Original source · versioned page text
Slope Stability Report - 22072-001-Rev0
21 March 2022 Project No. 22072-001-Rev0 H.M de Groot Email: [email protected] SLOPE STABILITY ASSESSMENT FOR PROPOSED SHED 195 TOP FORESTRY ROAD, RIDGEWOOD Dear Mick 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment for a proposed shed at 195 Top Forestry Road, Ridgewood. This report presents the results of our slope stability assessment, together with geotechnical advice for the development. In summary, subject to implementation of the recommendations made herein, there would be a Low Risk of slope instability affecting the proposed development in accordance with the Australian Geomechanics Society Guidelines for Landslide Risk Management, dated March 2007 (AGS 2007). 1.1 Details of Site and Development The property is described as Lot 1 on RP170386, covers an area of 5 ha and is located within the Noosa Council (NC) area. The proposed shed site was located mid-slope, in the centre of the property, below Top Forestry Road which follows a local ridgeline. The allotment was densely vegetated and generally fell in a south-easterly direction. Rural residential properties surrounded the site. Text Figure 1 below illustrates the site layout. A detailed description of the site is given in Section 2. Proposed Shed Location Text Figure 1: Aerial image of the site, courtesy QGlobe June 2021 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
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 the other 14 pages
Page 2
H.M de Groot 22072-001-Rev0
21 March 2022
At the time of the assessment an existing relatively level building platform (approximately 20 m long by 20 m
wide) had been previously formed by cut and fill earthworks. It was estimated that fill in this area ranged up
to 2.5 m high, with cuts up to 3.0 m depth. It is anticipated that minimal additional earthworks will be
required (i.e. less than 0.5 m depth/height) for the proposed shed building pad. An existing carport was
located on the eastern side of the pad, with the proposed shed is to be located on the western side. The
shed footings are proposed to be wholly found within the exposed weathered rock (phyllite) cut area,
approximately 10 m off the crest of the existing fill batter.
Information supplied by the client indicates the shed will have a plan area of 9 m by 9 m and will comprise a
steel portal frame with light weight corrugated steel cladding on a concrete floor slab. Text Figure 2 below
illustrates the proposed shed design.
Text Figure 2: Proposed shed design (The Shed Company)
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, council overlays and aerial photographs.
A walk-over survey was conducted to make an appraisal of the general condition of the site regarding
topography, drainage, vegetation cover, geology, erosion and condition of existing structures. The
walkover was undertaken by an experienced geotechnical engineer from Tectonic on 1 March 2022.
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 26 years’ experience in
geotechnical engineering, including numerous slope stability projects.
2/10
Page 3
H.M de Groot 22072-001-Rev0
21 March 2022
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
The 1:100,000 scale Gympie Surface Geology Map (Sheet 9445) indicates that the site is underlain by early
to middle Triassic age phyllite (Rk) of the Kin Kin beds. A fault (black line) is indicated on the mapping at the
southern end of the property approximately 150 m south of the proposed shed site. An extract from the
geological map is shown in Text Figure 3.
Approximate site location
Text Figure 3: Extract from geological map
At the time of the assessment, the shed pad cutting ranged up to about 3.0 m deep and typically exposed
residual clays up to 0.3 m below ground level (BGL), overlying very low to low strength phyllite tending
towards medium strength with depth. Text Figure 4 below shows the cut batter at the time of the
assessment.
Residual clay soils
Very low to low strength
phyllite grading to
medium strength
Approx.
45°
Text Figure 4: Upslope building pad cut batter
3/10
Page 4
H.M de Groot 22072-001-Rev0
21 March 2022
2.2 Topography
As illustrated in Text Figures 5, the proposed shed site is located in the centre of the property, mid-slope and
to the south of Top Forestry Road which generally traces a local ridgeline. The allotment generally falls in a
south-easterly direction to a natural gully at the southern end of the property. Downslope to the south-east
of the proposed shed site is a dam that catches surface runoff.
The topographic features observed during the site walkover and ground slopes recorded using a hand-held
clinometer are shown in Text Figure 5. The inferred natural ground slope across the development site (prior
to earthworks) was about 12°, with slopes of 11° to 15° observed above the development area, and slopes of
12° to 25° downslope of the proposed shed site.
