey
gravel. Reference images of proposed Lot 3 are presented below in Text Figures 7 & 8.
Text Figure 7: Site layout (courtesy MWA) existing residence (centre) and general view looking north
over spur (RHS)
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Text Figure 8: View of concrete water tanks to east of house (LHS) & spearated apron slab (RHS)
The back garden behind the residence (to the north) generally dipped down at about 14°.
5.1.4 Proposed Lot 4
The building area for proposed Lot 4 dips down from Black Mountain Range Road at about 15° to 16°.
The slope continues away from the building envelope at up to 18°, before flattening slightly at the
confluence of two drainage lines to about 13°. The remainder of the block to the north consists mostly of
a broader scale convergent landform, which includes a dam.
The proposed building area is located on a broad, consistently defined and relatively linear slope
between the two spurs on the adjacent lots to the east (Lot 5) and west (Lot 3).
This type of feature would tend to promote sheet flow, and indeed there was little sign of channelised
erosion around the gully inverts downslope. Some minor exposed soils (described as pale brown silty
clay) was observed in the lower flanks, however this was assessed to be caused mostly by livestock action
across the slope. Some medium size trees were present, clustered around the gully inverts and the dam
downslope.
Spurs on proposed Lots
3 and 5
Linear slope forming
building area for
proposed Lot 4
Text Figure 9: Site layout (courtesy MWA) and oblique aerial view looking south
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5.1.5 Proposed Lot 5
Proposed Lot 5 is dominated by a well defined, elongated spur, which extends from the south-eastern
corner, through the centre of the site; dipping ultimately in to the north-eastern corner of the block. A
broadly defined drainage feature was present to the north-western corner, which contained a dam, which
was holding water at the time of our assessment.
A shallow relief drainage line head was located in the south-western corner of the lot, but with
moderately inclined flanks (~10°), and the invert reaching about 15°. The drainage line was dry at the
time of our assessment, and would offer an opportunity for some upslope protection and diversion of
drainage for the proposed building area.
The proposed building location was positioned on top of a rounded spur crest, which was sloping at 8°
to 10° just off Black Mountain Range Road, then reducing to gently sloping ground of no more than 5°.
The crest at the building area was at least 20 m wide. Further to the north, the dip of the spur crest
reduced to 3° to 4°. The spur would be slightly divergent naturally, helping to spread surface water flows
down and away from the site. While the crest was gently sloping, and therefore may locally have poor
drainage characteristics (e.g. allowing ponding) the scale of the landform allows ample space for the
creation of effective diversion drainage.
Text Figure 10 below illustrates the key site features and building location on proposed Lot 5
Text Figure 10: Site layout (courtesy MWA) and looking north along spur within proposed buiding
area (RHS)
5.2 Geology
Published geology mapping1 (Ref. Text Figure 11 overleaf) indicates that the site is underlain by the
middle Triassic age Kin Kin Beds (green shading, denoted “Rk”). The Kin Kin Beds are a low grade
meta-sedimentary unit containing phyllites, and “strongly cleaved and commonly kinked mudstone with
thin siltstone laminae; [and] minor volcaniclastic sandstone.”
An inferred high angle thrust fault (black dashed line in Text Figure 11) is indicated in the mapping,
trending approximately north-west to south-east through the centre of the site.
1
State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, 1:100,000 scale, Sheet 9445, 2018
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Indicative site location
(Kin Kin Beds)
Text Figure 11: Extract of Gympie Surface Geology Map
As mentioned above in Section 5.1, significant exposures of the underlying soils and rock were not
observed during our walk-over. However, in livestock tracks across some of the slopes and near the gully
inverts, grey, brown and pale orange brown, silty clays with varying gravel content were noted. These
types of soils are consistent with the weathering of the Kin Kin Beds.
5.3 Subsurface Conditions
The test pits were positioned generally around the upper and lower parts of the anticipated building
envelopes (Ref. Text Figure 1) and all produced similar results.
