Original source · versioned page text
Att 3 - Landslide Risk Assessment
Original PDF page 1, section 1. The text section number identifies this passage in the reader.
Job No.: 23-458
Our Ref.: 4602
Author: K. Donkor
October 26, 2023
K & S Dahl,
P O Box 6215,
Kincumber NSW 2251.
ATTENTION: KURT DAHL
Email: [email protected]
Dear Sir,
RE: LANDSLIDE RISK ASSESSMENT
PROPOSED RESIDENTIAL REDEVELOPMENT
#399 SUNRISE ROAD, TINBEERWAH QLD 4563
1.0 INTRODUCTION
It is understood that the property owner plans to construct a new residential redevelopment for the site
at #399 Sunrise Road, Tinbeerwah. It is noted that the existing building will be demolished to make way
for the new structure. Given that the site is located on land subject to Noosa Shire Council Landslide
Hazard Overlay Code, a site-specific landslide risk assessment report is required to demonstrate that
the proposed development would not trigger a landslide event within or outside the property boundary.
Mr Kurt Dahl commissioned Qualtest to undertake the required landslide risk assessment for the site.
This assessment was conducted by reviewing the following documents and a visit to the site to assess
existing conditions.
• Site Plan/Preliminary Design Plans – Aurora Residence – Lots 6, (#399) Sunrise Road,
Tinbeerwah, Prepared by Reitsma & Associates, Project No. 4635, Dated 18/09/23;
• A Site Investigation Report by AW Geotechnics, Project Number ES 228869, Job No.: AWT
77775, Dated 19/09/23; and
• Contour and Detailed Survey over part of Lot 6 on RP207543 – Prepared By Skyline Surveyors
Plan No. 9853 Det, Dated 06/05/21.
This report presents the assessment of the site slopes associated with the existing site slopes and post-
earthworks and recommendations to reduce the residual risk to a Low (or better) category.
Figure 1 shows the geographical location of the site.
1.1 Proposed Development
A two-storey residential dwelling is proposed for construction at the site. This new building is to replace
an existing building, which is planned for demolition. The Reitsma Proposed Site Plan shows the layout
plan of the dwelling, showing new concrete driveways, a detached garage/shed and a swimming pool
at the rear.
An extract of the Reitsma Proposed Site Plan is presented in Figure 2.
Qualtest Laboratory Pty Ltd
2/40 Boyland Avenue
Coopers Plains QLD 4108
PO Box 733 Archerfield QLD 4108
(07) 3875 1898 [email protected]
www.qualtestgeo.com
GEOTECHNICAL AND LABORATORY SERVICES ABN 74 010 752 815
See the original
The page text is free to read. Viewing or downloading an original file needs an account.
Log in to see the originalLog in to download the original file
Large file: the download may take up to a minute.
Searchable page text hides email addresses. Original files are unchanged and may show email addresses.
Full text of sections 2 to 13 (of 43)
Section 2
Job No: 23-458
Page 2
Figure 1 - The Geographical Location of The Site Highlighted in Red
Figure 2: The Proposed Layout Plan for The Site
1.2 Purpose
This report aims to establish the landslide risk of the existing slopes under current and post-construction
conditions.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 3
Job No: 23-458
Page 3
1.3 Scope of Work
The site has been identified as subject to unstable soils/landslip hazards and, as such, requires a slope
stability report from an RPEQ geotechnical engineer.
This site-specific slope stability assessment report should be prepared in accordance with the following:
• Section 8.2.8 of the Noosa Plan 2020 - Landslide Hazard Overlay Code and steep slope area
overlay code for works proposed within the overlay;
• Assessing the suitability of the proposed development based on existing geotechnical
conditions of the site;
• Identify all risk mitigation measures required to ensure the development remains geologically
stable in the long term; and
• A certification by a Registered Professional Engineer of Queensland specialising in geotechnical
engineering that the development of the subject will not initiate instability in or around the site.
