UNDERPINNING
7th Semester-CAHS72-P
Bachelor of Architectural Technology and Construction Management
ÁLVARO NOGUEIRA SAN ROMÁN
Dissertation Topic: Underpinning
Index
1.1.
1.2.
1.3.
1.4.
1.5.
Introduction
Definition and Uses
Previous studies and Work Process
Underpinning Methods
Conclusion
References
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7 Semester
CAHS-72P
Dissertation Topic: Underpinning
1.1.
7 Semester
CAHS-72P
Introduction
During the last years increasing needs for construction of multilevel basement,
mainly intended to parking space have raised all around the world. Excavations
with vertical cuts that may go to depths of 12-15m necessary for the construction
of these basements, have required to perform shoring and underpinning works of
existing structures.
1.2.
Definition and uses
Underpinning is a broad term to describe the process of modifying an existing
foundation by adding support. This can be done by several methods such as
concrete caissons, piles or grouting. Each one has its own advantages and
disadvantages depending on the specific characteristics of the Project. Different
Underpinning solutions are presented in the next pictures.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Furthermore, existing buildings, like historical constructions, sometimes experience
excessive settlement under their design load or face the prospect of excessive
settlement in the future if a change of building use is required and increased
foundation loading because of the change of the building use.
Various reasons may result in the need for underpinning. However in general,
underpinning of a structure is required:
-
To support a structure that is sinking of tilting due to poor soil or instability
of superstructure
As a safeguard against possible settlement of a structure when excavating
close to or below its foundation level
To support a structure while making alterations to its foundations or main
supporting members.
Underpinning of a structure is required for the reasons above and, in addition:
-
To enable the foundations to de deepened for structural reasons, for
example to construct a basement.
To increase the width of a foundation to permit heavier loads to be carried,
for example when increasing the height of a building with new levels.
To enable a building to be moved bodily to a new site.
Underpinning works are highly skilled operations and should be undertaken only by
experienced companies. No one underpinning job is like another because of
different technical points and each one must be given specific consideration for the
most economical and safest scheme to be worked out.
That´s why, the improvement and development of underpinning techniques have
been growing fast.
Traditional Underpinning methods entail large amounts of heavy and unhealthy
manual labor. New regulations regarding working conditions and adaption to
ergonometric research results in other branches of the building and construction
industries have made the poor conditions on underpinning sites even more
unacceptable. For this reason only, new methods were needed.
So, a number of methods for underpinning heavy structures have been developed
or adapted to these conditions during the last few years.
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Dissertation Topic: Underpinning
1.3.
7 Semester
CAHS-72P
Previous studies and Work Process
Before undertaking any scheme of underpinning
whether in connection with adjacent construction
operations, or to prevent further settlement as a
result of ground subsidence or overloading of
foundations, it is important to carry out a careful soil
investigation by means of boring or trial pits and
laboratory tests on soil samples to determine
allowable bearing pressures for the new
foundations.
If underpinning is necessary to stop settlement, it is
essential that the underpinned foundations should
be taken down to relatively unyielding ground below
the zone of subsidence. The underpinning must be
taken down to a deeper and relatively
incompressible stratum, if necessary by piers or
piles.
If the structure to be underpinned is close to the excavation, it is often convenient
to combine the underpinning with the supports to the excavation. The load of the
building can then be transferred to the tops of the piles. Alternatively, a system of
close-spaced bored piles can be used.
In all cases where underpinning is provided close to excavations, it is important to
design the underpinning members to carry any lateral loads transmitted to them
from the retained earth or ground water.
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Dissertation Topic: Underpinning
1.4.
7 Semester
CAHS-72P
Underpinning Methods.
Conventional pit method underpinning has been used for centuries. This method
consists of enlarging and/ or deepening existing foundations by removing soil from
beneath the foundations and replacing it with concrete, reinforcement, and a grout
material. In some cases the structure is temporarily shored to prevent settlement.
The pit method often results in moderate deformation of the structure and unsafe
working conditions. That´s why during the last 20 years, several less disturbing
methods developed to underpin structures that result in much less deformation
and a faster, less expensive and safer operation.
