The deep foundation describes a construction method in order not to direct the structural loads directly below the structure into the subsoil (as with the shallow foundation ), but to dissipate them deeper into the earth via additional vertical elements and transfer them there. A deep foundation is necessary if the layers near the surface are not stable enough.
One differentiates the deep foundations according to their supporting elements:
Piles are the oldest deep foundations. There are many different types, which are classified according to the following criteria:
- Wooden pole
- Steel pile
- Concrete pile
- Reinforced concrete pile
- Prestressed concrete pile
- Steel fiber reinforced concrete pile
Type of introduction
Type of force application
- via skin friction
- over peak pressure
- Single pile, e.g. B. Monopiles in the offshore area (mostly vertical loads)
- Post gratings (for large structures)
- Pile wall (with earth pressure load )
- Drainage post (for withdrawing or returning groundwater)
- Precast piles (manufactured in factories - wood, steel, prestressed concrete)
- In-situ piles (wood and concrete with steel reinforcement or steel fibers)
In the case of well foundations, a circular or elliptical cross-section with the floor plan dimensions of 2 to 10 m is created (sunk) into the subsoil to great depths. This makes it possible to transfer very high structural loads into the deeper subsoil. Well foundations are particularly suitable for the additional discharge of horizontal forces such as earth pressure and also bending moments from columns and supports. A wide variety of methods are used in production, the main difference being the way the soil is supported.
- Well rings are placed on top and connected to the ones below. The excavation is continued within the rings, the well rings sag due to their weight and thus support the soil.
- Support with segments . The excavation is carried out within the well in such a way that the individual well segments (segments) can be installed under the existing segment rings. After all segments have been offset at one level, the ring is closed and fixation (screwing, wedging and pressing) can take place.
- Shotcrete protection . A shotcrete ring is installed from top to bottom, opening small fields that are then closed with shotcrete. The shotcrete can be reinforced with reinforcement mesh. Steel fiber concrete has recently been used as shotcrete because it is easier to process.
- Piles or high-pressure soil cementation (HDBV) can be arranged along the perimeter of a foundation well so that a closed ring is created.
When the well is founded, the loads and moments to be transferred are introduced into the well body in accordance with the static requirements. For this purpose, the interior is filled with reinforced concrete or steel fiber concrete.
In addition to pile foundations , caissons are the oldest type of deep foundations. Port structures and bridges were built using caissons in ancient times. Today caissons are mainly used for structures in and below the groundwater area. In the final stage, they will be used for a wide variety of tasks. For example, pumping stations of a groundwater works were built using caissons. Entire underground lines (e.g. Amsterdam) were also built using caissons.
There are two different types of production:
- open caisson , the excavation is carried out from above with a rope excavator. The box sinks due to its own weight in the course of the excavation. A dewatering is not necessary because it can also be excavated under water. Once the caisson has reached its final position, underwater concrete is used to create a seal on the underside. The groundwater remaining in the box can then be pumped out and the interior construction can be carried out according to static principles.
- Compressed air caisson or caisson - these have been used for around 170 years. A pressure-tight ceiling is installed in the caisson above the cutting edge reinforcement (about three meters above the lowest point). An entrance, which is coupled with a pressure lock for personnel and material, leads through this ceiling. The cavity in the area of the top of the caisson is the working chamber in which the deeper subsoil is excavated. So much air is pressed into the working chamber that no more water can penetrate (effect of the diving bell). The work team and all material must be led into and out of the work chamber via the compressed air locks. This is relatively time-consuming, as the times for pressure equalization must be adhered to by the work team.
- Gerd Holbach, Anna Loewe, Sebastian Ritz: Flushing method for installing offshore foundations . In: Hansa , issue 11/2013, pp. 22-24, Schiffahrts-Verlag Hansa, Hamburg 2013,