DISCUSSION ON UNDERPINNING (28/3/12)
The following are opinions of the writer related to various forms of underpinning and foundation
stabilization. It should be noted that every situation requires an inspection / investigation process
to determine foundation movement cause, the priority for remediation and process required.
Foundation movement comes in varying degrees. An experienced professional should be
consulted to assess the movement to determine the severity and provide remediation solutions as
may be required. Abarent Construction Ltd. is experienced and qualified to assess your
foundation’s movement. Please enquire with us as to our qualifications, experience and any
technical resources relevant to your project.
To fully appreciate how foundation movement can be remediated, one must understand the cause
of the movement. Foundation settlement and movement can be caused by one, or a combination
of the following:
1. Dehydration of the soil creating shrinkage of the soils causing settlement
2. Expansion of the soil due to too much moisture or frost action causing uplift of the foundation
and it’s components
3. Existing pile failure due to piles not being designed properly or improper installation
4. “Wash Out” of bearing soil due to water erosion and/or water and sewer line issues
5. Existing Foundation components bearing on soft soils not capable of handling loads
There are other causes for foundation movement, but the above five are the most common. Once
again, it is very important that all relevant information be analyzed by an experienced professional
to determine what is occurring. Only then can an effective remediation plan be put together.
One of the most common solutions to stabilize the foundation of a building is to underpin the
foundation down onto a more stable soil. This soil layer should be suitable by strength and type to
accommodate the loads exerted on it. It should also be well below the “active soil layer”. “Active
Soil Layer” is the layer of soil that is affected by seasonal changes in moisture levels. Most
commonly this soil layer is within the top 20 feet of soil. Periods of drought or high periods of
precipitation can drastically affect expansive clay or sensitive soils.
Underpinning is typically done by installing a pile under the existing foundation. There are many
forms of piles available. The following are some of the underpinning piles used in our industry:
Helical Piers “Screw Piles”
Helical piers are a very basic product. They consist of a shaft/pipe and a helix (which is much like
a single rotation of a screw). Helical Piers “Screw Piles” were first used in England, invented and
patented by Alexander Mitchell sometime around 1830. Some of the original installed piles
installed in that era have been analyzed and have been found to have not lost any of their integral
strength.
“Screw Piles” are installed at relatively low RPM with an increase in torque during installation.
Sections are added as the pier is screwed into the ground. Once the predetermined depth and
installation torques are reached the pile can be cut off and a load bearing plate or bracket is
attached. When underpinning, this bracket can also be used to accommodate lifting the
foundation to level building elements. The latter is a definite advantage over some of the
underpinning systems available. Installation of the piles can be done with anything from small
portable equipment to varying sizes of machinery such as a backhoe or skid steer. During the
installation of helical screw piles, crews will record the pressure on the hydraulic motor and with
that, the torque can be calculated. This installation torque has been shown to have direct
Why Helical
Piers?
correlations
to installed pile capacity. A reputable company will only use this installation torque as
a confirmation to an already engineered pile design based on a soils investigation.
omparison of alternative foundation supports
Aside from little to no vibration and very little soil disturbance being a definite advantage of Helical
Screw Piles, these piles also resist upward forces. This is especially important in expansive
simple answer
is price
andhelix
performance.
In many cases
helical piers are
the easiest
to install
soil.
The
is anchored
in competent
load
bearing
soil and
and the frictional forces along the shaft
eads to lower cost. They also have the most predicable load carrying capacity. However, this is not
are
negligible
compared
to
the
end
bearing
force.
On
that
note, the biggest advantage of Helical
ys the case. Discussing these exceptions is the purpose of this article.
Screw Piles has to be the fact that they are designed as a “True End Bearing Pile” only. Any
article is aimed
at installers
andfriction”
designers who
are unfamiliar
andisare
trying
to
minimal
“skin
on the
side ofwith
thehelical
pile piers
shaft
not
calculated
into the piles required capacity.
cate themselves to this increasingly popular form of foundation support. To do this, I will discuss the
The
amount
of
weight
the
pile
can
handle
is
governed
by
the
bottom
of the pile (Helix plate). This
ngth and weaknesses of all the varieties of foundation support. Then I will summarize them in a table
creates
in the
Helical
System
to perform
ase of comparison.
