MCU14/0024 - Response to Information Request - Attachment 3

Page 2

Vegetations : Vegetation consists of grasses & scattered trees.

Slope Shape/Location : The surface profile across the allotments slope faces are generally

LINEAR PLANAR.

Drainage Fi The surface drainage is good, sub-surface drainage is good. There was
no indication of ground water seepage or rising water pressure.

Stormwater flows from upstope of the allotment will be intersected and
captured by the road reserve.

Earthworks : There is no existing earthworks apparent.

Adjacent Structures: Adjacent structures on the neighbouring properties appear to be a mix of
slab on ground and suspended timber framed floors.

Evidence of previous

Soil Movements é Surface cracks

LANDSLIP MECHANISMS

Landslip is generally defined as the movement of a mass of rock or soil down a slope.

It is generally considered to occur when the strength of the soil or rock mass to support itself is exceeded
causing movement to occur within the mass.

This loss of strength can occur as a result of numerous circumstances. These include, but are not
limited to the following:-

 

-  Siesmic Activity

-  Erosions effects such as wind or fluid scour

- Soil Saturation from rainfall events or defective drainage measures
-  Construction/Earthworks activities

- Groundwater variations/movements

- Previous Landslips

The nature of landslips vary with site specific conditions and failure mechanisms that cause the slips to
occur. Appendix B provides some diagrammatic examples of various landslips.

The speed of landslips can vary greatly from millimetres per year to a number of meters in a matter of
minutes. Once again this characteristic is governed by site specific conditions and the failure
mechanism causing the slip to occur.

The allotment is situated within a massive aeolian sand bed of Quaternary age. As a general observation
dunal horizons such as these are stable where;

- The natural slope is less than the angle of repose for sand (30°)
- Vegetation exists to protect from wind generated scour
- The drainage pattern is such that there is no risk of fluvial scour.
These horizons can become unstable when aeolian or fluvial scour erode slopes to angles greater than
30°, particularly when coincident with vegetation removal.
LANDSLIP HAZARD ASSESSMENT
For our assessment we utilize the Guidelines For Landslip Risk Management Framework as provided by

the Australian Geomechanics Society, in their journal of March 2007. Descriptions used by this
framework can be found appended to this report.

SCG Consulting Engineers 2 SCG Ref: 23277(SL)

Page 3

Using the Qualitive Risk Analysis Matrix (refer appendix) we have established the following
characteristics for the allotment.

Landslip Mechanism : Translational, Rotational or Creep Flow
We consider a potential landslip mechanism will consist of either a Rotational or Translational
Landslide or Creep movement within the sloping profile. Diagramation representation of these
mechanisms are appended to this report. We are of the opinion that this movement could occur
following over excavation or scour.
Likelihood of Landslip ‘ Likely
This is due to the evidence during our site visit which the surface grade within the allotment.
Consequences to Property i Minor
With these factors considered we classify the allotment as:-

MODERATE RISK OF LANDSLIP

Although we consider the allotment has a moderate risk of landslip, building work undertaken on the site
should follow accepted methods which assist in maintaining slope stability.

Such measures are discussed in the following section.

HAZARD MITAGATION — CONSTRUCTION RECOMMENDATIONS

We consider the primary risks for landslip occurring within this allotment is over excavation or,
stormwater scour and vegetation removal.

The construction of any cut/fill platforms within the site will lead to an increased risk of landslip.

Any over excavated (steep) cuts may lead to landslip. Similarly, steep fill batters may be susceptible to
slip or erosion.

Any exposed fill batters are susceptible to weakening from saturating rains and stormwater scour. This
risk is compounded, where poor compaction is achieved when placing the fill. Any fill batter profiles
should be limited to a maximum grade of 1 to 3 (vertical to horizontal). Where possible the fill platform
should be benched into the natural profile after the removal of all organic material i
Surface stormwater runoff should be diverted around fill batters. Vegetating the fill batter can improve
the batters ability to resist slip and or scour. Where these recommendations are unachievable,
benching, shoring or retaining measures should be implemented.

 

Any permanent exposed cut batters should be limited to a maximum slope of 1 to 2.5 (vertical to
horizontal). The face should be protected from stormwater scour with cut off drains above the cut. Catch
drains should also be provided along the base of the cut. The drains must dissipate all stormwater from
above and below the cut. Vegetating the cut batter can improve the batters ability to resist slip and scour.
Steeper batters should be retained with engineer designed retaining walls.

