|1. A method of driving a pile or like 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 detected pressure characteristics 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.
2. A method as in claim 1 where a change in the vibrational rate in
response to detected vibrational rate status is effected manually.
3. A method as in claim 1 where a change in the vibrational rate in
response to detected vibrational rate status is effected by an electrically generated signal which effects a change in the vibration rate.
4. A method as in claim 1 where a change in the vibrational rate in
response to detected vibrational rate status is effected by reference to a computer algorythm to indicate that the actual vibrational rate is either above or below a resonant vibrational frequency.
5. An apparatus for driving a pile or drill string into the ground which apparatus includes a vibrational driver secured to an upper end of the pile or drill string and adapted 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, means to measure hydraulic fluid drive pressures of the driver, and means to effect a raising or lowering of the vibrational rate in response to detected pressure characteristics 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.
6. An apparatus as in the immediately preceding claim in which there are means to effect a control of velocity of hydraulic fluid passing through the driver.
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. J
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.
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. 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 I
4 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 electrically 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. t
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, J
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. 1
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. J
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.
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 I
9 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 J:
10 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. i
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 J
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.