Anchoring Reinforcement in Base
Piles are often required to take tension loads. Because the Franki pile has an enlarged base a considerable tension capacity can be generated provided the steel reinforcement is anchored into the base. To achieve this the following variation to the installation technique takes place after the plug has been expelled.
Using a 20 mm slump concrete a 1.5 to 2.0 metre length of pile shaft is formed using the rammed shaft technique described hereunder. A slightly wetter mix is used and the concrete is extruded from the tube with the hammer as the tube is slowly withdrawn. The tube is then re-plugged with normal zero slump base concrete and driven back through the shaft. As a result of this the shaft concrete is forced sideways and an enlarged base is formed. The plug is then expelled and further enlargement of the base is achieved using the standard basing technique. The reinforcing cage is then placed into the tube and the shaft concreted iRthe normal way.
Franki piles made in this manner can resist considerable tension loads and tension load tests in excess of 1000 kN have been successfully carried out.
Rammed Shaft
The rammed shaft has an uneven surface which results in about fifty percent greater shaft friction than that of the smooth standard shaft formed using high slump concrete. The only reason for using the rammed shaft would be to increase shaft friction and as the Franki pile is generally considered an end bearing pile it is not widely used.
The rammed shaft is formed by expelling successive charges of zero slump concrete out of the tube as the latter is gradually withdrawn. Each 'measured concrete charge is placed in the tube and the hammer is lowered to rest on the concrete. The tube is then withdrawn about one
determined by the test blow. The concrete is given another three to four blows of the hammer by which stage the toe of the hammer should be level with the toe of the tube. Another charge of concrete is lowered into the tube and the process is repeated until the shaft has been cast to the correct level. Because of the stiffness of the concrete mix pile cut-off levels well below ground level are possible with the rammed shaft method.
It should be noted that the reinforcing cage is placed in the tube prior to the concreting of the shaft and that the hammer operates within the reinforcing cage. The cage has to be well made so as to prevent damage from the hammer. There is also limited control on the concrete cover with this type of shaft.
Problems have been experienced when using this shaft technique in very soft soil profiles. The resistance to the concrete being expelled from the tube is very low with the result that it tends to flow out in one direction more than another. This can result in abnomally shaped piles and piles with no cover to the steel in places. Similar problems can occur where the piles have been predrilled and soft saturated soil fills the annulus when the piling tube is installed.
Predrilling, Jetting and Coring
In the hinterland of South Africa the soils are normally not saturated and are often of dense or stiff consistency. It is thus very difficult and in many cases impossible to drive the piling tube into these soil profiles. To achieve the required penetration the pile position is first predrilled using an auger rig. The depth to which the pile is predrilled will depend on whether there is any tendency for the predrilled hole to collapse, the consistency of the soil profile and any tendency for pile heave to take place. It is standard practice to attempt to drive the tube beyond the predrilled depth so as to ensure that the pile's end bearing is not affected by the predrilling.
In the coastal areas where the soil profiles are generally saturated, predrilling is often not possible. In these circumstances jetting with a water jet pipe can be used to assist the penetration of the tube through a dense stratum. A water jet is typically a 100 mm pipe with a nozzle on the toe end and connected to a high pressure high volume pump. The jet pipe is lowered down the side of the piling tube as the latter. is driven into the ground. By keeping the jetting action close to the toe of the piling tube the driving resistance is reduced considerably. To keep the piling tube plumb it is common to use two jet pipes, one either side of the tube. It is standard practice to drive the tube two to three metres beyond the level at which jetting was discontinued ceased so as to ensure full end bearing capacity.
Conditioning the Soil
In sandy soil profiles the mere act of driving the piling tube has a conditioning affect on the soil. As more piles are driven so the soil strength increases and the ground is said to "tighten up". A greater degree of conditioning, however, can be achieved in softer soil profiles by first making a sand pile and then driving back through this. This achieves a double volume displacement and thus a greater degree of conditioning.
Another way a sandy soil can be conditioned is through the process of forming the enlarged base. By expelling material from the toe of the tube a sandy soil can be compacted. The material expelled can be concrete or it can be sand or gravel. If the latter, a concrete base is finally formed in the compacted zone in the normal way.
Additional Depth
One of the limitations with the Franki system is the limited depth to which the standard pile can be installed. This is 6 metres for the Mini and 15 metres for the Light, Medium, Heavy and Super Heavy. If the water table is not high these depths can be increased using an extension tube. This is merely an additional length of piling tube which is attached to the top of the normal piling tube and which enables additional depth to be achieved. An additional one to two metres for a Mini is common and up to 5 metres for the other sizes.
Using an extension tube does, however, slow down the production rate so the cost per metre will rise. Comparison of costs with other piling systems will determine whether the use of extension tubes is an economical proposition.
Because it is difficult to seal the joint between the extension tube and the main piling tube the use of this technique is not possible in areas with a high water table.
Permanent Liner to Pileshaft
In cases where the ground water is polluted with chemicals that are harmful to concrete it is
desirable to have a permanent liner which protects the shaft concrete from attack. Liners made out of steel or plastic can be incorporated into the shaft of the pile. These liners are normally fixed to the reinforcing cage and lifted up and lowered into position with the cage. The shaft is then concreted in the normal way.
Precast Concrete Shaft
A Franki pile can also be formed using a precast concrete shaft with the resulting pile being referred to as a Franki Precast Composite. A precast concrete shaft is a high quality product and the Franki Precast Composite pile is ideally suited to sites with aggressive groundwater conditions. The precast shaft is also smooth and in the formation of the pile there is an
A small quantity of sand/cement grout is placed in the tube once the enlarged base has been formed. The precast shaft is lowered into the piling tube and penetrates into the grout to bear on the enlarged base. The piling tube is extracted and the gap around the precast shaft is filled with loose sand so as to provide some lateral support to the shaft.