SCANDINAVIAN STEEL CORE PILES
Hakan Bredenberg, Dr Tech, Bredenberg Geoteknik Ltd, Sweden
ABSTRACT
Steel Core Piles used in Scandinavia consist of a permanent steel tube casing which is
drilled through soil down to bearing bedrock, and a steel bar placed in the casing. The space
between the bar and the casing is filled with concrete. This pile offers very good conditions
for quality assurance for every part. The load capacity varies between 400 to 4000 kN. If the
core is cast into a drill hole in the rock, the same pile works both in compression and tension.
Steel Core Piles are installed using light weight drilling rigs. Such rigs, as for example
equipped with ODEX, can penetrate many piling obstacles excluding other piling methods.
There is no need for ground water lowering and only limited vibrations are generated. The
equipment can be effectively noise protected. Altogether, this is obviously a pile type suitable
for use in urban environment.
Steel Core Piles has a very long life time due to the double corrosion protection that is
given to the steel core by the casing and the concrete surrounding it. This pile type is
sometime referred to as the Rolls Royce of piles, which gives a hint of the major disadvantage
of the pile : the high quality and the other advantages comes with a rather substantial price
tag, if cost per meter is considered. However, as is described in this paper, the total cost of a
pile foundation is many times not obtained using the cheapest type of piles.
Steel Core Piles has been used since the beginning of the 1960:s. The use has since then
increased continuously. Codes for the design, installation and control of this pile type are
issued by the Swedish National Highway Administration and the Swedish National Railway
Administration. Building authorities of other Scandinavian countries, as Norway and Finland,
also accepts Steel Core Piles.
INTRODUCTION
Steel Core Piles are mainly used where obstacles in the ground renders convention pile
driving methods difficult or impossible. Another typical area of application for these piles is
where only limited environmental impact, such as vibrations, noise and soil displacement, can
be accepted. Steel Core Piles are usually more expensive if price per unit length is considered,
but if all costs related to the foundation are brought into the picture, the total cost may be
minimized using these expensive piles. Further, Steel Core Piles are many times favorable
when it comes to obtain the shortest production time for a given foundation.
Most components of a Steel Core Pile can be carefully inspected, as for example the installed
casing tube and the core lowered into it. This means that the strength of the steel can be used
without need for large safety margins. For piles driven into the soil larger safety factors have
to be used, since eventual damage due to driving, or other factors, are buried in the ground.
Steel Core Pile requires rock with a compressive strength not less than 50 -100 MPa, in order
to yield sufficient bearing capacity justifying the cost of the core and the drilled casing. The
strength of granite and gneiss found in Scandinavia is often 200 MPa or even higher, which
explains why this pile type has been developed there.
The main parts of a Steel Core Pile are illustrated in fig 1. They are
-
a permanent steel casing tube drilled through the soil down a short distance in rock
a drill hole in extending into rock
a steel core installed in the casing and the rock hole
load transfer details and joints for the casing and the core
Fig 1a Point bearing pile
Fig 1b Shaft bearing pile
As illustrated in fig 1, the Steel Core Pile comes in two variations: A pile with a point bearing
core (fig 1a) and a pile with a shaft bearing core (fig 1b). Today, the shaft bearing core is the
more common variation, due to reasons explained later in this paper.
From Fig 1, it is obvious that the Steel Core gets a very good protection against corrosion.
The core is surrounded by the concrete, similar to a rebar in a reinforced concrete element,
and in addition the concrete is protected by the casing through the depth of the soil. Thus, a
very long life span can usually be expected for Steel Core Piles.
HISTORY
Steel Core Piles have been used in Sweden since around 1960. At that time, they were
called ‘Steel Pile Columns’. The present name was introduced around 1970.
One of the first project where the piles were used was a harbor store house in Stockholm. The
subsoil consisted of a rock fill on a deep very soft clay deposit. The clay rested on glacial till
and bedrock. Due to on going settlements and very high column loads for the new building,
there was a need to bring all the loads down to the rock. The fill could not be penetrated by
driven piles, due to the size of the boulders. Neither were bored piles considered a realistic
choice, by the same reason.