All excavations both temporary and permanent must be adequately battered or retained to prevent
collapse. Temporary excavations for retaining walls can have exposed cut batters of 1 to 1.7 (vertical to
horizontal) taken from the base of the excavation. These earthworks must incorporate shallow surface
spoon drains at the base and along the high side of the cut. Drains must have an efficient outflow down-
slope and clear of the construction area. Any exposed soil face should also be protected against the
potential for stormwater erosion. Saturation of the exposed cut batter from prolonged rainfall will
potentially de-stabilise the cut batter. Programming of the works must consider this possibility.
Temporary backfilling of the slope may be required.Should these recommendations be un-achievable the

 

 

SCG Consulting Engineers 3 SCG Ref: 23277(SL)

Page 4

builder should seek guidance from this office on temporary shoring, benching or other suitable
techniques.

The allotment will support a typical form of residential construction. Structural foundations should be
designed by a competent person in accordance with AS 2870, accepted engineering theory and the
geotechnical conditions reported by this office.

All earthworks should be constructed in accordance with AS3798-1996 ‘Guidelines on earthworks for
commercial and residential developments’ and sound engineering theory.

The builder must ensure the stability of neighbouring structures is maintained through not
undermining their foundations. This is achieved via restricting any earthworks to above a line
radiating downwards at an angle of 1 in 1.5 (vertical to horizontal) from the base of the
foundation/structure to be protected.

The proposed method of construction is for a blockwork dwelling supported on ground slabs. The
allotment is suitable for this form of construction, any excavations for lower floor areas must adhere to
the recommendations contained herein.

Retaining walls for landscaping purposes would need to be constructed with consideration of maintaining
stability of the surrounding ground during construction. Such measures would include limiting the slope of
any excavations to a slope of 1 vertical to 1.7 horizontal and protecting the works against stormwater
scour. The retaining wali designs must allow for the lateral soil pressures expected including additional
surcharging from sloping backfills.

Although we consider the allotment stable in its current form there will always be a risk of landslip under
severe stormwater conditions.

Measures that will reduce this risk include the following but are not limited to.

The finished ground surface should be leveled and graded to ensure no concentration of stormwater
runoff leading to ground scour can occur.

Any collected stormwater from areas other than the roof should be disposed of downslope from the
proposed dwelling and should also ensure no ground scour can occur.

All stormwater runoff from hard landscaped areas should be directed around the perimeter of the house
and contained within the site via diversion bunding, scour proof drains and field inlets.

All stormwater collected from roofed areas should be directed to storage tanks for re-use. Overflow from
the tanks should be disposed of via rubble soakage pits downslope of the dwelling.

No stormwater runoff should be allowed to flow over any exposed excavated embankment faces or filled
batters.

Areas adjacent existing gullies must be protected against stormwater scour are susceptibie to instability
from erosion effects and over saturation. With this considered, structures should not be located adjacent
s. Also, the area adjacent the flanks of the gullies should be protected from scour via vegetation
and limiting surface grades.

 

Alt existing cross allotment flows from neighbouring properties must also be considered, managed and
maintained. Clear unhindered drainage paths must be provided and maintained during and beyond
construction. All constructed retaining walls must have effective drainage measures upstope and beyond
the walls to relieve any sub-surface water pressure/build-up. These drains must be maintained and
flushed out regularly. To achieve this we recommend bringing the upstream end of the drain to surface
and capping off to allow future flush out.

We have attached guidelines for hillside construction extracted from Australian Geomechanics vol. 35

March 2000 — “Landslide Risk Management”. These references provide commentary on good design
and building practice.

SCG Consulting Engineers 4 SCG Ref: 23277(SL)

Page 5

 

Sean’Richardson RPEQ 5835
For and on behalf of
SCG CONSULTING ENGINEERS

Encs Appendix A -
Appendix B -
Appendix C -

 

SCG Consulting Engineers

Some Guidelines for Hillside Construction
Landslip Mechanisms
Landslip Risk Assessment Matrix

SCG Ref:_23277(SL)

Page 6

 

Rotational landslide Translational landslide Block slide

 

i

fn ctay

tn oe
ee weer es
aed oes

Lateral spread

 

Appendix B

Page 7

LANDSLIDE RISK MANAGEMENT

AGS SUB-COMMITTEE

SOME GUIDELINES FOR HILLSIDE CONSTRUCTION

GOOD ENGINEERING PRACTICE

POOR ENGINEERING PRACTICE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 

 

