Helical Piles 101: Strengthening Foundations - J.F. Brennan Company

2.1. Displacement piles

When the piles are installed, the existing soil is compacted and displaced laterally. By the compaction of the soil in the immediate vicinity of the pile, the bearing capacity is increased. According to EA-Pfähle (Recommendations of the working group "Piles"), displacement piles are divided into:

• Prefabricated driven piles consisting of reinforced concrete, prestressed concrete, steel, cast iron or timber,

• Driven cast in place displacement piles, 

• Grouted displacement piles and 

• Screw piles (full and partial displacement piles).

Reinforced concrete / prestressed concrete driven piles

Solid and hollow piles are used. Solid piles are made with slack reinforcement or with a prestressing. Possible cross-sections are square, rectangular, polygonal or jointed, with a square or circular cross-section usually being selected.
The cross-sections can be adapted to almost any static requirement, and almost any combination of cross-section, length and profile is possible during manufacture. Many connection possibilities with the building above are conceivable and are shown in the Geotechnical Engineering Handbook.

A disadvantage of this type of driven piles is their low load-bearing capacity in transverse bending. A heavy pile-driving rig is required for driving the piles into the ground, which involves great vibrations. If so-called coupling piles are used, they can be adapted to the soil mechanical properties of the soil with the resulting driving length during driving into the ground. 

Prefabricated driven steel and cast-iron piles

Prefabricated steel driven piles are manufactured and installed as steel tubes or H-sections, as joined sections or sheet piles with a variety of cross-sections and wall thicknesses.

While steel tube piles are produced from commercial supplies (seamless or welded), joined section piles are produced by welding together several individual piles of common sheet piling systems. Steel tube piles are also used in mixed sheet pile walls by welding locks onto the walls and inserting filler piles.

All driven steel piles have in common that they can usually be driven into the ground without the formation of a wedged pile tip. In tubular or joined section piles, a driving plug is formed at the base of the pile, which has the same characteristics as a pile tip. However, to create this plug, it is often required that the piles be driven into the ground for the last 3 meters. If the pile is vibrated into the ground (by vibratory pile driving), the load-bearing capacity of these pile tips formed by soil will be reduced.

If symmetrically arranged steel sections are welded onto the steel pile used (box, tubular or H-sections) in the footing area, the load-bearing capacity of such a pile can be increased significantly. The shaft area is significantly increased, and a much larger driving plug can be formed between the individual sections at the base.  The driving capability of the piles is hardly adversely affected by the welded sections, but the driving energy to be applied must be increased accordingly. The length of these welded-on sections depends on the type of pile and can have a length between 2 m and 3 m.

Steel piles have a high material strength combined with elasticity. They are insensitive during transport and can be extended during driving to suit the soil properties.
Analogous to the considerations and technical possibilities for driving sheet piles, the soils can also be classified according to their driving capability for driving the driven piles.

Cast-in-place displacement piles

Unlike the prefabricated driven piles presented in the previous two chapters, cast-in-place displacement piles, like bored piles, are produced directly at their final location. They are considered full displacement piles because they are driven into the ground under the protection of a closed-top tube until the load-bearing soil is reached. However, as the tube is pulled, the soil may relax slightly again. A well-known pile type is the “Frankipfahl”.

With the Franki pile driver, a tube is driven into the ground by means of free fall impact driving within the tube (internal driving). At the base of this tube a concrete plug, approx. 1 m high, is formed, which is strongly compacted by the tamping of the hammer. The plug drags the tube into the ground during further (internal) driving onto the plug. The soil is completely displaced and compacted, lateral stresses in the soil increase. Once the final depth is reached, the tube is held at its depth and the concrete plug is displaced from the pipe by further driving, thus forming an enlarged base installation. Further concrete of high plasticity is poured into the tube after setting a reinforcement cage and further compacted with internal driving. At the same time, the tube is withdrawn, creating a rough pile shaft. The result is a pile with a large pile base and with a large load-bearing capacity due to the compacted soil. Pile inclinations of up to 4:1 can be produced.

Prefabricated driven timber piles

Prefabricated driven timer piles should be briefly mentioned here for the sake of completeness. They have the advantage that they can be loaded immediately after driving into the ground and that they can be manufactured and installed in a previously known quality. If these piles are to be used permanently, they must be below the lowest groundwater or tidal water level. A simultaneous presence of oxygen and water causes the wooden piles to rot.

Pressure-grouted piles

A special type of driven piles is the pressure-grouted pile. Pressure-grouted piles are driven into the ground as a prefabricated pile with a wedge shaped enlarged pile shoe while adding grout. During driving the shoe forms an annulus around the steel shaft which is filled with cement grout during the installation process. 

Displacement bored piles

Displacement bored piles are divided into partial displacement bored piles and full displacement bored piles. In the installation of these piles, a drill tube sealed at its tip is driven into the ground with an auger attached. The difference between the two methods is mainly in the method of recovery of the drill tube and with the associated amount of drill cuttings, and is described below. Both methods have in common that, due to the screwing of the drill pipe into the ground, they disturb the existing ground only insignificantly and can be used well in built-up areas due to lowest vibrations or noise generation during their production.

Screw piles - Full displacement bored piles

In the production of full displacement bored piles, the soil is completely displaced. The tube, which is closed at the bottom, is screwed into the ground. At the base of the tube there is a cutting head that ensures the helical driving of the tube into the ground. The soil in contact with the tube is completely displaced laterally, thus compacting it. When the target depth is reached, a reinforcement cage is placed and high plasticity concrete is poured in. The tube is recovered by unscrewing it, the tip of the cutting head remains in the ground and the concrete completely fills the void created by the cutting head. The finished pile shaft possesses a helical, concrete bulge and the completed pile resembles a screw. 
In contrast to an Atlas pile, where only the sacrificial tip is lost in the soil after excavation, the tube of a Fundex pile is then withdrawn under oscillation, whereby the tip disengages from the tube. It remains in the ground and forms the pile base.

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Partial displacemenet piles