US3808820A

US3808820A - Pile driving utilizing standing wave vibrations

Info

Publication number: US3808820A
Application number: US00293479A
Authority: US
Inventors: A Bodine
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-09-29
Filing date: 1972-09-29
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07

Abstract

A pile is driven into the earth by means of vibrational energy which is applied to the pile near one end thereof, at a frequency such as to set up resonant standing wave vibration in the pile. Means are provided to assure that the standing wave pattern has a velocity node shifted toward the driving end thereof and a velocity antinode near the driven end thereof. This end result is achieved by first driving the pile into the ground a short distance with a limited amount of vibrational force and with limited bias force being applied by the pile against the earth. When the pile has penetrated to a depth where lateral vibrations thereof are damped by the earthen material, the vibrational drive power is increased to cause the standing wave vibration with velocity nodes and antinodes as mentioned above.

Description

22 Filed:

United States Patent [191 Bodi ne [54] PILE DRIVING UTILIZING STANDING WAVE VIBRATIONS [76] Inventor: Albert G. Bodine, 7877 Woodley Ave., Van Nuys, Calif. 91406 FORElGN PATENTS OR APPLICATIONS 387,473 2/1933 Great Britain 173 49 [451 May 7,1974

Primary Examiner-Jacob Shapiro Attorney, Agent, or FirmEdward A.'Sokolski 57] ABS'IRACT A pile is driven into the earth by means of vibrational energy which is applied to the pile near one end thereof, at a frequency such as to set up resonant standing wave vibration in the pile. Means are provided to assure that the standing wave pattern has a velocity node shifted toward the driving end thereof and a velocity antinode near the driven end thereof. This end result is achieved by first driving the pile into the ground a short distance with a limited amount of vibrational force and with limited bias force being applied by the pile against the earth. When the pile has penetrated to a depth where lateral vibrations thereof are damped by the earthen material, the vibrational drive power is increased to cause the standing wave vibration with velocity nodes and antinodes as mentioned above.

3 Claims, 2 Drawing Figures PILE DRIVING UTILIZING STANDING WAVE VIBRATIONS I This invention relates to the use of vibrational energy in driving piles, and more particularly to a pile driving method and apparatus in which an optimum standing wave vibration pattern is developed.

In my U.S. Pat. No. 2,975,846, an acoustical technique and apparatus for driving piles is described in which a pile is vibrationally elastically driven by means of an orbiting mass oscillator at a frequency which sets up a resonant standing wave vibration pattern in the pile. In this device, the vibrational energy is applied to the pile such that a velocity antinode of vibration, i.e., a region of maximum velocity and amplitude of vibration, appears near the upper end of the pile, the driving end of the pile generally being biased strongly against the earth so that there is very often a velocity node at or near this position. Thus, in the device of my US. Pat.

No. 2,975,846, the top end of the pile elastically ,vi-

brates with a relatively large stroke, while the bottom end thereof typically has a relatively small vibrational stroke.

It has been found that there are several disadvantages to operating a pile driver with this type of vibrational pattern. First, the high velocity vibration in the higher portions of the pile fully exposed to the air tend to generate a good deal of noise which is obviously undesirable. Further, the high amplitude vibration at the upper end of the pile tends to cause considerable vibration of the surrounding structures from which the pile is subtended, as well as in the oscillator drive and isolation mechanism, resulting in wear and tear thereon and the undesirable dissipation of vibrational energy. Further, where it is sometimes desired to use high impedance oscillators, such as those of the piezoceramic type, for driving the pile it is easier. to match this type of vibrational source to a high impedance point in the standing wave vibrational pattern rather than to the high velocity, low impedance afforded by a velocity anitnode. It is thus desirable to have a velocity node near the top end of the pile where the oscillator can be conveniently coupled thereto.