The subsurface conditions encountered in the test pits are summarised as follows:
▪ Topsoil – comprising silty clay with rootlets to between 0.1 m and 0.2 m BGL; then
▪ Residual Soils & XW Material – mostly very stiff (occasionally grading to hard at the transition to
the underlying ‘rockhead’), high plasticity, silty clays with varying angular, fine to coarse sized gravel
content. The residual soils and XW materials extended to depths of between 0.8 m and 1.3 m BGL;
underlain by
▪ Weathered Rock – described mostly as highly and distinctly weathered, very low to low strength,
phyllite. The rock material was highly fractured, and therefore could be excavated under slow
digging conditions with a 5.5 tonne excavator with 600 mm toothed bucket. This was progressed to
depths of up to 3.1 m BGL, with ‘refusal’ only logged in test pit TP2 (upslope side of proposed Lot 1).
In test pit TP2 the test pit was terminated on a quartz vein, with some broken quartz also reported in
test pit TP3A. Based on our experience in the regional geology, the occurrence of quartz veins are
not uncommon within the Kin Kin Beds, especially in the vicinity of potential fault activity.
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.
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5.4 Groundwater
No signs of groundwater seepage at the surface (‘springs’) were observed during our walk-over around
the proposed development areas, nor was groundwater encountered in any of the test pits during the
investigation, which extended on the downslope side to a maximum depth of 3.1 m BGL.
Additionally, the potential reservoir for groundwater would be limited because of the topographical high
point in close proximity to the south/south-west along Black Mountain Range Road.
It should be noted that groundwater conditions are expected to be reflective of prevailing weather
conditions, and seepage may develop following periods of prolonged or heavy rainfall (e.g. along
geological faults, degraded quartz veins or at the contact between the soil and the underlying
‘rockhead’).
5.5 Geotechnical Laboratory Test Results
Table 1 summarises the results of laboratory testing conducted on soil samples recovered from the
boreholes. Laboratory test certificates are attached in Appendix B.
Table 1: Summary of Soil Classification & Shrink-Swell Index Testing
Atterberg Limits Percent
Fines (%)
Shrink-
[material
Depth Linear Swell
Location Liquid Limit Plastic Limit Plasticity passing
(m BGL) Shrinkage Index Iss
(%) (%) Index (%) 75 µm
(%) (%)
sieve]
TP2 0.2 – 0.3 57 30 28 9.5 95 -
TP5 0.1 – 0.4 - - - - - 2.5
Atterberg limits and particle size distribution testing indicates that the residual soil sample tested from
test pit TP2 consisted of a high plasticity, silty clay, with a very high ‘fines’ content (95 %).
Shrink-Swell Index testing indicated an Iss value of 2.5 %, for the sample of residual soil from test pit TP5.
The residual soils at this site would therefore be expected to have a moderate to high potential for soil
reactivity to normal seasonal moisture content changes.
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6. Slope Stability Assessment
6.1 Existing Conditions
NC landslide susceptibility mapping (Ref. Text Figure 12) indicates that extensive parts of the site are
affected by areas denoted as Moderate to High susceptibility.
Existing overall lot boundary
Text Figure 12: Extract from NC landslide hazard overlay mapping
It should be noted that the NC mapping 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 visible indicators of global slope instability at surface (e.g. tension cracks,
unusual landforms, 'hummocky’ ground or the like). Proposed lots 1 and 5 were dominated by well
defined, rounded spur crests, which extended up to the road frontage with Black Mountain Range Road.
This type of morphology is favourable from a slope stability perspective. A building area on the crest of
the spur to the centre of Lot 2 could also be established, where access can be created in accordance with
the recommendations contained within Section 7 of this report. The building location to the south-west
of proposed Lot 2, and over the southern part of Lot 4, were located on relatively broad, linear slopes.
While moderately steep, with appropriate controls and limitations applied (per Section 7 of this report)
building in these locations would be feasible.
The subsurface conditions included very stiff to hard residual soils and XW materials from surface, and a
consistently shallow ‘rockhead’ at all test pit locations.
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6.2 Slope Stability Assessment
6.2.1 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 2 and the
Qualitative Terminology for Use in Assessing Risk to Property which is also attached in Appendix C.
Table 2: Stability Risk Levels
Risk Level Example Implications (1)
VH Very High Risk Unacceptable without treatment. Extensive detailed investigation and research, planning 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 Risk May be tolerated in certain circumstances (subject to regulators’ approval) but requires 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 Risk Acceptable. Manage by normal slope maintenance procedures.