 

 

ADVICE
GEOTECHNICAL Obtain advice from a qualified, experienced geotechnical consultant at early Prepare detailed plan and start site works before
ASSESSMENT. sage 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.
atising from the identified hazards and consequences in mind.
DESIGN AND CONSTRUCTION
HOUSE DESIGN Use flexible structures which incorporate properly designed brickwork, timber | Floor plans which require extensive cutting and
of steel frames, timber or panel cladding. filling.
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, Indiscriminant bulk earthworks.
Cuts | Minimise depth, Large scale cuts and benching.
Support with engineered retaining walls or batter to appropriate slope. Unsupported cuts.
Provide drainage measures and erosion control. Ignore drainage requirements
FILLs | 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.
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 ete i
ROCK OUTCROPS | Remove or stabilise boulders which may have unacceptable risk, Disturb or undercut detached blocks or
& BOULDERS | Support rock faces where necessary. boulders.
RETAINING Engineer design to resist applied soil and water forces, Construct a structurally inadequate wall such as
WALLS Found on rock where practicable. sandstone flagging, brick or unreinforced
Provide subsurface drainage within wall backfill and surface drainage on slope | blockwork.
above. Lack of subsurface drains and weepholes.
Construct wall as soon as possible after cut/fill operation.
FOOTINGS 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,
Design for lateral creep pressures if necessary.
Backfill footing excavations to exclude ingress of surface water.
SWIMMING POOLS | Engineer designed.
Support on piers to rock where practicable.
Provide with under-drainage and gravity drain outlet where practicable.
Design for high soil pressures which may develop on uph.
may be little or no lateral support on downhill side.
DRAINAGE . z
SURFACE | 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.
Provide general falls to prevent blockage by ion and incorporate silt traps.
i inimi: tion and make flexible where possible.
Special structures to dissipate energy at changes of slope and/or direction.
SUBSURFACE | Provide filter around subsurface drain. Discharge roof runoff into absorption trenches.
Provide drain behind retaining walls.
‘Use flexible pipelines with access for maintenance.
Prevent inflow of surface water.
Septic & | Usually requires pump-out or mains sewer systems, absorption trenches may | Discharge sullage directly onto and into slopes,
SULLAGE | bepossible in some areas if risk is acceptable. Use absorption trenches without consideration
Storage tanks should be water-tight and adequately founded. of landslide risk.
EROSION Control erosion as this may lead to instability. Faifure to observe earthworks and drainage
CONTROL & Revegetate cleared area, recommendations when landscaping.
LANDSCAPING
DRAWINGS AND SITEVISITS 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.
Ifseepage observed, determine causes or seek advice on consequences.

 

 

 

 

 

Australian Geomechanics ~ March 2000 94

Page 8

LANDSLIDE RISK MANAGEMENT AGS SUB-COMMITTEE

 

EXAMPLES OF GOOD HILLSIDE PRACTICE

                 

ROOF WATER

PIPED OFF SITE ig

£5 — MANTLE oF son

 

CUT & Fit
& ROCK FRAGMENTS
PAMIMISED
VEGETATION ICOLLUVII
INTO ROCK

SUBSON ORAWAGE MAY
BE REQUIRED INSLOPE

SEWERAGE EFFLUENT PUMPED-GUT
OR CONNECTED TO SEWER.
TANKS ADEQUATELY FOUNDED

 

GINEERED RETAINING WALLS
WITH SURFACE & SUBSURFACE
DRAINAGE CONSTRUCTED
BEFORE HOUSE

 

EXAMPLES OF POOR HILLSIDE PRACTICE

UNSTABILISED ROCK [DPPLES

 

 

SLOPE £ ACTIVATES SUP

POSSIBLE TRAVEL DOWNSLOPE WHITH
MPACTS OTHER DEVELOPMENT

 

 

 

Figure J1 Illustrations of Good and Poor Hillside Practice

92 Australian Geomechanics - March 2000

Page 9

APPENDIX C; LANDSLIDE RISK ASSESSMENT

OUALITATIVE MEASURES OF LIKELIHOOD

PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007

QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY

 

Appretimate Aneual Probability

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

   
 

 

  

 

