\ K INGBOROUGH C OUNCIL
B LACKMANS B AY B LOWHOLE AND ENVIRONS G EOTECHNICAL R EPORT
J anuary 2020
2 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
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Cover image
View west towards th e coastal opening of the B lackm ans Bay Blowhol e (centre left), 24 December 2019 . The cliff is composed of P e rmian - age siltstone , sandstone and mudstone dipping approximately 12 0 WSW .
Refer to this report as
Cromer, W. C. (20 20 ). Geo technical Re port , Blackmans Bay Blowhole and Environs . Unpub l i shed re po rt for Kingborough Council by William C. Cro mer Pty . Ltd. , 21 J anuary 2020 .
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S UMMARY
I n rece nt geotechnical investigations commissioned by K ingborough Council, t hree types of geotechnical hazards are recognised at and near Blackm ans Bay Blowhole. They are rockfalls/ topples of a range of sizes, small landslides , and tree falls.
T h e hazards present no unacceptable risk s to infrastructure ( the c liff top fence and Talone Road : no houses are at credible risk) , and no management is required.
T h e hazards present varying (mostly low ) risks to life. In this report, the se are compared to acceptability criteria recommended by th e Australian Geomechanics Society Landslide Risk Management 200 7 c , but Kingborough Council as regulator should assess acceptability for the se risks against its own organisational criteria . If risk treatment is contemplated , re ducing th e likelihood of the hazards occurring is impra cticable. Treatment (if any) should focus on reducing consequences of the hazards , which in turn may mean regulating public access to parts of the area .
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1 I N TRODUCTION 1 .1 Background , sc ope an d perso nnel
1.1.1 Background
In December 2019, W illiam C. C romer Pty . Ltd. (WCC) was commissioned by K i ngborough Council (KC) to conduct geotech nical investigations at the northern end of Black m ans Bay Beach, including the sea cliffs and Blackmans Bay Blowhole , a nd the land seawards of Talone Road as fa r nor th as 36 Talone Road.
Council ’ s background notes , instructions and t he study area ar e in Attachment 1 .
R e levant background observations are:
In January 2017 a fatality occurred from a cliff - top fall near the Bl ackmans Bay Blowhole.
Following a coron er ’ s re port i nto the fatality , KC acquired land adjacent to the site of the fatality, an d commissioned a Public Safety Risk Assessment by IPM Consulting Services.
IPM reviewed the public safety risks, the existing ris k management , and any additional improvements to ris k management which might be appropriate.
IPM note d the known activities in the area were sightseeing, general re creational use, abseiling (by tourist operators, community groups and schools) and cliff jum ping.
The most significant hazard s associated with t hese activities were identified as falls from th e e dges of the cliff and blowhole, injury from rockfalls and falling t ree limbs, injuries from pedestrian - traffic interaction, and slips and trip s on uneven and unofficial paths.
Collective ly, the public safe ty risk was regarded by IPM as significant.
IPM made several recommendations, including :
o upgraded fencing along the cliff top,
o engineered and certified anchors for abseiling,
o warnin g signa ge ,
o public educa tion , and
o a “ Geo technical review of the blowhole and cl iffs in accordance with the Land slide Risk Management standard released by the Australian Geomechanics Society in 2007. ”
1.1. 2 Scope, person nel and dates
WCC ’ s scope of inves tigations paraphrased from Attachment 1 was to :
d escribe the relevant geologi c al conditions in the study area ,
identify and characterise geological / geotechnical risks , suggest ( as required ) risk management measures, and
where appropriate , provide geotechnical specifications for the control measures recommended b y IPM.
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To address th i s scope , the following in vestigations were conducted:
D e sk - top study
A review was done of on - line geotechnical and geological information relati ng to th e study area and environs . Most is presented in Atta chment 2.
Fiel d work
Field work included :
a n initial i nspection and photography on 4 D e cember 2019
off - s hore ( kayak ing ) site inspection and photography on 24 D e cember 2019
mapping, drone photogr aphy , and informal doorknocking to meet local residents on Tal one Road on 2 January 2020,
dynamic cone penetromete r (DCP) profiling at 1 6 locations along the exis ting and proposed extended fenceline in the study area on 2 January 2020 , and
separate on - site discussions about abseiling with Mr. P. Harris of Aardvark Ad ventures (AA) , and Mr. Luke Hamilton of the Tasmania n Climbing Instructor s Association (TCIA) on 2 Jan uary 2020.
The geological and geotechn ical inspections , drone phot ography and abseiling me etings we re conducted by engineering geologist B il l Cromer . C on sulting geologist Genevieve Bremne r an d field assist ant Ric hard Mackintos h carried out the DCP profiling .
1.2 P resentation of results
Most of the results of investigation are pres ented in the six Attachments a ccom panying this report .
1. 3 P revious geo technical ins pection
At the request of KC, I conduct ed a brief geotechnical inspection of th e Blackmans Bay Blowhole in Apri l 1999, and in a one - page report 1 made s ome minor sugges tions in relation to fencing and stormwater control.
1 Cromer, W. C. (1999). Re: Blackmans Bay Blowh o le – geotechnical inspection . Letter/report to Mr. S. Kaczmarsk i, Prog ram Engineer, T e chnical Ser vices, Kingborough Council. Environmental & Technical Services Pty. Ltd., 4 May 1999. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 8 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
2 R ESULTS
2 .1 G eology
2 . 1 .1 Published geology
T he published 2 b edr ock geology of the district (Attachment 2 ) shows P e rmian - age sedimentary rocks underlie the whole area, with the exception of beach and dune sand at and near sea level on Blackmans Bay Beach. The Permian rocks are described as unfossiliferous glac iomari ne 3 interbedded mudstone, siltstone and sandstone of the Abels Bay Formation .
2. 1.2 Observed geol ogy
Site observations support the published geology. R e levant ones are:
Excellent exposures of well - bedded si ltstone , sandstone and mudstone occur on the sea cli ffs in the study area. T h e cliffs range from less than a few metres high at the northern end of Blackmans Bay Beach, to about 15 – 17m high at the entrance to the blowhole , to about 40m high at the north ern end of the study area (Figure s 1 and 2 ).
Blowhole entrance
Figure 1. V iew west to wa rds the southern pa rt of the st udy area, showing w ell - bedded Permian - age silt stone, sandsto ne and mudstone dippi ng approximately 12 0 southwest .
Bed thickness ranges from abo ut 0.1m to almost one metre, and dips appro ximately 12 0 to the southwest.
2 Calver, C . R . (compiler ). 2007 . Digital Geological Atlas 1 :2 5,0 00 S ca l e Series. Sheet 522 3 . Blackmans Bay . Mineral Resources Tasmania 3 Glaciomarine sediments are deposit ed in a marin e environmen t fed by glacial meltwater and/or glaciers and icebergs. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 9 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Jointing is well develop ed in the sedimentary rocks. Most is subvertical, with the m ajor joint direction , including tha t of the mouth t o the blowhole , approximately striking (trending) 70 – 250 0 T. A set of similar joints strikes at roughly right angles and is subparallel to th e bedding strike (Figur e 3). Joint spacing varies from less than 0. 1m, to more than a metre.
T h e main joint set creati ng the blowhole and its mouth extends further to the west southwest towards Blowhole Road, an d the blowhole itself ten ds to preferentially erode in the same direction.
Northern end of study are a
Figure 2 . View north west to the sea cliffs in the northe rn half of the study are a, showing w ell - b edded 0 Permian - age silt stone, sandstone and mudstone dipping approximately 12 southw est
Weathering of the be drock is variable, ranging from slight to extr eme. Slight weathering results in little mineralogical change and a relatively high strength roc k. Extreme weathering results in chemical b reakdown (typ ically of binding cement between grains) which produces a highly erodible material usually exhibiti ng soil p roperties 4 .
At the base of the cliffs is a shore platform of variable width (depending on tides) , which dips inland at the same angle as the bedding . Platform w idth ranges from as little as a few metres, up to 40m or so at low tide . In place s , th e p latfor m is absent, and has either been cut through by marine erosion along the major joint di rection (Figure 3) or is locall y replaced by a cobble beach ( eg at the northe rn end of the study area). The platform permits pedestrian access , particularly at lo w tide , but access is difficult where it is disrupted along the major eroded joint set (eg the blowhole mouth).