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 7, it is assessed that there should be a Low 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 3 overleaf is our
qualitative assessment of landslide risk. A summary of qualitative terminology for use in assessing risk to
property is attached in Appendix C (taken from AGS 2007).
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Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
Hazard Likelihood Consequence Assessed Risk Comments
Failure through Unlikely Minor Low Given the gentle to moderate gradients
future fill and/or across the building areas on proposed Lots 1,
natural residual 2, 4 and 5, and very stiff to hard nature of the
soils above the in situ residual soils/XW material, it is
foundation depth considered Unlikely that a landslide would
develop, subject to implementation of the
recommendations in Section 7. For a
potential Minor impact from such shallow
instability the resultant risk would be Low as
per AGS 2007.
Deep seated Rare Major Low The likelihood of a deep failure through the
failure within very residual soils/XW materials or very low
stiff to hard strength (or stronger) weathered phyllite,
residual soils/XW below the foundation depths is assessed as
materials or very Rare due to the strength of these materials,
low strength (or apparent lack of adverse structure in the rock
stronger) mass (e.g. persistently dipping layering) and
weathered phyllite no indications of persistent groundwater
below the seepage. There were also no signs of
foundation depth previous deeper seated instability on the site,
and the existing residence was dilapidated
and required some maintenance, did not
appear to show signs of distress that could
be attributed easily to slope instability.
Although the consequence of such a failure
could be Major, the resultant risk is Low as
per AGS 2007.
6.2.2 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 20 kPa surcharge,
to represent a two-storey residence (without fill) or 1 m height of fill.
The section was developed as a generic ground surface profile, allowing for the steepest slopes present
within the building areas and immediately downslope. The subsurface profile for this scenario was
modelled on the conditions encountered in the test pits, and assumes topsoil and unsuitable materials (if
encountered, Ref. Section 7) would be stripped from future development and building areas prior to
construction or filling.
Although groundwater was not encountered within the test pits, groundwater conditions were modelled
to assess the FOS during an adverse temporary (short-term) case with a piezometric surface at about
1 m BGL (close to the interface between the residual soils and the underlying ‘rockhead’). Results of the
numerical slope stability analysis are illustrated in Text Figure 13 overleaf. The key points from our
analysis are summarised as follows (overleaf):
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▪ Unit weight and shear strength parameters adopted for the soil and weathered rock 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.
▪ Analysis was undertaken for specified slip circle entry and exit zones to assess global slope
instability. The adopted ground surface profile was based on ground slopes measured during the
fieldwork
▪ The analysis returned an acceptable minimum FOS in excess of 1.5 for both short-term and long-
term conditions.
Text Figure 13: Slope/W Analysis output (critical plane indicated)
The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 7 have been designed to mitigate those impacts, so that a Low
Risk of instability (and factor of safety of ≥1.5) should be achieved, and can be maintained for the future
subdivision.
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7. 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.
7.1 Development Limitations
From a geotechnical and slope stability perspective the proposed site layout would be acceptable. To
achieve a Low Risk of slope instability (and factor of safety of 1.5 or more) the following is recommended
for future development:
▪ Unretained filling should be limited to 1 m height where ground slopes are 10° or greater, or should
be retained. Earthworks fill greater than 1 m height could be tolerated, subject to assessment and
approval by a 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 by the designer.
▪ With the relatively wide nature of the spurs present on proposed Lots 1 and 5, and in the alternative
building envelope on proposed Lot 2, conventional low-set construction is expected to be possible.
Given the moderately steeply sloping ground to the south-west of proposed Lot 2 and the south of
proposed Lot 4, high-set, lightweight suspended construction may be more practical.
▪ The building envelopes identified in the project brief (Ref. Section 2) would be feasible from a
geotechnical and slope stability perspective on each lot. As the building areas are relatively large,
site-specific ‘soil testing’ and slope stability assessment must be undertaken for future construction
on each allotment.
7.2 Earthworks
In construction areas, all vegetation and large roots must be ‘grubbed out,’ with any organic rich topsoil
stripped. Organic topsoil must not be reused as structural fill, however, may be reused for landscaping
purposes. Although trafficability should generally be good in ‘dry’ conditions, the soils could be subject
to strength loss from repetitive heavy vehicle loading (particularly from rubber tyred plant) or if they
become 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
building platforms.
