LANDSLIDE HAZARD ASSESSMENT - Kingborough Council · Interim Planning Scheme Overlay – Landslide...

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LANDSLIDE HAZARD ASSESSMENT

Adventure Bay Lodge

1005 Adventure Bay Road

Bruny Island

September 2018

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omissions. The author shall not in any way be liable for any loss, damage or injury suffered by the User

consequent upon, or incidental to, the existence of errors in the information.

lbromley

Stamp

© Geo-Environmental Solutions Pty Ltd

Introduction

Client: Susan Kay and Jonathon Jones

Date of inspection: 06/09/18

Location: 1005 Adventure Bay Road, Bruny Island (CT 102713/1 and CT 102713/2)

Land description: Approx. 1.6 ha rural residential site

Building type: Visitor Accommodation facilities

Investigation: Visual Inspection

Inspected by: J. P. Cummings

Background information

Map: Mineral Resources Tasmania 1:50,000 Dover Sheet

Rock type: Jurassic Dolerite with granophyre (Map Unit: Jdl)

Soil depth: Variable 1.5 m +

Planning Overlay: Landslide Hazard Area, Biodiversity Protection Area and Waterway and

Coastal Protection Area

Local meteorology: Annual rainfall approximately 715 mm

Local services: None

Site conditions

Slope and aspect: 17° above proposed roadway and 13° over development site area dipping to

the west-northwest.

Site drainage: Well drained

Vegetation: Native grass and bushland

Weather conditions: Dry, approx. 8mm rainfall received in preceding 7 days.

Ground surface: Moist sandy loam surface conditions

Investigation

Geo-Environmental Solutions Pty Ltd (GES) have been engaged by Susan Kay and Jonathan Jones

to carry out a landslide hazard assessment of the site at cadastral title (CT 102713/1 and CT

102713/2), located at 1005 Adventure Bay Road, Adventure Bay on Bruny Island as requested by

Kingborough Council. A site inspection was carried out by GES identify the distribution of, and

variation in soil materials on the site.


Previous site and soil evaluation reporting has been carried out at the site by ‘Richard Mason, Onsite

Assessment Tasmania’, which identified the following soil profiles:

Profile Summary

Hole 1

Depth (m)

Horizon Description

0.0 – 0.45 A2 Sandy loam, dusky yellowish brown 10YR 2/2; moist,

moderate structure, forms 10-15mm ribbon;

0.45– 1.15 B5 Sandy CLAY, greyish red 10R 4/2; damp, moderate

structure, forms 65mm ribbon;

1.15 – 1.50 + CR2/3 Clayey gravelly SAND, pale brown 5YR 5/2; damp,

moderately structured, forms 20-25mm ribbon.

Soil Profile Notes

Based on the above soil profiles, it is likely below 1.15 m, soils represent residually weathered

Dolerite bedrock.

Australian Building Code Board

This report presents a summary of the overall site risk to landslide hazards. This assessment has been

conducted for the year 2068 which is representative of a ‘normal’ 50-year building design life

category.

Per the Australian Building Code Board (ABCB 2015), when addressing building minimum design

life:

‘The design life of buildings should be taken as ‘Normal” for all building importance

categories unless otherwise stated.’

As per Table 3-1, the building design life is 50 years for a normal building.


The Tasmanian Building Regulations 2016

Building in hazardous areas

As outlined in the Department of Justice web site:

Hazardous areas include areas which are bushfire prone, comprise reactive soils or substances, or are

subject to coastal erosion, coastal flooding, riverine flooding, and landslip.

Division 5 - - Landslip. Section 59. Landslip hazard areas

(1) For the purposes of the Act, land is a landslip hazard area if –

a. the land is shown on a planning scheme overlay map as being land that is within a

landslip hazard area; and

b. the land is classified as land within a hazard band of a landslip hazard area.

(2) For the purposes of the definition of hazardous area in section 4(1) of the Act –

a. classification under a landslip determination as being land that is within a hazard band

of a landslip hazard area is a prescribed attribute; and

b. a landslip hazard area is a hazardous area.

Interim Planning Scheme Overlay – Landslide Hazard

The proposed site is within the low and medium landslide overlay (Figure 1). The majority of the

proposed lodge is located within a landside low hazard zone. The entire studio and

driveway/carparking area is also located in a landslide low hazard zone.

Figure 1 Landslide Overlay near the Site (The LIST)

LANDSLIDE LOW HAZARD

LANDSLIDE MEDIUM HAZARD


Site and Proposed Works

The site is located on a west-north western sloping coastal hillside, dipping by approximately 13° at

the proposed location of the lodge and the studio, which increases in steepness to approximately 17°

upslope of the proposed lodge (to the east) and to the south of the lodge where the proposed drive

intersects with the existing Grass Point and Flutes Cape Track. The site is currently occupied by

bushland and a recreational walking track.