Slow retreat of the sea cliffs is continual ly occurring via erosion by waves at its base, and wind . The mechanism of r etreat is mostly roc kfalls and to pples , and very small scale landsl iding . The sizes of the falls and topples is controlled by joint directions and spacings, and ranges from individu al joint blocks less than 0.1m in di ameter , u p to major falls involving hundreds of to nnes ( Attachment s 3 an d 4).
4 Mat erial with s oil properties can be remoulded in the hand , w ith o r with o ut adding water. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 10 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
C
B
A
Blackmans Bay Blackmans Bay Blo whole entran ce Blowhole entrance
KEY Cliff top (approximate)
0 Bedding (striking ≈ 340 – 160 T) Main subvertical joint set (striking ≈ 70 – 2 50 0 T) Conjugate to m ain subvertical joint set (striking ≈ 340 – 160 0 T) Grid North parallel to be dding Second ary subvertical joint set 0 50 (striking ≈ 35 – 215 0 T) Approx. metres Other subv ertical join t sets
Figure 3 . The dominant joint direction along the se a cli ffs in t he study area is app roximately 70 –
250 0 T . These are subverti cal, and mar ine erosion alo ng some o f them has produced th e blowhole an d its mouth , and re - entra nts A , B and C shown here. See also the photographs in Attachment 3.
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Soils developed in - situ on bedrock are mos tly light - coloured silty clay and clayey silt, usually with silts tone and sandstone rock fragments , and with a stiff to very stiff consistency .
Along the cliff top, so il th ickness ranges from about 0.2m to 1.6m , based on DCP profiling (Attachment 5). Soil properti es based on the DCP results ( Figure 5.1 in Attachment 5) are also variable:
A llowable bearing capacity typically (but not always) increases with depth, from ab out 2 00kPa near - su rface to more than 400kPa at depths below about 0.5m. Corresponding CBR values range from about 10% near - surface , to over 25% below abou t 0.5m.
At DCP 1, 2, 9 and 11, strength decreased with d epth down to bedrock .
Soil inter preted as t rans porte d (colluvial) and containing angular rock fragments was o bserved on south - facing slopes at the southern end of the study area.
2. 1. 3 Formation and age of Blackmans Bay Blowhole
Blackmans Bay Blowhole ha s bee n formed – and continues to be enlarged – by wa ve action pre ferentially eroding alon g a major set of subvertical joints trending in a west south west dir ection i n the Permian - age sedimentary rocks .
T h ere are many other examples of similar erosion along adja cent coasts.
We are currently in an interglacia l period, and ha ve been for the past 10,000 years or so , when sea level has been roughly the same as at present (Fi gure 4 ). It i s possible that the blowhole developed during this period, but it is also p ossible it is muc h older : T here was a period of over a hundred thousand years during the Last Glacial spanning about 6000 – 1 20,000 years ago when sea level was 30 – 130m lower than at present, and blowhole formation was not possible. But at around 120,000 – 130,000 years ago, sea level is thought to be up to 10m or so higher than current levels, and blowhole formation might have been active then as well.
3 D IS CUSSION 3 .1 G eotechnical hazards
3.1.1 Conceptual geological models
Figures 5 and 6 a re conceptu al geological c ross sections ( “ models ” ) , at natural scale, through the sea cliffs in the study area, at the locations shown in Figure 7 .
T h e detail in the sections is consistent with site observations.
Cross section A – B depicts interpreted geo logical co n ditions through the blowhole and its mout h. Cross section C – D is located some 120m further north , where the cliff is highe r and the shore platform wide r.
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Current sea level
L
A I L C
A A ? I ? L
s C s G A m m r R L r o E o f G f
T
R e e l N l E I o
o T h T h N I N w
w o E T o l l R S B LAST GLACIAL B R A U L
C
Figure 4 . Relative global sea lev el changes since the Last Interglacial, based on corals and other evidence. (Figure 1 of Waelbroek et . al . (2002) ). ka = thousands of years ago. Reference: Waelbroek, C. et. al . (200 2). Sea - level and deep water temperature changes derived from benthic fo raminifers isotopic records. Quaternary Science Reviews 21 p p 29 5 – 305.
3.1. 2 Identif ied geotechnical hazards
Three types of geotechnical hazards 5 are identified in the st udy area , and are shown in Figure s 5 and 6 :
Hazards 1 – 6 are roc kfalls/topples involving different volumes of material – from failures of hundreds of ton nes ( H azard s 1 and 2), to individual small boulders weighing a few k ilograms (Hazard 6) 6 . Examples of these hazards are depicted in Plates 2 – 6 in Attachment 3, and in Attachment 4.
Hazard 7 are very small landslides (earthslides 7 ) from at or near the t op of th e sea cliffs, involving failure of the soil profile . Example s are shown in Plates 2 , 3 and 5 i n Attachment 3 .
Ha zard 8 are tree and limb falls . Examples are depicted in Plates 2 , 3 and 5 in Attachment 3 .
5 I njuries from pedestrian - traffic interactio n, and slips and trip s on uneven and unofficial paths , as identified by IPM, are not consi dered to b e geotechnical hazards, and are the re fore not discuss ed further in this report. 6 The inset T ables on Figures 5 and 6 define landsl ide size and speed. Even rockf alls of hundr ed s of tonnes are “small ” . 7 Earthslides are landslides involving mostly soil , with o nly minor proportions of rock. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 13 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Figure 5 . Conceptual geolog ical cross section A – B through the Blackma ns Bay Blowhole, showing the types , and nominal sizes and speeds, of identified geotechnical hazards . Note the human scale ,
which is differ ent to that in Figure 6 . See Figu re 7 for th e location of the cross secti on.
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Figure 6 . Conceptual geological cross section C – D roughly through the centre of the study area , showing the types , and nominal sizes and speeds, of identified geotechnical hazards . N ote
the human scale, which is differen t to that in Figure 5 . See Figure 7 for th e location of the cross sectio n.
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C
D
Blackmans Bay Bl owhole B
A
Gri d North 0 100 Appr o x. m etres
Figure 7 . Location of section lines A – B a nd C – D . Source: www.t helist.tas.gov.au
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3.1. 3 La ndslide risk management
Attachment 6 is a landsli de risk management (LRM) assessment 8 of Hazards 1 – 8 , in accordance with the Australian Geomechanics Society Landslide Risk Management 2007 (AGS2007) .
Qualitat ive r isks to property
Property in this context is cons idered to be infrastructure – Talone Road an d the cyclone fence along the cliff edge.
It is regarded that houses along the western side of Talone Road are not at credible risk from any of the hazards 1 – 8.
Table 1 (reproduced from Attachment 6) sh ows that hazards 1 – 8 present Low r isk to identif ie d infrastructure in the study area , and no risk treatment is re quired.
Quantitative r isk s to life
Risk to life have been considered for f our examples of activities which are judged to be reasona bly repres entative of those ta king place in the study area :
E xample 1. A mo bile recreational i nd ivi dual walking o n the shore platform , subject to hazards 1 – 8 . Risk to life for an individual most at risk is the sum of all risks and is estimated to be 6 x 10 - 6 .
Exa mple 2. S tatic s ightseers ( 3 people at a time ) at th e fen c e line at the Talone Street car p ark , sub ject to hazard 1 only . Risk to life for an individual most at risk is estimated to be 3 x 10 - 9 .
Risk to life for all 3 people is 4 x 10 - 5 , which as an F - N pai r plots in the ALARP (As Low as R e a s o n a b l y P r a c t i c a b l e ) region of the societal risk graph .
Example 3 . A s mall s tatic abseiling group ( 5 people) at the base of th e cliff near the blowhole mouth , subject to hazards 1 , 3 and 4 . Risk to life for an individual most at risk is estimated to be 7 x 10 - 8 , 1 x 10 - 5 and 3.5 x 10 - 4 for hazards 1, 3 and 4 respectiv ely.
Risk to life for all 5 people is 3.5 x 10 - 7 for hazard 1, w hich as an F - N pair plots in the Acceptable region of the societal risk graph . The risk is 7 x 10 - 5 for hazard 3, which plots on the ALARP region of the societal risk graph .
Example 4. A n ind ividual mobil e kayaker or swimmer entering the blowhole v ia its mouth (calm seas are presumed) , subject to hazards 3 and 4 . Risk to life for an individual mos t at risk is estimated to be 2 x 10 - 5 and 5 x 10 - 4 f or hazards 3 and 4 respectiv ely.
