ESTIMATING THE OCCURRENCE OF ROCKFALLS IN
COLUMNAR BASALT
1
Peter G. Dahlhaus 1 and Anthony S. Miner 2
Dahlhaus Environmental Geology Pty Ltd, P.O. Box 318, Buninyong, Vic 3357, Australia
2
P.J. Yttrup & Associates Pty Ltd, 33 Roberts Road, Belmont, Vic 3216, Australia
ABSTRACT
The occurrence of rockfalls from a columnar basalt cliff at the Lal Lal Falls reserve, near Ballarat, Victoria has been
estimated using erosion rates and historical data. Erosion rates were calculated from the volume of material removed
over the past 3 million years, based on the geomorphological history of the site. Historical data included a rockfall in
1990, which caused the death of two high school students, and another rock topple observed in 1992. A reasonable
agreement is found between the volume of material involved in the observed events and the calculated annual erosion
rate. In the absence of more reliable data, the method provides a useful estimation of event size and frequency.
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INTRODUCTION
On March 28 1990 two high school students were
tragically killed by a rockfall while participating in a
physical education excursion at the Lal Lal Falls
Reserve. The site is a small recreation reserve about 18
km southeast of the City of Ballarat used for picnics
and educational excursions. The main feature of the
reserve is a waterfall approximately 35 m high, the
upstream migration of which has formed a gorge
bordered by steep cliffs along the Lal Lal Creek
(Figure 1). The walls of the gorge expose two layers of
basalt, both exhibiting well formed columnar jointing.
At the time of the accident, the students were
participating in an abseiling exercise on the upper part
of the valley wall.
Recent acceptance of risk assessment techniques in
engineering geology and geotechnical appraisals
require estimates of the frequency of such events, or
likelihood of occurrence of rockfalls to be evaluated.
Such estimates are inherently difficult and this paper
details a method using the geological and
geomorphological history at the site to calculate the
erosion rate, which is compared to rockfall event
frequency.
Figure 1. Lal Lal Falls reserve. The southern cliff face
is shown in the photograph.
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SITE GEOLOGY & GEOMORPHOLOGY
The geology of the area is significant and is listed as a feature of the National Estate (Joyce and King, 1980) and in the
excursion guide used by both schools and universities (Day, et al., 1988). The University of Ballarat's Geology
department regularly use the site for teaching and student projects.
GEOLOGICAL HISTORY
The geological history is summarised as follows (Krause, 1882; Yates, 1954; Day, et al., 1988).
Australian Geomechanics June 2001
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ROCKFALLS IN BASALT
DAHLHAUS & MINER
Palaeozoic
A thick sequence of mudstones, sandstones and siltstones were deposited in a deep marine environment during the
Ordovician. The lithified sediments were deformed during the Silurian into tightly folded rocks, subsequently invaded
by numerous quartz veins. The sedimentary rocks were intruded during the Devonian period by granitic magmas and
now form the basement geological unit of the Ballarat area. Following the intrusion of the granites the area underwent
regional uplift and extensive periods of erosion during the Palaeozoic and Mesozoic exposed the quartz reefs and parts
of the formerly deep-seated granites.
Cainozoic
From the late Mesozoic to the Cainozoic times further uplift of the Central Highlands caused renewed erosion in the
drainage systems. Many of these Tertiary river valleys were subsequently covered by the numerous lava flows of the
Newer Volcanics which flowed from the volcanos which were active in the region between two and three million years
ago. Over much of the country the lava spread beyond the valleys to flood the landscape resulting in the familiar flat
volcanic plains of the present day. Following the volcanic activity the planar landscape has been dissected by the
present-day drainage.
SITE GEOLOGY
Devonian Granite
The granite of the area was deeply weathered during the Tertiary to form thick regolith of kaolin clay with coarse quartz
sand (Roberts, 1984). The remnants of this weathered surface were preserved under the cover of basalt on the site.
Subsequent erosion has exposed the weathered granite at the base of the waterfall and along the bed of the Lal Lal
Creek downstream from the falls.
Newer Volcanics
The weathered granite is unconformably overlain by
dense, olivine-rich basalt of the Pliocene – Pleistocene
Newer Volcanics formation. The basalt at the reserve is
approximately 35 m thick and comprises two separate
flows. Near the waterfall the upper flow has a thickness
of approximately 14 m while the lower one is about 20
m thick. Both flows exhibit columnar jointing with the
columns in the upper flow being narrower (½ – 1 m)
than in the bottom flow (1½ – 2 m). A thin weathered
horizon between the two flows indicates a short time
interval between the two extrusions (Figure 2). Yates
(1954) examined the units in detail and concluded that
two separate lava flows issued from a vent at
Springbank, north of the site, and flowed
approximately 20 km southwards down the valley of
the ancestral Western Moorabool River.
