Original source · versioned page text
Appendix B - Driveway Slope & Stability Assessment (Tectonic)
6 March 2023 Project No. 23038-001-Rev0 Contour Consulting Engineers Attention: Natalie Barlow Email: [email protected] SLOPE STABILITY ASSESSMENT FOR PROPOSED NEW DRIVEWAY 182 UHLMANNS ROAD, FEDERAL Dear Natalie 1.0 INTRODUCTION Tectonic has undertaken a geotechnical assessment for a proposed driveway at 182 Uhlmanns Road, Federal. This report includes a slope stability assessment, together with geotechnical advice for the proposed development. In summary, subject to implementation of the recommendations made herein, there would be a Low Risk of slope instability affecting the proposed driveway, with the proposed driveway assessed to have a non-worsening effect on the existing slope stability of the site, proposed areas of work and the adjoining properties, in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). It is understood that this report will be submitted to Noosa Council (NC) to address Item 3 of an Information Request (NC Ref. RAL22/0032, dated 14 December 2022). 1.1 Details of Site and Proposed Development The site is described as Lot 147 on M371248, covers an area of about 9.3 ha, and has frontage to Uhlmanns Road in the north-western corner. At the time of the assessment two residences were located in the north- western part of the site, with the proposed driveway to extend between the two structures down to Uhlmanns Road. Two additional residential structures were located in the central part of the site, well away from the proposed development area. Semi-rural and acreage allotments with scattered buildings surrounded the site. A 2022 aerial image of the site and surrounds is shown in Text Figure 1 overleaf. A detailed description of the site is given in Section 2. Tectonic Geotechnical Pty Ltd PO Box 899, Buderim, QLD 4556 Office: (07) 5478 9016 Email: [email protected] www.tectonicgeo.com.au A.B.N. 83 165 727 828
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
The download may take up to a minute.
Full text of the other 14 pages
Page 2
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
Indicative Proposed Driveway Alignment
Subject Site
Text Figure 1: Aerial image of site and surrounds from 2022 (courtesy of Nearmap imagery), with inset
Preliminary drawings prepared by Contour Consulting Engineers (Ref. Plan No. 3225-SK01 & SKO2,
Revision P1 dated 1 August 2022) indicate that the proposed driveway will be located in the north-western
part of the site between an existing residence and shed. Provided sections indicate that the driveway will
be created mostly by excavating up to about 1 m deep into the slope, with filling <0.5 m high required for
the crossover, and edge batters to be formed at about 1V:4H. Text Figure 2 below shows the proposed
driveway layout and some cross-sectional views.
Text Figure 2: Proposed layout (LHS) and some cross-sectional views (RHS), courtesy of Contour Consulting Engineers
1.2 Method and Scope of Investigation
As part of our slope stability assessment, a desk-top study was carried out comprising a review of
published geological maps and slope stability studies.
A walk-over survey was conducted to make an appraisal of the general condition of the site in regard to
topography, drainage, vegetation cover, geology, erosion and condition of existing structures. The walk-
over survey was carried out by an experienced geotechnical engineer from Tectonic on 15 February
2023.
The results of the desk-top study and walk-over survey are included in Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been reviewed by Mr Ashley Davey, an RPEQ with more than 27 years’ experience in
geotechnical engineering, including a number of slope stability projects.
2/10
Page 3
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
The 1:100,000 scale Gympie Surface Geological Map 1 (Ref. Text Figure 3 below) indicates that the site is
underlain by materials derived of the early to middle Triassic age Kin Kin Beds (blue-green shading),
which are described as “commonly cleaved and commonly kinked mudstone…” The Australian
Stratigraphic Units Database describes the Kin Kin Beds as “arenite, argillite and phyllite.”
Approx. Site Location
Text Figure 3: Extract of Gympie Surface Geology Map
2.2 Topography
During the site walk-over, topographic features specific to the site were noted, and ground slopes were
recorded using a hand-held clinometer. As shown in Text Figure 4 overleaf, the proposed driveway
alignment is located on the western flank of a well rounded spur. The driveway alignment extends from the
crest of the spur. A northerly trending natural drainage line is located to the west of the proposed alignment,
with a culvert crossing along Uhlmanns Road further to the west.
