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20013-001-Rev0 Flooded Gum Pl - geotech report
4 February 2020 Project No. 20013-001-Rev0 Mr Andrew Jensen 28 Diggings Road Imbil QLD 4570 Email: [email protected] SLOPE STABILITY ASSESSMENT – RECENT EARTHWORKS & PROPOSED RESIDENCE 31 FLOODED GUM PLACE, BLACK MOUNTAIN Dear Andrew, 1.0 INTRODUCTION As requested, Tectonic has undertaken a slope stability assessment of recent earthworks and for a proposed residence at 31 Flooded Gum Place, Black Mountain. This report presents the results of our slope stability assessment, together with geotechnical advice for the proposed project. The report has been prepared to address Noosa Council’s (NC) Breach Notice (ref. COM20/008 dated 9 January 2020) supporting an application for a Development Permit for Operational Works and to provide advice for development of the future residence from a geotechnical viewpoint. 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 14 SP161828 and has frontage to the cul-de-sac of Flooded Gum Place to the north. To the south and east there are similar rural properties. There is bushland to the west and to the north, beyond Flooded Gum Place, are the banks of a gully line and dam. The proposed building site is located towards the south-eastern boundary of the property. Text Figure 1 illustrates the ground conditions of the site and surrounds. A detailed description of the site is given in Section 2 Site Boundary Indicative proposed building site Text Figure 1: 2017 Aerial image of site and surrounds (courtesy NC IntraMaps) 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
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Mr Andrew Jensen 20013-001-Rev0
4 February 2020
Earthworks have recently been carried out to form a building pad in the south-eastern part of the site. The
building pad is approximately 25 m wide and 50 m long. We are advised that the future building works will
comprise construction of a retaining wall less than 1 m high to support the lower part of the cutting along the
southern side of the building pad. A conventional residence will be built, likely single or double storey with a
slab on the ground floor.
1.2 Method and Scope of Investigation
As part of our geotechnical 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 by an experienced geotechnical engineer from Tectonic on 17 January
2020, to make an appraisal of the general condition of the site regarding topography, drainage, earthworks,
vegetation cover, geology, erosion, and slope stability. The results of the desk-top study and walkover
survey are included in Section 2 below.
1.3 Qualifications of Responsible Engineer
This report has been prepared by Mr. David Pollock, an RPEQ with more than 14 years’ experience in
geotechnical engineering, including numerous slope stability projects.
2.0 DESCRIPTION OF EXISTING CONDITIONS
2.1 Geology
The published geological map for the region 1 indicates that the underlying geology comprises the early to
middle Triassic age Kin Kin Beds (Ref. Text Figure 3), noted to comprise “arenite, argillite and phyllite.”
Approximate Site Location
Text Figure 3: Extract from Gympie Special Geology Map
The cutting for the building pad exposed a shallow cover of about 1 m depth of residual soil comprising very
stiff, gravelly clay which graded to highly weathered phyllite of very low strength. The rock was noted have a
sub-vertical, well developed fabric (Ref. Text Figure 4).
1
1:100,000 Gympie Special Geology Map, Sheet 9445, DME, 1999 (1:100, 000)
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Text Figure 4: Exposed phyllite rock in cutting on southern side of building pad
In the central part of the cutting, there appeared to be an altered zone which was inferred to be associated
with the intrusion of a dyke. The reisdual soil in this area presents as a finer grained, silty clay of very stiff
consistency with the rock noted to be in places extremely weathered (ref. Text Figure 5).
Dyke altered zone
Text Figure 5: Altered zone where dyke appears to have penetrated into phyllite rock in cutting
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No groundwater was observed on site during the site walk-over assessment.
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 inclinometer.
As illustrated in Text Figure 5 below, the site generally slopes down to the north at inclinations of between
11° and 18°, with the most gently sloping terrain located in the south-eastern part where the ground surface
likely sloped at about 5° to 8° to south (prior to recent earthworks to form the building pad). A ridge with a
relatively level crest runs east to west above the southern boundary of the site.
There is a broad central drainage line which propagates as a generally linear, grassed, surface depression
(as shown in Text Figure 6).
Earthworks have recently been carried out to form a building pad in the south-eastern part of the site where
natural ground slopes were the flattest. The building pad was approximately 25 m wide and 50 m long. The
southern side of the pad was formed in cut up to about 3 m deep with fill estimated to be up to about 2.5 m
high on the northern side. The lower part of the cut was formed at about 70° and will be supported by a
retaining wall 0.9 m in height, with the upper part of the cutting laid back at 20° (ref. Text Figure 5 above).