1.4 Site Description
The subject site, #399 Sunrise Road, is located in the rural residential suburb of Tinbeerwah, within the
Noosa Council land area. Access to the site is via a driveway easement from Sunrise Road, which forms
part of the site’s western boundary. On all other sides, the site is bordered by existing rural residential
properties.
At the time of the geotechnical site walkover, a residential dwelling exist, which was constructed on a
relatively flat cut/fill pad. This existing dwelling is proposed for demolition. At the upslope side of the
building, a block retaining wall, approximately 1.2 to 1.5m high, was noted, while on the downslope side,
an unsupported grassed batter sloping at about 18o was recorded.
A shed and an unsealed access track were observed on an adjacent cut/fill pad to the west of the
dwelling. The cut batter upslope of the track/shed was unsupported and was measured to be 0.5m to
1.5m in height and battered at about 30o.
Overall, the natural ground slopes were measured to be approximately 15o, falling in an easterly
direction. However, areas of prior earthworks contain slopes shallower or steeper than 15o.
Vegetation observed onsite consisted of a landscaped garden including shrubs/small trees, mown grass,
and isolated large-sized trees. Given the site slopes, it is expected that site drainage would be good.
Photographs of the site conditions at the time of the walkover are presented in Figure 3.
a b
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 4
Job No: 23-458
Page 4
c d
Figure 3a, b, c, and d: Photographs of The Existing Site Conditions
1.5 Previous Geotechnical Report
The AW Geotechnics Site Investigation report included drilling four (4) boreholes onsite. In boreholes,
BH1, BH2, and BH3, sandy clay soils that extend down to the termination depths were encountered. In
BH4, however, a ground profile consisting of upper-level silty sand overlying an intermediate zone of
sandy clay, which in turn overlies an extremely weathered rock layer, was recorded.
The investigation assigned a site class of P due to the abnormal moisture condition, the low bearing
capacity and the steep slopes on the site in accordance with AS2780-2011, Clause 1.3.3 (a & c) and
Clause 2.5.3 (a & d).
The report further found that the expected ground surface movement due to moisture changes
corresponds to a design classification of Class H1
1.6 Attachments
The following Appendices are included in this report:
Appendix A – Site Plan of Proposal Development;
Appendix B – Previous Site Investigation Report by AW Geotechnics;
Appendix C – Contour and Detailed Survey by Skyline Surveyors
Appendix D – The Landslide Frequency Assessment Form.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 5
Job No: 23-458
Page 5
2.0 LANDSLIDE RISK ASSESSMENT / METHODOLOGY
Based on the information provided by the Client, our methodology in addressing the geotechnical brief
included the following:
• A geotechnical site walkover for landslide risk assessment of the existing site slopes, fill
conditions, and proposed earthworks;
• Review of the survey data for the site;
• Review of proposed development plans;
• Review of the Engineering Drawings;
• Review of the previous geotechnical report by AW Geotechnics; and
• Review of AS3798 – Guidelines on Earthworks for Commercial and Residential Developments;
No Level 1 certification report has been provided for the onsite fill. Hence, the fill is considered to be
uncontrolled until it is proven otherwise.
A suitably qualified and experienced geotechnical engineer undertook the detailed site walkover on
October 17, 2023.
Onsite drilling was not part of this slope stability risk assessment because a previous investigation report
for the site was made available to Qualtest for review.
2.1 Geotechnical Issues
Based on the site conditions, issues of geotechnical relevance are the following:
• The overall site slopes of approximately 15o, and the presence of localised site slopes of up to
30o generally falling to the east;
• The low bearing capacity of the site as identified in the AW geotechnics report;
• The planned removal of the existing building;
• The risk of failure of the existing unsupported cut and fill batters, as well as existing retaining
structures around the site;
• The potential and consequences of failure of the proposed new structures on the site, including
cut-to-fill batters, retaining structures and footing systems.
