Up here we have the source area.
We had the earthquake, we had all the rocks coming out the source area.
The source area is the steep basalt cliffs.
As the rocks came out in all directions,
they would've hit the bottom of the cliff, the ground there.
As they leave the cliff face they're in freefall,
they hit the ground and they start to roll and bounce.
This is an impact crater.
and this impact crater is about twenty percent of the total area of the boulder.
So this is the bounce here
and it's about five metres from here to the hedge,
and it clipped the hedge completely,
about two meters above ground level.
This is an impact mark from the boulder that came through the fence,
there's the second impact mark
and then it went straight through the garage.
You can see the hole in the roof there
and the boulder is actually sitting in the garage at the front of the house.
The purpose of this work is to try and better understand
what areas of Christchurch are likely to be impacted in the future from rockfall.
This is important and because of the rebuilding and
reconstruction activities that are being carried out at the moment.
The reason the Port Hills of Christchurch are susceptible to rockfall
is because they're built on an old extinct volcano.
And they have a series of quite steep rock slopes
which are essentially lava flows.
And these bluffs are susceptible to rock fall
as we've seen from the the recent earthquake.
Castle Rock is a Christchurch landmark.
Climbers use it, walker's go past it, everybody can see it who is working in the area.
It's a beautiful part of the landscape.
Unfortunately, Castle Rock was given a big shake during the earthquake,
and there was a lot of boulders and rock loosened,
a lot of rock fell off and went down the slope.
We are on Castle Rock
and we've been observing and videoing
the scaling work that's been going on at Castle Rock.
This gives us a brilliant opportunity
to look at what the rockfall did during the earthquake,
because we are throwing the rock falls off
from the same locations that they sourced from during the earthquake.
By video in these, we can understand things
such as how high they bounce,
what their rotation is
-their angular momentum as they're going down the slope.
All this is really important because
if you're building a rockfall fence,
you want to make sure the fence is high enough for the boulders.
What this does is allow us to calibrate the numerical models
that will need to be done for design purposes,
but also to give us an understanding of just what happened during the earthquake.
And that's a really really important aspect.
We're quite lucky to be able to do this
so we can actually calibrate these models.
At the top of the cliff,
we're using the bars and other various mechanical methods
to actually prize these loose boulders off.
And as they start as they start to move,
the boulders essentially freefall.
They go from the source area, they fall
and their first impact is either on the rockface on the way down
or they impact the talus, area of rockfall debris at the toe of the slope.
Once they hit that, they can do one of two things.
They can either slide,
and if they slide they don't tend to go very far.
But if they start to roll,
they then start to accelerate and pick up momentum
and they start to increase their velocity.
These are the boulders that worry us
because these are the ones that travel many hundreds of meters down the slope
and these are the ones that have impacted housing
in the Port Hills areas as a result of the earthquake.
As the boulders travel down the slope, rolling and bouncing,
they tend to impact other boulders or other bits of rock on the slope
and they can shatter into three, four, ten or more pieces like a shotgun effect.
and all those much smaller boulders can then continue to roll down the slope.
We're using a GPS to try and understand the spread of the boulders from the one source area
they hit an area of ground that deflects them
into a different direction of movement.
What we're finding is that the these deflections
may be plus or minus about 30 degrees
from the centre for the average fall line.
These yellow lines here represent the trajectories of the mapped rockfalls
You can see there's trajectories all through here.
This is Heathcote Valley
and this is Lyttelton.
The red circles are actual mapped boulder endpoints,
so this is the source area, and this is where they have gone to.
This is the product of what we're doing,
we're trying to delineate zones
where rock falls are probable, or likely to impact.
This is really the first stage of the risk assessment.
What we then have to do is start looking at what the probability is
of a boulder impacting a house, or a road or other facility in these zones.
This is where we start getting into risk modelling.
One of the critical factors that we're doing
is trying to back-analyze and understand what controls the rockfall
and what controls the trajectories of the rockfall
and how that information can be used to
start facilitating a better understanding for reconstruction works.
We don't want to build, or allow people to build in areas that are prone to rockfall,
but we have to quantify what the rockfall risk is.
If the risk of being hit by a rockfall is way down the scale
compared to walking across the road or driving a car,
then we need to know that.