The technique and apparatus of this invention overcomes the aforementioned shortcomings of the prior art by making the wave pattern in the pile to be exactly opposite to that of my aforementioned patent, that is, by making the velocity node (or pseudo node) appear above the center of the pile with the top end of the pile moving elastically with only a small stroke, and the bottom end of the pile moving elastically with a large stroke. It is to be noted that in this type of system, the pile itself does not move up and down bodily but rather it shortens and lengthens elastically with the bottom end shortening and lengthening appreciably more than the top portion does. In this manner, the higher up portions of the pile which would tend to radiate sound most readily, have relatively low amplitude vibration and thus the noise is significantly lessened. Also, this has the additional advantages of lessening wear and tear on the parts directly coupled to the pile in view of the lower velocity vibration, and facilitating the coupling of the pile to a high impedance vibrational energy source.

It is therefore an object of this invention to reduce the noise involved in vibrationally driving piles into the earth.

It is another object of this invention to facilitate the installation of pile members in earthen formations.

It is still a further object of this invention to lessen the wear and tear and dissipation of energy in members utilized in conjunction with sonic pile drivers.

It is still a further object of this invention to facilitate the coupling of a high impedance vibrational driver to a pile member to be vibrationally driven into the earth.

Other objects of this invention will become apparent from the following description taken in connection with the accompanying drawings, of which:

FIG. 1 is an elevational view illustrating the practice of the technique of the invention, and

FIG. 2 is an elevational view partially in cross section illustrating an embodiment of the invention utilizing an electroacoustic sonic energy source.

It has been found mosthelpful in analyzing the operation of the device of the invention to analogize the acoustically vibrating circuit involved to an equivalent electrical circuit. This sort of approach to analysis is well known to those skilled in the art and is described, for example, in Chapter 2 of Sonics by Hueter arid Bolt, published in 1955 by John Wiley and Sons. In making such an analogy, force, F, is equated with electrical voltage, E; velocity of vibration, u, is equated with electricalcurrent, i; mechanical compliance, C,,,, is equated with electrical capacitance, C,.; mass, M, is equated with electrical inductance, L; mechanical resistance (friction), R,,,, is equated with electrical resistance, R; and mechanical impedance, Z,,,, is equated with electrical impedance, Z,..

Thus, it can be shown that if a member is elastically vibrated by means of an acoustical sinusoidal force, F sinwt (to being equal to 271' times the frequency of vibration), that Where mm is equal to (l/wC a resonant condition exists, and the effective mechanical impedance, Z,,,, is equal to the mechanical resistance, R the reactive impedance components wM and l lwC cancelling each other out. Under such a resonant condition, velocity of vibration, u, is at a maximum, power factor is unity, and energy is most effici'ently delivered to a load to which the resonant system may be coupled.

Just as in electrical circuitry, maximum acoustical energy can be transferred where a good impedance match exists, i.e., where the two elements between which the energy transfer occurs have like impedances. This fact becomes significant in the device and method of this invention in the transfer of energy from the driving end of a mandrel, casing or piling to the earthen formation in effecting the driving operations. Thus, in view of the resistive impedance characteristics of earthen formations it is desirable that the driving end of the piling exhibit corresponding good impedance characteristics with large stroke to assure maximum transfer of energy in fluidizing the formation.

It is to be noted that in the device of this invention the mass and compliance for forming the elastically vibrating system are furnished by the structural members of such system themselves such that the earthen formation is not incorporated in such system. The earth under such conditions acts as a friction load which provides no significant reactive components. This results in a random vibration of the earthen particles near to the pile, rather than a lumped coherent vibration, with a considerable relative motion between the separate grains. The net results is a high degree of fluidization' of the earthen structure with a thixotropic effect facilitating the penetration of the driven member and the eventual compaction of such structure around such member as the sonic energization is smoothly terminated.