It was therefore decided to use rock drilling equipment to install a permanent steel casing
down to the bedrock. In order to utilize the space created, as much steel section as possible
was installed in the casing, that is, circular steel bar. The bearing capacity of the bar was
tested by means of stress wave measurements, one of the first applications for this technique
in Sweden. The impact load on the steel core also drove the core through eventually left
drilling debris at the casing bottom, so that the core reached the rock surface. After acceptance
of the bearing capacity of the core, the space between the core and the casing was filled by
injection concrete by means of a hose brought down to the bottom of the casing.
Fig 2. Steel Core Pile for foundation of a store house at Stockholm Harbor, 1962
A cross section of the pile is shown in figure 2. Water well drilling equipment was used to get
the 300/8 mm diameter casing down. The diameter of the core was 160 mm. The allowable
load was 2 MN (200 tons), which correspond to a point contact pressure equal to 100 MPa (1
ton/cm2). The water/cement ration was 0.45.
As time went on, it became clear for the foundation engineers that there were many
applications for this type of pile. Therefore, the use of Steel Core Piles increased at a steady
pace. The development of new drilling methods, as for example the ODEX-method, supported
the growing number of these piles. The equipment necessary, mainly drilling rigs to bring the
casings down and to make the drill hole, is available at most locations. Further, the increased
number of building projects in urban environments makes it often necessary to install piles in
man made fills with many obstacles, and to limit vibrations and noise. The piling must many
times be made in very confined working spaces, as for example foundation reparations in
basements.
All those factors have sponsored the use of steel core piles. It has also been recognized that
the total cost for a foundation can be optimized using Steel Core Piles, as will be described
below in this paper. Today, the total annual amount of Steel Core Piles in Scandinavia is
around 100000 m, representing a production value of about 40 million $ US. Typical casing
diameters vary between 140 to 320 mm. Core diameters between 90 to 150 mm are most
common, but core diameters up to 210 mm have been used. The authorities responsible for
roads and railroads have included Steel Core Piles in their building codes.
Fig 3. Installing Steel Core Piles in a basement, replacing out aged wooden piles
There are also some other factors also in favor for this type of pile. For exa mple, a shaft
bearing Steel Core Pile (fig 1b) can take very large tensile loads. Thus, such a pile also
Fig 4 Installing casings with a KLEMM 806. Bridge foundation, Stockholm 1998
functions as an anchor. Further, the massive load bearing element, the core, is loaded to only a
moderate stress level compared to other types of anchors, meaning that pre-stressing to get rid
of elastic prolongation often can be avoided. This is an obvious advantage.
FURTHER DEVELOPMENT
The demand for quality control in building industry will probably continue to grow. Another
trend is the increasing need for short production time and the corresponding need to avoid any
delays in production. The environmental demands are another expanding area for builders.
Finally, the numbers of foundation objects to be carried out in confined urban areas are likely
to grow.
Since all these, and more, requirements are met by Steel Core Piles, the use of such piles will
probably continue to increase. One can expect that the pile type also will gain ground outside
Scandinavia : the need for competent rock within the reach for casing drilling is of course a
limitation at many places, but on the other hand there are several areas where hard rock is
situated within 20 – 40 m below surface, and Steel Core Piles are not yet used, as for example
Manhattan, San Francisco and many Canadian cities. When the foundation markets outside
Scandinavia fully realizes the multiple advantages with the pile type, the volume of Steel Core
Piles installed outside the Scandinavian countries will probably become the largest, by far.
Who will benefit from this ? Besides clients getting rid of delays due to foundation problems,
the winners are drilling companies with suitable equipment. Small companies with a high
degree of skill in installing casings, often water well drillers, can find a new branch for their
production. At least this has happened in the Scandinavian countries over the years. The
concept ‘Drilled Foundation Engineering’ is very much focused on Steel Core Piles.
APPLICATIONS
In order to find out if Steel Core Piles are suitable, a number of factors must be considered.
Conditions promoting these piles are :
-
piling obstacles in ground
piling close to existing foundations
piles in soil where corrosion is severe
piles have to reach rock surface
vibrations and noise must be restricted
only small, lightweight piling equipment can be employed
piles must take large tensile loads
large concentrated loads
well-defined dynamic response for the piles are required
especially important that delays in piling works are avoided
high required level of precision and quality control
The more of these conditions that are prevailing, the bigger are the probability that Steel Core
Piles is the best choice.
