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Franki Pile

Excavated Franki enlarged base

The Franki pile, known and used world wide, was developed at the turn of the century by the Belgian engineer Frankignoul. It is a cast-in-situ concrete pile with an enlarged base and a cylindrical shaft which, due to its powerful driving method during installation, can penetrate stiff soils and reach large depths. By expulsion of a dry concrete plug, the soil surrounding the pile base is improved and thus the initial soil bearing capacity can be increased significantly. In North America, the Franki system is known as "pressure injected footing". Although the application of the Franki system has decreased due to cost and environmental considerations, this system is still competitive and widely used when site conditions are suitable.

Typical Franki pile dimensions range from 0.275 mm to 0.7 m diameter, with design loads from 35 to 200 tons. Maximum lengths are of the order of 30 m, although the length usually used is less than 20 m.

The Franki pile may be vertical or raked and its bearing capacity varies with the diameter of the pile base and the driving tube being used, which can be changed to suit the loads specified. It requires only a minimum of site preparation and in some particular cases, e.g. when negative friction is encountered, a permanent steel casing or pipe may be placed without any difficulty. Such a cased shaft eliminates the curing time required for the concrete precast pile. The Franki pile can also be combined with pre-fabricated elements (Franki composite pile) or with enlarged base (VB pile or bored Franki pile ).

Execution of a Franki pile

Execution of a Franki pile - casing bottom driven
  1. Construction of the Franki plug with gravel.
  2. Bottom driving with an internal hammer. This operation causes compression of the soil by lateral displacement.
  3. Expulsion of the plug and starting to form the Franki base.
  4. Formation of the Franki base and anchoring of the reinforcement.
  5. Driving completed.
  6. Concreting of the shaft. Successive charges of zero slump concrete are rammed into the soil, simultaneously withdrawing the tube.
  7. The Franki pile. A driven cast-in-situ pile with a cast-in-situ pressure injected base.

Construction of a Franki pile

Driving of the casing

A thick-walled steel casing is placed vertically on the ground. A special concrete bucket is used to pour a certain amount of nearly dry concrete into the bottom of the driving tube. The concrete is rammed with a 2 to 8 ton hammer while the tube is kept in position by steel cables. This hammer can be dropped from a height of several meters. Under its impact the concrete forms a plug at the bottom of the casing which penetrates slightly into the soil.

Due to the compression of the concrete plug, a water-tight bottom plug is created which prevents soil or water from entering the casing.

Alternatively, the steel casing can be installed by "top-driving", using ordinary drop hammers. In that case, the driving casing must be provided with a lost bottom plate. The casing can also be installed using a large-stem auger (VB-pile).

Enlarged "Franki" base

When the tube has been driven to the required depth, the casing is very slightly raised and maintained in position with the aid of steel cables. The plug is then expelled by heavy blows of the hammer, making sure that a certain amount of rammed concrete remains in the casing in order to prevent any seepage of water or soil into the pile shaft. This operation is checked with marks made on the driving cable of the hammer and on the lifting cables. The expanded base of the pile is then formed by adding as much dry concrete as necessary to achieve a pre-determined "driving set". An enlarged concrete bulb, serving as a pile base is thus formed in the soil, which is heavily compressed and densified.

Concreting of the shaft

Pouring of concrete into casing

The shaft of the pile is formed by ramming successive layers of "dry concrete", raising the casing 0.2-0.5 m at a time. The hammer displaces the concrete laterally into the soil previously compressed by the driving of the tube. Due to the ramming process, the concrete is in close contact with the soil and in this way a cylindrical shaft is obtained, which is resting on an enlarged base, formed to refusal in the bearing layer. The pile shaft can also be made of "wet concrete", which speeds up the construction process. This process is similar to ordinary driven cast-in-situ piles. Alternatively, a pre-fabricated pile shaft can be used.

The reinforced concrete pile

Reinforced Franki piles

When Franki piles have to withstand important transversal forces or are subjected to pulling forces, they must be reinforced over their whole length or in part, by means of steel cages consisting of at least 4 bars, 12 -35 mm wide in diameter. The reinforcing bars are tied by 5-8 mm spirals at the pitch of 10 - 25 cm. The outside diameter of the cage varies according to the size of the tube being used.
The concrete of the shaft is compacted using the drop hammer, the diameter of which is smaller than that of the reinforcement. For piles subject to uplift forces, the reinforcement is anchored in the enlarged base, thus providing high pull-out resistance.

Raked (battered) Franki pile

Franki shafts may be installed raked. Depending on the equipment and depths to be reached, the leads can be tilted from 18 forwards to 25 backwards. The raked shafts are always reinforced over their whole length. They can withstand dynamic stresses and are particularly suitable foundations for structures containing machines and subject to dynamic forces.



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