8 Important notes. 1. LRM , an d particularly th e as sessmen t of hazard likelihood, is a subjective process . Likel ihoods f or hazards 1 – 8 are shown qualitatively in Ta ble 1, an d are listed qualitatively as P( H ) values in Table 6.4 in Atta chment 4. Tem poral and spatial prob abilit ie s are also subjective i np uts, an d r isk to life in particular is sensitive to these . As f ar as can be judged from available evidence, it is thought that the inputs are reasonable. However, i t is understood and readily acknowledged that other LRM as sessors might adopt different inp uts , an d hence estimate different risks to property and life. 2. It m ust also be understood that the accepta nce criteria for any risk shall be determined by regulators, not LRM risk assessors. Some guidance in this regard is offe red in Attachment 6. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 17 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Table 1. Likelihood , consequence and qual itative risk analysis of Sce narios 1 – 8. Risks to infrastructure are Low for all hazards.
1 2 3 4 5 6
d o o t f e
t t e e n w c o e d v
d e k k o s c n l o n d t s s n i e n i i m e b e o e t h r r e u a b s o a r e f f q m r s
Scenario e h s s b o o e i
t r a t u l m
l l t s e e d c e o e e o n s s c k n t k c v v i o s s s a o e e e i L a a C r L L R
o t
Rockfall/rock topple
d Small scale; in siltstone a nd sandstone, from any m u a
1 Unlikely i o d part of the cliff face; extremely ra pid; involving a r width of cliff face of at least 20m. e M Rockfall/rock topple S mall scale; in mudstone, from the lower parts of the 2 Likely cliff face; extremely rapid; involving a width of cliff fa ce of at least 20m. Rockfall/rock topple Very small sca le; in siltstone, sands t one or Almost 3 mudstone, from any part of the c liff face; extremely Ce rtain rapid; involving one or more bo ulders over a width of to Likely
cliff face of at least 5m. ) w o Rockfall/rock topple l e b
Very small scale; in siltstone, sandst one or s 4 mudstone, from a n y part of the cliff face; extremely t
e t s n
rap id; involving one or more boulders over a width of s a a
c
c liff face of at least 1m. i f o i n
n Low None Rockfall/ro ck topple (
g i e l Very small scale; in siltstone, sandstone or s b n I 5 mudstone, from any part of the cliff face; extreme l y a c i l
rapid; involving one or more boulders over a width of p p
cliff face of at least 0.5m. a
t Rockfall/rock topple o N Very small scale; in siltstone, sand stone or Almost 6 m udstone, from any part of the cliff face; extremely Certain rapid; involving a single boulder (joint fr agment) up to 0.1m in diameter. Earthslide Very small scale landslide, from at or n ear cliff top, 7 up to 5m wide and less than about 2m3 in vol ume; extr emely rapid, moist to wet, suspended to active n
Tree or tree limb fall a c i f
Very small scale fallen tree and/or limb, from at or i t
8 n
near cliff top, with a downslope affected zone up to g i s
abo ut 5m wide; extremely rapid. n I Notes Column 1: refer to cross sections (Figures 4 and 5 in report) for explanations of size and speed. Note definition of size is not fro m AGS2007c. Column 2: refer to Attachment 7 f or an explanation of Likelihood terms. Column 3: refer to Attachment 7 for an explanation of Consequence terms. Assets( ie property, infrastructur e) in this instance = Talone Road and fe nce Column 4: refer to Attachment 7 for an explan ation of Risk terms.
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3 . 2 Cl ifftop fencing
3.2.1 Location an d variable height
Cyclone fen cing extends close to the cliff edge on the seaward side of Talone Road the full length of the stu dy area. Its height ranges from 1. 1 m and 1.4m from the northern end south to the large eucalypt s opposite 5 3/5 5 Talone Road. From this point for a short distance south it is about 0.8m high. It increases in hei ght to about 1.3 – 1.4m , and then decreases again to about 0.8m opposite the blowhole (Figure 8 ).
Figure 8 . Different heights of the cyclone fence at the Talone Road carpark op posite the B lackmans Bay Blowhole (at DCP11) . At left, the fence is 1.4m high; at r ight, about 0.9m. Abseilers anchor from this location . Anecdotally, an old couch was positioned at one time over th e fence at left.
3.2. 2 Age and condition
Local re sidents say the fence is several decades old. Some of the posts are slightly loose. Apart f rom its vari able height, its condition is fair.
3.2. 3 Fence upgrading
IPM has recommended the f ence be upgraded. It is suggested here that i f it is replaced, it s hould b e of uniform height and material type. A minimum height of 1.2m is suggested as reasonable ( there are likely to be objections f rom local r esidents if it any higher , and height is not a serio us deterrent to people intent on scaling it ) .
There is very limited opportunity to change its locatio n – except perhaps at the Talone Road carpar k , where it c ould be moved a short distance into th e carpar k to move sights eers further
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 19 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
a way from th e risk of h azard 1. However, any slight reduction in risk to life would perhap s be offset by a larger number of people attempting to clim b over the fence if it offered a wider area on the seaward side.
3.2. 4 Fence footing design
Foo ting design for a new fence is an engineering issue, but it could be informed by the DCP res ults (Att achment 5) showing soil thickness and strength .
3.2. 5 Blowhole fence
T h e existing metal fence is about 1.8m high and in good conditio n . I n my view i t requires no upgrading apart from w here it has bee n damaged b y a leaning casuarina . The tree should also be removed.
3 . 3 Abseiling
3. 3 .1 Abseiling activities
Mr. L. Hamilto n of TCIA, an d Mr. P. Harris of A A , both conduct abseiling a ctivities. The former runs school groups, an d th e l atter school and other groups of all ages.
Other competent groups probably also use the site , an d it i s know n that some experienced individuals abseil alone. Mr. Hamilton noted that he has observed inex perienced people also abse il alon e or in small grou p s .
The abseiling sites are in the Talone Road car park, and one or two locations about 40m to th e south where the cliff is lower.
It is estimated that about 30 or more visits are made annually by abseilin g groups. Group size range from one or two people , u p to about 20.
Only one person abseils at a time from an anchor point .
3. 3 . 2 Abseiling anchor points
All groups apparently use existing anchor points : eg trees in front of or behind the fence , the fence itself , and presumably any point able to safely cope with th e weight of a single perso n. Several anchors might be used for each abseil.
There are no formal, recognised anchor points.
Both TCIA and AA agreed that formal, engineered anchor points would be be neficial. Typically, they would b e just on th e seaw ard side of th e fence, and might be eye bolts attached to a vertical rod or colum n cemented into demonstrable bedrock. All or some cou ld al so be located on th e land ward side of th e fence, althou gh these w ould be more likely to be vandalised.
Typically, the installed anchors would be between about 0.8 m and 1.2m apa rt, and an installed dept h of about 0.3m into bedrock would probably suf fice.
Installing engineered anchor points on either or both sides of an u pgraded fence might perversely increase rather than decrease the risk of abseiling incidents . While they would benefit all abseilers, they might encourage increased abseiling by inexp erienced peopl e.
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 20 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
3 . 4 Warning signs
IPM recommended warning signs be erected in the study area. From a geotechnical pers pective, signs probably do little to address risk s to life occasioned by hazards 1 – 8. But from a duty - of - care perspective, sign age along th e cliff edge and shore platform is appropriate.
T h e main eleme nts at risk are cliff jum p ers and other thril l seekers , and people intent on suici de, and it is difficult to regulate aga inst these activities.
3 . 4 Public education
IPM recommended a publ ic education program to raise awareness of public ri sk in the area.
This is lik ely to be beneficial in a general sense , but is unlikely to reduce geotechnical risk to life.
4 C ONC LU SIONS
T h e following conclusions arise from this report .
The sea cliffs in the study area comprise Permian - age siltstone, sands tone and mudstone di pping at approximately 12 0 to the west southwest. The rocks are or relatively high strength, and show prominent sub vertical jointing at vario us di rections and spacings. The Blackmans B ay Bl owhole has formed from marine erosion along a m ajor joint direction at least 6,000 years or so ago , but possibl y 120 ,000 – 130,000 years ago, and perhaps older.
Th ree types of geote chnical hazards exist at and near the Blackmans B ay Blowhole : rockfall s/topples, landslides and tree falls . Vario us volume s and sizes of rockfalls/topples are considered separately in this report. The hazards are:
1. Rockfall at least 20m wide (multiple boulders) in sandstone/siltstone
2. Rockfall at least 20m wide (multiple bould ers) in muds tone
3. Rockfall /topple up to 5 m wide ( one or more boulders)
4. Rockfall/top ple at least 1m diameter
5. Rockfall/topple 0.5m in diameter
6. Rockfall/topple 0.1m in diameter
7. Landslide (earthslide) up to 5m wide
8. Tree fall /topple
R isk to property ( ie assets s uch as Talone Road and th e c liff top fence, but no h ouses) from all hazards is regarded as Low and no risk treatment is required.
