The pile foundation is in the construction a variant of deep foundation . It can be used to transfer the loads from supporting structures into deeper, stable soil layers. Soil layers that are not or poorly load-bearing are bridged with piles. With pile foundations, piles (formerly also called pilots in Austria ) are drilled or rammed into the building ground until a sufficiently stable soil or rock layer is reached. The loads on the structure are then carried on the one hand by the pile's friction with the subsoil (skin friction) and on the other hand by the peak pressure of the piles.
Up until the end of the 19th century (and still today in smaller structures such as boat sheds), sharpened wooden stakes or tree trunks were driven into the ground. At the top of these piles there was often an iron fitting, the so-called pile shoe, which made it possible to drive the piles into solid ground. Sometimes the piles that had been sawn off at the same height were connected by means of applied beams, and the resulting pile grid then served as a support for the masonry.
Wooden posts are extremely stable and durable if they are constantly in the water, even posts from Roman times and undamaged to this day are known. Wooden posts in water change zones, e.g. B. in locations with changing groundwater levels, rot. This can also occur through long-term groundwater lowering, such as occurs, for example, through mining , river regulation or increasing overbuilding and thus sealing of an area, which is why historical buildings in particular must be checked for any pile foundations before such measures. The rotting of the pile foundation can lead to considerable damage to the building, such as the Trier Cathedral . The largest pile foundation is Venice , where the entire city was founded over hundreds of thousands of tree trunks over the centuries. But many old and new buildings in the Netherlands , namely the Royal Palace in Amsterdam and the old warehouses in the Speicherstadt in Hamburg, stand on wooden stakes.
Different types of stakes
Pile foundations can consist of different materials or be produced in different ways, depending on the purpose, soil and environmental conditions.
- Purpose: Reinforced concrete piles are advantageous for permanent structures, particularly because of the material costs. In the case of temporary (auxiliary) structures, on the other hand, steel piles can be pulled out of the ground and reused after they have been used.
- Soil conditions: Depending on how firm or loose the existing soil layers are, it is possible to press in, flush, screw or ram posts. Regardless of the soil conditions, it is possible to place piles in pre-drilled holes or to concrete them in pre-drilled holes on site.
- Ambient conditions: The appropriate installation method often depends on the environment and its development. For example, it is often not possible to drive piles into inner cities because the vibrations are too great.
Steel piles in the form of pipes are called full displacement piles because they displace the soil to the side during installation. If the steel pipe is not rammed, but screwed in and pressed, we speak of full displacement bored piles.
Steel piles in the form of double T-beams are often used as temporary piles, for example for temporary bridges and shoring. They can be drawn again after use. If the friction and the pressure arch between the flanges is sufficient to transfer the load, these piles can be driven in without further measures. If a higher load (including tensile forces) has to be absorbed, such a girder can also be provided with foot reinforcements and grouted after it has been driven in (“ram injection piles”, “RI piles”).
In contrast to rammed prefabricated reinforced concrete piles, rammed steel piles are characterized by their breaking strength when they hit obstacles such as B. large stones. This makes it possible to drive steel piles through hard layers of soil down to the rock.
Rammed steel piles are also used in port construction and in the offshore area . The vibrations that occur during installation are not disturbing here.
The ductile pile is a precast driven pile system made of ductile cast iron . The pile is designed for permissible working loads of 300 kN to 1100 kN depending on the diameter and wall thickness of the pile tube. The pile tubes are manufactured from ductile cast iron using the centrifugal casting process. The heart of the post is the socket, through which the post can be endlessly coupled and made in any length. When it comes to the lower end of the pile, the pile base plate, a distinction is made between two design options, the mortar-filled pile and the grouted pile. The mortar-filled pile is equipped with a pile base plate that seals the pile tube tightly. The pile is driven to the required final depth and then filled with concrete mortar. The load transfer takes place mainly via the peak pressure. A base plate that is larger than the pile tube cross-section is used for jacketed piles. This creates an annular space that is filled by the compression. For this purpose, mortar concrete is conveyed through the pile tube to the pile base during pile driving and pressed into the ground near the base plate, creating a concrete shell. The load transfer can take place via the skin friction. The individual pipe sections are rammed in with a quick-action hammer and a special insert tool. B. a hydraulic excavator. This flexible pile system can also be used in narrow and impassable construction areas where large pile devices cannot work. Due to its versatile and economic applicability for small construction sites, residential construction up to large construction sites, this pile system is gaining more and more importance in Germany.
Prefabricated piles made of reinforced concrete
Reinforced concrete piles are one of the pile systems in Germany with the highest material and quality standards. Currently, prefabricated concrete driven piles made of reinforced or prestressed concrete with a full square cross-section between 20 × 20 cm and 45 × 45 cm are used worldwide, which are reinforced or prestressed as standard for the stresses during transport, installation and structural loads (pressure, tension, bending). Round spun concrete piles are less common (e.g. in Austria and Switzerland) which, as a result of their manufacture, are hollow on the inside, so that only the reinforced concrete outer ring is available for load transfer.
The axial pile resistance (pile load-bearing capacity) of reinforced concrete piles is between 0.5 and 2.0 MN, depending on the cross-section and the subsoil conditions.
