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2.3. DRIVING SHEET PILE WALLS 15
Fig. 2.10 shows the principle of pile-pressing with plant supported on the sheet piles already
driven. In this method, only a single sheet pile is pressed into the ground in each pressing
operation. The self-weight and the sheet piles already driven provide the reaction. The pressing
plant moves forward on the wall itself to each next pressing position as the wall progresses.
2.3.3 Impact driving
Figure 2.10: Pile-supported pressing system Silent Piler
Impact driving involves driving the sheet piles into the ground with a succession of hammerblows
(Fig. 2.11). A timber driving cap is usually placed between the hammer and the sheet
pile. We distinguish between slow- and rapid-action systems. Slow-action plant such as drop
hammers and diesel hammers is primarily used in cohesive soils so that the ensuing pore water
pressure has time to dissipate between the individual blows. In a drop hammer, a weight is
lifted mechanically and then allowed to fall from a height h. Modern drop hammers operate
hydraulically. The number of blows can be set as required between 24 and 32 blows per minute.
The drop height of a diesel hammer is determined by the explosion of a diesel fuel/air mixture
in a cylinder. Depending on the type of hammer, the weight is either allowed to drop freely onto
the driving cap or instead the weight can be braked on its upward travel by an air buffer and then
accelerated on its downward travel by a spring. Using this latter technique, 60–100 blows per
minute are possible, whereas with the non-accelerated hammer the figure is only 36–60 blows
per minute. Rapid-action hammers are characterised by their high number of blows per minute:
between 100 and 400. However, the driving weight is correspondingly lighter. Rapid-action
hammers are driven by compressed air and the weight is accelerated as it falls.
The head of the sheet pile can be overstressed during impact driving if the hammer is too small
or the resistance of the ground is too great. Possible remedies are to strengthen the head or use a
larger hammer. In the case of a high ground resistance, excessive driving force or an incorrectly
attached driving cap, the pile can buckle below the point of impact. To avoid this, use thicker
sections or loosen the ground beforehand.