Welcome to the fun-filled, action-packed, geology
assignment for 3203.
What we're going to talk about today is some of the intro work
of what you might do before you go to a site just so you can
maximise the value of what you're going to get when
you're out there.
The first component of the site is Bronte.
So one of the things we need to do as geotechnical engineers
is to collect field information in a standardised way that
allows us to develop models of what we expect
the subsurface conditions to be, and also
to assign properties to the soil and rock that we encounter.
The purpose of this Bronte component of the assignment
is to allow you to actually experience
the process of collecting that data for rock engineering.
In this video, I'll just discuss what
you might do prior to going out and actually collecting
that data.
When we're doing any geotechnical engineering work,
as we're never actually going to see
a full exposure of the whole rock
until it possibly falls down and then it's too late,
we always work off a geotechnical model.
You can consider that model as being
an educated guess as to what we expect to be at the site.
As we learn more information, the model
is continually updated throughout that whole project.
So we learn more and more about the geological conditions.
We can add that to our model.
The first model, the first component of our model,
is done prior to going to the site
and it's called the preliminary geological model.
This model allows us to have a prediction of what
we might expect at the site.
Its job is to highlight any potential issues
we might encounter that might impact
on our particular engineering project.
This allows us to focus on what matters when we get to site
and we can ignore other things.
As we don't have any prior experience to going to a site,
our main source of information is
going to be the appropriate geology map.
In this case, the map we will use is the 1 to 100:000
Sydney geological map.
In most locations, the geology map
is going to tell us what the near-surface rock type is.
However, when we've got really deep soils,
it will give you the surface all time.
The map also provides you with some limited cross sections
and some valuable descriptions of the rock type
and the age of the rock type.
If we focus on our Bronte site, we
can observe that the site is dominated by Rh Hawkesbury
sandstone.
The map tells us this is a coarse sandstone
with occasional lenses of thin beds of shale.
To the north there are sands indicated.
They're probably the beach.
To the south, there's black lines on the map.
Those black lines are dykes.
Now, note these are to the north and south of the site,
they're not at the site, so we would not
include the beach nor the dykes in our preliminary model.
Also note the Hawkesbury sandstone is
approximately 250 metres thick.
So that's well beyond the limit of any civil engineering
project.
So as far as our model is concerned,
we don't worry about what's beneath our sandstone layer.
What I want you to do is to do some of your own research.
Research the properties of the Hawkesbury sandstone,
including the intact rock, the substance,
and the expected defects that you might encounter
in Hawkesbury sandstone.
I'd also like you to think about what soils might
lie on top of the sandstone.
You need to support your ideas.
So explain your thinking, give some references.
In our second part of your assignment,
what are you going to do is go on a self-guided field trip.
So what I want you be doing is seeing
real soils and real rocks.
In the classroom we're seeing lecture rocks, lecture soil.
I've travelled the world to try and find you the best pictures.
It's been very difficult for me.
But in engineering, we usually see weathered rock,
eroded soil, et cetera.
So you need practise at looking for those things.
So this assignment is all about that.
It's a good idea, as we did with Bronte,
to create a preliminary model before you get to the site
to maximise the value of your time at the site.
If you look at the North Bondi option for a field trip,
we can see the site is north of Bondi Beach.
We go to the geology map and we can see the rock type,
like Bronte, is dominated by Hawkesbury
sandstone, that Rh material.
In this case, however, it's intruded by a JV volcanic neck.
So the volcanic neck comes up through the sandstone.
And that contains a quote in the geology map, volcanic breccia,
and basalt.
The geology map also shows us a dyke
running parallel to the coast along outside
and three perpendicular to the coast
in the vicinity of our site.
So we need to consider looking for those.
So prior to getting to the site, you
should think about what effect the volcanic neck might
have on the sandstone around it, also do a bit of research
on what the dykes in Sydney typically look like.
So, again, once you're out there,
you've got a good idea of what you're looking for increases
your chances of finding things.
The Pyrmont excursion option covers most of Pyrmont
And if you look at the geology map for Pyrmont shows,
again, the areas underlined by Hawkesbury sandstone, the same
as Bondi and Bronte.
There's also a north-south trending dyke running right
through the middle of Pyrmont, so we'll keep our eyes out
for that when we're walking around.
We can also see to the northeast there's a section of reclaimed
land connecting to the harbour.
We won't to see that on our field trip,
but it's always good to note that that is at the site.
If we go on to the Long Reef site,
you're going to see quite different rocks.
And this is because the rocks at the site
are much older than the rocks at the other sites.
So being older these are the rocks that
would be underneath the Hawkesbury sandstone,
essentially 250 metres below Bronte, for example.
The geology map for Long Reef suggests
that the majority of the site we'll be walk around,
that forms the head land and the rock platform we walk on,
is Rnbh Bald Hill claystone.
This comprises-- according to the geology map, at least--
red shale and fine-to-medium sandstone.
There's a small exposure on the map of Rnn Newport formation.
And that's to the north of the site.
Now, the Newport formation, apparently,
comprises in a bit of shale and sandstone.
It's a younger unit than the Bald Hill claystone.
So I'd expect it to be above the Bald Hill claystone.
And so if we're looking at it on site,
it's got to be the higher elevations, somewhere where it
hasn't eroded previously.
At the very eastern tip of the site, so off the rock platform,
we can say there is Rnbu Bulgo sandstone.
And this comprises sandstone with shale inter-beds
within it.
This rock is the oldest unit at the site
and it's found beneath the Bald Hill claystone.
As the layer underneath that claystone.
The geology map shows that on the western half of the site,
it sits on marine sands, the yellow area.
This area was probably a river channel,
so it eroded through the rock.
And then, as the sea level rose from the last Ice Age,
it would have been in-filled with sand.
When we are developing a preliminary model
we'd like as much information as we can,
so we'll often look at aerial photographs
to see if we can observe other features.
One of the things we can look for
are lineaments or straight lines.
Lineaments are unusual in natural topography.
Nature likes curves.
And so if we see lineaments, they
might often indicate things like joints, fault lines,
might be areas where there's land sliding,
or it might indicate dykes.
If we have a look at the aerial photograph for Long Reef,
we zoom in, have a look at that, particularly on the rock
platforms, we can see there's a number of lineaments cutting
across, in particular, the rock platform.
Some of these seem to also control the shapes
so the rock platform actually cuts in along those lineaments.
So put that in your head, put it in your preliminary model,
all right There is the lineaments, where
they are, so when you go to a site,
you can try and locate them and try and identify what they are?
Are they faults?
Are they dykes?
Or are they just a line on the map?
OK.
So that's all from me, the rest is up to you.
Make sure you do some of your own research
before going to a site, don't just believe me.
Make sure you're well prepared for what you might find.
Remember to enjoy yourselves, definitely stay safe,
and I hope you learn some useful things.
See you back in class.