So this time what I want to show you is how to calculate the amount of
Spoil we're going to excavate and
the amount of Concrete we're going to have
To put back into our strip footing
for our
building so here's our
view of our strip footing with our dwarf wall
The reason I'm going to show the dwarf all is when we first get our plans all their dimensions are taken
To the outside of the building not necessarily outside of the footing
So the first thing we need to do is actually work out what length our footing is
Okay, so what
we need to do is add a certain amount to each end of
our
dimension
Well find the right spot, Okay, the way we work that out is
now footing in this case is 350mm wide
our dwarf wall is
230mm wide
so 350 minus 230
gives us
120 and
half of that because this is centrally this is position central on the footing so half of
120 is 60so it's going to have 60mm here which means are also going to have 60mm here
so we have to go all the way around our footing and add 60mm to each end or
120mm
to each dimension
So that's what we do there, and then we get our new dimensions for our footing
Then what we can do, just while we're at it our footings 350 wide we've already discussed that and it's also 500mm deep
And we'll need that a little bit later on
so now we can
start to work on the calculations first thing we need to do is to break the footing up into individual links and
We don't want to
repeat the corners or duplicate the corners, so for each length we're going to cut off one of the corners
as you can see they're just done that just drawn a line across there to cut off each corner and
We can label them so we know which ones which
generally, we start at the top and work our way around clockwise for labeling, and then we can start to work it out, so
Our formula for volume is length x width x depth is the first thing we need to know is the length of the footing
So for Side A it's going to be our overall dimension - the width of the footing
that will give us the actual length of Side A
which happens to be 4.640
and we work our way around doing that for all sides. Side B we are going to minus 350
now when we get to Side C
We'll notice that our dimension line
actually lines up with
our our marks, so there's no need to deduct anything for Side C
So no deduction and the way we work this length that was our overall length up here. Which is 4.990
4.990 minus
2m
4.900 gives us 2.5m
Side D happens to be the same
There'll be different if you line up this
dimension here, it actually looks like we've got to add
the width of our footing but if you line this one up, you've actually got then take it off again, so
By adding and taking off. We're not changing anything, so it stays the same as well
once again this dimension was determined by taking the
2.490 away from the 4.990
There to get two and a half again
keep going around
Side E and
Side F and we're now done all our sides so we can add that up and get a total length of our footing
Which is 18.560 we then times the length by the width and the depth
to get the total cubic metres
so 18.650 times 0.350 for our 350 width and
0.5 for our
500mm Deep, 1/2 metre gives us3.280cubic metres of
Concrete now
We have to allow a little bit waste on top of that because yeah ultimately there's going to be stuff left in the truck is
Going to be youknow, maybe you'll spill a barrow so
Yeah, you're always going to need a little bit more than the actual exact amount so going to allow 10% waste
The way I commonly do that is just by multiplying the original cubic metres
By 1.10 which will give add
10% to the original figure
So it gives us3.573 cubic metres of concrete to order
Now concrete's only available in increments of 0.2
So we have to round that up to the next increment of point two which is 3.6
So we're going to order 3.6 cubic metres of concrete for this job
Now before we can actually pour the concrete we would have had to
excavate for these footings and now there might be the situation we actually have to remove
the dirt that you've excavated which is referred to as spoil and
truck the spoil away off your site so you might have to actually work out how much
spoil you're going to have when you finish digging the footings now
generally the footings are
deeper in the ground or they're the top of the top of the concrete doesn't sit flush with the top of the ground
Generally, it's in the ground a little bit
possibly at one course of Brickwork. Which is 86mm it could be 100mm in the ground
in this case I've nominated that the footing is 100mm below the surface of the ground so
to work out the excavation of the spoil the first steps are same, we still need to know the length of the footing
The second step is essentially the same except this time instead of using
500 for the Depth we're now going to use 600 because we've added this the extra 100mm
So it actually works out that we need 3.898 cubic metres of soil is actually going to come out of that
excavation now
Interesting thing with spoil is when it comes out of the ground
It actually occupies more area or more volume out of the ground
Than it did in the ground because by excavating it digging it up
we've added air into the stockpile
so you have to add what they call a Bulking Factor because if we ordered enough trucks to carry
3.898
cubic metres of spoil we will actually have spoil left on the site
So this case I've indicated we need 20% bulking
so we times that by 1.20 to add 20% to it and
It gives us
4.678 cubic metres of
spoil to be carted away
so you need to hire truck that's big enough, or
several trucks to cart that away, so
that's it. That's how we calculate the volume of the spoil and the concrete for a strip footing