Excavations in the materials encountered to the test pit termination depths should be achievable using
conventional earthmoving equipment (e.g. excavators with augers and toothed buckets). Deeper
penetration (particularly in confined excavations) would require ripping attachments on possibly larger
machines for efficient excavation.
Temporary, unsurcharged batter slopes should be constructed at maximum grades of 1V:1H in stiff (or
stiffer) natural soils. Flatter batters may be required if groundwater seepage is encountered.
Permanent batter slopes in the stiff (or stiffer) natural soils may be formed at a maximum grade of 1V:2H,
and in ‘controlled fill’ at 1V:2.5H. Permanent batters will require erosion protection such as covering with
erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
slopes, batters would need to be supported by RPEQ engineer designed retaining walls.
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Placement of heavy ‘builders plastic’ or filter ‘cloth’, pinned to batters may be required if heavy of prolonged rainfall occurs during construction. Exposure of temporary batters should be minimised in order to limit shrinkage cracking in clayey soils and potential development of temporary instability. Ground surfaces should be graded to shed surface water runoff away from construction areas. It should be noted that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion and sediment control measures must be installed prior to commencement of earthworks at this site. The above recommendations do not supersede any existing safety regulations or legislation applicable to excavations (e.g. limits on personnel entry into trenches). The residual soils and weathered rock where broken down to <100 mm particles, as encountered in the test pits would be suitable for use as structural fill, subject to appropriate moisture conditioning and compaction. It is noted that placement of any site won materials as fill would have an effect on the site classification (Ref. Section 7.3.1 below). All fill must be ‘benched’ into the natural slope, over-filled by at least half-width of the compaction plant (horizontally), and then trimmed back to the well compacted material. It is recommended that any imported fill should comply with the following: ▪ Minimum CBR of 10 % ▪ Maximum plasticity index of 15 % ▪ Shrink-swell index ≤ 1 % ▪ Maximum particle size of 100 mm Fill should be compacted at moisture contents within the range of ±2 % of optimum moisture content for Standard Compaction. Select fill should have a maximum particle size of 100 mm for an uncompacted layer thickness of 200 mm and shall be compacted by repeated rolling with a small compactor to achieve a dry density ratio of at least 95 % of the Standard Maximum Dry Density for cohesive soils, or 70 % Dry Density Index for any imported cohesionless soils. Confirmatory compaction testing should 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. 7.3 Building Foundations 7.3.1 Preliminary Site Classification Based on the results of the investigation, the site classification for the proposed new lots is estimated to be Class M in accordance with AS 2870-2011 Residential Slabs and Footings. It should be noted that the predicted order of movement is based on the existing natural soil profiles, and moisture content changes within soils due to normal seasonal moisture variations only. Where up to 1 m of fill was placed as part of an earthworks building pad, for example, the site classification would extend to Class H1. 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, together with any removed/retained trees (yt factor). Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 18
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7.3.2 Footing Design
Footings supporting any structures (including retaining walls) are to penetrate through fill and unsuitable
natural soils (if encountered) to be founded a minimum of 0.5 m into stiff (or stiffer) residual soils.
Due to the depths of foundations recommended, conventional high-level footings (e.g. slab on ground)
should be suitable for some of the lots. Bored piles (or similar) would likely be required where fill
earthworks were undertaken, or where slightly steeper slopes were present (e.g. proposed Lots 2 and 4).
High level footings, so founded, may be designed using a maximum allowable bearing pressure of
100 kPa. A maximum allowable end bearing pressure of 150 kPa could be adopted for steel reinforced,
concrete bored piles. Footings taken to found in the underlying very low strength (or stronger) phyllite
rock could be designed adopting a maximum allowable bearing pressure of 400 kPa.
Footings should be designed for the site classification (at least) nominated above in Section 7.3.1.
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 14.
Text Figure 14: Footing depth required to minimise 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.
7.4 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 4 overleaf. These values 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 4: Retaining Wall Design Parameters
Unit Friction Lateral earth pressure coefficients
Retained material weight angle Ka Ko Kp
(kN/m3) (Degrees) (Cantilever wall) (Non-yielding wall)
Very Stiff to Hard
Residual Soils/XW 19 27 0.38 0.55 2.66
Material
Very Low Strength
(or stronger) 19 36 0.26 0.41 3.85
Weathered Phyllite
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 7.3 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.