2.2 Landslide Assessment Criteria and Methodology
With a view to managing risks to life and property associated with natural disasters such as landslides,
etc., posed to occupants in a development, it is Council's requirement that the susceptibility of such
developments to natural disasters should be addressed. The following criteria are considered as part of
Council's Operational Work condition for approving a Site:
• That the landslide risk susceptibility of a site should be deemed to be 'low' in accordance with
'Landslide Risk Management' Australian Geomechanics Journal Vol. 43, No. 1 March 2007.
• Part B of the Council's requirement indicates that where the average grade of a finished lot
surface exceeds 15%, the site must be certified by a geotechnical professional as having a 'low
risk' in relation to landslide.
The methodology adopted for the landslide risk assessment at this site was to the requirement of
Australian Geomechanics (AGS) Landslide Risk Management guidelines as contained in the Australian
Geomechanics Journal Vol. 42 No. 1, March 2007. Due to the broad scope of the AGS journal, it does
not specifically cover small-sized developments. The work of Hargreaves and Kidd (2012), which is in
line with AGS Vol. 42, No. 1 2007, has conceptualised landslide risk management in a way that is
amenable to small-sized developments and has been adopted for this assessment.
This study by Hargreaves and Kidd (2012) provides an evaluation of the level of site hazards in relation
to landsliding. The hazard rating is based on a five-level system and classified into Very Low, Low,
Moderate, High, and Very High categories.
The implications of this hazard rating classification are indicated in Table 1.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 6
Job No: 23-458
Page 6
Table 1: Implications of Hazard Classification
Hazard
Description Implications
Rating
Very Low The event is conceivable but only
Accepted. Managed by routine procedures
(VL) under exceptional circumstances
The event might occur under very Can be accepted. Treatment to maintain or reduce risk level
Low (L)
adverse conditions should be defined
Moderate The event could occur under May be acceptable provided treatment plan is implemented
(M) adverse conditions to maintain or reduce risk level
Detailed investigation, planning and implementation of
The event will probably occur
High (H) treatment options essential to reduce risk to acceptable
under adverse conditions
levels.
Very Extensive investigation, planning and implementation of
High The event is expected to occur treatment options essential to reduce risk to acceptable
(VH) levels
2.3 Assessment of Site Features and Relative Frequency Analysis
Noosa Council’s Noosa Plan 2020 Landslide Hazard Overlay Map has identified the site as having a
landslide risk potential (> 15% slope).
An extract from the Landslide Overlay map showing the site is presented in Figure 4, and highlighted in
green.
Figure 4: Extract from Noosa Council Overlay Map for Landslide Hazard
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 7
Job No: 23-458
Page 7
2.4 Geotechnical Site Walkover
A site inspection was undertaken to confirm the geological, geomorphology, fill conditions, and to assess
the likely hazard rating based on site features that contribute to the initiation of a landslide.
The slope stability walkover assessment was undertaken on October 17, 2023.
A site report and frequency analysis were undertaken during the visit. The site report is a standard form
that identifies the site features that contribute to the occurrence of a landslide.
The major site features relevant to this assessment for natural and man-made slopes are:
1. Slope Angle: In general, the steeper the average surface slope angle, the higher the risk of
slope instability. Slope angles were directly measured onsite as well as calculated based on the
detailed survey plan;
2. Slope Shape and Features: The shape of the slope provides an indication of the method of
slope development and the materials below the surface. Concave shapes often indicate past
movements;
3. Engineering Properties of Sub-surface Materials: The engineering properties of the materials
forming the subsurface profile contribute to the risk of slope instability. Most slope failures occur
in soil-strength material, and deeper soils increase the likelihood of slope failure.
