corangamite cma soil health strategy identification and ... · landslides are a threat to natural...

CORANGAMITE CMA SOIL HEALTH STRATEGY

Identification andmanagement of landslides

Produced for the Corangamite Catchment ManagementAuthority by the Victorian Department of Primary Industries

Cover Photos: Landslides are a threat to natural and built assets in theCorangamite region.

Compiled by: Tony Miner, Ian Crook and Troy Clarkson.

2

CORANGAMITE CMA SOIL HEALTH STRATEGY 2008-2012

Disclaimer:

This publication may be of assistance to you, but theCorangamite Catchment Management Authority, itsemployees and other contributors do not guarantee thatthe publication is without flaw of any kind or is whollyappropriate for your particular purposes and thereforedisclaim all liability from error, loss or other consequencethat may arise from you relying on any information in thispublication. Further, specific references to funding levelsin this publication are for indicative purposes only. Thelevel of Government investment in this publication iscontingent on budgets and Government priorities.

8556 Landslides Field Guide 16/4/08 8:48 AM

Contents

Introduction – Landslides 4

Known and possible locations of landslides 6

Key points to remember about landslides 7

Susceptibility to landslides 8

Action flow chart for landslides 10

Hazard identification for landslides 12

Further information 21

A FIELD GUIDE: LANDSLIDES 3

CONTENTS

TITLE PAGE

4

Introduction – LandslidesA landslide can be described as:

The movement of a mass of rock, debris or earth (soil)down a slope (under the influence of gravity).

As such, it should be noted that the term “landsliding” isneither limited to “land” nor to sliding and a morecomplete description of the possible landslide types isprovided in the Landslides section of the accompanyingManual.

Landslides are a form of erosion and are an importantprocess in the shaping and reshaping landscapes andlandforms. Landslides re-distribute soil and sediments in aprocess which can be extremely rapid or very slow.

Other terms used such landslip, mass wasting, slippageand falling debris have also been commonly usedalthough the term landslide is generally used.


Fig. 1: Simplified block diagram of a typical, but idealisedlandslide showing commonly used technical names given thevarious parts of the landslide (from Highland, 2004, based onVarnes, 1978). Definitions are provided in the glossary of terms.

Crown cracksCrown

Surface ofruptureMain body

Toe of surfacerupture

Surface of separation

Foot

Toe

Radialcracks

Transverseridges

Transversecracks

Side scarp Head scarpHead


Landslides are unpredictable. They can be slow orextremely rapid, occurring ‘without notice’ - to cause lossof life and property damage simply because there isinsufficient time for people to evacuate. A large, slow-moving landslide is less likely to cause loss of life but maybe difficult or impossible to control, resulting in damage toproperty, assets and infrastructure.

The factors that interact in the development and initiationof a landslide are described in detail in the Manual.These can be grouped into:

• Preparatory factors such as steepness, rock typeand structure, de-forestation, weathering, erosion andundercutting by river flow, waves, animal or humanactivity of various kinds: unsupported cuts, slopeloading (surcharge) by filling, and uncontrolled waterdischarges. The formation of earth dams, excavationand mining, irrigation, construction, services (such asstorm water, sewers, etc.), pilings, can all bepreparatory factors in landslide development.

• Triggering factors such as intense or prolongedrainfall, rapid snowmelt or sharp fluctuations inground-water levels, shocks or vibrations caused byearthquakes (M 3-4 or greater) or construction activitycan loosen granular soils, even when they are dry. Inconditions where the soil is saturated, granular orotherwise, even light vibrations can trigger arearrangement of the soil particles, resulting in atemporary increase of pore pressure and a reductionof the frictional forces in the material, destabilising theslope. Human Intervention can trigger slope failures –for instance construction activity that undercuts oroverloads dangerous slopes, or that redirects the flowof surface or ground-water.