The factors against the use of Steel Core Piles are :
-
the specific cost ( cost unit/load unit/length unit) is higher than most other piles
some obstacles are impossible to drill trough, e g steel debris
rock is to soft, pre-grouting may be needed
the rock surface is situated very deep, resulting in excessive drilling depths
soft rock, or rocks with open cracks may require too expensive grouting
Correspondingly, the more of the factors mentioned in the last list relevant for the project, the
less suitable are Steel Core Piles.
Among other projects where Steel Core Piles have used, below are some of those where this
pile type provided substantial advantages:
Tunnel and ramp for South Link Highway, Stockholm, 2001
This project recently (2005) finalized is about 6 km long and most of it is in underground
tunnels in the bedrock. The approach to the rock tunnel at the northern end was made in soft
clay, with the ground water table close to ground surface. A concrete ramp with walls and raft
was built from surface level down to entrance to the rock tunnel at about 14 m depth below
surface. Therefore, the ramp was subjected to large uplift forces due to the high ground water
level (fig 5). Further, the ramp was also designed assuming the ground water will be gone in
the future, for one reason or another.
Fig 5. Ramp for the Southern Link, Stockholm 2001
The original suggestion was to make use of gravity concrete in order to balance the uplift of
the water. Assuming loss of all groundwater meant that the weight of the gravity concrete
must be taken by piles. The cost of concrete and piles was considerable. At the tender stage an
alternative was presented to the client, Swedish Road Administration. The uplift as well as the
load of the structure without ground water uplift should be taken by Steel Core Piles with a
shaft bearing core. A cast-in length equal to 6 m in the granite bedrock was chosen. The core
diameter was 150 mm. The design load in compression and tension was 3 MN and 1.4 MN,
respectively. A number of the piles were tested in static tension tests. This alternative design
was considerably less costly and time consuming then the original one. The savings came
from less excavation, sheet piling, anchoring, gravity concrete an piling.
Office building in downtown Oslo, Norway
In 1999 a new office building for the Norwegian insurance company Uni Storebrand was built
in Oslo, the capital of Norway. The new building covered a whole city block. The ground
conditions varied a lot, with rock at the street level in one corner, whereas the depth to rock
on the opposite side was about 30 m. The rock consisted of lime stone, compressive strength
100 MPa. For deeper parts foundation was made on precast driven concrete piles.
Fig 6. Foundation of Uni Storebrands new office in Oslo, Norway, 1999
For the parts with the rock surface at more shallow depths, the load of the building was to low
to counterweight the uplift of high water levels in the adjacent Oslo River. The variation in
level was about 4 meters from low to high level. Therefore the designers suggested that that
the foundation at this part should be made on bored piles with large diameter. After that the
bottom floor of the building was completed, anchors should be made through the bored piles.
In that way, both compressive loads corresponding to low water level as well as tensile loads
from high water levels would be taken care of.
However, there were several disadvantages with this solution: The sloping rock at the bottom
of the bored shafts had to be flattened before concreting. The anchors must be made, tested an
prestressed at a late stage, when the building works were going on. Further, the bored piles
must be designed to take the full load of the test load of the anchors.
Instead it was decided to use Steele Core Piles. The cores of the piles were cast 5 m into the
lime stone and a number of the cores were test loaded in tension up to 2 MN. Using Steel
Core Piles made it possible to finalize the foundation works in one operation, without need for
anchor installation intermixed with erecting the structure. About 3 months of production time
was saved.
Foundation of Paper Manufacturing Machine, Ortviken, Sweden
In 1998, the paper mill company SCA decided to replace an old paper manufacturing machine
with a new one in Ortviken in the North of Sweden. There were very narrow specifications for
the static and dynamic behaviour of the piles. The first design suggested 2.1 m diameter bored
piles. However, the equipment required to install such piles could no way operate under the
existing production line, which was required.
It was therefore decided to use Steel Core Piles instead. In this way, it was possible to install
the piles during paper production. Further, the dynamic properties of a steel core pile, being
essentially a steel rod cast into rock, is very well defined. Thus, unwanted vibrations of the
foundation could be avoided. This is a great advantage for the foundation design stage.
Foundation of new head office for Pfizer in Stockholm, 2005
The foundation of this 100 million US $ office is a state of the art example of use Steel Core
Piles for building foundation. First, all columns were supported by a single Steel Core Pile.