Recent alluvium and colluvium
Minor deposits of alluvial material are found along the
course of the Lal Lal Creek both upstream and
downstream of the falls. The alluvium comprises
weathered granite and basalt blocks. The colluvial
deposits occur as scree slopes along the foot of the
cliff. It comprises a thin veneer of angular basalt blocks
in a clay matrix, which covers the slopes as they
develop.
Figure 2. Upper and lower basalt flows showing
columnar structure.
GEOMORPHIC EVOLUTION
The geomorphic processes at the site are the key to understanding the present risk conditions. The associated
broadening of the valley by slope retreat is evident as the steep cliffs develop by mass wasting processes into the
smooth slopes of the creek valley downstream. Below the falls the creek follows a deeply incised valley that reflects the
original meandering course of the stream. Above the falls the valley of the meandering Lal Lal Creek is very shallow.
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ROCKFALLS IN BASALT
DAHLHAUS & MINER
Origins of the falls and gorge
During the Pliocene volcanism, the first lava flow to
reach the reserve flowed down the ancestral valley of
the West Moorabool River. The flow displaced the
drainage of the West Moorabool River to the east. A
second lava flow followed after a brief interval with a
contemporaneous flow from the north west down the
ancestral valley of the Lal Lal Creek. The effect of
these flows was to fill the landscape so that a lake
formed at the site of the present day Lal Lal Swamp
(Yates, c. 1945).
The sand brought down by the Lal Lal Creek into the
swamp eventually choked the drainage and the Lal Lal
Creek was diverted along the northern boundary of the
lake and was captured by a small tributary of the West
Moorabool River.
The West Moorabool River
cascaded over the edge of the basalt flow at a point
about 500 m southeast of the confluenceof the two
streams. The erosive power of the river had increased
by the addition of the Lal Lal Creek, resulting in
thedevelopment of a waterfall in the columnar basalt
flows. Headward erosion caused the falls to recede
upstream. At the point of confluence two distinct
waterfalls developed - the Lal Lal Falls and the
Moorabool Falls. Both have since receded upstream to
their present positions.
Figure 3. Location of relevant geomorphic features.
There is no doubt that the rock structure (i.e. columnar jointing of the basalt) and the undercutting of the weathered
granite at the base of the falls has allowed the knickpoint migration to be relatively rapid. As the knickpoint recedes
upstream it cuts a deep gorge, the walls of which are eroded back by the processes of slope retreat. At the Lal Lal Falls
Reserve these mass movement processes are responsible for the change in nature of the valley walls along the Lal Lal
Creek. Profiles along southern wall of the Lal Lal Creek valley downstream of the falls (Figure 4) illustrate how the
steep cliffs adjacent to the falls are gradually battered back to a slope representative of the natural angle of repose of the
material.
Figure 4 Changes to cliff profile downstream from Lal Lal Falls.
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ROCKFALLS IN BASALT
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Description of rock mass
Both basalt units within the Lal Lal Falls Reserve exhibit columnar jointing
which has segmented the rock mass into long prismatic units. The jointing
occurs due to thermal stresses produced during the cooling of the lava flow.
The following general observations about columnar jointing in basalt flows
(Spry, 1961) are also applicable to the Lal Lal Falls basalts.
In cross section many columns show hexagonal symmetry although five and
seven-sided columns are common and three and four sided ones are not rare.
The most common angle between the sides is 120o to 130o. The joints
bounding the columns are continuous and dominant whereas those across the
prisms are less prominent and generally terminate at the edges of the prisms.
The smooth straight sides of regular columns are often marked by "chisel
mark" grooves perpendicular to the length of the column. The cross joints
may be perpendicular to the prism axis but variations include oblique joints
and curved "ball and socket" joints. The distance between the fractures
varies and where they are closely spaced and highly weathered the typical
"pile of Dutch cheeses" is produced. Columnar jointing is considered to be
the result of a sequence of events rather than one instantaneous episode; the
cooling rock is broken into progressively smaller parts.
The joint orientations of the upper flow (Figure 5) naturally form two
distinct groups. Those bounding the columns vary from 75o to vertical with
the majority between 85o and 90o. The cross-joints are more variable; 55o to
horizontal with most dipping 15o to 25o.
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ROCKFALL FREQUENCY ASSESSMENT
FAILURE MECHANISMS
Two rockslope failure mechanisms have been recognised at the site viz: rock toppling and rockfall.
