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LANDSLIDES ON THE BELLARINE AND NEPEAN PENINSULAS,
VICTORIA
1
Robert Wilson1 and Anthony Miner2
Coffey Geotechnics Pty Ltd, Melbourne, 2A.S. Miner Geotechnical Pty Ltd, Geelong
1
INTRODUCTION
This paper provides a brief overview of landslides on the Bellarine and Nepean Peninsulas. The main types of
landslides are discussed and selected brief case studies are presented to illustrate the types of slope instability. This
paper follows a brief paper outlining the extent and nature of the landslides in and around the Greater Melbourne
metropolis that was presented in the Engineering Geology of Melbourne (Wilson, 1992 in Peck et al., 1992).
The Bellarine and Nepean Peninsulas lie at the southern end of Port Phillip Bay about 30 km SSW of the Melbourne
(Figure 1). The Bellarine Peninsula is about 30 km long (east–west) by about 15 km wide. Relief on the Bellarine
Peninsula is typically low with the exception being the prominent 20 m high cliffs that extend along the north side of
the peninsula (Figure 2). The highest point on the peninsula is Mt Bellarine (RL 137 m AHD). The Nepean Peninsula
is about 20 km long (east-west) by up to 2 km wide (Figure 8). The highest point on the Nepean Peninsula is Cheviot
Hill (RL 40 m AHD).
Figure 1: Geology of the Bellarine and Nepean Peninsula (modified after 1:250,000 Queenscliff Sheet).
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Figure 2: Aerial photograph of part of the Bellarine Peninsula looking to the southeast over Clifton Springs. Note the
general low relief and the cliffs along the northern side of the peninsula. Mt Bellarine is at the centre of the photograph.
2
GEOLOGY
Figure 1 is a geological map of the peninsulas and Figure 3 is a cross-section extending through both peninsulas.
2.1
STRUCTURAL SETTING
The landforms in the southern Port Phillip area reflect the centrally located and active sinking Sorrento Graben bounded
to the west and east by two more static horsts comprising the Bellarine and Mornington Peninsulas (Holdgate 1976).
The Bellarine Peninsula is an uplifted horst block bounded by an unnamed fault in the west, the Curlewis Monocline to
the north and the Bellarine Fault in the east. The Nepean Peninsula lies within the Sorrento Graben and is bounded by
the Bellarine Fault to the west and by the Selwyn Fault to the east. These two peninsulas are separated by a narrow
deeply incised submerged channel (The Rip) at the heads.
Figure 3: Section through the Bellarine and Nepean Peninsula (modified after Leonard, 1992).
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2.2
GEOLOGICAL SEQUENCE
Despite its small size, the Bellarine Peninsula hosts the complete sequence of Quaternary and Tertiary geological units
that make up the surface geology of Melbourne, including:
•
Quaternary (Holocene) age alluvium including coastal and inland calcareous and siliceous dunes and sand
sheets, as well as alluvium, lacustrine and swamp deposits.
•
Quaternary (Pleistocene) age Bridgewater Formation calcareous and siliceous dunes and Newer Volcanic basalt
with minor scoria and ash.
•
Tertiary (Pliocene) age non-marine to marginal marine, partly ferruginous sands, clayey sands, clay, and gravels
of the Moorabool Viaduct Sands (equivalent to Brighton Group in Melbourne).
•
Tertiary age marine sediments including limestones, marls, calcareous clays, sand and silt of the Jan Juc
Formation, Curlewis Limestone and Fyansford Formation; overlying Tertiary age basalts and tuffs of the Older
Volcanics and non-marine gravels, sands and clays of the Werribee Formation.
The main difference with Melbourne is that the oldest rocks exposed on the Bellarine Peninsula comprise Cretaceous
age sandstone, arkose, shale and mudstone (Eumeralla Formation, Barrabool Sandstones), as opposed to Silurian –
Devonian age turbidites.
