1 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 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.
2 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 At that time, they were called Steel Pile Columns. The present name was introduced around 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.
3 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 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 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.
4 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.
5 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 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
6 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
7 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
8 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 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.
9 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
10 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 calculation 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, such 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.
11 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.
12 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
13 2. ENV Eurocode 4 : Design of composite steel and concrete structures Part 1-1 : General rules and rules for buildings. CEN Bredenberg Hakan, Steel Core Piles, Design, Construction and Control, Swedish Pile Commission, 2001, pp 1 56 (in Swedish)