INTRODUCTION - LANDSLIDES

6

Known and possible locationsof landslidesA diverse range of landscapes and soil units exist withinthe CCMA region and when combined with highly variableclimatic conditions resulting in average annual rainfallranging from 470mm to in excess of 1900mm, almost alltypes and forms of land degradation are possible. Theland degradation processes including landslides havebeen persistent throughout geological time and continueto be active, although they are generally episodic innature.

Major areas of landslide susceptibility and activity withinthe CCMA include

• the northern coast of the Bellarine Peninsula,

• the Otway Ranges and coast,

• the dissected plains of the Heytesbury Region and

• the flanks of the major river valleys including theBarwon, Moorabool and Leigh Rivers.


Fig. 2: Extent of known landslides in the CCMA region. Full details in the accompanying manual.

Corangamite Catchment Management AuthorityLandslide Inventory


KEY POINTS TO REMEMBER ABOUT LANDSLIDES

Key points to rememberabout landslides

Municipality Gully & Sheet LandslidesStreambank & Rill

Erosion Erosion

City of Ballarat 93 228 20

City of Geelong 178 288 117

Colac Otway 153 139 3,189

Corangamite 49 27 931

Golden Plains 1,603 777 48

Moorabool 709 1,125 379

Surf Coast 128 119 224

Other shires 11 32 36adjacent to the CCMA region

Totals 2,924 2,735 4,944

Overall total of erosion & landslide features = 10,603

• As at April 2007 there are 4944 mapped landslides inthe CCMA Region

• As a guide, positional accuracy may range from+/- 25m to +/-200m

• Also as a guide, there may be many more incipient orpotential landslides that have not been identified bythe surveying techniques used so far

• Many of the mapped sites intersect roads, railways orother infrastructure where human activity has createdpredisposing factors

• Further human activity may create triggering factors;climatic factors may also create ‘triggers’.

Whilst the capture and collation of information and data isongoing, the current number of mapped occurrences (as of April 2007) of erosion and landslide by municipalitywithin the CCMA region is shown in the following table.

Fig. 3: Individual landslide inventory maps have been produced byCorangamite Catchment Management Authority for each shire atboth local government area scale and at 1:25,000 scale forindividual map sheets.


Susceptibility to landslides

Fig. 4: Modelled LandslideSusceptibility in the

CCMA Region.

As one component of the Soil Health Strategy, the CCMAhas produced Landslide Susceptibility maps.

An important aspect to remember at all times when usingthese susceptibility maps is that the susceptibility depictedis only a modelled version of reality and there is nosubstitute for detailed on-site appraisal by a qualifiedgeotechnical practitioner experienced in the assessment ofthe potential susceptibility to landslides for a specific site.

Further detailed discussion - “Landslide and ErosionSusceptibility Mapping in the CCMA Region” Report No306/01/06, 30 June 2006, prepared by A.S. MinerGeotechnical. All landslide susceptibility maps are freelyavailable on the Corangamite Soil Health web site atwww.ccma.vic.gov.au/soilhealth

Corangamite Catchment Management AuthorityLandslide Susceptibility

8


SUSCEPTIBILITY TO LANDSLIDES

Colac-Otway Shire Landslide Susceptibility

Fig. 5: Colac Otway Shire Landslide Susceptibility Map.

10


STE

P 1

RE

FER

EN

CE

STE

P 2

GO

/NO

GO

STE

P 3

ON

SIT

E

HA

ZAR

D ID

SE

E S

EC

TIO

N ?

SE

E S

EC

TIO

N ?

NO

YE

S

NO

YE

S

NO

YE

S

NO

YE

SS

EE

SE

CTI

ON

?

Is th

e pr

opos

ed w

ork

site

in a

know

n ar

ea o

f hig

h or

ver

y hi

ghla

ndsl

ide

susc

eptib

ility

?

Is th

e si

te o

ne o

f rec

ent a

ctiv

ityor

failu

re?

ie. r

ock

falls

,co

llaps

ed r

oad

emba

nkm

ent

Can

a L

ands

lide

Ris

kA

sses

smen

t (LR

A)

be c

ondu

cted

by te

chni

cal s

taff

befo

re w

ork

crew

s re

ach

the

site

?