In contrast to the geologically diverse Bellarine Peninsula, the surface geology of the Nepean Peninsula is very simple.
In essence, it is a Pleistocene sand barrier (Bridgewater Formation) that has retained its form to the present day.
A summary of the landscape evolution of the southern end of the Port Phillip sunkland is presented as in Table 1.
Table 1: A summary of the landscape evolution of the southern end of the Port Phillip.
Geological
period/time
(approximate)
Palaeozoic to Early
Mesozoic
Early Cretaceous
Mid Cretaceous
Tertiary
Early to Mid Eocene
Late Eocene to
Early Miocene
Mid Miocene
Late Miocene
Early Pliocene
Late Pliocene
Pleistocene
Quaternary
3
Event
General area part of the north-south trending Lachlan Geosyncline with ongoing sedimentation.
Australia moves away from Antarctica and the first east-west faults trend across the Lachlan
Geosyncline. Period of extensive sedimentation from the north.
Extensive block faulting warping and minor folding occurred. Otway and Barrabool highs
uplifted whilst several areas of subsidence occurred including the Sorrento Graben and Port
Phillip Sunkland, with associated deposition (Eumeralla Formation, Barrabool Sandstones).
Subsidence along faults accompanied by deposition in the Sorrento Graben (Werribee
Formation).
Minor tectonic uplift and extrusion of basalt over Bellarine Horst (Older Volcanics).
Sea advances into Port Phillip bay via Moolap Sunklands and Sorrento Graben with period of
deposition. The sea depends and ongoing deposition of marls and calcareous clays (Fyansford
Formation).
Sea retreats and further deposition of sandy marl and calcarenites (Jan Juc Formation, Curlewis
Limestone).
Renewed tectonic activity. Gradual uplift on horsts but increased period of subsidence in the
Sorrento Graben allows further sedimentation.
Advancing sea deposits gravels, sands and sandy clays (Moorabool Viaduct Sands).
Retreating sea terminates further sedimentation.
Due to sea level changes, thick deposits of wind blown materials on the southern margins of the
Port Phillip Sunkland are transported from a broad plain in what is now Bass Strait.
Further uplift to present elevations. Ongoing dramatic climate and eustatic sea level changes.
INSTABILITY OF THE CLIFFS ALONG THE NORTHERN SIDE OF THE
BELLARINE PENINSULA
3.1
OVERVIEW OF LANDSLIDES
Landslides are common in the cliffs that extend along the north side of the Bellarine Peninsula, between Curlewis in the
west and Portarlington in the east (Ladd, 1971).
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The landslides near Curlewis in the west have developed within the Fyansford Formation and tuffaceous soils of the
Older Volcanics, both of which are characterized by fissured clays of high plasticity and low residual angles of friction.
As such these slides tend to be translational or shallow rotational and are capable of undergoing large displacements.
Glennon (1980) studied the 1978 landslide at Avila Road and determined a depth to length ratio of 1:20 with strength
parameters at failure of c’=0 and Ø’ = 10 indicating residual properties and probable reactivation of an older landslide.
A series of rotated medium sized (order of 10,000 to 100,000 m3) landslides are present between The Dell at Clifton
Springs and Portarlington including the Edgewater Landslide (Figure 4) and the large earth slide at McAdams Lane
(Rosengren 1988). The landslides at The Dell (Figure 6) and at the second Dell in front of the Clifton Springs golf club
have developed along the alignment of recessed drainage features perpendicular to the coast and now form natural
amphitheatre type features. Detailed investigations at the Dell (Parsons Brinckerhoff, 2003; Coffey, 2006a) indicate
these areas are in fact landslide complexes developed in the tuffaceous soils of the Older Volcanics.
A study by Ladd (1971) noted a series of back tilted rotational slides directly on the coast and postulated that whilst
much of the displaced materials were ferruginous Moorabool Viaduct Sands the failure had in fact occurred in the
underlying Miocene age Fyansford clays. A later study by Coffey (Coffey, 2006b) confirmed the locations of a number
of these back tilted blocks (Figure 5) in addition to a number of smaller rotational slides composed solely of clays.