The fill batter along the building pad northern edge is formed at about 1V:2H. The fill batter has been re-
surfaced for erosion protection with topsoil and grass cuttings presumably striped from the site prior to cut/fill
operations. We are advised that earthworks were conducted with a Catepillar D6 Dozer, with fill being
entirely sourced from on site cuts and tracked rolled in 200 mm thick layers.
The driveway from Flooded Gum Place is located on the eastern side of the central drainage line and is
formed with relatively minor earthworks less than 1 m in height/depth. At the top of the driveway an
embankment had been formed providing access over the head of the broad central drainage line to the
western part of the site. The fill there appeared to be up to about 2 m in height and was battered at 30° to
35°.
Surface depression associated with
broad central drainage line
Ridgeline above southern boundary
Text Figure 6: Image showing ground surface contours (courtesy NC IntraMaps)
General site conditions at the time of the walkover are recorded in Text Figures 7 to 9 on the following
pages.
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Building pad fill batter at 1V:2H
Steeper fill in driveway embankment
over head of central drainage line
Text Figure 7: Looking east across to building pad in southern part of site
Building pad fill batter at 1V:2H with
topsoil and grass cuttings coverage
Text Figure 8: Looking north-west over building pad fill embankment
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Text Figure 9: Looking south up driveway to building pad above
2.3 Groundwater
No seepage or ‘springs’ were observed in the vicinity of the proposed house.
2.4 Surface Drainage
The proposed house site has a very small stormwater catchment, with slopes falling to the north. Natural
surface drainage follows surface contours, mostly to the north. The proposed building area appeared to be
relatively well drained at the time of the walk-over. Surface water is expected to drain rapidly from the site
considering the moderate gradients, relatively low permeability clay soils and shallow depth to rock.
2.5 Vegetation
The site was generally clear and open with grass extending down slope, and occasional trees and shrubs.
Trees were not observed to be unusually bent or tilted.
2.6 Buildings and Other Structures
There were no structures on the site at the time of walkover. Houses on adjacent blocks to the east had
been constructed at a similar point in the topography (i.e. on cut/fill building pads formed in the northern
facing slope).
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3.0 ASSESSMENT OF LAND STABILITY
3.1 Existing Conditions
An extract from the NC Natural Hazards Overlay is shown below (Ref. extract in Text Figure 10). The
mapping indicates that much of the allotment is classified as being a “Landslide Hazard Area”, although not
the northern and southern parts including the area where building works are proposed.
Text Figure 10: Extract from Noosa Council Plan Natural Hazards Overlay Mapping
It is noted that this designation has been assigned at a coarse resolution, and largely records that the ridge
at the south of the site is not classified as a “Landslide Hazard Area”. The mapping is a broad-scale
indication of the potential landslide susceptibility only and is superseded by site specific investigation and
analysis works.
At the time of our visit, the ground did not exhibit any significant recent or historic indicators of slope
instability. No landslide back scarps, groundwater springs, or tension cracks were observed.
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 also attached.
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.
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Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given in Section 4, it is our opinion that there is a Low (L) Risk of global slope instability
affecting the proposed residence. 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 below
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 residual soils in the vicinity of
1: Shallow failure the proposed residence is assessed as Unlikely due to the
through residual soils moderate natural gradients, shallow depth to rockhead, apparent
above foundation Unlikely Minor Low lack of groundwater, and topographic position near the crest of
depths the ridge. The consequence of such a failure would be Minor
considering the anticipated limited effects of such shallow
instability, with the resultant risk being Low as per AGS 2007.
The likelihood of a deep failure through the weathered rock is
2: Deep failure assessed as Barely Credible due to the strength, and shallow
through weathered occurrence of this material (and shallow transition to very low
rock below the Barely
Major Very Low strength rock expected), moderate ground slopes, and no evidence
foundation depth Credible
of such instability in the area. Although the consequence of such a
failure could be Major, the resultant risk is Very Low as per AGS
2007.
The likelihood of a failure through embankment fill in the vicinity of
3: Embankment the proposed residence is assessed as Unlikely due to the relatively
failure through fill near flat batter slopes (1V:2H), the quality of the fill materials used
the residence Unlikely Minor Low (gravelly clay), apparent lack of groundwater, and topographic
position at the crest of the ridge. The consequence of such a failure
would be Medium considering the anticipated effects of instability,
with the resultant risk being 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.
