Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PILE DRIVING
This invention relates to pile driving or similar such as drill string driving
and is
directed both to an arrangement for facilitating pile driving or the like and
to a
method of pile driving or the like.
This description is being given in relation to pile driving but it is also
applicable
to drilling where the unit to be driven is a drill string with a drill head at
its lower
end.
DESCRIPTION OF THE PRIOR ART
It is well known to drive piles into the ground for subsequent support
purposes
by using an impacting hammer.
In such a technique, a pile is held aligned in a direction in which it is to
be
embedded within the earth and there is then a conventional impacting hammer
that is also aligned in the direction of the pile and it is raised and then
allowed to
fall on to an uppermost end of the pile to effect an impact loading thereon
and
thus cause the pile itself to enter further into the ground.
Such a technique is traditional but it has some difficulties.
A significant one of these is the ground vibrations that are caused by the
process which in some cases can be felt quite some distance away and which
can sometimes be considered destabilising to other structures.
Secondly, however, the noise itself can be very intrusive and to some extent
disruptive in many environs.
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Finally, the process implicitly requires substantial equipment and is
relatively
slow depending of course upon the nature of the earth into which it is being
driven.
Many years ago I was an inventor of a vibration power generator which I
together with Stewart George Page filed as a patent application which was
granted in various countries among which is Australia, under Patent No.
609165.
In this original patent specification, there was disclosed firstly a
vibrational
power generator which provided a velocity source and also a method by which it
could be determined as to whether a rate of vibration was above or below a
resonance frequency.
Since that time, experiments have been conducted on piles and drill strings,
in
an attempt to effect a useful pile driving effect from such a vibrational
power
generator with success, with the following discoveries.
Upon close examination of the various attempts to achieve effective pile
driving,
I have now established that several phenomenon or issues contribute greatly to
the success or failure of driving a pile with the apparatus described.
What was observed was that a vibrational generator, when securely attached to
the pile, was effective, or most effective when driven at the resonance of the
combined system consisting of: the mobilized mass of the vibrational
generator,
the connective device attaching the vibrational generator to the pile, the
pile and
the substrate into which the pile penetrates, particular at the tip of said
pile.
Through alteration of the mass and geometry of the connective device attaching
the vibrational generator to the pile, a substantial effect may be had upon
the
movement of the pile into the ground.
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The benefits of the piston cylinder style vibrational generator for pile
driving and
drilling became evident as the mechanism provides a velocity source, as
opposed to a force source, for effecting the advance of the pile or the like
such
as a drill string.
In addition, through rotation of the pile, which is made possible by the
simple
rotation of the moving cylinder of the vibrational generator, the penetration
rate
of the pile is improved substantially.
My discovery has been that greater efficiency of effect than has hitherto been
the case is achievable.
OBJECT OF THE INVENTION
An object of this invention therefore is to provide an improvement in high
frequency pile driving or drill stem driving or at least provide the public
with a
useful alternative.
SUMMARY OF THE INVENTION
In one form there is proposed a method of driving a pile or drill string into
the
ground which method includes the steps of securing a vibrational driver to an
upper end of the pile to be driven so that vibration of the driver in an
elongate
direction of the pile is effected to both push and pull the pile in
correspondence
to the vibrational drivers effort, and the driver is then caused to vibrate at
a
vibration rate sufficiently high so that the rate of vibration will be for at
least
some period of the vibration higher than a resonant frequency of the pile in
its
institu position together with its attached driver, and being sufficiently low
so that
the rate of vibration will be for at least some period of the vibration lower
than a
resonant frequency of the pile in its institu position together with its
attached
driver, measuring hydraulic fluid drive pressures of the driver, and effecting
a
raising or lowering vibrational rate in response to detect pressure
characteristics
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indicating whether the vibration rate is above or below resonance, and
effecting
a change in the vibrational rate in response to the detected pressures to
effect a
vibrational rate which is closer to the resonance frequency of the pile in its
institu position together with its attached driver.
In preference a change in the vibrational rate in response to detected status
is
effected manually.