In Germany and in the Scandinavian region, prefabricated reinforced concrete piles are manufactured in the pile work in compliance with defined quality criteria and continuous external controls. For logistical, economic and installation reasons, the length of a single pile is limited to 15 m. For the use of pile lengths> 15 m, sections can be connected to each other using tested couplings approved by the building authorities and extended as required. In Sweden, pile lengths of over 80 m have already been achieved in this way.
In the inner-city area, driven piles are rarely used today. The noise and, above all, vibrations caused by pile driving are no longer acceptable in densely built-up areas. The damage to neighboring buildings caused by ramming is usually so great that this type of foundation is no longer economical due to recourse claims from neighbors. Bored pile foundations have established themselves as an alternative to this. Although these piles have a lower load-bearing capacity, they only cause minor vibrations.
As with all types of piles, with precast concrete piles, loads are transferred via skin friction and tip resistance. This pile system can be used in almost any type of soil and any stratification of the building site. Due to their dimensional and cross-sectional stability, prefabricated piles made of reinforced concrete are particularly suitable in pulpy or soft, highly water-containing or contaminated soil layers. When designing in solid ground or on rock, a steel point must always be provided for spun concrete piles, which is not necessary for reinforced concrete piles with a full cross-section due to the high concrete strength.
The range of uses for prefabricated reinforced concrete piles ranges from single-family houses to industrial plants and large infrastructure projects. However, one of the most important areas of application has been the deep foundations of wind turbines in Germany and throughout Europe for decades .
The versatility of the application of prefabricated reinforced concrete piles is also shown in the fact that special piles can be produced in the factory, which e.g. B. contain injection pipes or lines for the use of geothermal energy through so-called energy piles.
Bored piles / in-situ concrete piles
If the concrete is poured on site (today mostly ready-mixed concrete), one speaks of in-situ concrete piles. The diameter and length of the piles can be designed according to the respective requirements.
Particularly when verifying the load-bearing capacity , in-situ concrete piles must be differentiated into large and small bored piles (“micropiles”). Micropiles are less than 30 cm in diameter. Correspondingly, large bored piles are used for piles with a diameter greater than or equal to 30 cm.
One example of the use of in-situ concrete piles is the Kaispeicher A in Hamburg. The 108 by 85 meter building, erected in the sixties, stands in the soft Elbe mud and is founded on exactly 1111 reinforced concrete piles. Since the calculated weight of the storage facility, which was supposed to be completely filled with cocoa sacks at times, was very large, the piles were designed for a load-bearing capacity of 160 tons each and had a diameter of 50 cm. The reinforced concrete piles conduct the weight through layers of clay and peat in the soil into more stable layers of sand.
With a bored pile, the earth is not displaced, but a steel pipe that is open at the bottom is drilled into the earth. The earth is removed from the inside of the pipe, then a steel reinforcement or steel support member is introduced and the pipe is filled with in-situ concrete or mortar. As the concreting progresses, the steel pipe is pulled out step by step, and any water that is standing in the depths or penetrating it is flushed upwards from the heavier fresh concrete until it finally stands as a "cushion" on the surface of the pile and separates the fresh concrete somewhat. This type of in-situ concrete pile is used for statically demanding foundations such as high-rise buildings or bridges. Another possible use is the lining up of bored piles to create a continuous bored pile wall , which then z. B. serves as excavation protection .
Another method of producing bored piles is the auger drilling technique . Here a hollow endless auger is "turned" into the ground. Once the desired depth has been reached, concrete is pressed through the hollow auger, pushing it upwards. The steel reinforcement is shaken into the fresh concrete after the auger and the upwardly transported soil have been removed.
In the case of press concrete bored piles or mortar injection piles, the gaps are grouted with mortar after production in order to create additional hold.
After the pile has been concreted and the concrete has sufficiently set, the construction pit is dug so that the pile heads can be seen. The bored pile heads are then chiselled to the required level while retaining the reinforcement, so that the pile head ends at the lower edge of the foundation or the base plate. If the pile is reinforced, the reinforcement is integrated into the component to be connected with the required length. Breaking off the protruding pile head initially appears to be a waste of material. The reason for this "concreting over" the piles lies in the concreting process. In order to avoid segregation of the fresh concrete, concreting is carried out using the contractor method, i.e. H. the concreting pipe always ends below the fresh concrete surface. The “bad”, less stable concrete floats on top and has to be chiselled off after it has set.
The piles are subjected to various tests depending on the requirements. The tests must be carried out by a certified pile inspector. The currently most common examination procedures are:
In the past, pilots used to work together with a hand-swung mallet - often to fortify rivers or lakes. In order to achieve rapid progress, the division of labor worked together and two men took turns beating a pilot. The popular pilot's song testifies to the rhythm and the necessary coordination and stamina of the opposing hammer-swinging people.
- Klaus Rheidt : Pile Foundation in the Anatolian Mountains - Wrong Technique at the Wrong Place? (PDF). In: Karl-Eugen Kurrer , Werner Lorenz , Volker Wetzk (eds.): Proceedings of the Third International Congress on Construction History . Neunplus, Berlin 2009, ISBN 978-3-936033-31-1 , pp. 1219-1226