Theconfidence
methods can roughly
be divided
intoPier
light and
heavy structure
supports.in soils where moisture levels change
(which would change the parameters upon which any friction pile would work). Keeping in mind
Light to moderate
supports
that the structure
most common
reason for foundation settlement is shrinkage of the soils due to moisture
loss, the Helical Pier is the most practical choice for the Engineering community.
Collins, P.E.
ical Piers
are different
manufacturers
of Helical
and they
are not all equal. Connection stiffness
cal piers can There
be used almost
anywhere traditional
deep foundations
can be Piers
used according
to Don
bitt PE, an experienced
helical
pier engineer.[1]
are better
suited
to the lower
capacity
is of great
importance
asTypically,
is pilethey
design
and
stringent
adherence
to the manufacturing process.
ications that make it less economical to use the larger install equipment required by the more
Our
manufacturing
company,
Helical Pier
Systems
(HPS),
ventional deep
foundations.
They also tend
to be more economical
in limited
access sites
or for dedicates considerable resources to
ation or noisetheir
free applications.
economics of each process.
case generally controls
researchHowever,
and the
manufacturing
HPS the
has offered engineering, manufacturing,
dation selection.
equipment design, creation and distribution for helical piling products since 1977.
cal piers are installed
screw them into the
d mounted on anything
ll spaces up to large
where one has a torque driver machine that
ground. Usually this is a hydraulic torque
from a portable torque frame that can fit into
backhoe mounted devices.
cal piers screw
y into bedrock. The
er torque. This is
recorded by the
ughly 10 times the
oking for a significant
se, firm load-bearing
age 20 to 30 feet below
themselves through the many layers and
layers are usually revealed by the varying
monitored by a torque pressure gauge read
machine operator. The pier bearing capacity
“kips” indicated on the gauge. The operator
increase in torque indicating he has hit
strata. For many locations, this is on
grade.
ions are added as the
pier is screwed into the ground. The final
on is cut off at a level
even with the other piers and capped with a
-bearing plate. It is
immediately ready to receive a load. There
o cleanup. This process
is quick if everything goes as planned and is
atter of hours for a
multiple pier job. If difficult soil is
ountered and pre-drilling
is necessary to break into hard rock, it can
a matter of several
days. A very good comparison with
tional drilled piers can
be found at the Helical Pier World’s
http://helicalpierworld.com/articles/taleof2part2.aspx. This running account of two side-by-side jobs
aks volumes about many factors in pier installation.
An advantage of helical piers in expansive soil is th
forces. The helix is anchored in competent load-be
and the frictional forces along the shaft are negligib
end loading force. This means the helical pier is ve
upward or downward loads. This is not the case wi
support without secondary operations or modificatio
with grout.
There are different manufacturers of helical piers, a
equal. Connection stiffness is an issue. It needs to
since a weak joint under compressive forces will bu
installation techniques to stiffen the connections an
on Helical Pull-Down Micropiles), but it is better to u
connections to begin with.
One advantage of helical piers is that if a rock is en
forward progress, the pier can be withdrawn and dri
away. Don Bobbit has also written a very comprehe
many difficulties and non-technical factors involved
installation and use of helical piers. [2]
Hydraulic push piers
Hydraulic push piers are essentially a helical pier
These are steel rods driven down into the ground
which is pushing up against the foundation. To w
with grout.
There are different manufacturers of helical piers, and they are not all
equal. Connection stiffness is an issue. It needs to be paid attention to
since a weak joint under compressive forces will buckle. There are post
installation techniques to stiffen the connections and shafts (see section
on Helical Pull-Down Micropiles), but it is better to use piers that have stiff
connections to begin with.
An advantage of helical piers in expansive soil is that they resist u
One advantage of helical piers is forces.
that if a rock
encountered
that stops
Theishelix
is anchored
in competent load-bearing soil or be
Hydraulic Push Piers
forward progress, the pier can be withdrawn and drilled several feet
and the frictional forces along the shaft are negligible compared to
away. Don Bobbit has also written a very comprehensive paper on the
end
loading
force.