In the alternative, a change in the vibrational rate in response to detected
status
is effected by an electiically generated signal which effects a change in the
vibration rate.
In this case in the alternative an hydraulic pressure wave is detected with
characteristics which can be interpreted either manually or by reference to a
computer algorithm to indicate that the actual vibrational rate is either
above or
below a resonant vibrational frequency.
A resonant vibrational frequency is a natural frequency which is determined by
the characteristics of the pile itself, its interchanging relationship with
the soil or
other earth through which it has been driven, together with the attachment
mechanism and the mobilized body of the vibrational generator.
The resonant frequency required is a natural frequency which is determined by
the characteristics of the assembly of the generator, the connection mechanism
and the pile but also includes factors such as the influence of the soil into
which
the pile is being driven, particularly at the pile tip, and also the sides of
the pile
in terms of the their adhesion or even stickiness of the soil as they pass
through, which may be reduced or overcome substantially through the rotation
of the pile.
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As such, a vibrational frequency required for resonance will not be actually
known necessarily in advance although some estimates of the required
frequency range can be made.
The advantage of the resonant vibrational frequency detection technique
coupled with an efficient vibrational driver is that it can be assumed that
such a
vibrational driver can be caused to vibrate at least through a range where it
is
able at times to be higher than a vibrational frequency required and at other
times lower than a vibrational frequency required.
By attaching the vibrational generator to a pile so that it will cause both
the pile
to lift and lower in response to the vibrational driver means that it is the
total
combination which will exhibit the characteristics needed to assess a resonant
frequency appropriate for that time.
It is known that any complex mechanical apparatus will have a number of time
constants providing separate resonant frequencies which are distinctly
different
one from the other.
When reference however is made to a resonant frequency of a combination of
pile, connection device and vibrational generator, there will be in our
experience
thus far a prominent or very dynamic resonant frequency which incorporates as
a part of its time constant the combined mass, and geometry, of the generator,
connection device and pile and it is this frequency to which we refer.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of this invention, it will now be described with
the
assistance of drawings in relation to an embodiment wherein,
Figure 1 is a perspective view of a pile partially inserted into the ground
and
having securely affixed to the top of the pile, a vibrational pile generator,
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Figure 2 is an exploded view of the elements by which in this case the
vibrational power generator is secured to the top of the pile,
Figure 3 is a perspective view of the combination of pile driver and pile as
in
figure 1,
Figure 4 is a cross sectional view of a vibrational power generator according
to
an earlier patent disclosure of which I was an inventor, and
Figure 5 is a cross sectional perspective view of the combination of the
generator coupled to a pile so that they will be constrained to act together.
DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
Referring to Figure 1, there is a pile 1 which comprises a conventional pile
constructed of a single metal member and being of elongate length and
constant cross sectional shape and size along its length.
Positively and rigidly secured to a top of the pile 1 is a vibrational power
generator 2 which is secured in this way so that when the vibrational
generator
causes an uplift, this has a lifting effect on the pile 1, and likewise when
the
vibrational generator is effecting a downward pressure, likewise the effort is
also
then on to the head of the pile 1 but also so that in both cases, the
direction of
the vibrational effort is in the elongate direction and aligned with the
elongate
direction of the pile 1.
There can be alternate forms of securing the vibrational power generator 2 and
the illustration is simply one of these.
The distinction here is that there is a positive and fixedly secured
connection
which in fact can be tested by testing whether there is both up lift and
downward
force exerted by the vibration power generated to in relation to the pile 1.
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The vibrational power generator 2 is shown in more detail in Figures 4,5 and 6
where there is a rotary valve 6 which is rotated at a frequency selected in
response to perceived criteria in relation to the vibrational effect of the
generator, there being hydraulic fluid being caused to pass in alternate
directions by the rotation of the valve 6 to either lift or lower the piston
element 7
with respect to a body 8.
This has been described in my earlier patent and it is shown here as one
example only of a means for effecting a vibrational power generator which is
able to effect a frequency that in practice can be both higher than and lower
than a combined resonant frequency of the power generator and pile when
embedded institu in a driving situation.