Rock toppling
Toppling involves the overturning of interacting columns. Block toppling, as described by Goodman and Bray (1976),
is the most appropriate model of failure mode within the reserve. Block toppling occurs when the toe of the slope with
shorter columns receives load from the overturning longer columns above. The shorter columns are thrust forward
allowing further toppling. The base of the mass typically exhibits a stairway structure, the steps formed by the crossjoints. The mechanics of failure may involve both sliding and toppling or simply toppling (Hoek and Bray, 1981). The
mode depends on the angle on which the base of the block rests and the ratio of block width to height.
Rockfall
Rockfall occurs when a rock is free to drop vertically
from the cliff face. The mechanism involves the
removal of the column base, thereby undercutting a
portion of the column that remains attached to the cliff
by friction imparted by the roughness on the joint face
(often provided by the "chisel marks"). Eventually the
gravity forces overcome the frictional forces that may
be weakened by water ingress or weathering and the
column simply drops.
The entire cliff face surrounding the falls is prone to
both block toppling and rockfall. Open joints, large
columns leaning out from the cliff face and undercut
columns are evident (Figure 6). Water ingress is
particularly important, as the water stored in a vertical
joint can impose a considerable force on the base of the
block due to the hydraulic head; or uplift on a cross
joint due to pore pressure. Heavy rainfall was
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ROCKFALLS IN BASALT
DAHLHAUS & MINER
implicated in the rockfall that caused the 1990 fatalities (Cooney, 1990). The trees growing at the edge of the cliff also
assist in destabilising the rock mass. Not only do their growing root systems exploit the joints and wedge them apart,
but also their movement due to wind can dislodge rock pieces.
Although the 1990 event was triggered by one of the rock climbers, the column was regarded as at the point of failure at
the time. This judgement was based on the observation that a length of about 0.8 m had previously fallen from above
the same column and that the column had been partially undermined by a previous small wedge failure (Cooney, 1990).
The steepest section of the cliff is more susceptible to block toppling, as the profile of the cliff changes away from the
falls to a typical "stairway" structure. Downstream from the falls, the rock mass is weathered into increasingly smaller
blocks which decreases their stability. The risk of a large-scale failure decreases away from the falls although the rate of
the rockfalls increases.
CALCULATION OF EROSION RATES
The volume of material eroded from the landscape since the emplacement of the basalt was calculated using a digital
terrain model. The model was constructed to represent only the slopes of the Lal Lal Creek valley and a portion of the
southern slopes of the West Moorabool valley, from the original edge of the basalt to its confluence with the Lal Lal
Creek. Volumes were calculated by subtraction from the original (assumed flat) surface.
In calculating the erosion rates, several assumptions have been made. It was assumed that the terrain immediately after
the extrusion of the basalt was essentially flat. While it is not possible to accurately reconstruct the initial post-volcanic
landscape, the assumption is been based on regional observations. Another assumption was that the volcanic flows
occurred 3 million years ago. Although the actual age of the basalt has not been ascertained, dating of the nearby
Dunnstown flow (2.60 Ma), Mt Rowan flow (2.97 Ma) and Alfredton flow (2.94 Ma) place the volcanism within this
range (Taylor, et al., 1996). Since both the volume of material and the time frame are large, these assumptions have
little effect on the calculation.
However, the assumption that the erosion rate has been constant over the past three million years remains problematic.
Climatic variation would have affected the volume of water in the creeks, and the rate of chemical weathering of the
basalt. Additionally, the sequence of events over the geologic past logically suggests that the initial erosion rate would
have been accelerated once the Lal Lal Creek was captured to join the West Moorabool River. The rate then must have
decreased once the confluence of the rivers was reached and the knickpoint divided into the valleys of the Lal Lal Creek
and the West Moorabool River. This assumption has the effect of "smoothing" the calculation to represent an average
value.
Despite these unavoidable difficulties in reasoning, the calculated rate serves as a useful estimate, as it represents the
best possible "guess". The Lal Lal Falls have retreated a distance of approximately 1650 m in the past 3 million years
which averages 0.55 mm/year. By comparison the Moorabool Falls have retreated about 1400 m over the same time
period - an average of 0.46 mm/year. Assuming a constant erosion rate, the valley slopes have receded away from the
streams at 0.0146 mm/m2/year. Based on these calculations, a volume of 0.04 m3/year has eroded from the upper
columnar basalt cliff of the entire southern slope over the past 3 million years. Based on the observed average column
diameter of 0.8 m, this equates to a column height of 80 mm per year.
HISTORICAL DATA
Information on previous accidents at the Lal Lal Falls Reserve is difficult to research due to the absence of records.