4. The Concentration of Surface/Ground Water: Water is one of the most important factors in
landslides' occurrence. The majority of slope failures occur during or following rainfall events
when there is a combination of surface erosion and saturation of subsurface materials. The
concentration of Groundwater: The presence of a high groundwater table can provide a similar
long-term situation to a rainfall event and contribute to slope failure without a major rainfall event;
5. Evidence of Instability: In many cases, slope failures occur due to the reactivation of previous
landslides. The presence of features that indicate past slope instability provides evidence that
could initiate future slope movement;
6. Regional position on a hillside; and
7. Site geology
8. Cut depth;
9. Cut batter angle;
10. Cut batter support;
11. Fill depth;
12. Fill batter angle;
13. Fill Batter Support;
14. Type of fill;
15. Wastewater, sewerage, etc;
16. Stormwater disposal;
17. Expandable/mechanical stormwater and drainage pipe connectors;
18. Footing system;
19. The foundation strata;
20. In-ground tanks (any type); and
21. Landscaping.
Additional observations onsite include:
• Minor distress of a small section of the block retaining wall;
• No groundwater or seepage was observed throughout the site;
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 8
Job No: 23-458
Page 8
• No tension cracks were observed within the site;
• Hummocky or bulging soils were not identified on the site slopes. There were sections of uneven
ground downslope of the house, which is attributed to previous earthworks and the removal of
trees.
• Physical observations made on existing structures such as Trees and Crib retaining walls
indicate signs of soil creep;
Evaluation of these features in a Landslide Frequency Assessment Form provided an indication of their
relative importance to slope failure. The assessment of the features included the allocation of a weighted
factor for each feature based on site observations, judgment, and experience. This allows a Relative
Frequency to be calculated by multiplying the selected factors together for each of the above site
conditions highlighted in blue and green.
From the assessment of results from their study (Hargreaves and Kidd 2012) and other studies in similar
terrain, a correlation between Relative Frequency and Hazard Rating has been established, as shown
in Table 2.
Table 2: Correlation between Relative Frequency and Hazard Rating
Relative Frequency (Output) Hazard Rating (Susceptibility)
< 0.2 Very Low (VL)
0.2 – 0.6 Low (L)
> 0.6 – 2.0 Moderate (M)
> 2.0 – 6.0 High (H)
> 6.0 Very High (VH)
The above major site features listed in Section 2.4 have been considered in determining a likelihood
estimate of landslides in this property. The features relevant to this site under consideration, as
recommended by Hargreaves and Kidd (2012), apply to Case 1 (1 - 7) and Case 4 (1 – 22).
With knowledge of the proposed development (earthworks, retention systems, fill, etc.), the assessment
has been carried out for the pre and post-development stages. The assessment indicates that the
relative frequency (Output) pre-development is 1.09 for Case 1 (items 1 to 7), which implies that the
landslide risk on the site is "Moderate", which may be tolerated by regulators; however, requires
investigation and implementation of treatment options to reduce the risk to Low or better.
To reduce the potential landslide risk level from “Moderate” to "Low" or better, the recommendations
presented in Section 2.5 should be implemented during and after construction.
2.5 Landslide Risk Mitigation Strategy
#399 Sunrise Road, Tinbeerwah, was identified as a "Moderate” landslide risk site per current site
conditions. The following recommendations are therefore provided to improve the landslide risk level
onsite during and post earthworks.
2.5.1 Fill
The borehole logs have not disclosed any fill at the drilled locations onsite. However, given the landform,
it is expected that some areas of fill would be present. Any material identified as fill is deemed unsuitable
to support structural loads. In areas subject to structural loads, these materials must be removed and
replaced with fill that is engineered and certified.
2.5.2 Cut and Fill Batter Support
Cut/fill batters must be cut to a grade no greater than 14 degrees (1V:4H). Batters steeper than this
must be appropriately retained. An engineered post and concrete sleeper or block retaining wall is
deemed appropriate for the site conditions.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 9
Job No: 23-458
Page 9
2.5.3 Type of Fill
Site won or imported fill must be placed in accordance with AS3798-2007. Imported select fill material
should be a good quality select fill material with a soaked CBR of at least 10%, a maximum aggregate
size of 50mm, and a maximum Shrink/Swell Index of 1.0%. Fill must be keyed into the existing slopes.
2.5.4 Stormwater Disposal
Stormwater collected within the site should be directed into designated drains and directed to Council's
stormwater system. Disposal of stormwater on the surface is not permitted in the immediate vicinity of
the proposed building area;
2.5.5 Site Drainage
Surface run-off should be diverted away from the proposed retaining walls and dwelling to prevent
scouring, riling, and erosion. Drains should be lined to prevent water from entering behind the wall. The
site drain shall be designed and inspected by a qualified person.