The proposed development comprises a single storey lodge, studio and associated car parking and

driveway which will following the existing alignment of the Grass Point and Flutes Cape Track to

within approximately 120 m of the proposed lodge. The road will follow the existing track and

ground surface for most of the access road but will start cutting into the natural slope for the last 55

m of track/car park. Cutting is expected to be up to 2.5 m in height.

South East 2011 Geoscience Australia LiDAR elevations have been considered accurate for display

purposes and localised adjustments have not been made to the digital elevation model given the

relative accuracy is +/- 50 mm.

Table 2 presents a summary of elevation ranges and overlay encroachment for aspects of the

proposed development.

Table 1 Summary of Proposed Development Works Falling Within Landslide Hazard Zones

Site Details Elevation Range (m

AHD)*

Landslide (E3) Overlay Low

Hazard Zone

Landslide (E3) Overlay

Medium Hazard Zone

Proposed Lodge 22.8 – 29.5 76% -

Proposed Studio 17.1 - 19.2 100% -

Proposed Car Park and Driveway 13.9 - 31 100% -

- Outside of Overlay

*Elevations estimated based on LiDAR elevation data

Development & Works Acceptable Solutions

Where applicable, the need for further performance criteria compliance is outlined in Appendix 1.

Landslide Hazard Code (LHC)

Given that the excavation works resides in the Landslip Hazard Area, and there are no acceptable

solutions for ‘Buildings and Works, other than Minor Extensions’ in a Landslip Hazard Area, the

E3.7.1 P1 performance criteria will need to be addressed.

Development Performance Criteria

The following performance criteria need to be addressed:

• E11.7.1 P1.


Geotechnical Assessment of slope stability

The geotechnical assessment of slope stability at the property takes into account the following

parameters:

Potential for instability in soil deposits

The site has moderately steep slopes of approximately 13 - 17° with mature eucalypt tree species,

native grasses and shrubs present over the site. Previous investigations at the site indicate the site is

underlain by high plasticity ‘CLAY’ to ~ 1.15 m depth, which is then underlain by ‘clayey gravelly

SAND’, likely to represent residually weather Dolerite bedrock ( Map Unit: Jdl). Observations made

by GES during the site inspection indicates along the track, bedrock is likely to be encountered at a

shallow depth. The lack of terracing or hummocks on site suggests that the natural soils are generally

stable on the moderate slopes on site.

The site appears stable in its present form with no evidence of recent or active land instability,

therefore, the geotechnical risk associated with instability in the natural soils is low. The soil

examined would be subject to limited erosion if exposed, and therefore considerable care would need

to be taken during and after excavation to maintain a stable land surface. Modification of drainage on

site could also cause localised instability as excess water destabilises sediments - therefore drainage

design should avoid water accumulation in the construction area – with cut-off drainage at the base of

the site cut strongly recommended - Please refer to the extract on good hillside construction practice

from the Australian Geomechanics Society and CSIRO BTF-18.

Potential for foundation movement

The relatively steep slope and presence of slightly reactive clay subsoils must be considered in the

design of the footings, but both factors do not preclude the design of serviceable footings. GES

recommend footings of all structures to be placed into the underlying bedrock to reduce the risk of

foundation movement. Surface water and possible groundwater flows on to the site should be

carefully managed with appropriate drainage controls.

Provided that the footings are placed below any site fill into underlying natural bedrock on site and

designed in accordance with AS2870-2011 the geotechnical risk relating to potential foundation

movement is low and acceptable. I also suggest that attention is paid to suitable backfill surrounding

footings and drainage to avoid water accumulation in the foundation area.

Potential for vegetation removal to cause instability

Vegetation is likely to be cleared at the location of the proposed building and of the car parking areas

adjacent to the property. Removal of vegetation downslope of the proposed development is not

advised and may result in destabilisation of the surficial clay materials overlying the bedrock.


Potential for runoff to cause instability

Given the sloping nature of the site and the site cutting there is potential for excess water flow onto

the site to accumulate and cause shallow seated instability if the construction does not make

allowance for appropriate drainage. In particular care should be taken to correctly channel any

possible accumulated water from sealed areas or any future driveway away from any foundations.

Cuttings to the north of the car parking area should incorporate a toe drain at the base of the slope.

Geotechnical Risk Assessment

The following risk assessment is based upon the Australian Geomechanics Society Sub-committee

report (March 2007) Landslide Risk Management Concepts and Guidelines. Australian

Geomechanics Journal 35 (1) p49-92. The geotechnical risk associated with residential development

on the site is classified as Low according to Australian Geomechanics Society Guidelines and minor

according to AS1726-1993 Geotechnical Site Investigations.


