Hello everyone I’m Peter Bower welcome to
Pete’s Lab
today , we have another
one of our short little lectures vignettes if you will on subjects related to
ground fields
and groundwater
today's topic is
porosity
Porosity is the
percentage of open space
in rock or regolith
usually we think of rock as being completely solid the but in many cases there are fractures
or cracks
sometimes open spaces
and that can
provide some
porosity
Regolith on the other hand is the lose Material at the surface of the earth
mud sand gravel and so forth
and it is composed of particles and those particles have spaces between them
which can give
you a rather large
amount of open space or
as we will see porosity
there's some other subjects that we're going to touch upon
permeability bulk volume bulk density density of particles
volumes of the particles and so forth
we’ll make some measurements and we'll discuss their significance
I have here
a pebble
quartz
and if one were to try to determine the volume of this pebble most people understand that
if we have
a graduated cylinder
in this case a hundred and ten milliliters of water that by dropping the pebble into the water
the water level will rise
and we should be able to determine
the volume of the pebble by difference
so let me drop this in here
and we can see that the water level has gone up
and the new water level is a hundred and thirty milliliters
meaning that the difference one thirty minus one ten
twenty milliliters is the volume
of a pebble
milliliters we should also understand is the same volume as a little cube one centimeter on a side
that is a cubic centimeter or CC is exactly the same volume as a milliliter
while most understand that one can determine the volume of a pebble by
volume different by dropping it into a
graduated cylinder with some liquid in it
it gets a little more confusing when
we have more than one particle
but it's really the same
principle
if we were to dump all these particles in this case
a nice pile of half-inch
plastic bedes
into a liquid surely the volume would rise
and what would we have done we would have measured
the volume of all of these particles
not just one but many
here for instance is a beaker of sand consisting of
hundreds and hundreds of thousands of
particles
we could do the same thing by dropping sand into a water and watching the water level rise in determining
the volume of all particles
that's exactly what we're going to do
okay so let's begin the experiment
before you
begin the experiment you should all have
a copy of
the handout particles
porosity bulk density and particle density
if you don't have it pause the video
and
go to the web site and print it out
and then start watching
the video
again so lets begin
the first thing that we’ll want to do is to
determine the
weight of this beaker
okay the weight of the beaker is one hundred and seventy one point
three grams
so if you would please enter that
data on
you're sheet
in item number two the way of the beaker
and the next thing
we’re going to do is determine the weight of the beaker plus the particles again the particles are the plastic
that is styrene beads the is one half inch in diameter
and we're going to use a weight
to assist us
they're we’re getting close
and the
wait of the beaker
plus the beads
is five hundred and thirty nine
point eight
grams
so please enter that on your sheet
five thirty nine-point eight
by subtracting the weight of the beaker
from the weight of the beaker plus the particles we can find out the weight of the particles
and that turns out to be three hundred and sixty six
point seven
grams se we’ve determined
one thing
and that is the weight
of the particles
the next thing that we could do that would be
Fairly straightforward
it would be to determine the
bulk volume that is the volume of the particles plus the spaces between the particles
and we can do that
by putting the particles into the graduated cylinder
try to get them level 0:06:32.720,0:06:37.999 the divisions on this graduated cylinder are
in increments of ten milliliters
the volume of the beads is
four hundred and forty
Milliliters so
please enter that on your sheet
and now
we’re going to determine the
volume of the particles
I have here some 0:07:07.389,0:07:08.979 water that I put some dye in
and I'm going to put this
into the graduated cylinder
and the
volume here is
three hundred and ninety milliliters
and the next step
the dramatic step is to
pour the beads
into this graduated cylinder exactly as we did for the single quartz pebble before
only were doing it
maybe
a hundred times over
and we can see that the water level has risen of course
and we can also note that
the spaces between the particles are completely filled with liquid
here
the pour space that is the open space between the particles is connected and fluids
water in this case can move freely
between the particles
this is an important
term called permeability it means simply
the ability of
fluids to be
transmitted by rock or regolith and clearly
the spaces between these particles are connected and fluid moves
easily between them
the
new volume
is six hundred and forty milliliters so if you enter that on your sheet
and then you can see
by looking at your sheet that we can now determine the volume of the particles by subtracting
the initial volume of the water
from the final
that is
three ninety from six forty gives us two hundred and fifty
milliliters
that's the volume of the particles
now since the bulk volume was the volume of
the particles plus the volume of the pour space
we can also now determine the
amount of open space parting these particles
the original bulk volume was four hundred and forty milliliters
the volume of the particles was two hundred and fifty so subtracting two fifty from
four forty gives us
one hundred and ninety
milliliters
which is the volume of the open space or
pour space between the particles
the rest of this is
really some simple calculations
we can determine the porosity that's number nine on your sheet
which is the percentage of open space
and we do that by dividing one ninety
the volume of the
pour space over the total volume the bulk volume of four forty and we get forty three percent
so forty three percent of the volume of these particles is open space
and conversely the particles take up fifty seven
percent
of the space
we can also now determine the density remember density is
the weight divided by the volume
so we could do the
bulk density which would be the
weight which is
we determined right at the beginning three hundred sixty six point seven grams
divide that by the
bulk volume of four hundred and forty milliliters and we find out that the bulk density
is point eight three grams
per milliliter or grams per
cubic centimeter that’s
particularly
useful much more useful is
the density of the particles themselves if we know the density of the particles themselves it
may give us
some information
that would be very useful in determining what the particles are made of
in this case
we divide three hundred sixty six point seven of weight of the particles by
the volume 0:11:01.290,0:11:03.960 of the particles two hundred fifty
milliliters and we find out that the density
of these styrene beads is one point
four seven grams per cubic centimeter
which makes sense
the density of water is one
gram per cubic centimeter
and of course these beads have
sunk to the bottom
if they were less dense than water they would float
and we wouldn’t be able to do the experiment
okay so
we have now shown you
how to measure porosity the
percentage of open space
in
a collection
of particles
in this case the percentage of open space in the
pile of plastic beads is about forty three percent
we weighed the particles we determine the bulk volume of the particles
and pour space
we determine the volume of the particles the density of the particles
and so forth
all these measurements are
a very important
for studying groundwater and the flow of fluids
through regolith
on the surface of the earth
we could have done the same experiment with this
with this sand and determined it’s porosity
and also the
density of the particles
permeability to remind you again is the ability of
fluids
to flow
through rock or regolith
in order for that to happen of course there has to be porosity
but the porosity must also be connected so that the liquid can move from one place to another
a good example of a material that
has very high porosity but
dosen’t
allow fluids to move is Styrofoam
take a Styrofoam cup and if you look
at the walls of the styrofoam cup you'll see that it's made of very tiny bubbles foam
made from styrene and
those bubbles are not connected
% they're all individually wrapped in plastic
and of course we know the Styrofoam cup
t does not was allow liquids to pass through it
there are some rocks such as pumice that has similar properties
very very light and airy lots of open space but the
air space is not connected at all
so permeability is the ability of
um fluids to move within
sand lets say
or even through rock where you have
fractures
and this number of porosity we'll see later on
the very important um
piece of information that we need in order to use Darcy’s law which allows us to
calculate the rate
at which water flows
in sub surface
until next time
this is
Peter Bower in Pete’s Lab