7.5 Drainage
Adequate site drainage shall be installed to ensure that stormwater is directed away from proposed
building sites, including retaining walls and batters. This may be achieved using measures such as
surface swales, diversion bunds or grated channels.
All collected stormwater (including overflow from rainwater tanks) must be directed by lined channels or
pipes to discharge a minimum of 5 m downslope from any structure or earthworks, preferably towards
the existing drainage lines. Surface dissipation, such as a rock weir or similar, must be installed at the
stormwater outlet(s) to prevent soil erosion.
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 assess hydrostatic pressure to be accounted for
in the design of the retaining structure. This should be collected and discharged as discussed above.
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).
7.6 Geotechnical Certification of Operational Works
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.
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8. 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 configuration and probable wastewater flow;
▪ Evaluation of the terrain-soil capacity to accept treated wastewater; and
▪ Consideration of NC wastewater treatment and disposal system construction and siting
requirements.
8.1 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) supplied by on-site rainwater tanks.
We therefore estimate a typical daily household wastewater flow rate of 840 L/day, based upon an
allowance of 7 x 120 L/person/day.
It should be noted that wastewater flow rates may be 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.
8.2 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.
Likely land application areas associated with the proposed future lots were up to about 5° (10 %) on Lot
5, however for the remainder of the proposed lots ground slopes of between 6° and 15° (10.5 % to 27 %)
were present. Therefore, sizing of EAA’s will need to consider a slope reduction factor as described in
Table M2 of AS1547:2012 or should be effectively terraced as necessary to reduce slope to < 10%
negating the effect of natural slopes. The addition of a slope reduction rate will result in an increase in
the size of necessary land application areas, which are described in Section 8.4.
It has been assessed from the results of the investigation that the soil profile across the proposed lots
predominantly comprises clayey sands/sandy clays, with sands ranging to coarse throughout, which is
equivalent to Strongly Structured Medium to Heavy Clay (Soil Category 6). This is a categorisation
and has been derived via the method described in Table M1 of AS1547:2012.
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Good quality topsoil obtained from excavations could be used to supplement the soil profile across the proposed irrigation areas, forming narrow mounds/swales along the contours. 8.3 Existing Wastewater Infrastructure Existing sewerage treatment and disposal infrastructure servicing the existing residence on Lot 3 was not observed during the walk-over. While no indications of inadequate operation or failure (e.g. odours, wet/soft/boggy ground, leaking effluent, or irregular grass growth) of any system or application areas was observed at the time of our site visit. Should the owner wish to retain the existing system to service the existing residence, the system (including disposal field(s)) must be inspected, and serviced as required, by a licensed plumber to ensure that it remains fit for purpose and conforms with NC requirements. If the existing system is deemed unsuitable, it is noted that sufficient space is available within proposed Lot 3 to site a new wastewater treatment system as described in Section 8.4. All wastewater installations must be completed by a suitably qualified and licensed plumber, experienced with on-site wastewater systems. Structures associated with any disused or discontinued wastewater treatment systems and trench areas should be fully removed (grubbed out) as part of operational works and replaced with controlled fill so as to not conflict with future site use. 8.4 Recommendations 8.4.1 System Type It is recommended that wastewater from each of the future residences be directed to a suitably sized household 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 (e.g. to the gully invert on Lot 2) it may be necessary to use an Advanced Secondary treatment system (Ref. Section 8.3.2 below). The STPs should be connected to either a surface or shallow subsurface drip irrigation system. To demonstrate feasibility of the lot layout, the use of shallow subsurface irrigation has been assumed for all lots as this provides a more conservative design. The use of surface irrigation is not supported on sites with slopes greater than 10 %. Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 2 mm/day for Category 6 soil. Therefore, a minimum irrigation area of about 420 m² (840 L/day / 2 mm/day) would be theoretically required. However, an additional 20 % reduction in DIR shall be applied for LAAs with slopes of 10 % to 20 %, and 50 % for slopes between 20 % and 30 % inclination (as described in Table M2 of AS1547:2012). The calculated EAAs for respective lots based on encountered slopes are set out in Table 5 overleaf below. This may be reduced at final design stage depending on the final location of the building and land application area and associated slope configuration. Geotechnical Investigation 10 June 2026 | 26073-001-Rev0 22
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Table 5: Required EAAs
Min. Land Typical Slopes Slope Reduction Required Land
Proposed Lot Application Area Present across Rate Application Area
(m2) EAA (%) (m2)
Lot 1 420 10 % to 20 % 20 % 470
Lot 2 420 10 % to 20 % 20 % 470
Lot 3 420 20 % to 30 % 50 % 840
Lot 4 420 20 % to 30 % 50 % 840
Lot 5 420 <10 % N/R 420
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 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 irrigation pipework can also be easily repaired in-place as required, therefore the
addition of a separate reserve area is not warranted.