Landslide Risk Management Model

Adapted from AGS Sub-committee (March 2007) Landslide Risk Management Concepts and Guidelines. Australian Geomechanics Journal 35 (1) p49-92.

Date 17/09/18

Site 1005 Adventure Bay Road

Project Proposed Lodge and Studio and Driveway

Scoping Lodge and Studio placed on Dolerite coastal hillslope with slope angle ~13°

Bedrock expected to be shallow (<2m deep) landslide develops downslope of proposed dwelling in residual soil.

Hazard and risk to be quantified.

1. Hazard Identification

a. Type of potential instability Shallow Slide and Debris slide/flow with residual soil.

b. Location Down slope of proposed buildings

c. Estimated area affected(m2) 400 (40m across and 10 m downslope)

d. Estimated volume (m3) 800 (soil 1.5 m deep)

e. Initiating event(s) Extreme heavy/prolonged rainfall or earthquake

f. Estimated velocity of movement Moderate to Rapid (5 x 10-1 to 5 x 101 mm/sec)

g. Estimated travel distance 90m

2. Frequency Analysis

a. Estimated frequency of event (PH) 0.0005 (1 in 2000 yr event)

b. Justification of frequency Low frequency of events within area. MRT landslide inventory.

3. Consequence Analysis

a. Element at risk Property, services & occupants

b. Value at risk (E) $500 000 (building)

c. Temporal probability (PT:S) 0.7 (probability of occupation)

d. Property vulnerability (VP:S) 0.5 (proportion of property value lost)

e. Probability of effect (PS:H) 0.5 (probability of slump affecting building)

f. Human vulnerability (VD:T) 0.5 (probability of loss of life)

4. Quantitative Risk Calculation

a. Property [Rprop = (PH) x (PS:H) x (VP:S) x (E)] = $62.50 (annual loss of dollar value)

b. Loss of life [RDI = (PH) x (PS:H) x (PT:S) x (VD:T)] = 8.75x 10-5

5. Semi-quantitative risk estimation for property

a. Likelihood of event Level D- unlikely (extreme conditions required)

b. Consequence to property Level 3 – moderate (remedial works required)

c. Combined level of risk Low – risk acceptable

6. Sensitivity Analysis

Most uncertainty surrounds frequency of event (item 2a)

7. Risk Evaluation (should the risk be accepted, reduced, avoided or rejected?)

From the assessment in 4a&4b the risk to life and property is acceptable

8. Risk Treatment

a. Options

Accept risk Recommended

Avoid risk Cut slopes to be graded to 1V:2.5H. Fill placed under the

building should be terraced into the existing hillslope to prevent instability. The building should be founded on the

underlying bedrock.

Reduce likelihood Yes – utilise drainage controls on site

Reduce consequences Yes – footing design based upon best practice

Transfer

b. Treatment Plan

Appropriately designed footings into bedrock in line with best practice recommendations

Installation of appropriate cut-off drains upslope of the construction area and at base of upslope cutting.

Stormwater and wastewater correctly connected to disposal areas

Excavation on site to have appropriate stabilisation

c. Implement Plan

Yes

d. Monitoring

No formal monitoring required – standard maintenance of building and drainage applies


Construction recommendations

The geotechnical risk associated with development of the proposed building is classified as Low and

acceptable according to Australian Geomechanics Society Guidelines. The development is not

expected to have any significant effect upon land stability on the subject or neighbouring properties

provided that the recommendations contained within this report are followed. Namely:

• That all stormwater should be immediately directed to disposal areas upon the construction of

hard surfaces to minimise any possible water accumulation and excess flows onto the steep

slopes below;

• That recommendations of the Australian Geomechanics Practice note guidelines for Landslide

Risk Management 2007 attached as Appendix 1 – Some guidelines for hill side construction

be followed;

• That any site cutting/filling is appropriately retained or battered in accordance with

engineering design recommendations (max soil batter 1V:2.5H);

• Careful attention should be paid to foundation design, with all foundations placed into natural

bedrock to further eliminate the potential for foundation movement; and

• All earthworks on site must comply with AS3798-2007 and sediment and erosion control plan

should be implemented on site during and after construction.

It is my opinion that the risk of land instability on either the subject site or neighbouring sites will not

increase substantially as a result of development of the proposed building provided that current best

practice for construction on sloping sites and soil and water management practices are followed as

discussed in this report.

I do however recommend that during construction that I and/or the design engineer be notified of any

major variation to the foundation conditions as predicted in this report.

Dr John Paul Cumming B.Agr.Sc (hons) PhD CPSS GAICD

Environmental and Engineering Soil Scientist


Appendix 1

Guidelines for hillside construction

Extract from

AGS Sub-committee ( 2007) Landslide Risk Management Concepts and Guidelines.