Risk to L ife for individuals an d group s has been considered for four kinds of activities exposed to all or some of th e hazards :
1. walking on the shore platform at and near the base of the cliffs,
2. sig htseers at the top of the cliff at the Talone Road carpark,
3. abseiling parties , and
4. kayakers and swimmers entering the blowhole via its mouth .
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 21 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
For these examples, r isk t o l ife to ind ividuals and ranges from about 3 x 10 - 9 to 6 x 10 - 6 for the first two examples , from about 7 x 10 - 8 to 3.5 x 10 - 4 for the third activity , and from 2 x 10 - 5 to 5 x 10 - 4 for the last activity.
From a group perspective, risk s to life for two of the activiti es plots in the ALARP region of th e societal risk gr aph. T h e remaining risks to life plot in the Acceptable region of th e grap h.
T h e highest risk to life (5 x 10 - 4 ) is to individuals in the blowhole itself, occasioned by hazard 4.
The regulator determin es risk acceptability c riteri a. If risks to life are required to be treated, reducing the likelihood of hazards 1 – 8 is not considered practic able. Reducing the c onsequences should be the focus . So me consequence - reducing actions could include:
m oving th e existing fence a metre or two inland at the Talone Road carpark , and at the same time l imiting t he number of sightseers able to stand close to the edge of the cliff (eg by constructing a narrow , partly enclosed walkwa y ) to no more than (say) 2 people , and
preventing or discouraging access to th e base of th e blowhole by kayakers and s wimmers .
In relation to fence upgrades, abseiling anchors, signage and public education, no specific geotechnical risk treatments are re quired .
W. C. Cromer Principal
T his report is and must rema i n acco mpanied by the followi ng At tachments
At tachme nt 1 . Kingborough Council Re quest for Quote for this job ( 3 page s )
A ttachm ent 2. Location, cadastre, contours, aerial imagery, hill shading , publish ed ge ology, and landslide, coas tal erosion and coastal inundation hazard areas ( 8 p age s )
A ttachment 3 . Site photogr aphy ( 3 page s )
Attachme nt 4. Comparative satellite image ry sh owing aspects of slope instability over 9 .5 year ( 3 pag es)
A ttachm e nt 5 . Dynamic cone penetrometer (DCP) res ults (7 pages)
Attachment 6. Landslide Risk Manag em ent ( 1 3 pages)
A ttachm e nt 7 . Qualitative terminology used in assessing risk to property ( 2 pages)
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 22 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Atta chment 1 ( 3 page s ) Kingborough Council Re quest for Quot e for this jo b
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 23 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 24 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 25 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Attachme nt 2 ( 8 pa ge s ) Location, cadastre, contours, aerial imagery, hill shading , publish ed ge ology, and landsl ide, coastal erosion and coastal inundation hazard areas Source: www.thelist.tas.gov.au
GDA94 Location , cadastre , 5m contours 5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m R I V E R
D E R W E N T
T alo ne
Road 3 5 m
B L A C K M A N S
B A Y 30 m
Blowhole 25 m Roa d
2 0 m Blackmans Ba y Blowhole
GDA94 5 2 39 2 00 m N 15 m
10 m
Study Area
Bl owhole Road
Grid No rth
0 250
Appro x. m etres Conto ur interval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 26 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Aer ial image ry GDA94 5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m R I V E R D E R W E N T
Talo ne Road 3 5 m
B L A C K M A N S 30 m B A Y
Blowhole 25 m
Road
2 0 m Blackmans Bay Blowhole
GDA94
5 2 39 2 00 m N 15 m
10 m
Stud y Area
Bl owhole Road
Grid North
0 250 Appro x. m etres Contour interval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 27 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
GDA94 Hill shad ing (fr o m NW) 5 2 6 7 00 mE
T al o ne Road
45 m
4 0 m R I V E R D E R W E N T
Talo ne Road 3 5 m
B L A C K M A N S 30 m B A Y
Blowhole 25 m Road
2 0 m B lackmans Bay Blowhole GDA94 5 2 39 2 00 m N 15 m
10 m
Study Area
Blowhole Road
Grid North 0 250
Appro x. m et res Cont our interval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 28 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
G DA94 Published geo logy Publi shed geology Calver, C . R. (compiler ). 2007 . Digital 5 2 6 7 00 mE Geological Atlas 1:2 5,000 S cale Serie s.
Sheet 522 3 . Blackmans Ba y . Mineral R esources Tasmania. Key t o colours Blue (symbol Pua ) = Permian s edimentar y rocks – unfo ssiliferous glaciomarine inter bedded mudstone , siltsto ne and mi nor sandstone (Abels Bay Formation) ; Y ellow (symbol Q) = Quaternary unconsolidated sediments ; Y ellow w i th 45 m b rown dot s (symbol Qhw) = Q uaternar y dune sand; Light yellow (sym bol Qhb) = 4 0 m Holocene bea ch sand. R I V E R
D E R W E N T Talo ne Road 3 5 m Pua
B L A C K M A N S B A Y 30 m
Pua Blowh ole 25 m Road
2 0 m B lackmans Bay
Blowhole
GDA94
5 2 39 2 00 m N 15 m
Pua 10 m
Study A rea
Q Blowh ole Q h w Road Q hb
Grid North
0 250
Appro x. m etres Contour interval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 29 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Landslide Hazard Bands GDA94 5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m R I V E R D E R W E N T T alo ne Ro ad 3 5 m
B L A C K M A N S
B A Y 30 m
Blowhole 25 m Road
2 0 m Blac kmans Bay Blowhole
GD A94 5 2 39 2 00 m N 15 m
10 m
Study Area
Blowhole Road La ndslide Hazard Bands Grid North
0 250 Appro x. m etres Contour interval 5 m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 30 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Coastal E r os ion Hazard GDA94 5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m
R I V E R Talo ne D E R W E N T Road 3 5 m
B L A C K M A N S
B A Y 30 m
Blowhole 2 5 m R oad
2 0 m Blackmans Bay Blowho le
GDA94 5 2 39 2 00 m N 15 m
10 m
Study Area
B lowhole Road Coastal Erosion Hazard
Grid N orth
0 250 Appro x. m etres Cont our in terval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 31 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Coa stal Inundation Hazard GD A94
5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m R I V E R D E R W E N T
Talo ne Road 3 5 m
B L A C K M A N S B A Y 30 m
Blowhole 2 5 m Road
2 0 m Blackmans Bay
Bl owhole
GD A94
5 2 39 2 00 m N 15 m
10 m
St udy Area
Bl owhole R oad Coasta l Inunda tion Hazard
Grid North
0 250
Appro x. m etr es Contour interval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 32 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
GDA 9 4 I nterim Planning Scheme 5 2 6 7 00 mE
Talo ne Road
45 m
4 0 m R I V E R
D E R W E N T Talo ne Road 3 5 m
B L A C K M A N S B A Y 30 m
Blow hole 25 m Road
2 0 m Blackmans B ay Blowhole GDA94
5 2 39 2 00 m N 15 m
10 m
Study Area
Blowhole Road
Grid North
0 250
Appro x. m etr es Co n tour in terval 5m
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 33 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Attachme nt 3 ( 3 pa ge s ) Site photography Hazard num bers correspond to th ose in Figure s 4 and 5 in the report. Drone imagery by Bill Cromer, 2 January 2 020
P1
S iltstone
Mudstone
Plat e 1 (above). View WSW towards t he mouth of the Bla ckma ns Bay Blowhole . The P ermian sedimentary rocks di p at about 1 2 0 away from the camera . The s hore platf orm is devoid of fallen rocks (r e moved by wave action) .
Earth slide ( Hazard 7) Earth slide ( Hazard 7) Fallen tree (Hazard 8)
Rock falls ( Hazard 4 - 6) Silts to ne
P 2
Mudstone
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Earth sl ide ( Hazard 7) Fallen tree (Hazard 8)
P 3
Rock falls (Hazard 4 - 6)
Pla te 3 (abo ve). View WSW towards a major set of subvertical joints trendin g in the same direction .