To explain this further, there have been attempts in the past to effect
vibrational
driving of piles but in my experience, they have relied upon rotating weights
and
it is very difficult indeed to achieve, without damaging bearings and other
mechanical parts, a sufficiently high vibrational rate with sufficient power
input.
In relation to the vibrational power generator 2 as described, as this is
subjected
to hydraulic pressure which provides the power of each vibrational stroke, I
have found that there are pressure wave characteristics in the pressure of the
hydraulic fluid which distinguish the status of the vibrational rate as
compared to
a resonant rate as to whether the vibrational rate is lower than the resonant
rate
or higher than the resonant rate.
I therefore attach a pressure meter in the hydraulic circuit and feed the
resultant
pressures as compared to time into an analyser which is a computer based
analyser-which simply compares a wave shape being received with that which
is typical at resonance and will then effect a control of the rotary valve
speed in
the vibrational power generator 2 so that when it is lower than a perceived
resonant frequency, it will then increase the speed, and when it is higher
than a
perceived resonant rate, it will then lower the vibrational frequency rate.
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To assist in further understanding the invention there is provided this
further
description. The resonance of a pile, using an eccentric mass vibrational
generator, referred to as a force source, can result in the uncontrolled or
galloping oscillation of the pile and vibration generator system, due to the
lack of
control of the force,(which is proportional to the square of the frequency and
thus cannot be altered at a given frequency, generated by the system at a
target frequency, which may result in destructive accelerations and forces
upon
the system.
lo
In relation to the vibrational generator 2, which is a velocity source, the
resonance force and amplitude of the pile, connector and vibrational generator
can be controlled and limited by the operator, independently of frequency, in
a
number of ways including the limiting of the capacity of the hydraulic fluid
volume or restriction of the hydraulic fluid pressure, resulting in the
ultimate and
infinite control over the amplitude and force of vibration in safe, efficient,
productive and controlled operation of the system for pile driving.
The advancement of the pile at resonance is hindered by the shape of the
standing wave developed in the pile at resonance, consisting of a node of zero
movement at the centre of the pile at the natural frequency corresponding to
the
first mode of vibration of the system, which will experience friction of the
soils
and will be thus restrained from advancement into the ground without
additional
benefit of any of the force of gravity, a translational force applied to the
vibrational generator in the direction of desired advance or the application
of
rotation to the pile via the vibrational generator.
The vibrational generator 2 exhibits a simple exterior slidably movable
cylinder
which may be rotated via the biasing isolator, mounted upon a rotary bearing,
capable of generating high torque and velocity to the pile for the overcoming
of
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soil friction at the node and along its entirety of length, including the tip
of the
pile.
The combined vibration and rotation of the pile can be synchronised such that
the tip, or attached bit to the pile tip, will rotate and strike, at an
optimal radial
advancement, a fresh piece of soil or rock, which under this timed and clocked
advancement, the bit features will maximise their effectiveness in striking
and
breaking the soil or rock.
There are two common methods of generating cyclic excitation (vibrations),
commonly referred to as in one case a force source and in the other a velocity
source. As has been described conventional vibrators use rotating eccentric
masses to generate cyclic excitation. Two horizontally opposed (opposite
rotation) eccentric mass mechanisms will produce a sinusoidal excitation.
Force produced is a product of eccentric mass, moment arm and square of
frequency produced. A peak cyclic force is locked to the frequency of
operation
and cannot be altered, thus the system is a force source.
The mechanism has an advantage of producing a sinusoidal excitation with
very high peak force as the frequency increases. However, this same force
must be resisted by shaft bearings. This proves to be a downfall of the
mechanism in that present bearing technology to the best of our knowledge is
not capable of withstanding high forces at very high frequencies, in a range
of
interest of this application and patent.. For a device designed to operate at
a
single constant frequency this downfall may be managed and a functional
system, at limited power, might be constructed. However, for a system that=
requires a variable or wide frequency range this technology fails for a number
of
reasons.
