At first glance, it seems that the pile length calculated in the design should always be sufficient. But in practice, things are a little different. Soil is a capricious material: in one place it's dense loam, while two meters away it's quicksand or peat. Therefore, when driving the pile, you have to adapt to the actual situation.
The main
reasons for extending piles are:
1. Not reaching the solid soil layer.
When driving, the operator feels that the pile still moves easily, indicating
that there is no supporting layer yet. If left untreated, the foundation may
"wander" or settle.
2. Elevation differences on the site.
On slopes, lower piles must be longer to ensure the foundation pad is level.
3. Drilling or calculation errors.
Sometimes the designer failed to take into account soil characteristics, or
site conditions have changed due to rain, backfill, or machinery use.
4. A heavier structure is planned. If
the design has changed (for example, brick walls have been chosen instead of
frame walls), the supporting piles must be deepened and reinforced.
5. Replacing old piles or repairing the
foundation. During reconstruction, it is often necessary to reach a more stable
horizon, in which case the piles are also extended.
Screw Pile Extension
Methods
Pile extension can be accomplished in various ways, but the
goal of all methods is the same: to ensure a strong, watertight, and coaxial
connection between the main pile and the extension. This directly affects
whether the foundation can withstand the design load.
There are two main methods:
1. Welded connection
This is the most common option. A metal sleeve or extension
pipe of a suitable diameter is welded to the end of the pile.
Pros:
•
High connection strength;
•
Sealed joint;
•
Suitable
for piles of any diameter.
Cons:
•
Requires an experienced welder;
•
Misalignment
may cause distortion, especially when screwing deeply.
To avoid errors, the pile is leveled before welding, and the
seam is welded all the way around with a continuous weld. After this, the joint
is coated with an anti-corrosion coating—usually a bitumen mastic or zinc-based
primer.
2. Bolted connection (on a sleeve)
This is used when welding on-site is not possible—for
example, near a house or in subzero temperatures. The extension pipe is
inserted into the sleeve and secured with bolts.
Pros:
•
Installation
is possible even in hard-to-reach places;
•
Easy
to remove if necessary.
Cons:
•
Less
rigidity of the connection;
•
Requires
regular inspection and tightening of bolts.
How the extension
process is performed - step by step
Step 1. Determining the Need for Extension
While screwing, the operator feels resistance. If there is
no resistance, it means the solid layer has not yet been reached. The pile is
driven deeper than planned, and the decision is made to extend it.
Step 2. Preparing the Pile End
The end of the pipe is cleaned of dirt and soil, and the cut
is checked for evenness. This is necessary to ensure the extension fits snugly
and without gaps.
Step 3. Installing the Extension
An extension—a section of pipe of the same diameter and wall
thickness—is either pushed or inserted onto the pipe. The length depends on how
far the pile is from the solid layer, typically 0.5–2 meters.
Step 4. Connection—Welded or Bolted
•
For
welding, the connection is welded all around, using a continuous weld.
•
For
a bolted connection, the coupling is secured with 2–4 bolts.
After this, the connection is checked for alignment using a
level or template. Step 5. Corrosion Protection
The connection is coated with an anti-corrosion
compound—most often, cold galvanizing or bitumen mastic. For more reliable
protection, apply two coats, drying between them.
Step 6. Continued Screwing
The pile with the extension is screwed in to the design
level. If the pile still slides easily, the screwing process is repeated.
Sometimes, two or even three sections must be made, especially on soft or
marshy soils.
Mistakes when
installing screw piles and how to avoid them
1.
Misalignment during welding
The most
common mistake is welding the extension at a slight angle. Visually,
this is almost imperceptible, but as the pile is tightened further, it arcs. As
a result, the pile heads are at different levels, and the foundation is at an
angle.
How to avoid: Use a spirit level and a template to align the
axes. On-site, professional installers check the vertical alignment immediately
after each tack weld before welding the entire seam.
2.
Incomplete or uneven weld
Some
"experts" save time and metal by welding only half the circumference.
This results in microcracks appearing in the joint, and after a couple
of years, the pile may simply crack.
How to avoid: The weld should be continuous all the way
around, without gaps or overlaps. After cooling, clean the weld and coat it
with an anti-corrosion mastic.
3. Using a
pipe of a different diameter
When
there's no suitable extension, some people decide to "use whatever's
available." This is a serious mistake: the load is distributed
unevenly, the pipe begins to play, and over time, it can bend.
How to avoid: The extension must be exactly the same
diameter and wall thickness as the pile. Check the match down to the
millimeter.
4. Lack of
anti-corrosion treatment
If the
metal is left unprotected after welding, corrosion will appear within a season.
This is especially dangerous for the joint, where rust weakens the metal
most quickly.
How to avoid: After welding, immediately clean, prime, and
apply a protective layer. For bolted connections, additionally treat the areas
around the bolts.
5.
Incorrectly selected extension length
Too short
will not reach the solid layer; too long will make screwing difficult and
increase the stress on the joint.
How to avoid: Use actual soil conditions as a guide. If you
are not sure, do a test screwing before ordering the entire set of piles.
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