P late 4 (belo w). View SSW . Note rock falls , and accelerated marine erosion WSW alo ng major join t s
Earth slide ( Hazard 7)
P 4
Ro ck fall (Hazard s 3 - 6 ) Siltstone S o urce for Hazard 3
Mudston e Ac celerated erosion
Rock fall (Hazard 3 )
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 35 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Earth slide ( Hazard 7) P 5
Relatively rapid e rosion Siltstone along m ajor WSW joint
s et
Fallen tree (Hazard 8)
M udstone Rock fall (Hazard s 3 - 6 )
Plate 5 (above). View W towards a major set of sub v ertical joints trending WSW . Boulder (ie joint s pacing) is smaller than elsewhere along the cliff line.
Plate 6 (belo w). View N . A majo r rock fall involving mostl y mudstone occurred s ince March 2008 (s ee Attachment 4).
Source for Hazard 1
R ecent (post - 2008 )
r oc k fall (Hazard 2) Siltstone P 6 Largest boulder approx. 9m x 8m x 5m = 360m 3 = 8 00 t
Boulde rs from older rockfalls removed by wave action
Mudstone
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A ttachme nt 4 ( 3 pa ge s ) Comparative satellite image ry sh owing aspects of slope instability over 9 .5 y ears Hazard numbers correspond to th ose in Figures 4 and 5 in the report. Sour ce: Google Earth . Note varying angu lar distortion between ima ges ; tides and sun angles differ
2008, 31 Marc h 20 17, 13 Nov ember
Rock fall (Hazard s 3 - 6 )
Grid North
0 50
Appro x. m etr es
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2008, 31 March 20 1 7, 13 Nov em ber
Rock fa ll (Hazard s 3 - 5 )
Gri d North 0 50
Appro x. m etres
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 38 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
2008, 31 Ma r ch 20 17, 13 Nov ember
Rock fall (Hazard 2 )
Largest boulder ap p rox. 9m x 8m x 5m = 360 m 3 = 8 00t
Gri d North
0 50 App ro x. m etres
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 39 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Attachme nt 5 ( 7 pa g e s ) Dynamic cone penetrometer (DCP) results
The Dyna mic Cone Penetrometer (DCP) Test is a standard m ethod of assessing the strengths of subsurface materials. A steel hammer weighing 9 k g falls 510mm down a steel rod onto a stop, driving the rod (with a 20mm dia meter steel cone tip) into the ground. The num ber of hammer blows t o pene trate each 100mm of d epth is recorded. The method is described in Australian Standard AS 1289.6.3.2 – 199 7 Method 6.3.2: Soil strength and consolidat i on tests – Determinatio n of the penetr ation resistance of a soil – 9 kg dynamic co ne penetrometer te st.
D CP locations are show n on th e satellite image below. DCP p rofiles and site photographs are on t he follow i ng five pag es. Published correlations betwe e n DCP values and some soil pr op erties are shown on Figure 4.1 (last page of th is Attachment).
D CP1
D CP 2
DCP 3
DCP 4
DCP 5
DCP 6
DCP 7 DCP 7 A
DCP 8 DCP 9 DCP 9 A
DCP 10
DCP 1 1
DCP 1 2 DCP 16 DCP 15 DCP 1 3
DC P 1 4
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DCP1 DCP 2
Bouncing on bedrock
Bounci n g on bedrock
DCP 4 DCP 3
B edroc k
B ed rock
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DCP 5
Bouncing on bedrock
DCP 6
B e drock DCP 7
Bouncing on be drock
DCP 7 A
Bouncin g on bedrock
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 42 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
DCP 8
Bouncing on bedrock
DCP 9
Bouncing on bedrock
DCP 9 A
Bouncing o n bedrock
DCP 10
B edrock
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DCP 1 1
Bouncing o n bedrock
DCP 1 2
Bounc ing on be drock
DCP 1 3
Bouncing on bedrock
DCP 1 4 B edro ck
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DCP 15
Bouncing o n bedr ock
DCP 16
Bouncing on bedrock
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(a) Allow able bear ing ca pacity from DCP results
Reference: Look, B. (2014). Handboo k of Geotechnical Invest igation a nd Des ign Tables (2 nd edition). CRC Press. The Netherlands. Table 5. 15. Th e Table applies to sh allow footings. Fa ctor of Safety =3. For high and low plasticity clays the allowable bearing capacity may be lower and higher, r e spectively.
(b) S oil and rock parameters from DCP results
Reference: Loo k, B. (2014). Handboo k of G eotec hnical Investiga tion and Design Ta bles (2 nd edition). CRC Press. The Netherlands. Table 5.11.
(c) CBR values from DCP results
Reference: Look, B. (20 14). Hand book o f Geotechnical Investigation and Design nd T ables (2 edition). CRC Pr ess. The Netherlands. Table 5.13. Figure 5 .1 . Published correlations between DCP values and some soil pr o p erties Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 46 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Attachme nt 6 (1 3 pages) Landslide Risk Manag ement
This At ta chment addresses slope stability is sues ( rockfall /topple, landslide a nd tree falls ) for Hazards 1 – 8 descri bed in Section 3 of the report . Hazard locations are along and adjacent to the se a cliff in the study area d epicted in Attachment 2. This Attachm en t is in accordance with Australian Geomechanics Society (AGS) Landsli de Risk Management (200 7) 9 . The process is depi cted in Figure 6 .1 . The main types of landslide movement are shown in Fig ure 6 .2 and Tab le 6 .1 .
Figure 6 .1. Framework for Landsli d e Risk Management Source: Rep roduced withou t amendment from AGS (20 07a). Gu idelin e for Landslide Susceptibility, Hazard an d Risk Zoning. Austra lian G eomechanics, Vol 42 N o 1 March 2007
9 The five A GS d ocuments are: AGS (2007a). Guideline for Landslide Suscept ibil ity, Hazard and Risk Zoning. Australian Geomecha nics, Vol 42 No 1 March 2007 AGS (2007b). Commentary on Guideline for Landslide Susceptibility, H az ard and Risk Zoning. Australian Ge omechan ics , Vol 42 No 1 March 2007 AGS (2007c). Practice Notes Guidel ines for Landslide Risk Manageme nt. Australian Geom echanics Vol 42 No 1 March 2007 AGS (2007d). Commentary on Practice Notes Guidelines for Landslide R isk Mana gement. Australian Geo mec hanics Vol 42 No 1 March 2007 AGS (2007e). The Australian Geoguides fo r Sl ope Management and Maintenan ce. Australian Geom echanics Vol 42 No 1 March 2007 Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 47 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Figure 6 .2 Main types of landslide movement Source: From Append i x B of AGS (2007c). Practice Notes Guideli n es for Landslide Risk M anagement . Aus tralian Geomechanics Vol 42 No 1 March 20 07
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Table 6 .1 Main types of landslide movem e nt Source: From Ap pendix B of AGS (2007c). Practice Notes Guideli nes for Landslide Risk Management. Australian G eomechanics Vol 42 No 1 March 2007
L ANDSLIDE R I SK M ANAGEMENT (LRM)
6 .1 Preliminary 6 .1.1 Desktop r eview of slope instabili ty A review of the Mineral Resources Ta smania lands lide map and database revea led no known and c atalogued instances of slope instability in or adj ac ent to the study area .
Attachmen t 2 shows the parts of the cliff line are in the Low and Medium Landslide Ha zard B an ds 10 .
6 .1.2 Field evidence and mechanisms/trigger s of slope instability Rockfalls and t opple s Rockfalls and rock topples are the mai n for m s of slope instabil ity in the study area. Aerial and sate llite imagery sho ws the foresh ore is locally littered with rock debris . A ll sizes are involve d, from failures involving single rock fragments weighing less than a kilog ram , to in d ividual cobbles wei ghing a few tens of kilograms, to in dividual boul ders weighing several tonnes, to compos ite falls of thousands of we ll - graded rock material or boulders collectively involving hun dreds of tonnes . Exa mples are shown in the photographs in Attachments 3 and 4.
Earth slides V ery small to small e a rth sli des i nvol ving only the soil profile are evid ent at several locations along the cliff line. Most are focussed where ma jor joints trend WSW, and s ome extend down t h e cliff face (Attachment 3).
Mechanisms for rock falls/topples are primarily gravity - induced movem ents on bedding surfaces , joints and ot her defects, c aused by slow weakening or erosion of fine particles by physical and chemical wea t hering by wi nd and water . Triggers for movement are related to the frequ ency and intensity of ra in events, an d less commo nly to earthquake vibrations. T h e earth slides move over bedrock, and are triggered also mainly by rain.