Gradients along the proposed driveway alignment were relatively gentle across the crest of the spur being
<7°, steepening progressing down the flank to between 10° and 15°, before reducing to become relatively
level along Uhlmanns Road. An inferred road cutting about 1 m high was present near the toe of the slope
which was battered at about 20°. Downslope of Uhlmanns Road the ground surface continued at about 20°
to 25°.
Areas immediately around the proposed development area had been altered by previous cut and fill
earthworks, presumably during construction of the existing structures and driveway. Cut and fill in the order
of about 1.5 m deep and 2 m high appeared to have been undertaken, with batters between about 20° and
25°.
1
State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, Sheet 9445, 2018
3/10
Page 4
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
Indicative Proposed Driveway Alignment
Text Figure 4: Ground surface contours, 1 m spacing (courtesy Noosa Council ‘Intramaps’)
Text Figures 5 and 6 illustrate site conditions across the proposed development area at the time of the
assessment.
Proposed Driveway Alignment
Text Figure 5: View of the proposed development area, looking west
Proposed Driveway Alignment
Text Figure 6: View on the proposed development area, looking east from Uhlmanns Road
2.3 Groundwater
No signs of groundwater seepage (‘springs’) were observed around the proposed development area
during our walk-over.
4/10
Page 5
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
2.4 Surface Drainage
Natural surface drainage follows the ground surface gradients from the proposed development area to the
west, ultimately into the drainage system along Uhlmanns Road. Generally, surface water is expected to
drain well from the area considering the positive gradients. No signs of significant erosion were observed
around the proposed development area at the time of the assessment; however, scouring was observed
within the natural drainage line and at the culvert crossing outlet further to the west along Uhlmanns Road.
2.5 Vegetation
During our walkover assessment, vegetation across the proposed development area and most of the site
comprised grassed areas, bushes, and mature trees with a relatively open understory. Mature trees around
the proposed development area were relatively straight, exhibiting no obvious signs of tilting/leaning which
could be attributed to global slope instability.
2.6 Buildings and Other Structures
At the time of the assessment an existing residence and shed were in close proximity to the proposed
driveway alignment. Sandstone block walls <1 m high were observed around terraced areas adjacent to the
residence. Based on an external visual assessment, the existing structures appeared to be in relatively good
structural condition, with no obvious signs of distress (e.g. cracking, misalignment etc.) which could be
attributed to slope instability.
The asphalt seal along Uhlmanns Road and an existing driveway adjacent to the proposed development
area were noted to be in good condition, with no obvious signs of cracking or distress which would indicate
slope instability. Other structures and driveways on the property were well away from the proposed
development area and were not assessed.
Text Figure 7: View of the existing residence (LHS) and Uhlmanns Road adjacent to proposed driveway (RHS)
5/10
Page 6
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 8),
which indicates that most of the proposed driveway alignment has a Moderate (pale brown shading) and
High (orange shading) landslide hazard.
Indicative Proposed Driveway Location
Text Figure 8: Extract from NC landslide hazard overlay mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility only
and is superseded by this site-specific assessment. At the time of our visit, the proposed driveway
alignment and immediate surrounds did not exhibit any indicators of global slope instability. No landslide
back scarps, groundwater springs, tension cracks, or areas of naturally ‘hummocky’ ground were observed.
The existing structures appeared to be in good condition.
Surface cracking was noted in an inferred fill batter to the north of the proposed driveway alignment, which
appeared to be related to soil shrinkage due to drying, rather than slope instability. Irregular, semi-circular
and convex shaped features were observed downslope (south-east) of the existing shed; however, these
features were well away from the proposed development area, on the opposite side of the spur, and not
relevant to the proposed works.