The means and methods of supporting a structure foundation depend on many
factors including:
-
Foundation loads: static and dynamic; permanent and temporary
State of existing foundations.
Type and magnitude of allowable structural movement i.e. deformations.
Subsurface soil conditions.
Subsurface groundwater conditions.
Condition of the structure.
Access and mobility to the foundations.
Potential for environmental hazards.
Seismic loading.
This list is by means exhaustive and each of these factors must be considered in
making the evaluation of which underpinning method can best satisfy the project
needs.
With the new methods developed below, underpinning can be achieved not just by
load transfer, but also by soil treatment, or by a combination of these two
mechanisms. Load transferring methods literally take structural loads and transfer
them to an underlying stratum that is more suitable for support. Soil treatment
changes the physical properties of the ground to make it stronger and more
supportive, often without any change to existing foundations. In some cases,
ground treatment can be utilized to strengthen the ground while also acting as a
load transfer.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Hundreds of underpinning methods exist around the world. Each underpinning
technology has its own specific soil and loading conditions where its application is
most effective. In some cases, constructability can best determine the system to
be used.
In the sections that follow, I will describe the different techniques that are mostly
used nowadays in typical projects.
A. Jet Grouting
Jet Grouting is a load transferring system for the underpinning, often also serving
as an excavation support and groundwater control system. This is an in situ
method of construction undertaken beneath foundations.
Simply stated, high velocity injection of fluids, often enclosed in air, erodes the
soils and replaces the soil with and engineered grout, forming a cementations
product known as soilcrete that is capable of attaining unconfined compressive
strengths in excess of 70,30 kg/cm2. Groups of 1 to 1,20 m diameter soilcrete
columns are constructed to transfer foundation loads to underlying suitable
bearing material.
This technology requires specialized
equipment and experience to construct
the soilcrete.
Work is accomplished safely above grade,
and sequenced so that little or no
structural deformation occurs. In fact, it is
the most widely used system for
underpinning historical and sensitive
structures.
Jet grouting for underpinning has been applied to construct deep foundation
systems, in site gravity wall structures, and groundwater cutoff barriers.
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Dissertation Topic: Underpinning
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CAHS-72P
B. Compaction Grouting.
Compaction grouting can be either a soil treatment system or a combination of soil
treatment and load transfer. Soil in a loose or soft state can be densified by
injecting a very viscous san-cement grout into these soil zones. Compaction
grouting uses displacement to improve ground conditions. With adequate
confinement stress and slow injection rates, the low mobility grout will displace the
soil into a denser arrangement, this increasing its bearing capacity and reducing its
compressibility. In case where ground dropouts are a problem, settled structures
can be pushed back to near original position as part of the grout treatment
program.
Depending on the location and arrangement of the grout injection and the quality
of the cured compaction grout, the grout element can be used as a load transfer
mechanism as well as a soil improvement system.
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Dissertation Topic: Underpinning
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CAHS-72P
C. Micropiles
This type of underpinning is used to stabilize or upgrade existing foundation by
installing micropiles through pre-drilled holes determined by load characteristics.
Micropiles are described as small diameter piles that can be installed in almost any
type of soil and that can carry loads up to 500 tons depending on the type of it.
These micropiles are steel reinforced placed into a small diameter hole and scaled
to the ground by grout injections under relatively high pressure.
Micropiles are widely recognized as a common remedial option for underpinning
structures having foundation problems after completion or during service period in
many countries. Some of the advantages of micropiles are high carrying capacity,
less site constraint problem, low noise and vibration and self-sustained operation.
Furthermore, a major advantage when using micropiles for underpinning is that
the system can be designed to have very low settlements. It is common for these
piles to develop settlements on the order of a few millimeters or less under
working loads. The only disadvantage of micropiles is the relatively higher cost as
compared to other pilling systems. There are different types of micropiles and
different systems depending on the solution needed; this is what will increase the
final price of the work.
In the last few years, micropiling techniques have developed greatly mainly due to
the bearing capacity improvement related with the use of high pressure grout
injection techniques and high resistance steel hollow tubes.