3.2
CAUSE OF LANDSLIDING
The landslides that are present along this section of the coast are strongly influenced by the presence of the Curlewis
Monocline. The monocline extends between Curlewis and Portarlington and dips to the northwest. It is considered to
represent the partial re-activation of a northeast-southwest striking fault in the underlying Otway Group rocks.
The main contributing factors to the landslides, specifically the presence of the cliffs, the dip of the weak materials
towards the Bay, and the presence of areas of groundwater discharge (with associated high piezometric pressures) are
linked in some degree to the monocline.
Figure 4: Edgewater Landslide
3.3
Figure 5: Back-tilted toe
THE DELL – AN EXAMPLE LANDSLIDE
The Dell is a public park located at Clifton Springs, about 15 km east of Geelong. It is located within a prominent
natural amphitheatre (Figure 6) that is about 150 m across (parallel to the coast) by 100 m long (perpendicular to the
coast). The Dell formed part of a mineral springs resort between the 1870s and the 1920s (Figure 7). Changes since
then include removal of material from the beach and construction of visitor facilities.
The Dell was closed in July 2002 as the result of an extensive tension crack opening up around the head of the site and
subsequent geotechnical investigations and monitoring which indicated a large landslide (or landslides) was developing
at the site (Garrard et al., 2003).
The geology of The Dell comprises lower Tertiary age Older Volcanics and interbedded Werribee Formation sands and
clays overlain by orange brown sands and clays of the Pliocene age Moorabool Viaduct Formation (Coffey, 2006 a).
The contact of the Moorabool Viaduct Formation and the underlying Older Volcanics, where these beds have been
unaffected by landsliding, dips northwest at about 4o at The Dell. The volcanic beds at sea level either side of The Dell
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dip northwest between about 20o and 35o. To date, no reliable in situ bedding dips have been recorded for the volcanic
beds within The Dell.
Figure 6: Aerial view of The Dell.
Figure 7: Historical photograph of The Dell (from
Bellarine Historical Society Photo collection).
The hydrogeology of The Dell is poorly understood. The Dell is in a groundwater discharge zone, which is confirmed
by the groundwater elevation data that indicate upward hydraulic gradients. The hydrogeological aspects of the
adjacent mineral springs are discussed in Shugg (2001). The total discharge from the springs is estimated to be of the
order of 1 megalitre per week. There are up to 7 different types of mineral water present and these differences in water
quality probably reflect the varying geology and flow paths.
Groundwater and ground movement monitoring has shown there is sometimes a delayed response to heavy rainfall.
This delay may be partly because some of the groundwater recharge areas are some distance from The Dell. For
example, Dahlhaus (2003) suggests that the main groundwater recharge areas are probably around Mt Bellarine and that
there may be a direct connection between the basalt at Mt Bellarine and the basalt encountered beneath The Dell.
The arcuate and irregular landforms that give The Dell its amphitheatre-like shape indicate that The Dell was formed in
geological times (probably many tens of thousands of years) by a series of landslides (some very large) and it can be
now regarded as a landslide complex. The landslide complex has formed because of the presence of weak material
(clay), the coastal slopes and high groundwater pressures. The Dell landslide complex has evolved over time by a
succession of multiple movements of different landslides which have resulted in a progressive increase in the size of the
overall complex. Twenty individual slides have been recognized at The Dell. Most are slides develop by a series of
slow small individual movements but there are also some extremely small flows which originated on the steeper slopes
and travelled much faster.
There has been extensive slope monitoring at The Dell including piezometers (for groundwater), inclinometers (for
subsurface movement), extensometers and survey (for ground movement) and systematic visual observations.