Text Figure 9: Inferred shrinkage cracking in fill batter near development area (LHS) and semi-circular feature to the
south of the existing shed (RHS)
6/10
Page 7
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
3.2 Stability 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 1 below, and
the Qualitative Terminology for Use in Assessing Risk to Property is attached.
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning and
Very High
VH implementation of treatment options essential to reduce risk to Low; may be too expensive and not practical.
Risk
Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
H High Risk required to reduce risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulators’ approval) but requires investigation, planning
M Moderate Risk and implementation of treatment options to reduce risk to Low. Treatment options to reduce to Low risk should
be implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level,
L Low Risk
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 (Ref. Section 2), and subject to the implementation of
recommendations given in Section 4, it is assessed that there would a Low (L) Risk of global slope instability
affecting the proposed driveway, with the works assessed to have a non-worsening effect on the existing
slope stability of the site, proposed areas of work and the adjoining properties. 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 2 is our qualitative assessment of landslide risk for the site. A
summary of qualitative terminology for use in assessing risk to property is attached (taken from AGS 2007).
This risk assessment excludes geotechnical risk associated with the existing structures, dams and areas of
the site outside of the proposed driveway area.
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk Comments
The likelihood of a failure through fill or natural soil is
assessed as Unlikely due to the gentle to moderate
ground slopes immediate to the proposed driveway
1: Shallow failure through fill or alignment and no signs of previous instability in that
Unlikely Minor Low
natural soil area. The consequence of such a failure would be
Minor considering the anticipated confined extent of
instability, with the resultant risk being Low as per
AGS 2007.
The likelihood of a deep failure through stiff/medium
dense (or stronger) natural soil or weathered rock is
assessed as Rare due to the inferred strength of these
2: Deep failure through
materials, apparent lack of groundwater, and no
stiff/medium dense (or stronger) Rare Major Low
evidence of such existing deep-seated instability across
natural soil or weathered rock
the proposed development area. Although the
consequence of such a failure could be Major, the
resultant risk is Low as per AGS 2007.
The potential impacts of slope instability on the proposed development have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts so that a Low Risk
of instability would be achieved for the proposed driveway.
7/10
Page 8
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
4.0 RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached.
4.1 Site Layout and Development Limitations
The planned extents of the proposed driveway alignment as described in Section 1.1 are acceptable from a
slope stability viewpoint. Should any revisions be made to the proposed extent of the development, Tectonic
must review these prior to construction to assess if there are any impacts on slope stability.
4.2 Earthworks
Provided sections indicate that the driveway will primarily be cut into the slope, requiring excavations up to
about 1 m deep, with filling <0.5 m high required for the crossover. Should proposed earthworks exceed
these quantities, Tectonic must assess the impacts on slope stability before construction.
4.2.1 Site Preparation and Trafficability
Any organic rich topsoil, uncontrolled fill, firm/loose (or weaker) soils and overly wet materials must be
stripped and removed from the construction area. Topsoil materials must not be used in structural fill;
however, may be placed across landscaping areas. It is recommended that the subgrade should be proof
rolled using a minimum 4 tonne pad foot roller to identify any ‘soft spots’. Any such soft spots should be
excavated and replaced with select fill material, constructed as recommended in Section 4.2.3.
The subgrade materials exposed after site stripping and excavations are expected to comprise stiff (or stiffer)
clays. The clayey soils could be subject to strength loss from repetitive 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 any laydown areas.
The site 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.
4.2.2 Excavations
Excavations should be achievable using small to medium sized conventional earthmoving plant
(e.g. 5 to 14 tonne excavators) with toothed buckets, or ripper attachments.
Temporary, non-surcharged batter slopes may be formed at 1V:1H in the stiff/medium dense (or stronger)
soils, provided groundwater seepage is not encountered. Flatter batters may be required if groundwater
seepage is encountered, or if earthworks are undertaken during a period of heavy rainfall. Permanent non-
surcharged batters in stiff/medium dense (or stronger) natural soils and controlled fill up to 1 m in height must
be formed no steeper than 1V:2.5H. We note that the design specifies maximum batters of 1V:4H which
would enable vehicular access, such as for mowing. Where such battering is not possible, temporary and/or
permanent retention will be required. Any permanent soil and fill batters will require erosion protection (e.g.
revegetation or surface protection).
