631
Technical Committee 102 /
Comité technique 102
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris2013
3. PILE IMPEDANCE
During pile installation, an impulse load from the hammer ram
is applied to the pile top, and dynamic longitudinal force in the
pile is transferred to the surrounding soil. According to Peck et
al. (1974) and Woods (1997), pile impedance affects the force
transmitted down the pile. Pile impedance characterizes the
pile ability to overcome the soil resistance to pile penetration
and develop required capacity.
One of pile impedance, Z, definition can be presented as
c/EA Z
(2)
where E is modulus of elasticity of pile material; A is pile
cross-section area; and c is longitudinal stress wave velocity. It
can be seen that impedance depends only on the pile material
and dimensions.
Recognizing the importance of pile impedance for
assessment of to the ground vibration, Heckman and Hagerty
(1978) proposed the equation for the peak particle velocity of
ground vibrations from pile driving as a function of the rated
hammer energy, W
r
, and the distance, D, from a driven pile
with the coefficient, k, which is dependent on pile impedance.
D
Wk v
r
(3)
The coefficient, k, is inversely proportional to pile impedance.
It means that driven piles with higher impedance generates
lower PPV of ground vibrations and vice versa.
Svinkin (2000) derived equations for PPV of pile
vibrations, V, and the maximum force, F, measured at the pile
head as
ZL
cW2 V
t
(4)
and
L
cZW2 F
t
(5)
where W
t
is the energy transferred to a pile.
Similarly to equation (3), equation (4) shows that the
velocity triggered by the hammer ram impact is an inversely
proportional function of pile impedance. However, equation (5)
displays that the force is proportional to the root square of pile
impedance. It means that pile impedance affects force and
velocity at the pile head in opposite ways.
Case histories presented in a number of publications, for
example Svinkin (2000), demonstrate higher ground vibrations
triggered by installation of high soil displacement piles
(concrete piles and steel pipes with closed ends) in comparison
with low soil displacement piles (H-piles and steel pipes with
open ends). A practical experience is the evidence that pile
impedance affects ground vibrations in the proximity of driven
piles, but this pile property does not affect the dynamic field at
some distance from driven piles in accordance with Saint
Venant’s principle.
4. HAMMER ENERGY
Pile installation generates ground vibrations due to the hammer
energy applied to a pile. Obviously, PPV of ground vibrations
have to be a function of the hammer energy transferred on a
pile. However, some case histories demonstrate no correlations
between the hammer energy and PPV of ground vibrations,
Hope and Hiller (2000). It happens due to the effects of soil
conditions, the pile penetration depth, and the soil resistance to
pile penetration into the ground. Nevertheless, the hammer
energy is the major cause of ground vibrations because without
the hammer energy there are no pile penetration into the
ground and ground vibrations.
5. PILE CAPACITY AND GROUND VIBRATIONS
Some authors, for example Robinson (2006), found enormous
scatter of PPV of ground vibrations as a function of the
hammer energy. For example, PPV of ground vibrations
changed between about 0.4-21.6 mm/s at the rated energy of
135 kJ and between about 0.9-17.8 mm/s at the transferred
energy of 40 kJ. It happened because other factors mentioned
above affected ground vibrations and in consequence that data
measured at various construction sites with different soil
conditions, pile types and pile driving implementations were
considered together. However, Robinson (2006) suggested a
correlation between ground vibrations and pile capacity
determined during pile driving. He believes that pile-soil
interaction, not energy, is the major influence in the generation
of ground vibrations from driven piles. Obtained conclusions
are not accurate because ground vibrations and pile capacity
are outcomes of the same pile driving process and only an
accidental correlation between them is possible.
It is necessary to say that ground vibrations and pile
capacity for sure depend on the hammer energy because pile
capacity cannot be mobilized without the sufficient hammer
energy. Moreover, during pile driving, the static pile capacity
is determined by signal matching software on the basis of force
and velocity measurements at the pile head. Unfortunately,
different software produces different results. It means that PPV
of ground vibrations are dependant on signal matching
technique used for analysis of testing data. Besides, during pile
installation, ground vibrations should be measured not
calculated because of possible detrimental effects of pile
driving operations and also measured ground vibrations are
more reliable than calculated ones.
6. CONDITION SURVEYS AND VIBRATION
MEASUREMENTS
Approximate calculation of expected ground vibrations and
even vibration monitoring yield relative information on vibration
effects on structures, and these results could be inconclusive.
Moreover, there is uncertainty in application of the existing
vibration limits for assessment of pile driving effects on soils and
structures. Therefore, it is imperative to perform condition
surveys of structures before, during and after pile installation
which provide complete information on structural responses to
vibration excitations. Obtained information can be much
beneficial for analysis of causes of damage to structures than
vibration assessment and measurements. Dowding (1996)
pointed out the necessity of professional performance of a
preconstruction survey.
Condition surveys during pile installation and after the
completion of pile driving are significant for analysis of
possible causes of damage to structures. Each construction site