The working loads went up to 3.5 MN (350 metric tons). The use of just one pile for such
large loads was possible due to very small tolerances (max 10 mm) for the installation of the
pile. A 80 mm thick concrete raft was cast over the footprint of the building. This provided an
excellent surface for the foundation works. At the location of the piles, a hole was cut in the
slab, leaving no opportunity for the drilling crew to put the casing for the pile in a position
outside tolerances.
Part of the building was built on a surface of excavated hard rock. Nevertheless, Steel Core
Piles were used here too. Why was that ? Well, there was ground water uplift at the basement
floor level equal to 5 meters. Some parts of the building were not heavy enough to balance
this. Therefore, Steel Core Piles were also taking the role as anchors.
Fig 7. New offices for Pfizer on Steel Core Piles, Stockholm 2005.
Further, the spacing for the columns was up to 12 meters. The resulting thickness for the
basement floor slab to take the uplift pressure without too large crack width was 1.6 meters.
Such a thick concrete casting creates considerable problems, as for example heat. Therefore
Steel Core Piles were installed between the columns in order to decrease the bending. In this
way, the thickness could be limited to 0.65 m. The cost for the extra Steel Core Piles was
much smaller that the saving of reinforced concrete. Again, the large tension capacity of Steel
Core Piles saved the client a lot of money.
DESIGN
Design of steel piles in soft soil has a long history in Scandinavia. As early as 1918 Mr Carl
Forsell at the Royal Institute of Technology in Stockholm presented the expression for the
buckling load Fk of a column surrounded by an elastic media :
Fk=2(EI/kd)1/2
... (1)
EI = flexural stiffness of the pile
kd = coefficient of horizontal subgrade for the pile
Forsell assumed an initially perfect straight pile, hinged at top and bottom. Further, the elastic
media around the pile was assumed perfectly elastic.
Since then many improvements have been on this expression in order to take real properties of
a pile into account. Today, a calculation of the capacity usually includes :
-
a measured or assumed initial deflection from the straight axis top to point
a bilinear stress-strain relationship for the soil around the pile (fig 8)
value of kd takes long time loading and short time loading into account
built in stresses in the pile material is accounted for
effects of pile joints are included in load bearing capacity calculation
effects on driving on material strength is considered
Fig 8 Main features of present calculation methods of axial load capacity for steel piles in
soft clay
A comprehensive description of the calculation is beyond the scope of this paper, but the main
lines are as follows ;
- Apply a part of the axial load on the top of the pile
- Calculate the corresponding increase in horizontal deflection
- Check if the pile will buckle
- Check if the capacity assuming no buckling is reached
- If applicable, add load to the top and make a new calc ulation run
The analysis is mostly made by means of a computer. Software for different pile types is
available. For Steel Core Piles, the point or shaft bearing capacity of the core in contact with
the rock must also be analyzed. Further, the stresses in the concrete surrounding the steel core
must also be checked. In case of a tension pile, the tensile stresses in the rock mass around the
shaft cast in are evaluated.
Design also includes calculation of corrosion of the casing. The core is not assumed to be
subjected to any corrosion. A lot of work has been made to determine the rate of corrosion on
steel in soil assuming different conditions. The resulting corrosion related decrease in wall
thickness for the casing is a function of several parameters, suc h as life time for the piles and
the corrosion characteristics of the soil and the ground water. However, often the value 2mm
for 50 years is assumed, unless there are special requirements for determination of rate of
corrosion. For thin walled casings, as for example 5 mm ODEX-casings, the resulting wall
thickness may be so small that a check of local buckling must be made.
Usually, only the core is assumed to take axial load. The casing and the concrete in between,
is assumed to contribute to the bending moment capacity. Bending moments occurs due to the
horizontal deflection mentioned above.
PILE DETAILS
The main parts of a Steel Core Pile are the steel core itself and the casing tube. In addition to
that, there are a number of accessories in a completed pile :
Top plate
The top plate shall transfer compressive or tensile loads from the super structure to the pile. In
its simplest form, the plate is spot welded to the top of the core.
Distances
The core is fitted with distances in order to keep the distance between the core and the casing
as the core is lowered into the casing. Some clients require the distances to be made by nonmagnetic material.