Any excavation required adjacent to existing structures will need to be specifically designed to ensure that
the structures are not undermined.
Care should be taken to ensure excavations do not cause excessive ground vibrations that could result in
damage to existing nearby structures. Should vibratory excavation or construction techniques be adopted,
ground vibrations should be monitored, and excavation methods may need to be altered should adverse
impacts be anticipated or encountered. It would be prudent to conduct a dilapidation survey of nearby
structures before commencement of construction.
The above recommendations do not supersede any existing safety regulations or legislation applicable to
excavations (e.g. limits on personnel entry into trenches).
8/10
Page 9
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
4.2.3 Filling
Site soils where free of deleterious materials (e.g. vegetation, oversize etc.) are expected to be suitable for
re-use as structural fill, subject to appropriate moisture adjustment. It should be noted that compaction of
highly reactive clays, as are anticipated at this site, could be difficult during wet weather and adverse impacts
on site classification for footing and pavement design should be considered when re-using such clays.
Should importation of fill be required, it is recommended that such fill should comply with the following:
Minimum CBR of 10%
Maximum plasticity index of 15%
Shrink-swell index ≤ 1%
Maximum particle size of 100 mm
Controlled fill materials should be compacted at moisture contents within the range of -2% to +2% of OMC for
Standard Compaction. Confirmatory testing must be carried out at regular intervals and further details for
control and testing of fill are given in Australian Standard AS 3798-2007 Guidelines on Earthworks for
Commercial and Residential Developments. 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.
Fill must be ‘benched’ into the natural soil, over-filled by 0.5 m (horizontally) and then trimmed back to the
well compacted material.
4.3 Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area.
Permanent drainage must be installed to ensure that stormwater runoff from the driveway is formally
collected (e.g. positive gradients along the pavement directing stormwater into side drains). As the driveway
alignment runs down a natural slope, it would be prudent to include surface water flow dissipation devices
such as ‘water bars’ and/or rock check dams at regular intervals to reduce runoff velocity and minimise
erosion.
All stormwater collected from the driveway should be directed by pipes or surface channels to discharge into
the existing stormwater infrastructure along Uhlmanns Road. Surface protection along channels should be
assessed by the design engineer on a case-by-case basis depending on the specific conditions and may
range from turf in low velocity flow areas to rock or concrete in high velocity flow zones. A surface
dissipation device such as rock weir must be installed at the stormwater outlet(s), where not discharging into
an existing pit.
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).
9/10
Page 10
Contour Consulting Engineers 23038-001-Rev0
6 March 2023
5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
affecting the proposed driveway subject to implementation of the recommendations given in this report, with
the proposed driveway assessed to have a non-worsening effect on the existing slope stability of the site,
proposed areas of work and the adjoining properties
6.0 LIMITATIONS
This assessment relates only to the proposed driveway and the associated impacts to surrounding areas.
Existing structures, slopes, and driveways away from the proposed development area may be subject to risk
of slope instability, but assessment of those matters was beyond the scope of our commission.
Your attention is drawn to the document Limitations, which is attached to this letter report.
Please contact the undersigned should you wish to discuss any of the above matters.
Yours faithfully
TECTONIC GEOTECHNICAL PTY LTD
Michael Cocks BEng NER RPEQ 27540 Ashley Davey BEng (Geological) RPEQ 8159
Senior Geotechnical Engineer Managing Director
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
10/10
Page 11
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Implied Indicative Landslide
Description Descriptor Level
Indicative Notional Recurrence Interval
Value Boundary
10-1 5x10-2
10 years The event is expected to occur over the design life. ALMOST CERTAIN A
20 years The event will probably occur under adverse conditions over the
-2
10 100 years
design life.