This
means
helical pier
is versatile
manyare
difficulties
and non-technical
factors
involved
in
successful
Hydraulic Push Piers
steel pipe
or rods driven
down
into
the
ground
bythe
a hydraulic
jack
which with eit
upward
or
downward
loads.
This
is
not
the
case
with
other types
installation
and use of helical
piers.
[2] be careful that the foundation that it is pushing
is pushing up against
the foundation.
One
must
support without secondary operations or modifications such as fill
piers with grout.
against is strong enough to take the concentrated load.
Hydraulic push
As each pier is driven, the friction between the soil and the pier accumulates until it exceeds the
Therea are
different
manufacturers
of helical piers, and they are no
Hydraulic
pushAt
piers
essentially
helical
pierstarts
without
load being placed on
the pier.
thisare
point
the foundation
tothe
lifthelix.
and the pier refuses to go
equal.
Connection
an issue. It needs to be paid atten
These are steel rods driven down
into the
ground by stiffness
a hydraulicisjack
any deeper with the available foundation weight. This is one of the disadvantages of this type
which is pushing up against thesince
foundation.
work,
therecompressive
needs to be forces will buckle. There a
a weakTojoint
under
of system, as on a something
lot of occasions
one should
go are
past
soft layer
just
underneath
a hard layer
to
to push against,
so installation
these
notatechniques
suitable
for new
to stiffen
the connections
and shafts
(see s
more competent soils.
Another
disadvantage
of the
Hydraulic
Push
Pierbutisitthat
since
theypiers that h
construction
where
the foundation
has
not Pull-Down
yet
been poured.
However
on Helical
Micropiles),
is better
to use
they are
suitable for remedial
foundation
lifting
wherecan
the lift them. Further to
are friction supported,
expansive
clay in work
the and
intermediate
layers
connections
to begin
with.
typical
weight
of
the
structure
is
in
the
neighborhood
of
one
ton
per
that, the main disadvantage is that the soil friction along the side of the pier can change it’s
lineal foot. However, one must be careful that the foundation that it is
characteristics creating
a situation where it’s
to handle
weights
In is
contrast,
Oneaability
advantage
helical
piers ischanges.
that if a rock
encountered tha
attached to is strong and can take
concentratedofload.
Helical Piers are end loaded, are not affected
by
a
light
foundation,
and
are
unaffected
byseveral fe
forward progress, the pier can be withdrawn and drilled
intermediate expansive
soils.
away.
Donso
Bobbit
also weight
writtenisa very comprehensive paper o
The piers are typically driven one
at a time
the fullhas
building
available to drive the pier. As each
is driven, and
the friction
between factors involved in successful
manypier
difficulties
non-technical
the to
soilbe
andcareful
the pier when
accumulates
until
it exceeds
the
load
being this
placed
The consumer needs
hiring
contractors
that
system
as often the
installation
and use
of utilize
helical
piers.
[2]
pier. pipe
This isand
called
“driving
refusal”
the foundation
pipe being installedonisthe
used
there
is to
very
littlewhere
testing
or qualityjustcontrol with the pile
starts to lift and the pier refuses to go any deeper with the available
components.
foundation weight.
Hydraulic push piers
An advantage over helicals is that they can be installed without any Hydraulic
torque
push piers are essentially a helical pier without the he
equipment, generally closer to a wall, and can register the load capacity
These are steel rods driven down into the ground by a hydrauli
directly. The disadvantage is that since they are friction supported,
which
expansive clay in the intermediate layers can lift them up unless they
are is pushing up against the foundation. To work, there nee
something to push against, so these are not suitable for new
deep enough into bedrock or other layers unaffected by moisture. This
construction where the foundation has not yet been poured. Ho
means that they must have enough force on them to drive them to bedrock,
something that is not always done if the contractor is in a hurry or they are suitable for remedial work and foundation lifting where
careless. Refusal should happen at bedrock but won’t if there is nottypical weight of the structure is in the neighborhood of one ton
enough reaction force from a lightly loaded foundation. In contrast, lineal
helical foot. However, one must be careful that the foundation th
piers are end loaded, are not affected by a light foundation, and are attached to is strong and can take a concentrated load.
generally unaffected by intermediate expansive soil.