Australian Geomechanics Journal


Appendix 2 Acceptable Solutions

Landslide Code Areas

Sta

nd

ard

Code Acceptable Solution

Per

form

an

ce

Criter

ia

Use

E3.6.1

Hazardous Use

A1 Hazardous use relates to an alteration or intensification of an approved use. P1

A2 No acceptable solution. P2

E3.6.2

Vulnerable Use

A1 Vulnerable use is for visitor accommodation. A1

A2 No acceptable solution. A2

Dev

elop

men

t

E3.7.1

Buildings and

Works, other than

Minor Extensions

A1 A1 No Acceptable solution P1

E3.7.2

Minor Extensions

A1

Buildings and works for minor extensions must comply with the following:

(a) be in a Medium Landslide Hazard Area.

P1

E3.7.3

Major Works

A1 No acceptable solution. P1

Su

bd

ivisio

n

E3.8.1

Subdivision

A1 No Acceptable solution P1

A2 Subdivision is not prohibited by the relevant zone standards. P2


Appendix 2 Risk Assessment - Consequence Index

Consequence Details - Storm Erosion and Inundation Details – Waterways and Coastal

Protection

Catastrophic Loss of life, loss of significant environmental values due to a

pollution event where there is not likely to be recovery in the

foreseeable future.

Very serious environmental effects

with impairment of ecosystem

function. Long term, widespread

effects on significant environment

(eg. RAMSAR Wetland)

Major Extensive injuries. Complete structural failure of development,

destruction of significant property and infrastructure, significant

environmental damage requiring remediation with a long-term

recovery time.

Serious environmental impact effects

with some impairment of ecosystem

function. Relatively widespread

medium-long term impacts.

Moderate Treatment required, significant building or infrastructure

damage i.e. loss of minor outbuildings such as car ports,

garages and the like. Replacement of significant property

components. linings, hard paved surfaces, cladding, flooring.

Moderate environmental damage with a short-term natural or

remedial recovery time.

Moderate effects on biological or

physical environment (air, water) but

not affecting ecosystem function.

Moderate short term widespread

impacts (e.g. significant spills)

Minor Medium loss – repair of outbuildings and repair and minor

replacement of building components of buildings. Replacement

of floor/window coverings, some furniture through seepage

(where applicable). Minor environmental damage easily

remediated.

Minor effects on biological or

physical environment. Minor short-

term damage to small area of limited

significance.

Insignificant No injury, low loss – no replacement of habitable building

components, some remediation of garden beds, gravel

driveways etc. Environment can naturally withstand and recover

without remediation. Inundation of the site, but ground based

access is still readily available and habitable buildings are not

inundated, including incorporated garages.

Limited damage to minimal area of

low significance.

Source: AN/NSW 4360:2004 Risk Management

Likelihood Index

Qualitative Risk Matrix


Appendix 4 Quantitative Risk Assessment Tables

Likelihood & Consequence Index


Qualitative Risk Matrix

GES Pty Ltd – Geotechnical Report 198 Dowling Road, Huonville


Appendix 5 Quantitative Risk Assessment

Performance Criteria E3.7.1 P1

Buildings and works must satisfy all of the following:

Relevance Management Options

Managed (treated) Risk Assessment Further

Assessment

Required Consequence Likelihood Risk

(a) no part of the buildings and works is in a High Landslide

Hazard Area; NA

(b) the landslide risk associated with the buildings and works is

either:

(i) acceptable risk (means a risk society is prepared to accept as

it is. That is; without management or treatment); or

(ii) capable of feasible and effective treatment through hazard

management measures, so as to be tolerable risk.

The residual tolerable risk may be assessed using either qualitative or

qualitative methods in the landslide risk assessment either:

(a) if using the AGS qualitative risk assessment method apply

the "As Low As Reasonably Possible (ALARP)" principle with the

residual tolerable risk level no higher than a "moderate" risk level

under the AGS 2007(c) risk method; or

(b) if using the AGS quantitative risk assessment method then

the tolerable loss of life for the person most at risk as suggested by the

AGS 2007(c) to be:

(i) if existing slope / existing development: 10-4 / annum;

(ii) if new constructed slope / new development / existing

landslide: 10-5 / annum.

Building and road is positioned

within low landslide hazard

zone with shed located within

medium hazard zone.

Risk to residential

development is considered

acceptable. However, it is

recommended that

foundations of the

development be placed on

the underlying bedrock and

area of fill placement under

proposed development

should be keyed into

hillslope to ensure fill

stability.

Cutting to the rear of the

property and along the

access road to be cut to

1V:2.5H with a spoon drain

at the toe of cutting to divert

surface water away from the

buildings and car parking.

Medium Unlikely Low No

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