Is s

omeo

ne in

the

wor

ks c

rew

trai

ned

to m

ake

an O

nsite

Land

slid

e R

isk

Ass

essm

ent

(OLR

A)?

ON

GR

OU

ND

STA

FFS

UP

ER

VIS

OR

S

Con

duct

a L

RA

Action flow chart for landslides

Are

ther

e an

y si

gns

(old

or

rece

nt)

of p

revi

ous

inst

abili

ty a

t the

site

?


ACTION FLOW CHART FOR LANDSLIDES

STE

P 4

ON

SIT

E R

ISK

AS

SE

SS

ME

NT

STE

P 5

ON

SIT

E R

ISK

MA

NA

GE

ME

NT

STE

P 6

ON

GO

ING

DA

TAC

OLL

ATI

ON

STO

P w

ork

IMM

ED

IATE

LY.

Con

tact

you

rsu

perv

isor

AS

AP

SE

E S

EC

TIO

N ?

SE

E S

EC

TIO

N ?

SE

E S

EC

TIO

N ?

YE

SN

O

NO

YE

S

YE

SN

O

Are

ther

e an

y fe

atur

es o

r in

dica

tors

whi

chsu

gges

t lan

dslid

es c

ould

be

poss

ible

?

Con

duct

wor

k in

str

ict

adhe

renc

e to

you

ror

gani

satio

n’s

OH

S p

olic

y an

dbe

st w

orks

pra

ctic

e

Rec

ord

all d

etai

ls o

f obs

erva

tions

of l

ands

lides

and

inst

abili

ty (

incl

udin

g ph

otos

and

GP

S lo

catio

n) a

ndth

e “O

nsite

Ris

k A

sses

smen

t” if

con

duct

ed a

ndfo

rwar

d to

sup

ervi

sor

as s

oon

as p

ossi

ble

Are

ris

ks a

ccep

tabl

eto

allo

w th

eco

ntin

uatio

n of

wor

k?

Fina

l dec

isio

n on

whe

ther

cre

ws

atte

ndsi

te li

es w

ith s

uper

viso

r

Con

duct

Ons

iteLa

ndsl

ide

Ris

kA

sses

smen

t (O

LRA

)

12


Hazard identification forlandslidesBefore work orders are developed:-

Step 1: Check the landslide inventory maps for the site whereworks are to be undertaken.

• Consult the current CCMA detailed 1:25,000inventory maps

• Consult maps of known landslides held by theorganisation if they exist.

Step 2: Check to see what the modelled landslide susceptibility isfor the area.

• Consult the current CCMA landslidesusceptibility maps.

Field recognition and identification

It is very important to note that landslide hazards may bederived off site but the hazard may exist on the actualworks site.

Cracked walls androof, sinkingfoundation

Dead trees(water has

drained out ofcracked ground

Overtight powerlines

Tilted utility poles

Hummockyridges

RegolithSlip

surfaceBedrock Secondary

slump

Brokenfence

Cracked anddisplaced highway

Headscarp

Swampylow area

Fig.6 :The different features of a landslide.


HAZARD IDENTIFICATION FOR LANDSLIDES

Two sources of useful information will bepresented here:

Terrain morphology and Proxy landslide risk indicators.

The features of any landslide will be reflective of the typeof landslide and its age. For example, a rotational slide willbe characterised by a steep, near vertical headscarp,gentle mid-slopes and a convex toe. A slope undergoingrock fall will have scree (or debris) at the base of the slopewhich can range in size from small sand-like particles upto large boulders.

Be suspicious of flat areas intermediate between slopingground above and below in overall steep and slopingterrain, as they very often prove to be old landslide sites.Rocks or an accumulation of debris at the base of theslope indicates activity from above.