Also, micropiles can be connected to the structure in different way as I will
describe below. There are different solutions depending on the type of
construction, the space to work, the investment, the solution required and the
buildings around. On the basis of the above, the different solutions bases on
micropiles are:
Pile and Beam
Pile and Cantilever Beam
Knuckle Pile
Piled Raft
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Angle Pile
Twin Pile
Cantilever Ring Beam
Jack Down Piles
Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Pile and Beam
This system of underpinning can be used with different pile options. This method
consists on installing mini-piles to either side of the affected wall, a pocket of
brickwork is then removed below ground level then a prefabricated steel cage is
installed to span over the two piles. This is then inspected by local building control
and then concreted.
Loadings are based upon the bearing capacity of the underlying strata which will
determine mini-pile type and size for this system. Very high foundation loads can
be accommodated by reducing the span between needle beams:
General Guidelines for Mini-Pile Sizing
90 mm dia.
105 mm dia.
150 mm dia.
200 mm dia
220 mm dia.
250 mm dia
Up to 40 kN
40-60 kN
60-100 kN
90 – 120 kN
120 – 200 kN
150- 250 kN
Installation Procedure
Install piles in pairs, driven, drilled or augured, at longitudinal centers
approximately 1.0m - 1.5m; lateral centers 1.0m - 1.2m; excavate, break out for
and construct reinforced concrete needle beams.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Advantages
a) Because large excavations are not required, disruption and mess are kept to a
minimum.
b) System becomes cost effective when the depth of traditional underpinning
exceeds 1.5m.
c) Conservative design when pile centers do not exceed 1.0m.
d) High load capability.
e) Quicker when compared with dig-out systems.
f) Suitable for restricted access.
g) Needle beam can be
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Pile and Cantilever Beam
This system is based on the stabilization of an existing wall foundation by the
installation of RB mini-piles in pairs, one as a tension pile, one as a compression
pile, connected by a reinforced concrete or concrete encased structural steel
needle beam supporting the wall.
This solution is used when the foundation suitable bearing strata occurs at depths
in excess of 1,5 m.
Loadings are based upon the bearing capacity of the underlying strata which will
determine mini-pile type and size for this system.
General Guidelines for Mini-Pile Sizing
90 mm dia.
105 mm dia.
150 mm dia.
200 mm dia
220 mm dia.
250 mm dia
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Up to 40 kN
40-60 kN
60-100 kN
90 – 120 kN
120 – 200 kN
150- 250 kN
Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Installation Procedure
Install piles, either driven, drilled or augured, in pairs, at longitudinal centers
approximately 1.0m - 1.5m; lateral centers 1.0m - 1.2m, excavate, break out for
and construct reinforced concrete or concrete encased structural steel needle
beams.
Advantages
a) Because large excavations are not required, disruption and mess are kept to a
minimum.
b) Access from one side of a wall. Occupants may not necessarily need to be
relocated.
c) Quicker when compared with dig-out systems.
d) System becomes cost effective when the depth of traditional underpinning
exceeds 1.5m.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Knuckle Pile.
In this case the process consists on the stabilization of an existing wall foundation
by the installation of a series of knuckle piles connected to the existing wall from
one side.
This kind of solution is used with all type of shallow foundation, especially where
traditional underpinning is unsuitable or where access is restricted.
Knuckle pile has a capacity of loadings up to 100 kN/m, dependent upon
underlying bearing strata. Individual Knuckles are generally rated at 50kN.
Installation Procedure.
This process starts excavating a small area at each knuckle position. Break out
projection of foundation as necessary. Install either by driving casing or auguring,
a 150mm diameter pile at centers as required to accommodate loadings; break out
a 300mm x 300mm pocket in existing brickwork; install main knuckle
reinforcement; concrete knuckle ensuring adequate cover to reinforcement. Install
at 0,5m - 1.5m centers.
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Dissertation Topic: Underpinning
Advantages
a) Less disruption than traditional underpinning.
b) Fast compared with traditional underpinning.
c) Access required from one side of the wall only.
d) Occupants may not have to be relocated.