LIKELY B
-3
-3 5x10 200 years
10 1000 years The event could occur under adverse conditions over the design life. POSSIBLE C
5x10-4 2000 years
The event might occur under very adverse circumstances over the
10-4 10,000 years
design life.
UNLIKELY D
-5 5x10-5 20,000 years
10 The event is conceivable but only under exceptional circumstances
100,000 years RARE E
over the design life.
-6 5x10-6 200,000 years
10 1,000,000 years The event is inconceivable or fanciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Description Descriptor Level
Indicative Notional
Value Boundary
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
200% CATASTROPHIC 1
stabilisation. Could cause at least one adjacent property major consequence damage.
100%
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
60% MAJOR 2
stabilisation works. Could cause at least one adjacent property medium consequence damage.
40%
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
20% MEDIUM 3
Could cause at least one adjacent property minor consequence damage.
10%
5% 1% Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works. MINOR 4
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
0.5% INSIGNIFICANT 5
notional boundary of 0.1%. See Risk Matrix.)
Notes: (2) The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
(3) The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
(4) The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68039
68039 RL2Risk
Landslide August 2010
Management Issued 10/7/07
Page 12
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of 1: CATASTROPHIC 2: MAJOR 3: MEDIUM 4: MINOR 5:
Approximate Annual 200% 60% 20% 5% INSIGNIFICANT
Probability 0.5%
A – ALMOST CERTAIN 10-1 VH VH VH H M or L (5)
-2
B - LIKELY 10 VH VH H M L
C - POSSIBLE 10-3 VH H M M VL
D - UNLIKELY 10-4 H M L L VL
-5
E - RARE 10 M L L VL VL
-6
F - BARELY CREDIBLE 10 L VL VL VL VL
Notes: (5) For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
(6) When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
VH VERY HIGH RISK options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
H HIGH RISK
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
M MODERATE RISK implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
L LOW RISK
required.
Acceptable. Manage by normal slope maintenance procedures.
VL VERY LOW RISK
Note: (7) 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.
Australian Geomechanics Vol 42 No 1 March 2007
Form No. Landslide
68039 68039 RL2Risk
August 2010
Management Issued 10/7/07
Page 13
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX G - SOME GUIDELINES FOR HILLSIDE CONSTRUCTION
GOOD ENGINEERING PRACTICE POOR ENGINEERING PRACTICE
ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical practitioner at early Prepare detailed plan and start site works before
ASSESSMENT stage of planning and before site works. geotechnical advice.
PLANNING
SITE PLANNING Having obtained geotechnical advice, plan the development with the risk Plan development without regard for the Risk.
arising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
Use flexible structures which incorporate properly designed brickwork, timber Floor plans which require extensive cutting and
or steel frames, timber or panel cladding. filling.
HOUSE DESIGN
Consider use of split levels. Movement intolerant structures.
Use decks for recreational areas where appropriate.
SITE CLEARING Retain natural vegetation wherever practicable. Indiscriminately clear the site.
ACCESS & Satisfy requirements below for cuts, fills, retaining walls and drainage. Excavate and fill for site access before
DRIVEWAYS Council specifications for grades may need to be modified. geotechnical advice.
Driveways and parking areas may need to be fully supported on piers.
EARTHWORKS Retain natural contours wherever possible. Indiscriminatory bulk earthworks.
Minimise depth. Large scale cuts and benching.
CUTS Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
Minimise height. Loose or poorly compacted fill, which if it fails,
Strip vegetation and topsoil and key into natural slopes prior to filling. may flow a considerable distance including
Use clean fill materials and compact to engineering standards. onto property below.
FILLS Batter to appropriate slope or support with engineered retaining wall. Block natural drainage lines.
Provide surface drainage and appropriate subsurface drainage. Fill over existing vegetation and topsoil.
Include stumps, trees, vegetation, topsoil,
boulders, building rubble etc in fill.