2.5.6 Footing System
All footings must be designed in accordance with AS2870-2011. It is understood that piers and pier
supported slab on-ground footing systems are proposed for the site. This type of footing system is
considered appropriate for the site. However, the piers must be extended beyond any fill and sufficiently
socketed into extremely weathered rock or better materials. Under no circumstances should the piers
be terminated within fill.
The proposed swimming pool located downslope within the site should also be supported on piers that
are socketed into the basement rock.
2.5.7 Roof Water Control
It is understood that rainwater tanks would be used on site. Ensure overflow and other downspouts are
not flowing onto any fill/slopes but are collected and channelled into storm water drains. Leaks along
these lines should be avoided.
2.5.8 Erosion Control
Erosion protection of all exposed batters should be carried out at the site. All run-off at the site should
be appropriately channelled to ensure that run-off does not cause severe erosion that may result in
adverse ground profiles.
2.5.9 Retaining Walls
All proposed retaining structures should be designed by a qualified engineer and checked for global
stability, sliding, and rotational failure.
2.5.10 Wastewater and Sewerage System
Qualtest is unaware of the sewerage system planned for the site. However, to not adversely affect the
performance of the site slopes, wastewater land application area should be located at least 10m
downslope of the proposed residence.
Following the implementation of the above recommendations, the site has been reassessed, based on
Cases 1 – 22, to have a hazard rating of 0.46, corresponding to a “Low" landslide risk.
3.0 CONCLUSION
To progress the proposed development at #399 Sunrise Road, Tinbeerwah, Council requested a report
on the stability of the site, which has been identified as being within the map overlay for landslide hazard.
The slope stability (landslide) assessment has been conducted, and the site, in its current state, has
been assigned a "Moderate" landslide risk category. Following the implementation of the
recommendations in Section 2.5, the risk of landslide events has been revised to “Low”.
Based on our site inspection and sub-surface analysis, we conclude that the site is suitable for the
proposed development and has a “Low" risk to slope instability, provided the recommendations outlined
in this report are adhered to.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 10
Job No: 23-458
Page 10
Should you require further information regarding the above, please do not hesitate to contact this office.
Yours faithfully,
MICHAEL MORRISON DENNIS ALAZIGHA, RPEQ 22169
For and on behalf of
QUALTEST LABORATORY PTY LTD
Appendix A – Preliminary Site Plan of Proposal Development;
Appendix B – Previous Site Investigation Reports by AW Geotechnics;
Appendix C – Contour and Detailed Survey by Skyline Surveyors;
Appendix D – The Landslide Frequency Assessment Form.
Ref: 4602
399 Sunrise Road, Tinbeerwah
Section 11
APPENDIX A Preliminary Site Plan of Proposal Development
Section 12
Lot Descrip on Legend Final loca on of walls, gardens, paved Pool fencing and gates in accordance with local All slab levels to be verified by contractor Engineering Notes Do not scale dimensions. Removable door to WC.
Lot : 6 areas TBC. on site by builder/client authori es regula ons + A.S. 1926.1 Loca on prior to commencement. Discrepancies Check and verify all dimensions prior to commencement.
Plan : RP207543 Pool Water Concrete Refer to Engineers details for:
Gravel/ Sediment control measures to be TBC. on site by builder/client are to be reported to the designer. Foo ngs, Slab, Tiedown, Bracing, Framing These plans to be read in conjunc on with engineers report
Area : 14,280.52m2 All windows to be verified against floor plans and eleva ons by contractor.
Parish : MAROOCHY Pebbles taken on site during construc on All stormwater drains to be UVPC sealed system Nominated hardscaping/landscaping items Schedule, Floor Construc on, Roof framing and
Truss details, Wind Category, Pool, Retaining Walls Discrepancies are to be reported to the designer prior to commencement.