The piers are typically driven one at a time so the full building w
available to drive the pier. As each pier is driven, the friction be
the soil and the pier accumulates until it exceeds the load being
on the pier. This is called “driving to refusal” where the foundat
starts to lift and the pier refuses to go any deeper with the avail
foundation weight.
An advantage over helicals is that they can be installed without any torque
equipment,
generally
closer Shaft)
to a wall, and can register the load capacity
Concrete Drilled
Pile Caissons
(Drilled
directly. The disadvantage is that since they are friction supported,
in the intermediate
layers
can lift them up
are concrete and
A drilled pier isexpansive
a deep clay
foundation
system that
is constructed
by unless
placingthey
fresh
into bedrock
or other
layers unaffected
moisture.
This it with a rebar
reinforcing steeldeep
in a enough
drilled shaft.
This pile
is installed
by drilling by
a hole,
reinforcing
means
that
they
must
have
enough
force
on
them
to
drive
them
to
bedrock,
cage dropped down the hole, and then pouring in concrete. For Underpinning,
the work area is
something that is not always done if the contractor is in a hurry or
careless. Refusal should happen at bedrock but won’t if there is not
enough reaction force from a lightly loaded foundation. In contrast, helical
piers are end loaded, are not affected by a light foundation, and are
generally unaffected by intermediate expansive soil.
tight, so the most common method of drilling this hole is with a fabricated A-Frame that leans
against the structure and hydraulically drills the hole with an auger.
There are a multitude of disadvantages to this type of pile system. (1) It is very difficult to attain
depths over 25 feet deep due to drilling constraints. This is often necessary to accommodate
the soil issues that are the reason for being there in the first place. (2) The drilled concrete pile
relies on soil friction along the side of the pile to provide capacity. As discussed earlier, this
is a definite problem as the most common reason for Underpinning is due to changes in soil
moisture content. These changes alter the soils ability to provide friction along the concrete pile.
It is quite common to see failure of this type of pile due to that reason. (3) The pile system does
not generally allow for any lifting mechanism, so in most cases is just used to attempt
stabilization of the foundation. (See Engineering Paper Helical Piers vs. Drilled Concrete
Piers in Highly Expansive Soil Areas by Ryan J. Hardesty, P.E. for a case study on this pile
type– Attached to this handout – Much of the same principals discussed and illustrated in
this paper can be used in reference to Hydraulic Push Piers)
Expanding Foam Resin:
There is a proprietary process used by a company where a foam resin is injected into the soil
below the footing area. This foam is theoretically used to “stabilize and lift” the footing and
foundation. It is our opinion that any lift would not be possible without a mechanically method of
lifting (hydraulics etc.). Considering that the main reason for foundation settlement is dehydration
of expansive clay it is illogical to agree that injection and densifying of the soil at this level will
prevent any further settlement if expansive clays continue to dehydrate and shrink. If the main
issue were weak base soils or an issue with granular soils or permafrost there might be a positive
argument to using this method. We have personally seen too many failures with this method used
for foundation stabilization or “lifting” to endorse it.
Years ago Abarent Construction Ltd spent considerable time researching which piling system it
should use for Underpinning. Obviously we could have picked any of the methods available, but
chose to endorse the method that was most viable for our climate and soil type…The Helical
Screw Pile.
We have evolved a process for all Underpinning Projects:
1. Engineered Design of the project that includes:
 For Underpinning - Analyze pattern of cracking and location of cracks
 Calculate loads of structure and point load areas
 Calculate allowable spans between piles (This varies on foundation type and size)
 Obtain soils data from a Soils Bore Hole
 Have our Manufacturer (HPS) design the pile based on required Loads and Soils Data
 Provide an Engineered Stamped Plan for the project
2. Obtain a Building Permit
3. Excavate and Prep Pile locations
4. Install piles and record installation pressures (Torques)
5. For Underpinning – Install Patented Underpinning Brackets
6. For Underpinning – Lift Structure (as structure allows) to as close to level as possible
7. Backfill and clean-up project
8. Provide all installation Data to Engineer
9. Obtain Pile Certification from Engineer
**No part of this handout shall be used in any form without the express written permission of
Abarent Construction Ltd. Copyright 2012**