ROCK OUTCROPS Remove or stabilise boulders which may have unacceptable risk. Disturb or undercut detached blocks or
& BOULDERS Support rock faces where necessary. boulders.
Engineer design to resist applied soil and water forces. Construct a structurally inadequate wall such as
Found on rock where practicable. sandstone flagging, brick or unreinforced
RETAINING
Provide subsurface drainage within wall backfill and surface drainage on slope blockwork.
WALLS
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
Found within rock where practicable. Found on topsoil, loose fill, detached boulders
Use rows of piers or strip footings oriented up and down slope. or undercut cliffs.
FOOTINGS
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
Engineer designed.
Support on piers to rock where practicable.
SWIMMING POOLS Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uphill side whilst there
may be little or no lateral support on downhill side.
DRAINAGE
Provide at tops of cut and fill slopes. Discharge at top of fills and cuts.
Discharge to street drainage or natural water courses. Allow water to pond on bench areas.
SURFACE Provide general falls to prevent blockage by siltation and incorporate silt traps.
Line to minimise infiltration and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
SUBSURFACE
Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Usually requires pump-out or mains sewer systems; absorption trenches may Discharge sullage directly onto and into slopes.
SEPTIC &
be possible in some areas if risk is acceptable. Use absorption trenches without consideration
SULLAGE
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Failure to observe earthworks and drainage
CONTROL & Revegetate cleared area. recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITE VISITS DURING CONSTRUCTION
DRAWINGS Building Application drawings should be viewed by geotechnical consultant
SITE VISITS Site Visits by consultant may be appropriate during construction/
INSPECTION AND MAINTENANCE BY OWNER
OWNER’S Clean drainage systems; repair broken joints in drains and leaks in supply
RESPONSIBILITY pipes.
Where structural distress is evident see advice.
If seepage observed, determine causes or seek advice on consequences.
Australian Geomechanics Vol 42 No 1 March 2007 113
Form No. 68029 RL2 August 2010
Page 14
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 15
LIMITATIONS This document has been prepared for the purpose outlined in Tectonic’s proposal and no responsibility is accepted for the use of this document, in whole or in part, for any other purpose. The scope of Tectonic’s Services are as described in Tectonic’s proposal, and are subject to restrictions and limitations. Tectonic did not perform a complete assessment of all possible conditions or circumstances that may exist at the site referenced in the report. If a service is not expressly indicated, do not assume it has been provided. If a matter is not addressed, do not assume that any determination has been made by Tectonic in regards to it. Conditions may exist which were undetectable given that economic and time constraints limit the practical extent of geotechnical investigation. Variations in conditions may occur between investigation locations, and there may be special conditions pertaining to the site which have not been revealed by the investigation and which have not therefore been taken into account in the document. Where variations exist on site, additional studies and actions may be required. Tectonic’s opinions are based upon information that existed at the time that the work was performed. The passage of time, man-made or natural events, may alter the site conditions. It is understood that the Services undertaken allowed Tectonic to form an opinion of the actual conditions of the site at the time the site was visited and cannot be used to assess the effect of any subsequent changes in the quality of the site, or its surroundings, or any laws or regulations. Any assessments made in the preparation of this document are based on the conditions indicated from published sources and the findings of the investigation described. Actual subsurface conditions may differ from those indicated in the document (e.g. between boreholes or test pits). No warranty is included, either express or implied, that the actual conditions will conform exactly to the assessments contained in this document. Where data supplied by the client or other external sources, including previous site investigation data, have been used, it has been assumed that the information is correct unless otherwise stated. No responsibility is accepted by Tectonic for incomplete or inaccurate data supplied by others. This document is provided for the sole use by the Client and its professional advisers. No responsibility whatsoever for the contents of this document will be accepted to any person other than the Client. Any use which a third party makes of this document, or any reliance on or decisions to be made based on it, is the responsibility of such third parties. Tectonic accepts no responsibility for damages, if any, suffered by any third party as a result of decisions made or actions based on this document. Rev 1 (March 2016)