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
- Construction
of the Franki plug with gravel.
- Bottom
driving with an internal hammer. This operation causes compression of
the soil by lateral displacement.
- Expulsion
of the plug and starting to form the Franki base.
- Formation
of the Franki base and anchoring of the reinforcement.
- Driving
completed.
- Concreting
of the shaft. Successive charges of zero slump concrete are rammed into
the soil, simultaneously withdrawing the tube.
- 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
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
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.