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7 Semester
CAHS-72P
Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Piled Raft.
The idea of this type of underpinning is the stabilization of existing wall
foundations (to whole rooms) by the installation of mini-piles, capped with a
reinforced concrete raft incorporating needle beams and reinforced concrete ring
beams as necessary to support walls and to provide lateral restraint and a new
floor.
This method is useful when the bearing strata is at depths in excess of 1.5m where
complete rooms, or structures are to be underpinned and includes the provision of
a new, suspended internal floor slab. Alternative application of this system may be
used where expansive soils exist. (Clays - heave/shrinkage).
Loading is based upon the bearing capacity of the underlying strata which will
determine mini-pile size for this system. Pile sizes may generally be considered as
follows:
General Guidelines for Mini-Pile Sizing
90 mm dia.
105 mm dia.
150 mm dia.
200 mm dia
220 mm dia.
250 mm dia
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Up to 40 kN
40-60 kN
60-100 kN
90 – 120 kN
120 – 200 kN
150- 250 kN
Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Installation Procedure
Install piles, either driven, drilled, jacked or augured, internally at centers
determined by loadings. Break out and construct reinforced concrete needle beams
at 1.0m - 1.2m centers. Reinforce and construct ring beam and reinforced concrete
floor slab.
Advantages
a) Economic at depths greater than 1.5m. Additional costs are linear for increased
depth
b) Provides lateral and transverse ties throughout the structure with piled
foundations for internal walls and floors
c) Provides fully suspended replacement ground floor slab
d) Quicker when compared with dig-out systems
e) Especially suitable where access externally is restricted and lateral and
longitudinal restraint is required
f) Minimizes disruption to existing external services (i.e. drains and service pipes)
and consequential reinstatement
g) Recommended for total structure isolation from underlying strata, particularly in
clay heave/shrinkage situations.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Angle Pile
The description of the work is the stabilization of an existing wall by the use of
piles installed at an angle inside and outside through drilled holes in the existing
foundation.
This solution can be used with all types of concrete strip foundation or bases
founded at a depth in excess of 400mm below ground level.
Loadings are based upon the bearing capacity of the underlying strata, which will
determine mini-pile size for this system.
High foundation loads can be accommodated by reducing the spacing of the minipiles.
General Guidelines for Mini-Pile Sizing
90 mm dia.
105 mm dia.
150 mm dia.
200 mm dia
220 mm dia.
250 mm dia
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Up to 40 kN
40-60 kN
60-100 kN
90 – 120 kN
120 – 200 kN
150- 250 kN
Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Installation Procedure
The existing foundation is predrilled; normally using air flushed rotary percussive
equipment. Permanently cased steel driven piles are then installed through the
predrilled hole with the casing terminated at the underside of the existing
foundation. The pile is then concreted and reinforced up through the existing
foundation.
Advantages
a) Recommended low cost underpinning scheme.
b) Because large excavations are not required, the disruption and mess are kept to
a minimum.
c) Economical at depths greater than 1.5m. Additional costs are linear for
increased depth.
d) Conservative design when pile centers do not exceed 1.0m.
e) High load capacity.
f) Quicker when compared with all other systems.
g) Suitable for restricted access.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Twin Pile
The system comprises of a series of pairs of reinforced steel cased mini-piles
installed from one side of the wall only.
Connection to the existing foundation is made by bond within a hole drilled
through the foundation.
The requirements for this solution are the minimum thickness of foundation is
225mm for the 90mm diameter pile and 200mm for the 100mm diameter pile.
The Loading capabilities of his system have a maximum
load of 50kN per pair of piles. The minimum anchor pile
length is 3.3m actual required lengths vary with soil
parameters. The minimum compression pile length is
3.5m.
Installation Procedure
The piles are formed by bottom driving 90mm or
120mm thin walled steel casing to lengths dictated by
set and static calculation. The required reinforcement is
then placed in the casing after being filled with
concrete.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Advantages
a) Recommended low cost underpinning scheme.
b) Because large excavations are not required, disruption and mess are kept to a
minimum.
c) Economical at depths greater than 1.5m. Additional costs are linear for
increased depth.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Cantilever Ring Beam
This type of micro pile is used to stabilization of an existing wall foundation by the
installation of mini-piles in tension and compression connected by a reinforced
concrete ring beam incorporating needle beams to a support the wall.