Movements of up to 70 mm have been observed in the past three and a half years. Most of the movements have been
associated with heavy rainfall. Pumping of groundwater wells at The Dell appears to have reduced groundwater
pressures and ground movement generally although further movements did occur following heavy rain in January and
February 2005. Nearby landslides were also reviewed recently (Coffey, 2006a) to help in the assessment of how
existing landslides and potential new landslides at The Dell might behave.
Following a detailed landslide risk assessment (Coffey, 2006a), Council is currently working towards a partial reopening of The Dell to the public.
4
INSTABILITY ASSOCIATED WITH THE BRIDGEWATER FORMATION
4.1
GEOLOGY
The Bridgewater Formation, which is a Pleistocene sand barrier that has retained its form to the present day, forms the
Nepean Peninsula (Figure 8) and parts of the Bellarine Peninsulas. The Bridgewater Formation comprises well-bedded,
variably cemented dune sands that range from hard limestone to friable calcarenite to variable strength calcrete layers to
loose sand (Figures 9 and 10). The Bridgewater Formation is overlain by a thin, variable thickness of recent wind
blown sand.
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Figure 8: View across Nepean Peninsula at Sorrento showing rugged Bass Strait coast, dune-like landforms, low overall
relief, and Port Philip Bay (top left corner of photo).
Figure 9: Bridgewater Formation – Sorrento Back Beach.
Figure 10: Dune cross-bedding in Bridgewater Formation
The Bridgewater Formation was formed during a period (1.6 million years to 10,000 years before present) when there
were many changes in sea level associated with glacial/interglacial cycles. The sea level during this time varied from
several metres above the present sea level to at least 100 m below the present level. Because of these repeated changes
in sea level and climate, the Bridgewater Formation comprises a mix of marine sediments (mostly sand) deposited
during periods of higher sea level; wind blown sands deposited during periods of lower sea level; all of which have
been variably cemented by local solution and re-precipitation of the calcium carbonate (cementing).
4.2
SOIL SLIDES
The coastal strip of the Nepean Peninsula with its relatively steep slopes down to Port Phillip Bay experiences surficial
instability from time to time. This instability can range from relatively small and minor wind and slopewash erosion to
larger and faster surficial movements and displacements of landslides. The instability is a result of the relative steepness
of some of the slopes in combination with rainfall on the slopes, surface runoff arriving at the top of the slopes, wave
and tidal action at the toe of the slopes, the generally low strength of the parts of the Bridgewater Formation and the
materials overlying this formation (including fill materials), progressive loss of the natural vegetation from the slopes,
and building, earthworks and other activities by humans.
Most landslides are relatively small (less than a few hundred cubic metres) but they are often challenging and expensive
to repair because of severe access constraints. Further, the adopted repair technologies tend to reflect those used in the
sands of Perth, rather than those used in clay slides that typically occur elsewhere in Victoria.
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4.3
ROCKFALL
Rockfall hazards are associated with the ragged cliffs and steep slopes in the Bridgewater Formation at Barwon Heads
and Point Lonsdale on the Bellarine Peninsula, and along the back-beaches of the Nepean Peninsula. Occasional
rockfalls occur from these cliffs as the result of collapse of overhangs in the calcretes and topples of undercut
calcarenite. In 1996 a 7-year old boy was killed on the Sorrento Back Beach when a large rock fell on him (AGSO,
1999). Rockfalls from the same site struck but did not kill persons in 1950 and 1996.
Due to ongoing erosional processes associated with wind, rainfall and coastal storm surges such rockfall events will
continue to occur in the future.
4.4
FORT NEPEAN ROAD – AN EXAMPLE LANDSLIDE
Defence Road at Point Nepean is located within the Mornington Peninsula National Park and provides access to the
historic Fort Nepean complex for transported tours, as well as access for management and emergency vehicles. The
road was upgraded and sealed in 1988 as part of works to open Point Nepean to the public. Slope instability became
evident along a 50 m long section of Defence Road adjacent to the historic fort in 2003 following use of the road by
heavy vehicles (Figure 11).
