92 Piling equipment and methods
Because of difficulties in achieving a consistent energy of blow, due to temperature and
ground resistance effects, the diesel hammer is being supplanted to a large extent by the
hydraulic hammer, particularly when being used in conjunction with the pile-driving
analyser (see Section 7.3) to determine driving stresses.3.1.5 Piling vibrators
Vibrators consisting of pairs of exciters rotating in opposite directions can be mounted on piles
when their combined weight and vibrating energy cause the pile to sink down into the soil
(Figure 3.16). The two types of vibratory hammers, either mounted on leaders or as free hanging
units, operate most effectively when driving small displacement piles (H-sections or open-ended
steel tubes) into loose to medium-dense granular soils. Ideally a pile should be vibrated at or near
to its natural frequency, which requires 100 Hz for a 25 m steel pile. Thus only the high-frequency
vibrators are really effective for long piles,(3.2)and while resonant pile driving equipment is costly,
high penetration rates are possible. Most types of vibrators operate in the low-frequency to
medium-frequency range (i.e. 10 to 39 Hz). Vibrators mounted on the dipper arm of hydraulic
excavators have high power to weight ratios and are useful for driving short lengths of small
section tubular and H-piles, limited by the headroom under the bucket, say 6 m at best.
Rodger and Littlejohn(3.3)proposed vibration parameters ranging from 10 to 40 Hz at
amplitudes of 1 to 10 mm for granular soil when using vibrators to drive piles with low point
resistance, to 4 to 16 Hz at 9 to 20 mm amplitude for high point resistance piles. In fine soils
frequencies in excess of 40 Hz and high amplitude will be needed but care must be exer-
cised because of the potential changes in soil properties such as liquefaction and thixotropic
transformation. Predicting the performance of vibratory pile driving is still not very reliable.
Where specific test data are not available for the vibrator installing bearing piles or the pile
is not bearing on a consistent rockhead, it may be advisable (as is common in the USA) to
use the vibrator to install the pile to within 3 m of expected penetration and then use an
impact hammer to drive to the bearing layer. Vibrators are not very effective in firm clays
and cannot drive piles deeply into stiff clays. They are frequently used in bored pile
construction for sealing the borehole casing into clay after pre-drilling through the granular
overburden soils. After concreting the pile the vibrators are used to extract the casings and
are quite efficient for this purpose in all soil types (see Section 3.4).
Vibrators have an advantage over impact hammers in that the noise and shock wave of the
hammer striking the anvil is eliminated. They also cause less damage to the pile and have aTable 3.4Continued
Maker Type Mass of ram Maximum Maximum
(kg) energy per blow striking rate
(kJ) (blows/min)
I-62 6623 223.7 35 – 50
I-80 8030 288.0 35 – 45
I-100 10 000 360.0 35 – 45
422 a 1810 30.5 76 – 82
520 a 2300 40.6 80 – 84
640 a 2722 54.2 74 – 77
Note
a Double acting.