6 .1.3 Site plans and maps Sit e plans and maps are incl uded in Attachments to this repo rt.
10 Landslide Hazard Bands are computer - generated models from publish ed geology at various scales and slo pe angl es. Individual pixels on hazard band maps may be the size of a hous e or larger. The banding may need to be amended after site in spection. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 49 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
6 .2 Sit e sections and conceptual geological mode ls C onceptual geological cross section s are presente d in Figures 5 and 6 in the report, at the locations s hown in Figure 7 .
6 .3 Hazard Analys is 6 .3.1 Hazard cha racterisation As summarised in Se ction 3.1.2, t hree t ypes of geotechnical hazards are identifi ed in the study area , and are shown in Figure s 5 and 6 :
Hazards 1 – 6 are roc kfalls/topples involving dif ferent volumes of material – from fail ures of hundreds of tonnes ( H azard s 1 and 2), to ind ividual small boulde rs weighing a few kilograms (Hazard 6) 11 . Examples of these hazards are depicted in Plates 2 – 6 in Attachment 3, and in Attachment 4.
Hazard 7 a re very small landslides (earthslides 12 ) from at or near t he top of th e sea cliffs, involvi ng failure of the so il profile . Examples are shown in Plates 2 , 3 and 5 in Attachment 3 .
Hazard 8 ar e tree and limb falls . Examples are depicted in Plates 2 , 3 and 5 in Attachment 3 .
The hazards are char ac terised as follow s :
Scenario 1: Rockfall/ rock top ple Small scale; in siltstone and sandstone, from any part of the cliff face; ex tremely rapid; invol ving a width of cliff face o f at leas t 20m .
Scenario 2 : Rockfall/ rock topple Small scale; in mudstone , from the lower part s of the cliff face; ex tremely ra pid; involving a wid th of cliff face o f at leas t 20m .
Scenar io 3 : Rockfall/ rock topple Very s mall s cale; in siltstone, sandstone or mudstone , from any part of the cl iff face; ex tremely rapid; involving o ne or more boulders over a width of cliff face o f at leas t 5 m .
Scenario 4 : Rockfall / rock topple Very s mall scale ; in siltstone, sandstone or mudstone , from any part of the cliff face; ex tremely rapid; involving one or more boulders over a width of cliff f ace o f at leas t 1 m .
Scenario 5 : Rockfall/ rock toppl e Very s mall scale; in siltstone, sandstone or mudstone , from any part of the cliff face; ex tremely rapid; involving one or more boulders over a width of cliff face o f at leas t 0.5 m . . Scenario 6 : Rockfal l/ rock topple Very s mall scale; in siltstone, sandst one or mudstone , fro m any part of the cliff face; ex tremely r apid; involving a single boulders (join t fragment ) up to 0.1m in diameter .
Scenario 7: Eart h slide Very small scale landsl ide, from a t or near cliff top , up to 5 m wide and less than about 2m 3 in volume ; ext rem ely r apid.
Scenario 8 : Tree or tree limb fall Very small scale , from a t or near clif f top , with a downslope affected zone up to about 5m wi de; ext rem e ly rapid.
11 The inset Tables on Figures 5 and 6 define landsl ide size and speed. Ev en rockfal ls of hundred s of tonnes a re “sma ll ” . 12 Earthslides are landslides involving mostly soil , with o nly minor proportions of rock. Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 50 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Note that the nominal bould er s izes of Scenari os 1 – 6 are observ able in the st udy area. The sizes chosen are intended as examples only, an d any size c ould of course be represent ed .
6 .3.2 Frequency analysis Ta ble 6 .2 lists the su bjective like lihood of occurrence of the scen arios shown in F igures 5 and 6 under current conditions. T erminology for measu re s of likelihood are explained in Attach ment 7.
6 .3.3 Elements at risk Elemen ts at risk for Scenarios 1 – 8 include sightsee rs, recreational walkers along the top and base of the cl iff , a bseiling indi vidual s and groups, kayakers , swi mmers and cliff jump ers .
It is envisaged that no rockfall/ topple or any other f orm of landslide c an credibl y involve occupants of houses on the western side of Talone Road , or pedestr ians and occupants of vehicles on the road.
Table 6 . 2 Evidenc e for and subjective likelihood of occurrence of Sc enarios 1 – 8. 1 2 3 Current Scenario Fie ld Evidence likelihood
Rockfall/rock topple At least one source identified Small scale; in siltst one and sandstone, from any 1 northeast of study area (see P late Possible part of the cliff face; extremely rapid; involving a 6 in Attachmen t 3) width of cliff face o f at least 20m.
Rockfall/rock topp le One recent (<2 0 year old) example Small scale; in mud stone, from the lower parts of the 2 identified nort heast of study ar ea Likely clif f face; extremely rapid; involving a width of cliff (see Plate 6 in Attachment 3) face of at least 20m.
Rockfall/rock topple Very small s cale; in siltstone, sandstone or Almost 3 mudsto ne, from any part of the cliff face; extremely Certain to rapid; involving one or more boulders ove r a width of Likely cliff face of at least 5m.
Rock fall/rock topple Ver y small scale; in siltst one, sandstone or Within study area, at least one 4 mudstone, from any part of the cliff face; extremely source identified (see P late 4 in r apid; involving one or more boulders over a width of Attachment 3), and the results of cliff face of at least 1m. many rockfalls/topples ar e the Rockfall/rock topple range of boulder sizes observed on Ver y small scale; in siltstone, sandston e or shore platform (see P lates 3 - 6 in 5 mudstone, from any p art of the cliff face; extremely Attachment 3 , and Attachment 4) rapid; involving one or more boulders over a width of cliff face of at least 0.5m. Rockfall/rock topple Very small scale; in siltstone, sandstone or Almost 6 mudston e, from any part of the cliff face; e xtremely Certain rapid; involving a single boulder (joint fragmen t) up to 0.1m in diameter. Earths lide Several examples observ ed at and Very small scale landslide, from at or near cliff top, near cliff top along fu ll length of 7 up to 5m wide and less than about 2m3 in volume; study area (see Plates 2, 3 and 5 in extremely rapi d, moist to wet, suspended to active Attachment 3) Tree or tree limb fall Several examples observed a t and Very small scale fallen tree and/or limb, from at or near cliff top along full length of 8 near cliff top, wi th a downslope af fected zone up to study area (see Plates 2, 3 and 5 in ab out 5m wide; extremely rapid. Attachment 3 ) Notes Column 1: refer to cros s sections (Figures 4 and 5 in repo rt) for explanati ons of size and spee d. Note definition of size is not from AGS2007 c. Column 3: refer to Attachment 7 for an explanation of Likelihood terms.
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6 .4 Consequence ana lysis and qualitative ris k to property estimation
Table 6 .3 is a consequence an alysis and risk t o property (assets) assessment for the eight scenarios in Figu res 5 and 6 . The assets are Talone Road and the fence along the top of the cliff line . There are no assets on or along the base of the cliff line.
Current subjective risk t o assets is judged t o be Low for S cenarios 1 and 8 .
No risk treatment is re commended for any Scenario.
Table 6 .3 Likel i hood , consequence and qualitative risk analysis of Sce narios 1 – 8. Risks to infrastructure are Low for all hazards. 1 2 3 4 5 6
d
o
n d t o o t f
t t a e w n
o e e
d e k k o
c c e l d s s n v d s n n i i e m o t e t r r n e e o b
i e o a r f f u b m r s Scenario (Existing conditions) h h s e o o a i q t r u s e
l m l l t e e c a s b e o e e s t s c k k c v v i e n s s o e e e i s L a o r L L R s C a
o t
Rockfall/rock topple
d Small scale; in siltstone and sandstone, from any m a u
1 Unlikely i o d part of the cliff face; extremely rapid; involving a r width of cliff face of at least 20m. e M
Rockfall/rock topple Small scale; in mudstone, from the lower parts of the 2 Likely cliff face; extremely rapid; involving a width of cliff face of at least 20m.
Rockfall/rock topple Very small scale; in siltstone, s andstone or Almost 3 mudstone , from any part of the cliff face; extremely Certain rapid; involving one or more boulders over a width of to Likely
cliff face of at least 5m. ) w o Rockfall/rock topple l e b
Very small scale; in sil tstone, sandstone or s t
4 mudstone, fr om any part of the c liff face; extremely e s
rapid; involving one or more boulders over a width of s a cliff face of at least 1m. o
t Low n None ( n Rockfall/rock topple a e l c i b Very small scale; in siltstone, sandstone or f i a n c
5 mudstone, fro m any part of the cliff face; ext remely i g l i p rapid; involv ing one or more boulders over a width of s p n I a
cliff face of at least 0.5m. t o
Rockfall/rock topple N Very small scale; in siltstone, sandstone or Almost 6 mudstone, from any part of the cliff face; extremely Certain rapid; involving a single boulder (join t fragment) up to 0 .1m in diameter.