Figure 11: Site of works. Cheviot Hill, the highest point on the
Nepean Peninsula is in the centre of the photograph.
Figure 12: Screw pile repairs to Fort Nepean Road.
At the affected section, the road has been cut into steep sandy slopes. Open cracks up to about 80 mm wide in the
asphalt and extending about 11m along the outside edge of the road were present in December 2003.
The geology of the site comprises recent wind-blown sand overlying dune sand deposits cemented to form calcarenite
rock. Outcrops of calcarenite are present on the ocean beach platform, and occasionally on the down slope area about
10-15 m below the road. The sand varies from very loose to loose to a depth of 1 m below the ground level and
becoming medium dense or better with depth.
Fill comprising locally derived sand is present along the outside edge of the road. The maximum thickness of the fill is
about 1.2 m. Below the fill, the original ground consists of 1 m to 1.5 m thick of loose sand overlying medium dense
sand that extends to about 8 m below the ground level in both boreholes. The measured SPT “N” values for the
medium dense sand layer varied from 11 to 20. Below about 8 m depth, the sand becomes dense. Some cemented
bands were encountered below 10 m depth. The sands are poorly graded and comprise predominantly medium to fine
sand.
The geology of the site comprises recent wind-blown sand overlying dune sand deposits cemented to form calcarenite
rock. Outcrops of calcarenite are present on the ocean beach platform, and occasionally on the down slope area about
10-15 m below the road. The sand varies from very loose to loose to a depth of 1m below the ground level and
becoming medium dense or better with depth.
Fill comprising locally derived sand is present along the outside edge of the road. The maximum thickness of the fill is
about 1.2 m. Below the fill, the original ground consists of 1 m to 1.5 m thick of loose sand overlying medium dense
sand that extends to about 8 m below the ground level in both boreholes. The measured SPT “N” values for the
medium dense sand layer varied from 11 to 20. Below about 8 m depth, the sand becomes dense. Some cemented
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bands were encountered below 10 m depth. The sands are poorly graded and comprise predominantly medium to fine
sand.
The road needed to be designed for multiple daily trips by a tourist “Transporter” (small tractor towing 5 low covered
carriages), weekly use by a single-axle water truck, and occasional use by fire tankers in the summer fire season. The
stability analysis of the pre-failure condition indicated marginal factors of safety of the road without traffic loading and
factors of safety less than one with heavy vehicles travelling close to the ocean-side road edge.
Remedial options needed to consider the likelihood of damage to the adjacent historic site during construction as well as
visual and environmental issues associated with working in a national park. Remedial options considered included road
alignment as well as various retention and piling options. Eventually a below-the-road screw pile solution was adopted
(Figure 12).
4.5
THE BLUFF - AN EXAMPLE LANDSLIDE
The Bluff is a prominent coastal headland situated at the mouth of the Barwon River at Barwon Heads (Figures 1 and
13). The area is a popular holiday destination and attracts significant tourist and local community usage. Risk
assessment techniques have been employed at the site to assist with the understanding of the hazards and the interaction
with the elements at risk.
Figure 13: Cliffs in Bridgewater Formation calcarenites overlying Newer Volcanic basalt at mouth of Barwon River.
The Bluff is part of the coastal dune structure of the Bridgewater Formation and more typical of the Nepean peninsula
than the Bellarine Peninsula on which it is located. The Bluff represents the lee side of this Pleistocene dune system
and consists of a thick sequence of aeolian calcarenite interspersed with calcrete layers developed by pedogenic
processes associated with stabilisation and vegetation establishment during cessations in dune building (Holdgate, 2001;
Zhou, 1994). The Bluff is unconformably underlain by Newer Volcanics derived from Mt Dunned. Shore platforms of
basalt have developed to the north whilst Tertiary limestone platforms exist on the southern side of the feature.