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 Geotechnical Constraints
The results of our assessment indicate that construction should be avoided along central drainage line/
surface depression; and otherwise where slopes exceed 15° (26 %).
Construction should therefore be focused on the ridge on the southern boundary. Observations made on site
indicate that shallow rock is present in this area, at approximately 1 m depth (from the top of the ridge).
Both low-set and high-set type construction should be possible, noting that footings must penetrate through
any fill or natural soils to socket at least 300 mm into the underlying weathered rock.
The embankment located on the western side at the top of the driveway needs to be re-profiled to 1V:2H. It
is noted that the fill here appears to cut off surface water flows from the slope above so a culvert or some
form of adequate diversion drainage system should be developed to enable surface water flows to be
directed around or under the embankment.
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4.2 Earthworks
No further cut or filling should be carried out on site unless reviewed by Tectonic.
Permanent batter slopes should be constructed at a maximum grade of 1V:2H in natural soil, fill and
weathered rock (may be steeper in rock subject to inspection and confirmation by Tectonic). Permanent soil
or fill batters will require erosion protection (e.g. revegetation or surface protection).
Steeper batters such as the embankment on the western side of the top of the driveway should be re-profiled
at 1V:2H, or alternatively should be retained by engineer designed retaining walls (ref. Section 4.3 below).
4.3 Retaining Structures
Retaining structures must be founded as described in Section 4.4 below. Retaining structures greater than
1 m high will require engineer design and certification of construction.
We suggest the following parameters 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)
Gravelly Silty Clays 19 25 0.40 0.57 2.46
Weathered Rock 21 36 0.26 0.41 3.85
Fill * * * * *
*Depends on type of fill used, and level/quality of compaction
These parameters do not include allowance for surcharge above the wall, or additional loads imposed by
sloping ground.
4.4 Footing Design
Footings should penetrate through any fill, and residual soils, to found a minimum of 300 mm into weathered
rock. Footings so founded may be designed using an allowable bearing capacity of 250 kPa. Where
weathered rock is exposed in the southern, cut side of the building pad it may be possible to use normally
dimensioned high level footings to achieve this. However, to penetrate through the deeper fill on the northern
side of the building pad deeper footings such as bored piers or similar will be required.
The above footing advice would also apply to future construction adjacent to the house (e.g. a swimming
pool).
All footings should found such that they are not adversely affected by any adjacent excavations, batter
slopes, 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/batter slopes/excavations/retaining
walls, then rising up at 1V:1H, as illustrated in Text Figure 11 overleaf.
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Text Figure 11: 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 engineer should inspect footing excavations to confirm
adequate foundation materials are encountered.
4.5 Drainage
Adequate site drainage must be installed to ensure that runoff is directed away from the building walls and
footings (e.g. swales, mounds, grated drains).
Subsurface drainage must be installed behind any retaining walls in order to prevent the development of
hydrostatic pressure (e.g. slotted ‘aggi’ pipe wrapped in filter ‘sock’ placed in gravel backfill).
All stormwater collected around the residence (including overflow from rainwater tanks) must be directed by
pipes or lined channels to discharge at a point(s) at least 10 m down slope of any structures or fill batters,
preferably into the natural drainage line/gullies present on site. It would be prudent to construct a rock
dissipator or rock contour drain at the drainage outlet point(s) to minimise erosion.
The embankment located on the western side at the top of the driveway is noted to cut off surface water
flows from the slope above so a culvert or some form of adequate diversion drainage system should be
developed to enable surface water flows to be directed around or under the embankment.
Subsurface disposal of stormwater must not be undertaken.
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5.0 SUMMARY & CONCLUSIONS
Based on the findings of our assessment we consider that there would be a Low Risk of slope instability
subject to implementation of the recommendations given in this report, in particular:
The proposed residence is to be supported on footings founded at least 300 mm in to weathered
rock.
Diversion drains are established above cut and fill batters
Ensuring that stormwater collected by drainage, including any overflow from tanks, is directed to
discharge at least 10 m down slope of any fill and structure.
All cut/fill is battered no steeper than 1V:2H or supported by retaining walls.
Vegetation must be established and maintained to prevent erosion of currently bare soils.
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
David Pollock BE(Civil) RPEQ14121
Senior Geotechnical Engineer
Attachments: Qualitative Terminology for Use in Assessing Risk to Property
Some Guidelines for Hillside Construction
Limitations
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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 13
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 14
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
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PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
114 Australian Geomechanics Vol 42 No 1 March 2007
Form No. 68029 RL2 August 2010
Page 16
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)