Keywords: Deep foundations, foundation piles, prefabricated piles, monolithic piles, micropiles, foundation wells, caissons

Deep or vertical foundations transmit the load to the depth through vertical elements. Deep bases are proposed in the case of insufficient bearing capacity and high compressibility of the surface layers. Ground structures are mostly based on pilots. Less often on shaft pillars, foundations wells or caissons.

Foundation piles are rod elements of the circular or square section, which transfer the load of the building on the foundation soil in depth. Piles are elements whose length to transverse dimension is at least 5: 1.

Depending on the transmit the load to the subsoil, the piles are pushed, tensile, oblique, and piled loaded by bending and buckling. Most often there are pushed pilots (end bear, friction and bearing-cum-friction pile). End-bearing piles carry the load predominantly by a tip that is supported by a bearing suboil. Bearing-cum-friction pile carry the load on both the tip and the friction on the casing. Friction piles do not interfere with load-bearing soil and are all their length in the non-loading-bearing soil to which they carry the load only by friction on the casing.

Depending on the material piles are distinguished by wood, concrete, reinforced concrete, prestressed concrete and steel.

According to the relationship, we distinguish the solitary pilots and group pilots. Solitary pilots do not affect each other. The contours of the loaded areas are not intersected at their peak and their axial distance is at least 6 x the diameter of the pile. Group piles are made up of several piles arranged below the shallow foundation structure. Group piles are affected and are always considered as one.

According to the manufacturing process, we distinguish pilots prefabricated (driven) and monolithic piles (excavated).

Prefabricated driven piles can be wooden, reinforced concrete, prestressed concrete and metal. They are made as full or hollow. They are driven by ramming, flushing, pushing, vibrating or other methods. The most widespread method is ramming. The pilot's heads must be protected from damage by a protective shard. Flushing is based on the flooding of the soil under the pile tip. The pile penetrates into the ground with his own weight, or with a slight ramming, to the into the flooded ground. Pushing the pile is done by hydraulic presses. The vibration driven is mainly used for steel pilots.

Wooden piles are used in places permanently below the groundwater level. Parts of water must be impregnated. The most commonly used square or circular diameters of 200 to 400 mm in length are up to 10 meters. The tip of the wooden pilot is provided with a steel shoe; the head is protected by the shards. The advantage of wood pilots is long life underground water and easy adjustment of length (shortening).
 

Reinforced concrete piles and prestressed concrete piles are used to a depth of 20 meters, exceptionally up to a depth of 50 meters. The piles are produced with a hollow cross-section or full. Full pilots usually have a circular, polygonal or square cross-section with bevelled edges. Piles with cross sections of 250 x 250 to 600 x 600 mm are strongly reinforced with longitudinal reinforcement with stirrups or spiral-shaped reinforcement. The tip of the pile should be protected with the steel tip. Hollow pilots are not load-bearing capacity and are replaced by the pipe with steel piles.

Steel piles are made of molded steel profiles or steel pipes. Their advantage is high strength, easy adjustment and reduction, and especially easy pushed into the soil, steel piles are used up to 60 meters deep.

Monolithic piles are manufactured on-site into pre-drilled wells as either sheeted or non-sheeted (with or without a casing pipe). Monolithic pilots may have a fixed cross-section along their entire length or are expanded. Monolithic piles are made of concrete or reinforced concrete. Concrete piles are used in the case of stress only. Reinforced concrete piles are used for stress and tension and bending. We distinguish between three basic types of monolithic piles – non-sheeted piles, piles with a casing pipe withdrawn and piles with a casing pipe left.

Non-sheeted piles can only be carried out in cohesive soils and above the groundwater level. Digging is usually done by drilling with a diameter of 600 to 800 mm. The concrete mixture is stored directly into the borehole. The piles must be concreted immediately after the excavation. If necessary, the borehole walls can be strengthened with clay lather.

Piles with a casing pipe withdrawn are used in all soil types and below groundwater. The casing pipe is a steel tube which drives into the soil by ramming, bruising or vibrating. The casing pipe may be open or closed at the bottom.

Piles with a casing pipe left are used in an aggressive environment where it is necessary to protect concrete against harmful effects. Kept steel casing pipe reduce the value of surface friction. These piles cannot be used as friction piles. When using open casing pipe, the soil remains inside the casing pipe and is subsequently extracted, for example by drilling. Concreting is carried out in the prepared borehole. These pilots are referred to as pre-drilled piles. Closed casing pipe is fitted with a plug in the heel which prevents the soil from penetrating in. The closed casing pipe are used in the so-called pre-driven piles. Once the required depth has been reached, the plug will come out. The concreting is carried out under the protection of the casing pipe for its gradual pulling out. Piles with a casing pipe withdrawn have a rough surface and can be used as friction pilots.

Micropiles or root piles are short piles of small diameter (80 to 250 mm) which are reinforced with reinforced concrete or steel pipe. Micropiles are made using a variety of technologies. Pre-drilled holes can be filled with cement grout and a perforated pipe is inserted into the drill. After the bore is sealed, this mixture is injected. It penetrates under pressure into the lower part of the borehole and into the boundary in the soil to form an expanded root. The fifth micropile achieves high strength. Micropiles are used for reconstructions and for the capture of buildings. Micropiles may be vertical or oblique.

Large-dimensional piles are prismatic or cylindrical deep foundations with a diameter of more than 0.6 meters. In the case of a diameter of more than 1.2 m are referred to shaft pillar. Large-diameter piles are used as a single pile and replace the whole group of pilots. Large-diameter piles are made of reinforced concrete, possibly coupled with a steel pipe.

The shaft pillars are either dredged or drilled. They are used up to a depth of up to 4 m, to which the piloting is not economical and at a depth of more than 4 meters in case of higher load carrying. In larger buildings, only pillars are drilled. Dredged shaft pillars are suitable for dry or soils with little leakage of water.

The foundation wells are underground structures of cylindrical or prismatic shape with a minimum diameter of 1 meter. The foundation wells are mainly used for the foundation in water-borne and easily disconnectable soils that allow the wells to be quick submerged.

The lifting of the soil is carried out under the protection of the shell consisting of hollow prefabricated elements, usually from the rings provided at the bottom with the cutting edge. The soil is extracted from the interior of the foundation well, and the substructures are gradually undermined and their own weight enters the subsoil. The inner space is concreted after reaching load-bearing soil.

The caissons are used for building foundations in the water. The caissons are large-area wells enclosed by a ceiling structure that creates a working chamber secured against water ingress and allows construction work under water.

To dispose of water from a caisson, it is necessary to achieve a pressure equal to the pressure from the outside of the caisson. Afterwards, workers can enter the caisson, where the earth extracts, and so the caisson submerges. After lowering to the desired depth inside the caisson be cast. Caisson forms deep foundations overlying structure.