Fresh activity will be characterised by sharp edges andfeatures as well as distinct colour changes wherematerials have parted from the parent rock or slope. Older failures may have very degraded features includedrounded headscarps and worn edges and will bereflective of the ongoing weathering and erosionalprocesses which continually modify the landscape.

14


Tabl

e 1:

The

follo

win

g ta

ble

desc

ribes

mor

phol

ogic

, veg

etat

ion

and

drai

nage

feat

ures

whi

ch c

an b

e ch

arac

teris

tic o

f slo

pe in

stab

ility

pro

cess

es.

Terr

ain

feat

ures

Mo

rph

olo

gy:

Con

cave

/con

vex

slop

e fe

atur

es

Ste

plik

e m

orph

olog

y

Sem

icirc

ular

bac

ksca

rp a

nd s

teps

Bac

k-til

ting

of s

lope

face

ts

Hum

moc

ky a

nd ir

regu

lar s

lope

m

orph

olog

y

Infil

led

valle

ys w

ith s

light

con

vex

botto

m,

whe

re V

-sha

ped

valle

ys a

re n

orm

al

Veg

etat

ion:

Vege

tatio

nal c

lear

ance

s on

ste

ep s

carp

s,co

inci

ding

with

mor

phol

ogic

al s

teps

Rel

atio

n to

slo

pe

inst

abili

ty

Land

slid

e ni

che

and

asso

ciat

ed d

epos

it

Ret

rogr

essi

ve s

lidin

g

Hea

d pa

rt of

slid

e w

ith o

utcr

op o

f fai

lure

pla

ne

Rot

atio

nal m

ovem

ent o

f slid

e bl

ocks

Mic

rore

lief a

ssoc

iate

d w

ith s

hallo

w m

ovem

ents

or s

mal

l ret

rogr

essi

ve s

lide

bloc

ks

Mas

s m

ovem

ent d

epos

it of

flow

-type

form

Land

slid

e ni

che

and

asso

ciat

ed d

epos

it



Mor

phol

ogic

al fe

atur

es a

ssoc

iate

d w

ith L

ands

lidin

g.

Veg

etat

ion:

Irreg

ular

line

ar c

lear

ance

s al

ong

slop

e

Dis

rupt

ed, d

isor

dere

d, a

nd p

artly

dea

dve

geta

tion

Diff

eren

tial v

eget

atio

n as

soci

ated

with

chan

ging

dra

inag

e co

nditi

ons

Dra

inag

e:

Are

as w

ith s

tagn

ated

dra

inag

e

Exc

essi

vely

dra

ined

are

as

See

page

and

spr

ing

leve

ls

Inte

rrup

tion

of d

rain

age

lines

Ano

mal

ous

drai

nage

pat

tern

Slip

sur

face

of t

rans

ition

al s

lides

and

trac

k of

flow

s an

d av

alan

ches

Slid

e bl

ocks

and

diff

eren

tial m

ovem

ents

in b

ody

Sta

gnat

ed d

rain

age

on b

ack-

tiltin

g bl

ocks

, see

page

at f

ront

al lo

be, a

nddi

ffere

ntia

l con

ditio

ns o

n bo

dy

Land

slid

e ni

che,

bac

k-til

ting

land

slid

e bl

ocks

, and

hum

moc

ky in

tern

al re

lief o

nla

ndsl

ide

body

Out

bulg

ing

land

slid

e bo

dy (

with

diff

eren

tial v

eget

atio

n an

d so

me

soil

eros

ion)

Spr

ings

alo

ng fr

onta

l lob

e an

d at

pla

ces

whe

re fa

ilure

pla

ne o

utcr

ops

Dra

inag

e an

omal

y ca

used

by

head

sca

rp

Stre

ams

curv

ing

arou

nd fr

onta

l lob

e or

stre

ams

on b

oth

side

s of

bod

y

16


Fig. 7: Cracks in the ground are indicators that the ground is creeping.

One significant telltale sign of potential failure is thepresence of cracks in the ground. Such cracks are knownas “tension cracks” and indicate tension or pulling apartwithin the soil. Most soils are relatively strong incompression but only have limited strength in tension orshear.

