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This article can be also found at this link, DIY Retaining walls.
Building the
hybrid rock wall
This hybrid wall that we are going to build is a big job, it
starts at the same level, as we have dug the ground approximately level, but
because we are building the wall only to the height of the natural ground
level, it is going to start from almost nothing and at the highest point will
be 1.8 meter, it is really two walls that meet and forms a square at the
highest point.
Here we must say that today some people think that this
should not be done, as this is neither a rock wall nor a concrete masonry wall,
as it mixes about everything, but so what, if it is convenient, why not do it?
I must say that in my life time I have built walls like this with whatever was
available on the site; and I mean everything in fact I was using stones that
were laying in this small creek bed and pieces of concrete that we had broken
to do some alterations and old bricks, some concrete and some mortar, this has
happened a few year ago and last time I have seen the owner he said that the
wall is still there. So, most things can be done if you know that it has got a
good chance to last a long time.
Anyhow, let us go back to build our hybrid rock wall. Now, to build
this wall you need a few cubic meter of rocks, because you want the wall to
look as a rock wall, you don’t need very big rocks or special size rocks,
because you can use the cement wherever necessary to fill the gaps and this is
good, since the cement is less costly, you need a mixer, you need some gravel,
sand and cement; and of course, you must have the tools to do the job.
You got all this on site, and you can start to build. First
of all, drive a couple of pegs in the ground, exactly where you want the face
of the wall to be, tie and stretch a line on these pegs and see any adjustment
that you need to do before you start laying the first stone. What you are
looking for here is that the bank you are going to build the wall against has
been dug straight, it is almost vertical or a few degrees sloping, you have dug
a shallow foundation, because you want the wall to be strong and last for a
long time, so, you are satisfied that the foundation is going to be solid
enough, since it is well below the original natural ground, therefore, it can
hold a lot of weight and pressure, because the ground is very compact.
You have done all this, you have set the pegs again so that
your wall is set to be at least 10” thick overall, you have chosen this
thickness because most of the large rocks seem to be that thick, so now you are
ready to lay the first course of stones, mix a few batches of concrete and lay
it in the foundation and then before this concrete sets lay the first course of
stones on this concrete making sure that the bottom of the stones are below
ground level, so that the entire stone wall seems to start from below the
ground; once you have done that you can fill the back of the stone with
concrete a bit above ground level but below the full height of the stones,
because you want to leave enough room to lay next course of stone easily. Now
the wall is all set to go, but we need to reset how we must continue and
complete the hybrid rock wall, the right way with the right lean against the
bank. You need to lean the wall against the bank, so that the weight of your
wall by itself adds strength to the wall.
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Completing
the hybrid rock wall.
So, before we build this retaining wall, we must work out
how much lean against the bank we want, here we must keep in mind, how tall and
how thick is the wall, we don’t want the wall to lean too much and rely only on
the dirt behind it to hold it back, but at the same time we want to lean it
against the bank so that the weight of the wall is used to hold the dirt back.
Here we must go back to look at what happens if you lean a
monolithic object to the point it is going to fall; what happens here is that
the monolithic object is going to fall, only after its center of gravity falls
outside the area of the base. Now we don’t want to go this far, and we don’t
want to use the center of gravity to decide that, but we are going to lean the
wall about two third of the thickness, so that the very top layer is still
vertically one third of its thickness above the base; we believe that it is
very safe to go this far and the wall would stand up even if we don’t backfill
it. Anyhow, in our case we are not going to backfill it, because we have cut
the bank exactly as the wall is going to be, you see we are going to use the
bank as a template, so, we believe that we are not going to backfill it at all.
Now that we know how much we are going to lean the wall; we
are setting a leaning profile on every corner leaning approximately 10 to 12
centimeters per meter and we are ready to resume building our hybrid rock wall.
But there is still another important thing to work out; we need to make sure
that when it rains the water that seeps behind the wall can run out, so, we must
leave a way to drain this water out and at this level is the right place to do
that, so, we choose a few spots about a meter apart and mark them, then when
you lay next course of stones make sure that you leave a space between the stones
that the water can run out, you must also leave the back free of abstractions
for a course or two around this space, so, in this space after laying the front
stone, just lay some smaller stones dry against the bank to stop the concrete
blocking the drain, then you can resume to concrete the lot. Alternatively, you
can lay a few short pieces of pipe across the wall for drainage, but you have
still to make sure that they are clear of any obstruction at the back near the
bank.
