..............................................................................................................................................................
Pile Foundations
understandconstruction.com
Pile foundations are used in the following
situations:
When there is a layer of weak soil at the
surface. This layer cannot support the weight of the building, so the loads of
the building have to bypass this layer and be transferred to the layer of
stronger soil or rock that is below the weak layer.
When a building has very heavy, concentrated
loads, such as in a high-rise structure, bridge, or water tank.
Pile foundations are capable of taking higher
loads than spread footings.
There are two fundamental types of pile
foundations (based on structural behaviour), each of which works in its own
way.
End Bearing Piles
In end bearing piles, the bottom end of the
pile rests on a layer of especially strong soil or rock.
The load of the building is transferred
through the pile onto the strong layer. In a sense, this pile acts like a
column.
The key principle is that the bottom end
rests on the surface which is the intersection of a weak and strong layer.
The load therefore bypasses the weak layer
and is safely transferred to the strong layer.
Friction Piles
Friction piles work on a different principle.
The pile transfers the load of the building to the soil across the full height
of the pile, by friction.
In other words, the entire surface of the
pile, which is cylindrical in shape, works to transfer the forces to the soil.
To visualise how this works, imagine you are
pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream.
Once you have pushed it in, it is strong
enough to support some load. The greater the embedment depth in the ice cream,
the more load it can support.
This is very similar to how a friction pile
works. In a friction pile, the amount of load a pile can support is directly
proportionate to its length.
WHAT ARE PILES MADE OF?
Piles can be made of wood, concrete, or
steel.
In traditional construction, wooden piles
were used to support buildings in areas with weak soil.
Wood piles are still used to make jetties.
For this one needs trees with exceptionally straight trunks.
The pile length is limited to the length of a
single tree, about 20m, since one cannot join together two tree trunks.
The entire city of Venice in Italy is famous
for being built on wooden piles over the sea water.
Pile Foundations
Concrete piles are precast, that is, made at
ground level, and then driven into the ground by hammering - more on that
later.
Steel H-piles can also be driven into the
ground. These can take very heavy loads, and save time during construction, as
the pile casting process is eliminated.
No protective coating is given to the steel,
as during driving, this would be scraped away by the soil. In areas with
corrosive soil, concrete piles should be used.
HOW PILES ARE USED
As pile foundations carry a lot of load, they
must be designed very carefully.
A good engineer will study the soil the piles
are placed in to ensure that the soil is not overloaded beyond its bearing
capacity.
Every pile has a zone of influence on the
soil around it.
Care must be taken to space the piles far
enough apart so that loads are distributed evenly over the entire bulb of soil
that carries them, and not concentrated into a few areas.
Engineers will usually group a few piles
together, and top them with a pile cap.
A pile cap is a very thick cap of concrete
that extends over a small group of piles, and serves as a base on which a
column can be constructed. The load of this column is then distributed to all
the piles in the group.
HOW PILES ARE CONSTRUCTED
Cast-in-place piles are made in the following
steps:
hammer a thin-walled steel tube into the
ground
remove all earth left inside the tube
lower a steel reinforcement cage into the
tube
cast the pile by pouring wet concrete into
the tube
The thin walled steel tube is called the
casing, and only serves to form a secure mould for casting concrete that is
free from earth and debris. It has no structural role to play after the casting
is complete.
Some soils are highly cohesive, meaning that
if one drills a hole into the soil that is say 1 foot wide by 50 feet deep,
then the soil holds the shape of the hole and does not collapse into the hole
and block it.
If such soil is present at the site, then one
does not need to leave a casing in place: one can use the casing to drill the
hole for the pile, and then remove it, and then cast the pile in place.
This saves costs as the same casing tube can
be used to drill holes for all the piles.
Precast Driven Piles are first cast at ground
level and then hammered or driven into the ground using a pile driver.
This is a machine that holds the pile
perfectly vertical, and then hammers it into the ground blow by blow.
Each blow is struck by lifting a heavy weight
and dropping it on the top of the pile - the pile is temporarily covered with a
steel cap to prevent it from disintegrating.
The pile driver thus performs two functions -
first, it acts as a crane, and lifts the pile from a horizontal position on the
ground and rotates it into the correct vertical position, and second, it
hammers the pile down into the ground.
Piles should be hammered into the ground till
refusal, at which point they cannot be driven any further into the soil.
SPECIAL PILES
Pile driving is very noisy and causes massive
vibrations through the soil. For this reason, it is sometimes difficult to use
them in sensitive locations.
For example, if an operational hospital or
science lab is to be extended, driving piles would cause unwanted disturbance.
Their use is also restricted in residential
areas in many countries. The vibrations could also cause structural damage to
older buildings that are close by.
In such situations it is possible to use
micropiling or helical piling, neither of which rely on hammering.
Micropiles or minipiles are small piles that
are constructed in the following way:
Step 1: a hole a
little larger than the pile diameter and the full length of the pile is dug
into the ground using an apparatus like a soil boring machine.
Step 2: a precast
concrete pile is lowered or pushed into the hole.
Step 3: a concrete
grout is poured into the gap between the pile and the earth.
Helical piles are steel tubes that have
helical (spiral) blades attached to them.
These can be drilled into the ground, meaning
that the pile acts as a giant drill bit, and is rotated and pushed into the
ground from above, much like a screw drills into wood.
Once the steel pile is driven into the
ground, a pile cap is poured on top of the pile to prepare it for the
construction above.
This
is a site that explains the art and science of building construction in great
clarity and detail. Our goal is to make you understand concepts in
building construction.
Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
We are a free site - we cover our costs by advertising, so do feel free to click on ads that interest you!
Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
We are a free site - we cover our costs by advertising, so do feel free to click on ads that interest you!
No comments:
Post a Comment