TECHNICAL NOTE
DON’T SLIP OVER BOULDER FALLS
Peter C Wright
Associate, Jeffery and Katauskas Pty Ltd
ABSTRACT
Many of us undertake stability assessments for assessing hillside development. In most instances, the available fees are
quite limited and there could then be commercial pressure to complete fieldwork as quickly as possible. The example
below demonstrates the possible consequences of not taking the hour required to walk a sufficient distance uphill. We
do not know whether consultants completed stability assessments for the residences around this site.
EVENT DETAILS
We were recently requested to visit a site near Gosford on the Central Coast of New South Wales where a client
reported a boulder having fallen into their backyard in the early hours of the morning. There were no obviously
destabilising events, such as heavy rainfall within a reasonable period (about 1 month) prior to the boulder fall.
The site was located near the toe of a colluvial slope which had an average slope of approximately 34°. There were
several large outcrops of sandstone on the slope above the house and the uphill end of the slope terminated against a
near vertical sandstone cliffline which had a height of about 10 m. The geology of the region comprises Hawkesbury
Sandstone over the Narrabeen Group sandstone and siltstone.
The sandstone boulder was sitting in the back yard. A photograph of the boulder (Photograph 1) is attached, along with
a rough sketch of the dimensions of the boulder (Figure 1). Based upon this sketch, it was estimated that the boulder
would have had a final mass of about 22 tonnes. Just uphill of the resting place of the boulder was a tree of about 0.3 m
diameter which had been sheared off at ground level, and a crater with a depth of about 0.5 m resulting from the boulder
impact. This last impact of the boulder displaced approximately 3 m3 of soil (see Photograph 2).
To assess the cause of the boulder fall, we then walked uphill from the boulders’ resting place to find the path the
boulder had taken and its source. The boulder had originated from the cliff-line of Hawkesbury Sandstone at the crest
of the slope (see Photograph 3). The path of the boulder down hill has been sketched in Figure 2.
After dislodging from the cliff-line, the boulder bounced several times for a total distance downslope of 40 m.
Following this, it rolled a further 65 m downhill. A portion of the rolling path of the boulder is shown on the attached
Photograph 4; note the fragments of sandstone along the path. A tree which was impacted by fragments broken off the
boulder during its progress downhill, was offset about 30 m from the path of the main boulder and the impacts on the
tree were as high as 6 m above ground level. A piece of the boulder broken off on its journey was located just to the left
of the tree; this portion would have been about 1 m3 to 2m3.
After the downhill roll, the boulder hit another sandstone outcrop and again started bouncing for the remainder of its
way downhill. The distance of the last three bounces of the boulder before it finally came to rest were 23 m, 20 m and
14 m respectively. In total, the boulder bounced and rolled a distance of about 170 m down the 34° hillside. The final
resting place was about 25 m short of a two storey timber pole home.
Trying to imagine a 30 tonne block of rock tumbling 170 m downhill, with individual bounces of some 20 m, while
shedding fragments of several tonnes on the way is not an easy thing to do. Neither is imagining blocks of sandstone
weighing possibly 3 to 5 tonnes hitting 6 m up a tree trunk.
Well, try imagining if the boulder had taken a mere one or two more bounces; just another 15% of the distance it had
already travelled. With the energy in a 20 plus tonne block of sandstone bouncing 20 m at a time, how much would the
timber wall of the pole home slow its progress, and what would be the consequences of impact?
Also recently, we were called to a site on Sydney’s northern beaches. At that site, a sandstone boulder dislodged from a
small to moderate cliff line and rolled about 50 m down a slope of about 35°. The sandstone boulder was about 1 m3
initially and only stopped after shattering on impact with another boulder on the soil slope.
What I have tried to show here is that while such rock falls are often considered a very rare event, they can occur,
without having an obvious triggering event. The consequences of such an event would be catastrophic. When
undertaking stability assessments, make sure you take the time to look uphill and at least consider the risk of boulder
falls.
How many of your previous stability assessment projects are at risk from boulder falls?
Australian Geomechanics Vol 37 No 5 December 2002
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TECHNICAL NOTE
PC WRIGHT
Photograph 1: View of boulder at final resting place.
Figure 1: Dimensions of boulder at final resting place, estimated about 22 tonnes.
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Australian Geomechanics Vol 37 No 5 December 2002
TECHNICAL NOTE
PC WRIGHT
Photograph 2: Final impact crater, about 0.6 m deep by 3.0 m across and 1.7 m down slope.
Location of source
of boulder
400mm diameter
tree sheared close
to ground level
Photograph 3
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TECHNICAL NOTE
PC WRIGHT
Part of path of
boulder looking
downhill
Note:
Fragments of
broken off boulder
Photograph 4
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Australian Geomechanics Vol 37 No 5 December 2002