So, now you can go back to mix some cement to lay your
stones, you can mix a batch of river sand four to one that you can lay between
the rocks to give them a bit of grip, you are laying the rocks in a straight
line that you have tied on the profiles, when you do that make sure that this
cement does not come too far forward, because you still want the rock wall to
look like a rock wall, at the same time do not forget that you have to leave
these spaces with no cement at all for the seepage where you have marked, after
you have used the batch of river sand and cement and laid say a course of
rocks, you can mix one or two batch of concrete to fill the back of the stones,
then you repeat this procedure until you build the entire wall.
At the top of the wall, because the wall in not going to be
level, you may have to do some adjustment to follow the height of the ground,
here you have to use common sense the best way you can, if you happen to have
enough wide and long rocks that can be used for a coping, use them to finish
your wall, if not, just use your common sense and use anything that is
available, even if it is going to be a strip of concrete in the back of the
wall, so that all the wall is tied up.
I hope I have explained well enough for you to follow my way
of building this hybrid rock wall. Here I want to add that this is just one way
out of several other ways that can be used. A lot depends how the bank has been
cut and if it follows very closely the shape of the wall, so, you can use it as
if it is formwork, but if there is going to be some back filling to be done,
you may have to consider some temporary formwork that can be easily removed
before you backfill. All these things can vary from job to job, and if you see
that it is hard to build your hybrid rock wall, then you must consider other
ways to build your retaining wall.
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Why we need these retaining walls.
When the block of land is steep, it requires a lot of work to be done, if you want a small portion of your land flat, so that you can stand and enjoy whatever is there to enjoy and do whatever you want to do in the open air. See the photo here under, they have built a Besser block retaining wall with an indent in it, mainly to give extra strength to the block wall and to put this concrete slab and use this indent with whatever they want to do next. Note that above this block wall there is another sleeper retaining wall, these sorts of things are very costly, and we must plan it out before we start, anyhow, under below we are going to write about a Besser block retaining wall, first just a normal strong retaining wall and after that we are going to build a leaning Besser block wall, similar to the hybrid rock wall.
Building a Besser
block retaining wall.
Besser blocks retaining walls require a reinforced concrete
foundation with steel starter bars placed exactly in the centre of the hollow
part of the block, when you look at the wall from the front or back, and if possible,
a bit close to the side where the pressure of the soil is going to be, but that
can be asking too much from the concrete man and the steel fixers. So, a
profile must be in place before you can even concrete the foundations, or fix
the steel in the foundation, because all measurements need be known and easily
measured while working on it.
This is a very demanding job and it is not a do it yourself job. So, the name that I have given this article DIY retaining wall would not apply in this case.
Anyhow, when you start laying the first Besser block in most
cases, it is helpful if you can start the first course with an open block, this
block looks like a square C, and you lay it with the open side facing
away from the bank that it is going to hold back when everything is done, the
corners are built with normal blocks and most of the walls are built with
blocks that look like an H, so, it is easier to fill up with concrete,
once the wall has been built. Now while the block layer is laying the blocks,
he will have to lay horizontal steel bars between the courses of blocks as
specified, if there is a plan and specifications, or use common sense, or copy
from the specifications of a similar job he has done, the vertical steel bar
will be inserted from the top before or while pouring concrete into the blocks.
Now the reason why we need to use an open C shaped
block for the first course, it is because it is necessary that before we fill
up the blocks we remove the mortar that has fallen down at the bottom of the
blocks while we were laying them, you see, we want a solid base and we want the
concrete to bind around the starter steel bars that we have laid in the
foundation and this is the best way to make the retaining wall as strong as we
can. Once that is done, we can fix a plank against the openings, and we are
ready to pour the concrete in the wall.