County : CANNING Tiles/Paving Lawn Sewer line loca on to be confirmed in accordance with A.S. 3500.3 To disperse such as driveways/paths/gardens/ retaining All work to comply with the NCC, these drawings and all other relevant
Site Coverage : on site prior to construc on of pool 170m2 max. roof area (final size & loca on of site etc. Are design sugges ons only. Refer Plumbing and drainage technical literature for this wind category.
.34
and/or proposed dwelling drains tbc by plumber/builder/client) builders specifica ons for specific details To comply with the requirements of the local authority
133.71
133
132.5
.26
DUMPY PEG
133
.95
133
.00
.19
134
134
133
.86
.70
133
.52
.33
133
ENT
133
131
.21
.06
KM
134
133
133.5
.85
.55
BAN
134
132
EM
134
.28
.07
135
.92
135
.19
.75
.14
134
135
.85
135
135
135
.65
134.5
.29
.19
.80
.26
.88
133
.95
134
.24
135
135
135
134
135
.48
135
.26
.52
.09
135
.92
135
.98
.53
.53
.63
131.5
135
.64
.51
134
.11
.24
.31
.62
.68
.06
.68
135
.50
133
135
135
135
.68
.08
.63
.73
135
135
135
134
134
.77
134
.71
.26
135
.38
135
.27
135
136
135
136
.28
135
135
135
135
134
135
135
.63
135
.41
136
STAIR
134
135
.88
.30
.97
135
TG139.39
133
135
135.5
TG139.39
.69
.56
132
.26
.68
136
GARDEN PAVED PATIO
136
136
132
.21
.70
.70
.71
.70
.71
136
132
136
.79
.70
136
136
136
.42
136
CONCRETE
.71
.71
136
136
.56
136
136
TANK
136
136
BUILDING EXTENTS
.06
137
136.76
.40
136.5 134.27
.37
.45
FLOOR LEVEL
.40
132.5
.70
136
137
133
FLOOR LEVEL
136
.55
.16
136
EXISTING
133
Section 13
137
.50
136
DWELLING
.72
.30
.72
AWNING
.77
136
134
137
136
136.79 1 & 2 STOREY CONCRETE RENDERED DWELLING
FLOOR LEVEL APEX
ENTRY
.40
RL 143.12 133
.09
137
TG 142.0
.58
137
138
.01
RETAINING
.01
CONCRETE
PAVED
WALL 6
.85
.0
.08
CONCRETE DRIVEWAY
.60
.76
134
138
.05
136
.65
1
136
6.7
136
136
138
136
136
.73
13
.56
.65
136
134
.41
134
133.5
.22
134
.34
139
.44
.14
139
134
TG139.4
.70
139
.96
.49
.65
.38
.66
137
.44
.14
138
.86
.22
.59
137
136
.10
.70
136
136
137
139
.28
.33
137
138
139
.05
136
CONCRETE TANK
.72
136
.80
.72
136
.66
139
.62
.94
138
136
.46
134
137
136
137
138
.13
PAVED AREA 134
136
.63
136
CONCRETE DRIVEWAY
.94
138
.17
CLOTHES
138
IL1139.06
GARDEN
.78
136.76
.65
136
6
LINE
137
136
40.1
FLOOR LEVEL
.64
.66
136
.72
134
.74
.65
.65
139
.73
136
136
136
.53
134.5
.27
136
.13
.04
.88
.60
.64
140
135
.10
.60
140
139
138
.35
136
136
.31
.50
.95
140
136
.67
140
.70
0.375Ø CONC
140
.08
139
135
.52
.95
138
CON
135
140
.01
140
CRE
137
.12
139
CONCRETE RETAINING WALL
TE R
.57
140
.73
ETA
.00
.05
.04
.74
138
.67
.24
ININ
140
138
139
139
136
.10
.14
.06
CONCRETE
.81
G 135
136
139
WA
.04
136
STAIR
.34
139
139
140
SLAB LL
.83
136
138
137
The text continues on section 14.