Implied Indicative Landslide a :
Indicative Notional Recurrence Interval Pa ennai =
Value Boundary
107 5x10? 10 years ae The avemt is expected to occur over the dezign life ALMOST CERTAIN A
10? - 100 years ea toaeT The event will probably occur under adverse condoms over the [yp peeyy 3
. = design bie =a
oo are rs
10” oo : 1000 years = pi The evemt could occur under adverse condition: over the design life | POSSIBLE Cc
10+ 5210 10.000 year: The event might occur under very adverse cocumetances over the | 5.2, + n
a aetna 20,000 year; [Seen ee a :
10° 5210 a ae or tees The event js conceivable Dut only under excephonal cincumstance: =
100,000 vears gis tibdsicn He RARE E
° 0.020 vears + = — -- =
107 =e Toooood yams | 22M8 vers [The event is meoncelvable of fauciful over the design life. BARELY CREDIBLE F
Note: (1) The table should be used from left vo nghr, use Approwimate Acmual Probability or Description to assign Descriptor, not vice versa
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
elisa Notional Description Descriptor Level
Value Boundary
200% sarucsara(s) completely destroyed and or large scale damage requiring major enzineering works for CATASTROPHIC L
a 100" tian. Could cause at least one adjacent property major consequence damage Se aet errs |
60% al ve damage to mos of structure, and or extending deyond ste boundanes requimng sigmficant MAJOR a
ia stabilisation works. Could caase at least one adjacent property medium consequence damage. 7
% eae Moderate damage wo tome of sracture, and or significant part of site requiring large stabilisation works x
20% MEDIUM 3
ie Could cause at least one adiacent property minor consequence damage.
3 " Lamited damage to part of structure. and or part of site requ:ring some reinstatement stavikseton works MINOR, 4
os 7 Little damage Qvote for high prosatility event (Almost Certain), this category may be subdivided at a INSIGNIFICANT -
oe notions! Doundary of 0.1%. See Risk Matrix.) oe ee m
Notes: (2) The Approximate Cost of Damage is expressed a5 2 percentage of market value, being the cost of he improved value of the unaffected propery which inchides the land plus the
unafected structures.
G) wouiniate Cost is to be an estimate of the direct cost of the damage. such a: the cost of reinstatement of the damaged pordon of the property (land plus structures), szabiluanen
noted to tender the site to tolerable risk level for the landslide which has occurred and prowessional design fees, and consequential costs iuch as jezal fees, temporary
omemodation. it does not mchude addtronal sabiksation works to address other landslide: whach may affect the property.
Sy table should be weed from left w might: use Approximate Cost of Damage or Descnpcion to assign Descriptor, not vice versa
ot Australian Geomecmanics Vel 42 No 1 March 2007

 

Page 10

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 C ONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damags)
Indicative Value of | 1: CATASTROPHIC T MAJOR © MEDIUM + MINOR
Approximate Annnal 200% SO 2 ae INSIGNIFICANT
Probability os
A - ALMOST CERTAIN ir! H Mor L (5)
B- LIKELY lie ESne s L
C - POSSIBLE we AP M M VL
D - UNLIKELY it pecs M L it VL
E - RARE iw M o L VL VL
F - BARELY CREDIBLE 1g? L VL ¥L VL VL

 

 

 

 

 

 

 

 

 

Notes: (2) For Cell ‘ike may des ames such ch that a consequence ee Risk

& an

 

RISK LEVEL IMPLICATIONS

 

Rick Level Example Implications (7)
Unaccepable without reamert. Extensive dewiled investigation and research, planting and implementation of reamment

‘option: essential te reduce risk to Low, may be too expensive and not practical Work likely to cost more than value of the

   

 

 

 

 

 

 

 

 

 

 

et sed
H . HIGH ESE : Cnaccapable without eamment Detaled invesdgation, planaog and implementation of ieatment opnons required to reduce
= . tisk to Low, Work would cost a sudstamtial sum in relation to the value of the propery.
Mar De tolerated in certs circumstances (object to regulator’: approval) bur require: invevtigation, plancine and
M MODERATE RISK implementation of razment options to reduce the risk to Low. Treatment options to raduce to Low msi should be
implemented a: 3000 a3 practicable
L LOW RISK Usually acceprable to regulators. Where treatment has bean requited to reduce the nek to this level, ongoing manmtenance is
ee i ——
VL VERY LOW RISE Acceptable. Manage by aormal slope maiotenance procedures.
Note: (7) The anplications for 2 particular sinuation are to be determined by all parties to the risk assessment and may degend on the rature of che property at risk, these are only

20ve0 a5 a general made