8.4.2 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 overleaf.
Table 6: Setbacks for subsurface land application area for on-site sewage treatment plant
Site Feature Horizontal separation distance
Upslope Downslope Level
Property boundaries, pedestrian paths, walkways, recreation
areas, retaining wall, and footings for buildings and other 2m 4m 2m
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.
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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:
▪ Top of bank of permanent or intermittent water course,
lake, bay or estuary;
▪ Top water level of a surface water source used for 10 m 30 m
agriculture, aquaculture or stock purposes;
▪ Open stormwater drainage channel or drain;
▪ Bore or a dam.
Unsaturated soil depth to a permanent water table (vertically) 0.3 m 0.6 m
Note: Distances measured from the edge of the irrigated wetted area to any point of the feature.
Unsaturated soil depths of 0.6 m (i.e. depth to rock or wet soil) and 0.3 m are recommended for
Secondary and Advanced Secondary effluent, respectively. The test pits indicate that adequate
unsaturated soil depths are available across the proposed development area.
A conceptual wastewater layout plan has been prepared (attached as Figure 1) which demonstrates that
a new STP and wastewater irrigation area 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.
8.4.3 System Construction
Construction of wastewater land application areas must be carried out in accordance with Appendix M of
AS1547:2012 and NC 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.
▪ 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”).
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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.
8.4.4 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).
9. 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.
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EU \ i \ /ADVANGED, I hg . 1 lok CLIENT THREEDOM NOMINEES PTY LTD DRAWN BY DATE PROJECT GEOTECHNICAL INVESTIGATION & SLOPE STABILITY ASSESSMENT PROPOSED SUBDIVISION, 220 BLACK MOUNTAIN RANGE ROAD, BLACK MOUNTAIN DRAWING TITLE SITE PLAN NOT! ES BASEMAP COURTESY OF NEARMAP, DATED 8 MAY 2025 & MAX WATTERSON & ASSOCIATES RKG 09/06/2026 CHECKED BY DATE ACD 09/06/2026 SCALE SHEET SIZE AS SHOWN A3 PROJECT NO. 26073 DOC NO. 001 FIGURE NO. REVISION LEGEND HL APPROX. TEST PIT LOCATION EXAMPLE EFFLUENT LAND APPLICATION AREA (REF. REPORT SECTION 8.4)
Page 32
Appendix A
Test Pit Logs & Explanatory Notes
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Test Pit No.
TP1
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486176.0 m E 7080668.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, dark w= St
brown PL
CH
Silty CLAY, high plasticity, pale red brown,
with fine to coarse sized angular gravel
300
(RESIDUAL)
0.5
w<
VSt
DS_0.6-0.8m becoming grey mottled pale red brown, PL
grading to extremely weathered material with
depth 320
1.0
PHYLLITE, highly weathered, pale grey,
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
stained red brown, very low strength
Not Encountered
1.5
EX
2.0
becoming distinctly weathered, very low to low
strength
2.5
becoming slightly weathered, low strength
3.0
Hole Terminated at 3.10 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 34
Test Pit No.
TP2
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486134.0 m E 7080626.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, dark w= St
brown PL
CH
Silty CLAY, high plasticity, pale red brown
DS_0.2-0.3m (RESIDUAL)
300
w<
0.5 VSt
PL
350
becoming grey mottled pale red brown,
grading to extremely weathered material with
depth
PHYLLITE, highly weathered, pale grey,
Not Encountered
1.0 stained red brown, very low strength
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
EX
1.5
becoming moderately weathered
2.0
DS_2.0-2.1m QUARTZ VEIN, slightly weathered, pale grey,
stained orange brown, medium to high
strength
Hole Terminated at 2.40 m
2.5 Refusal on rock
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 35
Test Pit No.