Here we must say that for this retaining wall we have not
mentioned any drainage yet, so, what happen to the water that seeps behind the
wall needs to be worked out, you see, sometimes when we build walls this strong,
there could be a reason that we do not want any water past the wall, it can be
part of a base of a house being built, so, we have not even left any weep holes
in the wall, because there is going to be a drainage pipe laid behind the wall
surrounded by gravel. The wall might also be treated with special paints to waterproof
it.
This is one type of blocks retaining wall, and I am going to
write about another type, where we lean the retaining walls towards the bank for
future back fill that it is going to hold up, this is a large job that we did
years ago in a yard of a new large home in Redhill a Brisbane suburb. We are
going to write it here, to show you another type of retaining wall and the many
ways that can be built.
Redhill backyard problem solved.
Years ago we worked on this part brick house in Redhill, we
say part brick house because the top floor of the house and the interior was
made of timber, so, it was a brick veneer, the base was solid bricks and
blocks, it was a large house, on a large block of land had two street frontage.
It was on a hill and the difference between the front and the back yard was a
lot, but the two street frontages was good, because the house was being built
on the higher part of the block of land, where it was nearly flat there and
almost level with the road, so, there were no problem building the house; but
the back yard was very steep, in some places could have been up to 45 degrees.
So, when we finished the brickwork on the house we were
asked from the owner, if we could build some retaining walls, because the
backyard was useless as it was. So, after discussing the financial part how he would
pay us; we discussed how we could fix the back-yard building block retaining
walls. So, it was agreed that we had to lay these block retaining walls leaning
a few degrees towards the bank. We had to build concrete stairs on one side, so
that the house could also be entered from the back street, so, working together
with the owner we did the following.
Because you cannot build a very high wall on the road
boundary, first a section of about 4 feet wide (1.2 m) was dug and a foundation
was poured, with starter steel bars as needed and the main wall was set about a
meter from the road where the first retaining wall was to be build, this was
the main foundation for a start, some of the small foundations and other
adjustment we had to work out as the work progressed.
The decision was that we had to build the wall leaning
towards the bank for extra strength; you see, when building retaining walls the
weight of the retaining wall with the weight of the foundation must be more
than the weight that the backfill can move, if possible that you can build a
very strong monolithic wall and foundation together this is the best solution,
in this case it was not possible, because the job was too big, beside that you
need to have a very wide and heavy foundation and a very strong bond between
the foundation and the wall, since this bond between the foundation and the
wall is the weakest point unless it is concrete and poured together, but we
were using blocks to build the wall, therefore, it had this weakness at the
bottom of the wall, therefore, by leaning the wall towards the bank, the weight
of the wall becomes the first point or
resistance.
Now, how much we must lean the wall was left to us block
layers, because for sure there will be problems, and there were several
problems to overcome; the first problem is that if you have to return the
corners it cannot be done, but we had corners on the boundaries of the joining
block of land, so, we had to think how to overcome this drawback, for this it
was decided that we would not build the corners as usual, we should treat them
as separate, but we would place a corner steel rod every course, instead of
every second course, as in the wall itself.
There was also the possibly that while we were building the
wall, if we lean it too much it could fall on the inside, while we were
building it, so, we had to strike a balance. Knowing that this first wall was
going to be no more than 3 meters high, we worked out that if we lean the wall
5 centimeters per meter, the top of the wall was going to be about 15 centimeters back from the perpendicular, the blocks were 19 centimeter wide and it would be
okay, as once the wall was built it could stay up on its own without falling
inside, but while we were building we should be very careful not to apply any
extra weight on the leaning wall, just for fear that this could happen we added
a couple of block piers behind the wall, this would not only make the wall safe
for us while we were building it, but it also would make the whole wall
stronger.
Anyhow, we built this block wall placed the reinforcing rods
in place, we also built some concrete steps on one side of it, so, that if you
happened to be on the back road you could walk to the house, the ready-mix
truck came, and the wall was filled with concrete and that was the first stage
finished. We had to wait for this wall to become strong before we could
backfill it, and then we could build another retaining wall halfway the
backyard to finish the job.
Dear readers, be warned that this type of work can only done
from very experienced block layers, as it breaks all the rules in the building
trade, but we did it and it worked.
Anyhow, this article is becoming too long, so, see you in our next article, Mix your own cement.
To see more click on this link, DIY Retaining walls.
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