Local newspaper records provided three incidents of people falling from the cliff edge, but only the 1990 fatalities
involved a rockfall. The Coroner’s investigation on the two fatalities (Johnstone, 1990) reported that rockfalls had been
observed during a climb one month earlier and that “loose rock” was a common problem associated with climbing at the
locality. Discussions with the local municipality, who manage the reserve, revealed that there have not been reports of
major rockfalls or a requirement to clear rocks from the previous walking path in the past few decades.
Historic photographs, particularly those taken on previous geology student excursions, were more useful. Comparison
of photographs from 1975 to 1990 shows little change in the cliff faces, and major rockfalls are certainly not evident.
By coincidence the actual site of the 1990 fatality was photographed in 1972, which provided information on the size
and volume of rockfall material. In 1972 both the column involved in the accident and the 0.8 m length that had fallen
sometime prior, were in place.
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ROCKFALLS IN BASALT
DAHLHAUS & MINER
Another rockfall event at a different site on the upper cliff was observed in May 1992 and an estimate of the volume of
material could be made by comparison to a 1990 photograph. This fall involved a toppling failure of a column 1.5 m
high by 0.5 m diameter.
CALCULATION OF EVENT FREQUENCY
The volume of rock associated with the fatal rockfall in 1990 has been estimated at approximately 0.2 m3 (Cooney
1990). It is also known that approximately 0.2 m3 of rock had fallen from above the accident site between 1972 and
1990. Based on the calculated erosion rates, these falls each represent five year’s volume, or a 1 in 5 year event. The
rockfall observed in 1992 equates to a volume of 0.3 m3, which is 7½ year’s volume, or a 1 in 7½ year event. All three
events are of a similar size.
A total of 0.7 m3 is known to have fallen from the southern cliff face in three separate events between 1972 and 1992.
The photographic record and municipal records suggest that no other significant falls occurred during this time. The
observed annual rate (0.035 m3/y) over the twenty-year period compares well to the calculated erosion rate (0.04 m3/y).
As such it could be expected that a similar size fall would occur every 5 to 7½ years.
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CONCLUSION
In the absence of more reliable historical data, the method used at the Lal Lal Falls Reserve represents a logical and
sound approach to the calculation of event frequency for rockfalls in columnar basalts. Volcanic events can provide
relatively precise dates and the subsequent geomorphic development of a site offers the opportunity to estimate event
frequency through the calculation of erosion rates. At Lal Lal Falls Reserve, the observations over a twenty-year period
indicate that the estimates are plausible and the relatively short occurrence intervals have significant implication in the
further process of risk assessment and evaluation at this site.
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REFERENCES
Cooney, A.M. (1990). “Geological report on the rock fall at Lal Lal. A report for the Deputy Coroner.” Unpublished
report 1990/13. Geological Survey of Victoria.10p.
Day, P.L., Carey, S.P., and Harms, J.E. (1988). “Ballarat” in Victorian Geology Excursion Guide. Australian Academy
of Science. pp. 197-211
Goodman, R.E. and Bray, J.W. (1976). “Toppling of rock slopes” in Rock Engineering for foundations and slopes.
Proceedings of ASCE Conference. University of Colorado, Boulder, Colorado, August 15-18, 1976, Vol. 3, pp. 201235. ACSE publication.
Hoek, E. and Bray, J.W. (1981). Rock slope engineering. Institute of Mining and Metallurgy, London.
Johnstone, G. (1990). “Rock climbing inquests.” State Coroner Victoria Report, 5th October 1990, State Coroner’s
Office, South Melbourne.9p.
Joyce, E.B. and King, R.L. (1980). Geological features of the National Estate in Victoria. Geological Society of
Australia, Victorian Division. p.183
Krause, F.M. (1882). “Notes on the geology of Lal Lal” Annual Report, Ballarat School of Mines, p.51
Roberts, P.S. (1984). “Explanatory notes on Bacchus Marsh and Ballan 1:50 000 geological maps.” Geological Survey
Report, No. 76, 102p. Geological Survey of Victoria.
Spry, A. (1961). “The origin of columnar jointing, particularly in basalt flows.” Journal of the Geological Society of
Australia. Vol. 8, Pt.2. pp. 191-219
Taylor, D.H., Whitehead, M.L., Olshina, A. and Leonard, J.G. (1996). “Ballarat 1:100 000 map Geological Report”
Geological Survey Report, No. 101, 117p.Geological Survey of Victoria.
Yates, H. (c. 1945). “The basalts, granitic rocks and physiography of the Ballarat district.” Unpublished Report. Ballarat
School of Mines.
Yates, H. (1954). “The basalts and granitic rocks of the Ballarat district.” Proceedings of the Royal Society of Victoria,
Vol. 66, pp.63-102
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