Riplets
On the part of the core to be cast into a drill hole in the rock, 3 mm circular welding is made
at 50 to 100 mm distance on the core. This is done in order to increase the shear strength
along the part of the core cast into rock.
Joints for casing tube
The casing tubes are usually jointed by welding. For thick walled casings sleeve joints are
available.
Joints for steel core
Steel Core segments can also be jointed by means of welding. When Steel Core Piles are
installed in spaces with small working height, many short core elements must be jointed. In
order to avoid excessive welding the joints are often made of so called API-joints, se fig 9.
Fig 9. API-joints for steel Core elements.
Usually, the drilling crew finalizes a number of casings and rock holes before any cores are
installed. Then a corresponding number of ready-to- install steel cores cut in actual lengths
arrives from the manufacturing unit, with all accessories fitted. At the site the casings are
filled up with injection concrete, and the steel cores are lowered by means of a crane.
This is the normal production cycle for a reasonably large project. For smaller projects the
piles are often finalized one by one.
DRILLING
Drilling of the casing is often using the ODEX (Overburden Drilling Eccentric Method).
When suitable, a DTH (Down The Hole) hammer is used. This is usually the most efficient
and economical combination to install casings for Steel Core Piles. The principles for the
ODEX- method are shown in figure 10a.
There are however some limitations with this method. The casing wall thickness for standard
ODEX is limited to 5 to 6 mm. If a casing with a greater thickness is required, the solution is
the use centric drilling, as for example the Finnish method SYMMETRIX, fig 10b. The
disadvantage using centric drilling with this and similar methods is that the drill crown for the
casing is left in the rock. An advantage is that symmetric drilling gives more straight casings
and rock holes.
The DTH-hammer is usually driven by compressed air, which is released close to the bottom
of the casing. The used air is exhausted up in the casing tube, which is open at the lower end.
If the casing tip is situated below the ground water table, which is very common, the air
exhaust causes a pressure drop in the water pressure around the tip. In this way soil particles
are drawn into the casing and transported upwards in the casing. Drilling in fine grained soils
as silt, may therefore result in excessive transport of soil from the area around the casing
opening up to the ground surface. As a result, ground surface settlement can occur.
Fig 10
a) ODEX, eccentric recovered drill crown.
b) SYMMETRIX, centric left crown
In order to avoid this well known problem, top hammers are sometimes prescribed. The
efficiency of a top hammer is however generally lower than a corresponding DTH. Therefore
modified DTH hammers have been developed, in order to decrease the risk of uncontrolled
soil particle transportation. Another improvement is the water-driven VASSARA hammer,
where water is used as drive media instead of air. Use of drill mud is also sometimes
recommended to avoid the problem mentioned.
CONTROL
An advantage with Steel Core Piles is that the components are possible to control. The
straightness and integrity of the installed casing and the drilled rock hole are such examples.
The bearing capacity of point bearing Steel Core Piles can be checked by stress wave
methods, usually the CASE- method. A hammer made up by a core segment is then often
used. The blow to the core also makes the core penetrate soil particles left at the bottom of the
rock drill hole. If a pneumatic hammer is used the stop criteria is usually given as a maximum
permanent set per minute, as for example not more than 5 mm per minute during 3 minutes.
The weight of the piston shall be equal to the weight of at least 2 m steel core.
When shaft bearing cores are used, the hammering of the core is usually skipped for
production piles. Instead, limiting the average skin bearing to 1 MPa is considered to exclude
the need for load capacity testing of every pile. On a project including many piles, test loading
is made before the start of the actual piling job in order to avoid unnecessary cast- in lengths
for a large number of cores.
Static test loading of a shaft bearing core can be done by tensile or compressive loading.
Using a hollow hydraulic jack to apply a tension force at the top of the core is easier than the
build up required to perform a compression load test. The shaft capacity evaluated for a
tension test is usually accepted also as the compressive capacity of the core.
REFERENCES
1. Eronen Sami, Drilled Piles in Scandinavia, thesis Tampere University Finland, 1997,
pp 1 – 71
2. ENV 1994-1-1 Eurocode 4 : Design of composite steel and concrete structures – Part
1-1 : General rules and rules for buildings. CEN 1992.
3. Bredenberg Hakan, Steel Core Piles, Design, Construction and Control, Swedish Pile
Commission, 2001, pp 1 – 56 (in Swedish)