Either barrel type tubes or spirally manufactured tubes can be used provided the manufacturing process, and in particular the welding, has been carried out to high standards with stringent quality control. The tubes should be manufactured according to SABS 719 Grade B which is the equivalent of the American API 5L Grade B specification which is used worldwide.
Before installation can commence the sections of piling tube have to be prepared and welded up. The lead section of tube has a steel plate or rock shoe welded to the toe end and a splicing band welded to the top end. Follower tubes need only the splicing band to be welded to one end. The wall thickness of the tube will be in the 5 to 10 mm range. The smaller diameters will most likely have a 5 or 6mm wall thickness throughout whereas the larger diameters will have a 10 mm wall at the toe reducing to 6mm at the top. The splicing bands should be about 250 mm wide and should be made out of plate of the same thickness as the wall of the tube. If two tubes of different thickness are being welded together then the plate thickness should match the thicker of the two. Ideally there should not be more than a 2 mm difference in the wall thicknesses of two sections of tube to be welded together.
The diameter of the end plate should exceed that of the tube by the OD plus 12 mm. Alternatively, the plate can be welded on the inside of the tube in which case the friction component of the pile's bearing capacity will be greater. The end plate can have a rock shoe type arrangement welded to it. Rock shoes should be used if a boulder layer has to be penetrated or if the pile is to be driven onto a sloping rock face. An effective rock shoe can be manufactured using a central pin and four heavy gussets welded to the end plate as shown in Figure 7.2.1.
The tubes can be either top or bottom driven during the installation process. If bottom driving is used there is obviously more stress placed on the welds, both circumferential as well as longitudinal. Bottom driving is more efficient, however, so is often resorted to for the final drive even if the initial penetration is achieved with a diesel or hydraulic hammer driving on a helmet on the top of the tube.
INSTALLATION TECHNIQUE
Bottom Driven
The lead section of tube is lifted up and positioned in the leader of the piling rig. A leader guiding mechanism must be clamped to the tube and the leader adjusted for verticality or rake. With the toe of the tube resting on the ground on the pile position a measured quantity of semi dry concrete is discharged into the tube. The initial quantity should fill the tube about 3 to 4 diameters.
The hammer is a cylindrical drop hammer which operates within the bore of the piling tube. The plug of concrete is compacted using a few short drops of the hammer. The drop is then gradually increased but is normally not greater than 2.5 metres. The plug material becomes pulverised during driving and this lowers the efficiency of the blow of the hammer. For this reason and to prevent the tube from splitting the plug has to be continually refreshed by adding additional plug material throughout the driving operation.
The next section of tube is welded on when the top of leader tube is at a convenient height for welding. Further sections are welded on as and when required.
When the toe of the tube is approaching the founding stratum measurements of the penetration rate are made and the set checked occasionally. When the required set has been achieved this signifies the completion of the driving and the hammer is removed from the tube. It is possible to inspect the internal bore of the tube using a light or a mirror and reflected sunlight if necessary .
The reinforcing cage is then lowered into position and the shaft of the pile concreted using a high slump self-compacting mix. It is common practice to reinforce only the upper 12 metres of the pile shaft because of the permanent thick wall casing.
Top Driven
If top driving is used the tube is lifted into the leader of the piling rig as previously, the guides are fixed and the leader is adjusted for verticality or rake. The helmet and hammer are then lowered onto the head of the tube and alignment of the hammer and tube is checked. Driving commences with the operation of the hammer and is continued save for welding on follower tubes until the required founding stratum has been reached and an adequate set
Top driving of long closed ended steel piling tubes with a thin wall is not as efficient as bottom driving in hard driving conditions as a large proportion of the hammer's energy is absorbed by the tube itself. If the driving is particularly hard or the piles are heavily loaded then it may be necessary to resort to bottom driving to achieve the required depth or set. The efficiency of bottom driving is estimated to be between 15 and 25 percent.
Once the tube is driven it is a simple matter to place the reinforcing cage and concrete the shaft as described above.
A typical pile detail is shown in Figure 7.2.1. Plate 7.2.1 shows driven tube piles being installed on a river bridge foundation.