15° Proposed Shed
Location
Existing Residence
25°
and Workshop
Dam
12°
Text Figure 5: Image showing ground surface contours (10 m), courtesy QGlobe
Text Figures 6 to 8 below and on the following pages illustrate the general site conditions at the time of the
assessment.
Text Figure 6: Northern view across building platform
4/10
Page 5
H.M de Groot 22072-001-Rev0
21 March 2022
Text Figure 7: Looking south below proposed shed site
Text Figure 8: South-eastern view across proposed shed site
2.3 Groundwater
No seepage or ‘springs’ were observed in the vicinity of the proposed shed site.
2.4 Surface Drainage
Natural surface drainage from the shed site follows surface contours towards the south-east. The natural
slopes appeared to be relatively well drained at the time of the walk-over. Surface water is expected to
drain rapidly from the natural slopes considering the moderate to steep gradients, relatively low permeability
clay soils and shallow depth to rock.
At the time of our visit, the previously formed building pad graded slightly to the north allowing surface water
to pond on the proposed shed pad. It appeared that a small diversion swale had been installed above the
northern end of the cut batter, channelling runoff east away from the proposed shed site down into the
existing dam.
2.5 Vegetation
At the time of the inspection, the proposed development area was free from vegetation with exposed phyllite
at the surface or gravel fill across the access tracks. Dense high grass, shrub vegetation and numerous
large mature trees were observed around the shed site. Most of the trees appeared to be relatively straight,
with no obvious signs of distorted growth which could be attributed to slope instability.
5/10
Page 6
H.M de Groot 22072-001-Rev0
21 March 2022
2.6 Buildings and Other Structures
At the time of the assessment, a small brick veneer residence and a colourbond clad workshop were located
above the proposed shed site to the north. Based on an external visual assessment, the house and
workshop did not display any obvious signs of distress (e.g. cracking, misalignment, etc.) which could be
attributes to slope instability.
3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
As illustrated in Text Figure 9, NC’s landslide hazard overlay map indicates that the proposed shed site has
a Moderate (pale yellow shading) landslide susceptibility, whilst other areas across the site (typically gullies)
are mapped as having a High (orange shading) landslide susceptibility.
Proposed Shed Location
Text Figure 9: Extract from NC Landslide Hazard Mapping
It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility and
is superseded by this site specific assessment. The site did not exhibit any indicators of global, natural
slope instability at the time of our inspection. No landslide back scarps or areas of naturally ‘hummocky’
ground were observed in the vicinity of the proposed building area, with residual soil and exposed rock in
existing cut batters.
3.2 Stability Risk 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 overleaf and
the Qualitative Terminology for Use in Assessing Risk to Property is also attached.
6/10
Page 7
H.M de Groot 22072-001-Rev0
21 March 2022
Table 1: Stability Risk Levels
Risk Level Example Implications(1)
Unacceptable without treatment. Extensive detailed investigation and research, planning
Very High
VH and implementation of treatment options essential to reduce risk to Low; may be too
Risk
expensive and not practical. Work likely to cost more than value of property.
Unacceptable without treatment. Detailed investigation, planning and implementation of
H High Risk treatment options 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
Moderate
M investigation, planning and implementation of treatment options to reduce risk to Low.
Risk
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
L Low Risk
to this level, ongoing maintenance required.
Very Low
VL Acceptable. Manage by normal slope maintenance procedures.
Risk
Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
depend on the nature of the property at risk; these are only given as a general guide.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given in Section 4, it is assessed that there would be a Low (L) Risk of global slope
instability affecting the proposed shed. Regulators (NC) normally require that a Very Low or Low Risk of
landslide affecting the 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).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
Assessed
Hazard Likelihood Consequence
Risk Comments
The likelihood of a failure through the existing cut batters or fill
embankments adjacent to the building platform is considered Unlikely,
1: Failure through
given the favourable topography, lack of groundwater, lack of evidence
the existing cut
Unlikely Medium Low of existing natural instability and subject to implementation of the
batters or fill
recommendations specified in Section 4. The consequence of such a
embankments
failure would remain Medium, with the resultant risk being Low as per
AGS 2007.
2: Deep failure The likelihood of a deep failure through the weathered rock is assessed
through weathered as Rare due to the strength of such materials, and no evidence of deep-
Rare Major Low
rock below the seated instability in the area. Although the consequence of such a failure
foundation depth could be Major, the resultant risk would be Low as per AGS 2007.