An eccentric mass system fails to deliver high force or power at frequencies
much lower than the peak design frequency of the system. For example, a
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system designed to deliver 110 kW and 200 kN of peak power at 150 Hz will
produce only 22% of the power and peak force at 70 Hz. For pile driving and
drilling targeted at resonating the pile or drill tooling, a range of 70 to
150 Hz is
reasonable. Thus for much of the drilling or driving the machine will be
producing as little as 22% of the rated power of the system, which may prove
to
be too little to effectively advance the pile or still tool at the end of a
drill string.
A most troubling effect of the force source lies in response of the excited
system
to that cyclic excitation force. Should the coupled system (pile or drill
string)
come to be excited at the resonant or natural frequency of the source,
connective mechanism and the pile or drill string, then the system will act as
a
spring and multiply in energy as the system exchanges the elastic and dynamic
energy of excitation. This may be used to conduct work with great advantage,
provided the system can shed energy to the system upon which the work is
done. This expenditure of energy is referred to as damping. If the work or
damping is greater than the excitation energy the system is over damped and
lacks the true benefit of resonance. If a system is balanced with work done
equivalent to the energy introduced the system can be efficient and safe. If
the
system does not expend as much energy as is introduced then the amplitude of
excitation, strain and stresses within the material will continually raise
until
fatigue or failure occurs. This is referred to as galloping oscillation and is
the
reason mechanical engineering and physics professors instruct their charges to
avoid resonance in any form. Such an effect is not uncommon with present day
sonic drilling systems.
Finally, high frequency vibrators cannot use variable moment eccentric mass
mechanisms and thus start from 0 Hz and pass through all frequencies to arrive
at the target frequency. This generally results in the excitation of the pile
driver
head or drill mast, supports or other portions of the crane or base machine
experiencing excitation at its natural or resonant frequency. This can result
in
destructive galloping oscillation of the machine components, fatigue and
failure.
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This invention is directed to a mechanism that provides a velocity source of
cyclic excitation which therefore relies upon an entirely different mechanism
and
means of producing vibration. A velocity force is generated as described by
use
of a piston cylinder type device to generate a cyclic load. In this manner a
push
pull can be produced, which, with a control valve can generate varying cyclic
excitation with independent control over frequency, force and amplitude. The
detractions of such a device include a non sinusoidal loading history and a
limited range of peak force and amplitude. The advantage of the independent
control over frequency, amplitude and force far outweigh these detractions.
When independent control over frequency, amplitude and force are combined
with the ability to tune the device to the natural frequency of the driven
system
and then locking the generation of the velocity to the pile or drill string
the
overall apparatus offers significant benefits to the user. Through tuning to
resonance the user can now achieve the upmost efficiency in producing high
force and amplitude and thus work and this is efficiently transmitted for both
push and pull directions to the pile or drill string. With no coupling of
frequency
and force the user tunes to resonance and adjusts the amplitude to eliminate
the risk of galloping oscillation. The user can now improve the system
response
to the limit of the available power to maximise the effective work. Further a
failsafe feature emerges through the offering of a finite flow of any drive
fluid to
the piston cylinder device. By limiting the available flow, if the system
under
excitation is available to gallop and begins to do so, the mechanism will
implicitly limit the runaway increase in amplitude as it chokes displacement
possibly through cavitation or suction restraints. In addition the user can
tune
the work to limit the stress and or strain in the system and maximize the life
or
effectiveness of the tooling under excitation. For example, light wall pipe or
drill
tooling with discreet joints may be efficient to use for the purpose at hand,
be it
the desired final capacity of a pile or the handling of a light drill pipe.
The mass
and strength of the pipe may be limited to certain discrete stresses or
strains for
a limited number of cycles. Through control of the amplitude and excitation
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forces the system may be efficiently and effectively installed without over
stressing the components. The installation stresses and strains are often the
greatest a system ever experiences. Tools may now be inserted in drill holes
or
oil wells which require limited excitation to perform a given desirable task.