The lower section of the bluff is comprised of pale brown aeolian calcarenite composed of sand sized particles, rounded
quartz and some marine fossils cemented by calcium carbonate. The lee side of the Bluff displays northerly cross
bedding with dips up to 30 m with a variable strike ranging from northeast to north west and is reflective of the wind
blown source materials derived from sandy plains that once existed in the now Bass Strait.
The upper section of the bluff consists of variable thickness calcretes separated by loose aeolian sand units and weak
calcarenites. Whilst the lower layers are light brown and tend to be weathered and more massive some of the upper
calcretes are grey –white and relatively thin. Some layers of breccia are also located within the sequences and are
reflective of past erosion surfaces (Muller et al., 2006).
Hazards and Risks
Ongoing sub-aerial erosion was found to be the dominant slope forming process and numerous rockfalls and collapses
of overhangs have been observed at the site. Studies conducted by Muller (2003) identified 8 distinct modes of failures
which include calcarenite failure modes of dip slip along bedding planes, falls from overhangs and failure through the
intact rock and toppling. Calcretes were noted to fail through tensile failure of sub horizontal overhangs and
dislodgement of small irregular fragments from the underside of brecciated layers.
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The size and volume of failures is dependent on the failure mode and measurements indicate sizes can range from
pebbles and cobbles to large blocks of about 3 m3 in the lower calcarenite. Most of the failed material travels only a
short distance (approximately 10 m) from the base of the cliff. Where the failures do not reach the base they remain
perched on the face of the cliff or on overhangs and remain a hazard until they are subsequently re-mobilised.
Review of historical reports, photographs and anecdotal evidence by Muller suggested an ongoing but somewhat
episodic history of rockfall occurrences at the site. Historical photos dating back to 1905 provided confirmation of at
least 63 rockfall events in the past 100 years, although many more are likely to have occurred. Falls are known to have
occurred in both the upper calcretes and the lower calcarenites with a number of events being recorded since 2000
(Yttrup, 2001). Although further research is required to better define the likelihood of occurrence, it is clear from
recent observations that rockfalls can be expected after triggering events such as heavy or prolonged rainfall
The episodic nature of many of these events is reflected in the time taken for overhangs to develop. Muller (2003)
estimates that 70 years to 100 years are needed for some of the lower calcarenite undercuts and mid level calcrete
overhangs to develop.
Back calculations from a limited number of failed overhangs indicate strengths at failures well below those of the intact
rock strengths measured by Muller (Miner, 2004). Such results tend to indicate the significant effect of defects, the
three dimensional effects of rock mass in the observed failure mechanisms, and the substantial variability of the
substance strength of such materials.
Previous estimates of risk based on known public usage and likelihood estimates made from initial rockfall observations
indicate tolerable levels of risk although increased signage, ongoing visual and photographic monitoring and limited
removal of some perched rocks has been undertaken by the asset manager. Recent detailed observations of public usage
at the site (Aucote, 2006) have highlighted the access issues at the Bluff associated with high tides and generally
confirm most people tend to avoid the scree slopes at the base of the cliff when they have an opportunity.
5
CONCLUSIONS
Landslides on the Bellarine and Nepean Peninsulas are the result of geological and geomorphic settings and occur at
locations with known conditions and susceptibilities to such processes. Landslides have been a feature in the geological
past and will continue to occur. As such, they represent an ongoing hazard for both the pubic and the development of
infrastructure and housing at or adjacent to these locations. Continued development of these coastal foreshores and the
increased recreational usage of the sites require increased levels of awareness, understanding and management by asset
managers and the public.
6
ACKNOWLEDGEMENTS
We gratefully acknowledge permission from the City of Greater Geelong to use the aerial photographs presented as
Figures 2 and 6. We would also like to thank our reviewers, Mr Alan Moon of Coffey Geotechnics Pty Ltd and Mr
Peter Dahlhaus of Dahlhaus Environmental Pty Ltd.
7
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