The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts.
7/10
Page 8
H.M de Groot 22072-001-Rev0
21 March 2022
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 Earthworks
Based on the proposed development and the existing site conditions it is anticipated minimal cut and fill
earthworks will be required (i.e. less than 0.5 m depth/height). Should proposed cut/fill exceed 0.5 m
depth/height, the impacts on slope stability must be reviewed by Tectonic before construction.
The shed pad should be proof rolled to identify any soft spots, which should be removed and replaced with
select, compacted fill. The pad surface should be re-graded at a minimum 1V:100H ‘into’ the hillside (north)
to prevent surface water overtopping the fill embankment on the southern side.
Excavations in the natural soils and very low to low strength phyllite should be achievable using conventional
earthmoving equipment (e.g. small to medium sized excavators in the 5 tonne to 12 tonne range). However,
excavations into medium strength (or stronger) phyllite may be difficult and may require the use of ripping
tynes and/or rock hammer attachments. Where excavation into rock is required the contractor must consider
the effects of noise and vibrations.
Temporary batters in stiff (or stiffer) natural soils may be formed at 1V:1H; with permanent batters formed no
steeper than 1V:2H in natural soils or 1V:1H in weathered rock. Flatter batters may be required if
groundwater seepage is encountered. Permanent batters will require erosion protection such as covering
with erosion matting or vegetation. Where spatial constraints prevent adoption of the above recommended
batter slopes, cut or fill batters would need to be supported by retaining walls.
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 any further 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).
4.2 Retaining Structures
Retaining structures must be founded as described in Section 4.3 below. Retaining structures greater than
1 m high will require engineer design and certification of construction. Retaining structures should be
engineer designed in accordance with AS4678-2002 Earth-retaining structures.
We suggest the following parameters in Table 3 may be adopted for retaining wall design.
Table 3: Retaining Wall Design Parameters
Lateral earth pressure coefficients
Unit Friction
Retained material weight angle
Ka Ko
(kN/m3) (Degrees) Kp
(Cantilever wall) (Non-yielding wall)
Stiff (or stiffer)
19 25 0.40 0.57 2.46
Clays
Weathered Rock 21 35 0.27 0.43 3.69
The values in Table 3 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.
8/10
Page 9
H.M de Groot 22072-001-Rev0
21 March 2022
4.3 Footing Design
Footings for the proposed shed and any ancillary structures (e.g. tanks or retaining walls) should penetrate
through any fill, and residual soils, to found a minimum of 0.3 m into very low strength (or stronger) rock.
Footings so founded may be designed using an allowable bearing pressure of 400 kPa. It is understood that
the shed slab and footings will found directly on/into such rock.
Footing design should also account for potential soil reactivity, to be assessed by others.
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 building loads. 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, as
illustrated in Text Figure 10.
Text Figure 10: Footing depth required to prevent undermining or surcharge loading
Footing bases must be cleaned following mechanical excavation to expose undisturbed materials over the
full base area. A suitably qualified and experienced RPEQ must 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 above, Tectonic
must be notified immediately and an inspection carried out to determine if changes to footing design are
required.
4.4 Drainage
Adequate site drainage must be installed to ensure that surface water flows are controlled, and diverted
away from the building area and batters. Flows must not be allowed to form concentrated streams running
across the building platform, adjacent batter slopes or over top retaining walls. Lined (with turf, rock,
concrete, or similar) cut off/diversion drains should be constructed along the crests and toes of the existing
batters and directed via lined batter chute drains or pipes to discharge at least 5 m down slope of any
9/10
Page 10
H.M de Groot 22072-001-Rev0
21 March 2022
structures or fill batters and not as concentrated discharge upslope of any neighboring
structures/embankments. The platform should be graded slightly (minimum 1V:100H) back ‘into’ the
exposed rock batter slope to drain appropriately to the diversion drainage system and not allow over topping
of the downslope fill batters.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.
It would be prudent to construct a rock dissipator or rock contour drain at the drainage outlet point(s) to
minimise erosion. Drains should be directed to discharge towards the natural drainage lines.
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).
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 shed, subject to implementation of the recommendations given in this report.
6.0 LIMITATIONS
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
Jarrad McVey BEng (Civil) (Hons) Ashley Davey RPEQ 8159
Geotechnical Engineer Director/Principal Geotechnical Engineer
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)