The appearance of cracks at the soil surface usuallyprecedes full failure and is a sure sign that movement isoccurring within a slope. Whilst tension cracks may beassociated with slow movement (or creep), distinct sharpedges to tension cracks are a strong indicator thatmovement has been relatively quick and may signal theonset of even more rapid movement leading to overallfailure.

The nature and signs of instability can often varydepending on the type and scale of the failure. However,ground movement can be recognised by other featureswhich may not be immediately associated with slopeinstability.

These can include:

• Ancillary structures such as decks and patios tiltingand (or) moving relative to the main house

• Sunken or down-dropped road beds

• Tilting or cracking of concrete floors and foundations

• Soil moving away from foundations



Fig. 9: Deforestation of a slope can result in an increasedlandslide hazard.

Fig. 8: Bent trees and tilted fences indicate creep.

• Broken waterlines and other underground utilities

• Leaning telephone poles, trees, retaining walls, orfences

• Off-set fence lines or retaining walls

• Springs, seeps, or saturated ground in areas that havenot typically been wet

• New cracks or unusual bulges in the ground or streetpavement

• Rapid increase in creek water levels, possiblyaccompanied by increased turbidity (soil content)

• Sticking doors and windows, and visible open spacesindicating jambs and frames out of plumb

• Sudden decrease in creek water levels though rain isstill falling or just recently stopped.

18


2. Topples and Spreads

Sketch of lateralspreading

Sketch of a typical rock fall site

Diagrammatic representation of landslides

The key categories of landslides:

1. Falls

Fig. 10: Examples of a “Fall” landslide.

Rock toppling process

Fig. 11: Examples of rock toppling process and sketch oflateral spreading.



3. Slides - Rotational and translational

Rotational slide Translational slide

Rotational failure withmultiple planes

Translational failure

Fig. 12: Examples of rotational and translational landslides.

20


4. Flows

Debris flow Earth flow

Different types of landslides

Two views of debris flow environments

Fig. 13: Examples of different types of flows.


FURTHER INFORMATION

Contact your Supervisor who will have access to moredetailed information on landslides from the CorangamiteCatchment Management Authority.

Additional information also available on the CCMA SoilHealth website at www.ccma.vic.gov.au/soilhealth

Acknowledgement: Much of this information has beensourced from other documents and full reference is given inthe accompanying training manual.

CCMA Soil Health Web SiteThe Corangamite Soil Health Strategy contains a significantamount of information on landslides in the CorangamiteRegion, Also look at: www.ccma.vic.gov.au/soilhealth fordownloadable versions of previous reports on landslideswithin the CCMA region as well as information on upcomingevents, workshops, seminars and field days.

Other On-Line ResourcesAustralian Geomechanics Society’s site - downloadableversions of the 2000 guidelines and updated 2007guidelines:http://www.australiangeomechanics.org/index.htm

Geoscience Australia’s Natural Hazards:http://www.ga.gov.au/hazards/landslide/

Landslide facts, emergency response and preparedness atthe Australian Governments Emergency ManagementAustralia (EMA) website:http://www.ema.gov.au/agd/ema/emaschools.nsf/Page/RWPF98CABB36B90E441CA256C5D0015D69C?OpenDocument

The USGS - a wide range of landslide related informationand sites including fact sheets and frequently askedquestions: http://www.usgs.gov/hazards/landslides/

PublicationsNumerous books, publications and texts have beenpublished on the subject of landslides and slope instability.A selection are listed in the Manual.

Further information

GS

DM

_855

6

Corangamite Catchment Management Authority

64 Dennis Street, Colac. 3250

T: 03 5232 9100 E: [email protected]

W: www.ccma.vic.gov.au

For further information on Soil Health in the Corangamite region:www.ccma.vic.gov.au/soilhealth

corangamite cma soil health strategy identification and ... · landslides are a threat to natural...

Documents