Earthslide Very small scale landslide, from at or near cliff top, 7 up to 5 m wide and less than about 2m3 in volume; extremely rapid, moist to wet, suspended to active Tree or tree limb fall Very small sc ale fallen tree and/ or limb, from at or 8 near cliff top, with a downslope affected zone up to about 5m wide; extremely rapid. Notes Column 1: refer to cross sections (Figures 4 and 5 in report) for explanations of size and speed. Not e definition of size is not from AGS2007c. Column 2: refer to Attachment 7 for an explanation of Likelihood terms. Column 3: refer to Attachment 7 for an explanation of Consequence terms. Assets( ie property, infrastructure) in thi s instance = Talone Road and fence Column 4: refer to Attachment 7 for an explanation of Risk terms.
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6 .5 Quantitat ive risk to life estimatio n for various site activities
6.5.1 Exam ple situations in the study area
Risk to life is more i mportant th an risk to pr operty for t he study area. The following four examples ( Figure 6.3) ar e consider ed reasonable examples of existing activities in the stud y area . Other examples could of course also be generated.
Fi g. 6.3( a) . Example 1. A mo bile recreational Fig. 6.3.(b) . Example 2. Sightseers ( 3 people at i n d ividu al walking on the shore platform , subject a time ) static at the fen c e line at the Talone to hazards 1 – 8 . Street car p ark , subject to hazard 1 only . S ee Figures 5 and 6 for cross sections . S ee F igure 5 for cross section . See Table 6.2 for brief descriptions of hazards. See Table 6.2 for brief descriptions of hazards. Indivi dual makes a return trave rse ( red dotted line) On average 3 people stand as a group (red oval) at the once a fortnight along full fores hore length (35 0m) = 25 sightseeing spot in the carpark for 10 ho urs a day, 365 re turn traverses a year . days a year.
Top of Base of Top of Base of cliff cliff cliff cl iff Shore platform Shore platform
Track of individual ( return trip)
Cliff Water Cliff Water Sigh tseeing group Mouth Mouth
Carpark Carpark
Blowhole Fence Blowhole Fence
Fig. 6. 3 ( c ) Example 3. A s mall static abseiling Fig. 6 .3(d). Example 4. A n ind ividual mobile group ( 5 people) at the base of th e cliff near the kayaker o r swimmer ent ering the blowhole v ia its blowhole mouth , subject to hazards 1 , 3 and 4 . mouth (calm s eas are presumed) , subject to S ee Fi gures 5 and 6 for cross sections hazards 3 and 4 . See Table 6.2 for br ief desc riptions of hazards. S ee Figu re 5 for cross section U p to 5 people stand as a g roup (red oval) on the shore See Table 6.2 for b rief descriptions of hazards. platform nea r the blowhole mouth for 15 mins 25 times One kayaker or swimmer spends 0. 2 5 hours on a year. average each week at the site.
Top of Base of Top of Base of cliff cliff clif f cli ff
Shore Shore p latform platform
Cliff Water Cli ff Water
Mouth Mouth Carpark Carpark Track of A bseiling kayaker or group swimmer Blowhole Fen ce Blowhole Fence
Figure 6 . 3 Schematic plans for exampl es 1 – 4.
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6.5.2 Example 1 [ Figure 6. 3 (a) ]
In this exa mple, a mo bil e recreational i nd ividual walks along the shore platform , and is exposed to hazards 1 – 8 . He or she is as sumed to be able to navi gate across the mouth of the blowhole, and walks th e full length of the study area – about 350m. Travelling at 1 km/ ho u r, a si ngle traverse takes abou t 20 m inutes, and th e return trip a similar time. T h e individual does this once a for tnight on ave rag e , for a total of 16 hours per year, and a temporal probability - 3 [P ( T:S) ] of 2 x 10 .
The foreshore in this example is consider ed an existing site/slope.
F requency analysis inferred from sit e obser vatio ns provides a range of likelihood s [P (H) ] for the 13 haz ards , summarised in Table 6.4. T h e summed risk to lif e [ R (LOL) ] ] from all eight hazards for this individual is estimated at 6 x 10 - 6 .
Table 6 . 4 Risks to Life for Example 1
Scenario P [H] n i w f d V [D :T] s i R [LOL] 1 0.001 50 1 1 20 1 1 8E - 08 2 0.005 50 1 1 20 1 1 4E - 07 3 0.05 50 1 1 5 1 1 1E - 06 4 0.5 50 0.75 1 1 1 1 2E - 06 5 1 50 0.5 1 0.5 1 1 1E - 06 6 10 50 0.25 1 0.1 0.5 1 2E - 07 7 0.1 50 0.5 1 5 0.3 1 1E - 06 8 0.1 50 0.5 1 5 0.5 1 1E - 06 Total individual risk is sum of risks 6E - 0 6 Th e F - N pair s for hazard 1 , and all hazards com bined , for this individual most at ris k plot on the Acceptable region of the soci etal risk graph (Figure 6.4) .
6.5.3 Example 2 [Figure 6. 3 (b)]
In this static example, on average, there are 3 people sightseeing at the fen c e line at the Talone Street carp ark for 10 hours a day, 365 da ys a ye ar, for a total exposed populati on (e) of 43,800 people a year. Each pe rson spen ds 15 minutes at the sit e , and is subject only to - 4 H azard 1 where P( H ) = 1 x 10 . The site is an existing element at risk.
An individual is p resent for 0. 25 hours, whic h is 2.9 x 10 - 5 as a proportion of th e year. The annual risk to l oss of life to this ind ividual most at risk is 1 x 10 - 4 x 1 x 2.9 x 10 - 5 x 1 = 3 x 10 - 9 .
13 Explanation of symbols used in risk to life assessment
P [H] the annual probability that the rockfall/topple tree fall occurs
n i the number of traverse s the i ndi vidual most at risk makes annually w the proportion of the shore platform width affected by the rockfall/topple (range 0 – 1) a reduction factor to reflect probability of individual being present when rockfal l occurs (range 0 – 1; 1 = no f redu ction) d boulder diameter or tree canopy size (metres)
V [D:T] Vulne rability of the individual most at risk impacted by the rockfall/topple (range 0 – 1; 1 = certain death)
s i speed (km/hr) of the individual most at ri sk along the shore platform (1km /hr for 35 0m long foreshore = about 40 minutes spent on foreshore dail y by the individual) R [LOL] the annual probability of loss of life of the individual most at risk. Derived from equation 3. 1 in AGS2007c: R[LOL] = P[H] x P[S:H] x P[T:S] x V[D:T], where P [S: H] is probability of spatial impact . In example 1, it is = wd/l where l = length of shore platform, and where P[T:S] = temporal probability
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To calculate the F - N pair for this situation, the temporal probabil ity [P( T : S )] is 0.42 (10 out of 24 hours every day). The pro bability of t he hazard impacting the people is F = 1 x 10 - 4 x 1 x 0.42 = 4 x 10 - 5 . Ass ume all 3 people are killed i f the ha zard occurs (N = 3).
This F - N pair plots in the ALARP (As Low as R e a s o n a b l y P r a c t i c a b l e ) region of the soc ietal risk graph (Figure 6. 4) .
6.5. 4 Example 3 [Fi gure 6. 3 ( c )]
In this static example, 5 a bseilers at a ti me g ather at the base of the a bseiling site below the Talone Street carpark , and spend 15 minutes each on the shore platf orm befor e climbing back up the cliff (a ll under supervis ion). The group repea ts this ex ercise 25 times a year.
The si te is an existing el ement at risk.
The group is at risk o f life from exposure to Hazards 1, 3 and 4 . I t is assumed that death wo uld not r esult from hazards 5, 6 and 7, and haza rd 2 does not exist since th e mudst one at the site is below th e abseil e rs .