TP3A
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486239.0 m E 7080503.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, dark w= Fr
brown PL
CH
Silty Gravelly CLAY, high plasticity, pale red
DS_0.2-0.4m brown, fine to coarse sized gravel and quartz
(RESIDUAL)
250
0.5 w<
becoming grey mottled pale red brown, VSt
grading to extremely weathered material with PL
depth 350
1.0
PHYLLITE, highly weathered, pale grey,
stained red brown, very low strength
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
Not Encountered
1.5
becoming distinctly weathered, very low to low
EX
strength
2.0
with fine to coarse sized quartz gravel
2.5
3.0
Hole Terminated at 3.10 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 36
Test Pit No.
TP3B
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486347.0 m E 7080535.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity,
brown, with roots w=
St
PL
CH Silty CLAY, high plasticity, pale brown, with 220
fine to coarse sized angular gravel
(RESIDUAL)
Silty CLAY, medium plasticity, pale grey
0.5 mottled pale red brown, with medium to
coarse sized angular gravel (EXTREMELY
WEATHERED MATERIAL)
w<
VSt
PL 320
1.0
Not Encountered
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
EX
PHYLLITE, highly weathered, grey, stained
red brown, very low strength
1.5
becoming moderately weathered, low strength
2.0
2.5
Hole Terminated at 2.60 m
Target depth
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.
TP4
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486440.0 m E 7080380.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, dark w= St
brown PL
CH
Silty CLAY, high plasticity, red brown, with
medium to coarse sized angular gravel
(RESIDUAL)
280
w<
VSt
0.5 PL
becoming grey mottled pale red brown,
grading to extremely weathered material with
depth
PHYLLITE, highly weathered, pale grey,
stained red brown, very low strength
1.0
Not Encountered
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
EX
1.5
becoming moderately weathered, low strength
2.0
2.5
Hole Terminated at 2.80 m
Target depth
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 38
Test Pit No.
TP5
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486569.0 m E 7080408.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, w= St
brown PL
U50_0.1-0.3m CH
Silty CLAY, high plasticity, pale red brown, 350
trace fine sized angular gravel, (RESIDUAL)
0.5
w<
VSt
becoming grey mottled pale red brown, PL
340
grading to extremely weathered material with
depth
1.0
PHYLLITE, highly weathered, pale grey,
Not Encountered
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
stained red brown, very low strength
EX
1.5
2.0
becoming distinctly weathered, very low to low
strength
2.5
becoming slightly weathered, low strength
Hole Terminated at 2.80 m
Target depth
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 39
Test Pit No.
TP6
Page 1 of 1
Engineering Log - Test Pit Project No.: 26073
Client: Threedom Nominees Pty Ltd Commenced: 3/3/2026
Project Name: Proposed Subdivision Completed: 3/3/2026
Hole Location: 220 Black Mountain Range Road, Black Mountain Logged By: MM
Hole Position: 486551.0 m E 7080326.0 m N MGA2020 Zone 56 Checked By: RKG
Equipment Type and Model: 5t Excavator RL Surface: No survey
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 CLAY (TOPSOIL) medium plasticity, dark w= St
brown, with roots PL
CH
Silty CLAY, high plasticity, pale red brown,
DS_0.2-0.4m trace fine to medium sized subangular to
250
angular gravel (RESIDUAL)
w<
0.5 VSt
becoming pale grey mottled pale red brown PL
with medium to coarse sized angular gravel,
tending to extremely weathered MATERIAL 450
PHYLLITE, highly weathered, pale grey,
1.0 stained red brown, very low strength
Not Encountered
TECTONIC 3.00.2 LIB.GLB Log 1 TECTONIC BOREHOLE 26073.GPJ <<DrawingFile>> 31/3/2026 16:15 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
EX
1.5
becoming distinctly weathered, very low to low
strength
2.0
2.5
becoming slightly weathered, with low strength
Hole Terminated at 2.80 m
Target depth
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 40.