96 Piling equipment and methods
is to be driven. Single and double-acting hammers, hydraulic and diesel hammers are
effective in all soil types and the selection of a particular hammer for the given duty is based
on a consideration of the value of energy per blow, the striking rate and the fuel consumption.
The noise of the pile-driving operation will also be an important consideration in the
selection of a hammer. This aspect is discussed in Section 3.1.7.
A knowledge of the value of energy per blow is required to assess whether or not a hammer
of a given weight can drive the pile to the required penetration or ultimate resistance without
the need for sustained hard driving or risk of damage to the pile or hammer. The safety of
operatives can be endangered if sustained hard driving causes pieces of spalled concrete or
mechanical components to fall from a height. The employment of a dynamic pile-driving
formula can, with experience, provide a rough assessment of the ability of a hammer with a
known rated energy value to achieve a specific ultimate pile resistance to the time of driving
(see Sections 1.4 and 7.3 for a further discussion of these formulae). However, the manufac-
turer’s rated energy per blow is not always a reliable indication of the value to be used in a
dynamic pile equation. The efficiency of a hammer can be very low if it is poorly maintained
or improperly operated. Also the energy delivered by the hammer to the pile depends on the
accuracy of alignment of the hammer, the type of packing inserted between the pile and the
hammer, and on the condition of the packing material after a period of driving.
The increasing use of instruments to measure the stresses and acceleration at the head of
a pile as it is being driven (see Section 7.3) has provided data on the efficiencies of a wide
range of hammer types. Some typical values are as shown in the following table:
Hammer type Efficiency of hammer/cushioning system (%)
Hydraulic 65 – 90
Drop (winch-operated) 40 – 55
Diesel 20 – 80The wide range in values for the diesel hammer reflects the sensitivity to the type of soil
or rock into which the pile is driven and the need for good maintenance. Present-day practice
is to base the selection of the hammer on a driveability analysis using the Smith wave
equation (see Section 7.3) to produce curves of the type shown in Figure 3.17. They show
the results of an investigation into the feasibility of using a D100 diesel hammer to drive 2.0
m OD by 20 mm wall thickness steel tube piles through soft clay into a dense sandy gravel.
The piles were to be driven with closed ends to overcome a calculated soil resistance of 17.5
MN at the final penetration depth. Figure 3.17 shows that a driving resistance (blow count)
of 200 blows/250 mm penetration would be required at this stage. This represents a rather
severe condition. A blow count of 120 to 150 blows/250 mm is regarded as a practical limit
for sustained driving of diesel or hydraulic hammers. However, 200 blows/250 mm would
be acceptable for fairly short periods of driving. Commercial computer programs based on
wave equation models enable the piling engineer to predict driveability, optimize the
selection of hammer, select energy level which will not damage the pile, and ensure that the
correct dolly and adapters are used.
The American Petroleum Institute(3.5)states that if no other provisions are included in the
construction contract, pile-driving refusal is defined as the point where the driving resistance
exceeds either 300 blows per foot (248 blows/250 mm) for 1.5 consecutive metres or 800
blows per foot (662 blows/250 mm) for 0.3 m penetration. Figure 3.17 also shows the driving