F or i ndividual risk to li fe :
f or Haza rd 1, a n individual is present for 0. 25 x 25 = 6 hour s per year , which is 7 x 10 - 4 as a proportion of th e year. The annual risk to l oss of life to this individual most at risk is 1 x 10 - 4 x 1 x 7 x 10 - 4 x 1 = 7 x 10 - 8 .
f or H aza rd 3 , t he annual risk to l oss of life to this individual most at risk is 2 x 10 - 2 x 1 x 7 x 10 - 4 x 1 = 1 x 10 - 5 , and
f or Hazard 4, the annual risk to l oss of life to this in dividual most at risk is 5 x 10 - 1 x 1 x 7 x 10 - 4 x 1 = 3.5 x 10 - 4 .
For group ris k to life, it is ass umed that only Hazards 1 and 3 are capa ble of causing multiple deaths. T o calculate the F - N pair for g r o u p s o f five people at this site , the temporal proba bil ity - 3 - 3 [P( T: S )] is 3.5 x 10 ( ie 5 x 0.2 5 x 25 = 31 hours per year = 3.5 x 10 a s a proportion of a year ). The pro bability of hazard 1 impacting the people is F = 1 x 10 - 4 x 1 x 3.5 x 10 - 3 = 3.5 x 10 - 7 . The pro babi lity of hazard 3 impacting the people is F = 2 x 10 - 2 x 1 x 3.5 x 10 - 3 = 7 x 10 - 5 .
Assume all 5 people are killed i f eith er ha zard occurs (N = 5 ).
Th e F - N pair for hazard 1 plots in the Acceptable region of the societal risk graph (Figure 6.4) .
Th e F - N pair for hazard 3 plots on the ALARP region of the soci etal risk graph (Figure 6.4) .
6.5. 5 Example 4 [Figu re 6. 3 ( d )]
In this mobile exampl e, one in dividual kayaker or swimmer per week enter s the blowhole v ia its mouth (calm seas are presumed necessary for entry, but are not common ) . The individual is subject to hazards 3 and 4 . I t is assumed that hazard 1 is not credible inside the blowhole , death wo uld not result from hazards 5, 6 and 7, and hazard 2 does not exist since th e mudst one at the site is below th e abseil e rs . The blo whole and mouth is an existing element at ris k.
Each individual spends 0. 2 5 hours at the site. F or individual risk t o life :
f or Haza rd 3 , a n individual is present for 0. 25 x 50 = 12 hour s per year , which is 1 x 10 - 3 as a proportion o f th e year. The annual risk to l oss of life to this individual most at risk is 2 x 10 - 2 x 1 x 1 x 10 - 3 x 1 = 2 x 10 - 5 .
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f or Haza rd 4 , t he an nual risk to l oss of life to this individual most at risk is 0.5 x 1 x 1 x 10 - 3 x 1 = 5 x 10 4 .
Th e F - N pair for haz ard 4 for this individual most at ris k plot on the ALARP region of the soci etal risk graph (Figure 6.4) .
1E - 02
1E - 0 3 UN ACCEPTABLE F – N pair for Example 4 , Hazard 4
s e i
t i
l 1E - 0 4 a
t F – N pair for Example 3 , Hazard 3 f F – N pair for Example 2 , Hazard 1 N
t s
a e l
t 1E - 0 5 ALARP a
f F – N pair for Example 1 , all hazards o
y
t i l i
b a
b 1E - 0 6 o r
p
l F – N pair s for Hazard 1 a F – N pair for Example 3 , Hazard 1 u plot along this line n
n A 1E - 0 7 ACCE PTABLE –
F Intense scrutiny r egion F – N pair for Example 1 , Hazard 1 1E - 0 8
1E - 0 9 0.001 0. 01 0. 1 1 10 100 1000 10000 N – Number of Fa talities
Figure 6 . 4 S ocieta l risk graph for risk s to life estimated for Examples 1 – 4 in the study area . The acceptability show n here of the risks is for guidance only a nd is based on AGS2007c Section 8 Table 1 reproduced here in Figure 6.5. Kingborough Council as regulator shoul d assess the acceptability of the risks against its own organisational criteria.
6 .6 Comment on acceptability criteria for landslide risk 6.6.1 Acceptability of risk t o property
The regulatory author i ty sha ll determine whether or not a risk to prop erty is acceptable , tolerable or otherwise .
Guidance is provi ded in th e AG S (2007c) tables in Attachment 7.
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 56 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
6.6. 2 Acceptability of risk to life
The regulatory author ity shall determine whether or not a risk to lif e is acceptable , tolerable or otherwise . G uidance is provi ded in Figure 6. 5 , which is presented unamended from AGS (2007c ) Section 8.
Figure 6 . 5 Guidance on risk to life crite ria (AGS (2007c), Section 8 (Table 1 and its accompanying notes on pages 77 and 78) . In relation to Notes 1 – 5 , it is suggested that the sea clif fs in the study are a at Blackmans Bay would reasonably fall into “ Existing Slopes ” since a very large proportion of them show no recognisable landslid ing “ over at least several seasons … ” even though their very existence is due to slope instability ( albeit mostly over much longer time frames ). Categorising the cliffs as “ Existing Landslides ” woul d seem unreasonable , since in this report none of the observed landslides or rockfalls is regarded as requiring remediation.
6 .7 Adherence of this LRM to AGS (2007)
Table 6 .5 compares the items required by A GS (2007c) to be ad dress ed in LRM , with this report.
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 57 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Table 6.5. Extent of ad herence of this report to AGS (2007c) reporting requirements AGS (2007c) Section 10
REPORTING STANDARDS
"Section 10.1 The report o n the risk assessment is to document the data gathered, the logic ap plied and conclusion reached in a defensible manner.
The practitioner will gather relevant data, will assess the relevance of the data and will reach conclusions as to the app ropriate geo techn ical model and basic a ssessment of the slope forming processes and rates. Full documentation of these results provide s evidence of completion, provides transparency in the light of uncertainty, enables the assessment to be re - examined or e xtended at a late r date and enables the assessment to be defended against critic al review. The process often identifies uncertainties or limitations of the assessment which also need to be documented and understood."
Review of the current repor t
Stated explanation Data included in "Section 10.2 The data to be presen ted includes: for omission of report? missing data
a. List of data sources. Yes b. Discussion of investigation methods used, and any limitations thereof. Yes c. Site pl an (to scale ) wit h geomorphic mapping r esults. Yes d. All factual data from investigations, such as borehole and test pit logs, laboratory test results, groundwater level Yes observations, record photographs.
e. Location of all subsurface investi gations and/ or ou tcrops/cuttings. Yes
f. Location of cross section(s). Ye s g. Cross section(s) (to scale) with interpreted subsurface model showing investigation locations. Yes
h. Evidence of past performance. Yes i. Local history of in stability wi th as sessed trigger events. Yes j. Identification of landslides , on plan or section or both, and discussed in terms of the geomorphic model, relevant slope forming process and process rates. Landslides need to be considered above the site, below the s ite a nd adjacent to the Yes sit e.
k. Assessed likelihood of eac h landslide with basis thereof. Yes l. Assessed consequence to property and life for each landslide with basis thereof. Yes
m. Resulting risk for each landslide. Ye s n. Risk asse ssment in relation to tolerable risk criteria (e.g. regulator’s published criteria where appropriate). Yes
o. Risk mitigation measures and options, including reassessed risk once these measures are implemented." Yes
"Where any of t he above is not or can not be completed, the report should docum ent the missing elements, including an explanation as to why. The report needs to clearly state whether the risk assessment is based on existing conditions or with risk treatme nt measures imple mented. In some cases, the assessment for bo th existing and aft er treatment should be documented to demonstrate the effect of risk control measures on reducing risk. A report which does not properly document the assessment is of limited va lue and woul d app ear to have no reasona ble basis."
6 .8 Certificate of currency for Profe ssiona l Indemnity Insuran ce
A copy of the certificate of currency for Professional Indemnity Insurance for Willi am C Cromer Pty Ltd is included here as Figure 6 . 6 .
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 58 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Figure 6 . 6 Confirmation of insu rance for William C Cro mer Pty . Ltd.
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 59 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Attachment 7 ( 2 pages) Quali tative terminology used in assessing risk to property Source: Appendix C of AGS2007c.
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com 60 KINGB OR OUGH COUNCIL: BLA CK MANS BAY B LOWHOLE & ENVIRONS G eotechn ic al Repor t 21 January 2020
Wil liam C Cromer Pty Ltd 74A Cha nnel Highway T a r o on a T asma ni a 7053 Envir onmental, en gine ering an d groundwat er geolog ists M 0 40 8 122 127 E bil l crome r@bigpo n d. com W www. will iamccro mer. com