Cantilever Ring Beam system is used where bearing strata is at depths ins excess
at 1,5m and where longitudinal stability is also required.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Loading Capabilities
Loading capacities are based upon the bearing capacities of the underlying strata
which will determine mini-pile type and size for this system.
Installation Procedure
Install piles, driven, drilled or augured, staggered at approximately 1.0m centers
longitudinally and laterally, capped with a reinforced concrete ring beam with
integral concrete encased steel needle beams to support the wall.
Advantages
a) Economic at depths greater than 1.5m. Additional costs are linear for increased
depth.
b) Because large excavations are not required, disruption and mess are kept to a
minimum.
c) Quicker when compared with dig-out systems.
d) Access from one side of a wall.
Occupants may not necessarily
need to be relocated.
e) Provides longitudinal stability to
distressed walls.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
Jack Down Piles.
This system is based on the silent and vibration less installation of piles using
hydraulic jacking techniques for:
a) The upgrading and stabilization of existing foundations and column bases.
b) The provision of new piles in sensitive areas.
Because of the different advantages that this system has, you can use this method
in all types of foundation stabilization, upgrading and new works piling.
Loading Capabilities
The dead load of the structure, ground anchors and/or kentledge is used to
mobilize sufficient resistance to equal the working load of the pile plus its factor of
safety. Working loads available vary and are dependent upon:
a) Ground conditions
b) The fabric of the existing structure
c) Space available for the provision of kentledge
Piles can be installed to S.W.L. in excess of 1000 kN in head rooms of less than
2.5m.
Installation Procedure
After the provision of a suitable reaction medium eg, RC beams and rafts, steel
grillage, kentledge, ground anchors or existing foundations; heavy steel casings or
precast concrete piles are jacked in sections to the required load. The piles are
concreted and bonded to the structure to be supported.
Advantages
a) Recommended for use on listed buildings and for highly loaded structures.
b) Every pile is tested as it is installed.
c) Almost silent and vibration free.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
d) The jack rigs are operated by hydraulic power packs that can be sited away
from the area of works. Electric power packs are available.
e) Clean, dust free - piles have been installed in operational food factories and
operational printing press halls.
f) Piles can be installed within 260mm of a wall to the center line of the pile and
500mm into an internal corner.
g) Because of their unique size and adaptability, the jack rigs can be manhandled
into the most difficult areas, with maximum operating headroom of 1.8m but an
access size requirement of only 1.2m x 0.7m.
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Dissertation Topic: Underpinning
1.5.
7 Semester
CAHS-72P
Conclusion
The demand for underpinning has increased gradually in the last years as
renewals and renovation works have gained popularity because the quantity of
buildings built in the last years. This space problem united with the world crisis
have reduced the number of constructions, and increased the renovation works.
Furthermore the different solutions existing in this subject allow to solve any
construction problem, which does that underpinning is a really good solution.
On the other hand, it is important, with the different alternatives available, detailed
subsurface information and an understanding of the critical ground performance is
fundamental. An experienced geotechnical consultant can offer much to the
success of an underpinning system.
Finally, it is important point out that underpinning works requires experts in the
design and execution levels, along with safe working practices, because of the
difficulty of this kind of works and specially the dangerousness for the renovation
building and the buildings around it.
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Dissertation Topic: Underpinning
7 Semester
CAHS-72P
References
http://www.falconstructural.co.uk/
www.bigdig.com
www.haleyaldrich.com
www.engineering.com
www.schnabel.com
www.structural-design-solutions.co.uk
http://www.bdry.com
http://www.peirceengineering.com/
Foundation Engineering Handbook, Winterkorn & Fang, Van Nostrand Reinhold.
www.nhbcfoundation.org/
Underpinning : its practice and applications, Edmund Astley; Lazarus White, New
York: Columbia University Press
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