It is also important to note the significance of the attainment of high acoustical Q" in the casing 'member being driven, to increase the efficiency of the vibration thereof and to provide a maximum amount of energy for the driving operation. As for an equivalent electrical circuit, the of an acoustically vibrating circuit is defined as the ratio of the energy stored in each elastic vibration cycle to the energy used in each such cycle. Q is mathematically equated to the ratio between wM and wR,,,. Thus, the effective Q of the vibrating circuit can be maximized to make for highly efficient, high amplitude vibration by minimizing the effect of friction in the circuit and/or maximizing the effect of mass in such circuit.

Briefly described, the invention is as follows: A sonic vibration generator is coupled to a pile to be driven into the earth near one end thereof. The opposite end of the pile is placed against the earth with a limited amount of bias force between it and the earthen material. The oscillator is then energized to drive the pile with a limited amount of vibrational force to cause the end of the pile to penetrate into the earth a short distance below the surface to a position where lateral vibrations of the pile are effectively damped by the fluidized earthen material surrounding the pile. The oscillator is then driven to provide high vibrational energy to the pile and to cause longitudinal resonant standing wave vibration thereof with a velocity node of the standing wave vibration pattern above the center of the pile and a velocity antinode near the driving end of the pile. This end result can be achieved in several manners, including the adjustment of the vibration frequency and/or the coupling of a relatively large mass to the top of the pile to provide a high impedance load thereat. The pile is thus driven to the desired depth with high velocity and displacement vibration in the standing wave pattern appearing primarily in the portions of the pile in the earth or close to the surface thereof so that noise and the transfer of high amplitude vibrational energy to surrounding parts of the pile rig are minimized.

Referring now to FIG. 1, a first embodiment of the device of the invention is illustrated as being utilized in performing the operation of the method of the invention.

The device of the invention is utilized to drive a pile in earthen formation 11 in the following manner: With pile being tightly held by means of clamp12, the entire assembly is lifted into position for driving by suitable means, such as a crane assembly A which operates cables 18. With pile l0 vertically positioned with its end portion lightly abutting against the earthen formation to be penetrated, oscillator 12 is first turned on so as to .cause low power rotation of its eccentrically weighted rotor members as described inconnection with my US. Pat. No. 2,976,846. It is to be noted that in the event that a hollow pile is used, the end portion thereof should sometimes be capped to prevent the entry of unconsolidated earthen material therein.

The rotors are position phased as indicated in the aforesaid US. Pat. No. 2,975,846, and when rotatedin opposite directions produce vibrational forces that are additive longitudinally and which effectively cancel each other out insofar as transverse vibration is concerned. Pile 10 is thus longitudinally excited in accordance with this invention at a frequency determined by the speed of rotation of the rotors. When the end of the pile has penetrated into the earth a short distance, such that lateral vibrations thereof are damped out by the fluidized earthen material, this rotation speed is adjusted so as to produce increased vibration of pile 10 with a standing wave as indicated by graph lines 68 being set up therealong. As indicated by graph lines 68, maximum resonant vibrational activity is provided at the lower portion of pile 10 where it is most needed to accomplish the driving operation. Frequency adjustment for this new wave pattern can be further aided by having substantial mass in element 12.

Pile 10 is preferably fabricated of a material such as steel having relatively high Q characteristics to make for a resonant system having optimum efficiency.

During this operation, the earthen particles are fluidized and compacted so that they in effect stand back from the pile permitting relatively easy penetration. In view of the fact that the elastic vibration system is contained fully within the pile and the associated components have the wave pattern shown, the earthen material being penetrated tends to act as a resistive load iwth the particles being caused to vibrate randomly rather than as a unitary mass. It is believed that each of the individual irregular grains when energized by the sonic energy in this fashion separately vibrates in a random path with a relatively fixed radius of vibration which changes in direction but remains fixed in magnitude. Such random vibration effectively separates the irregular particles so that they do not adhere to each other and are thuskept in a highly fluid or thixotropic condition. This high degree of fluidity, with the large amplitude at the bottom of the pile as indicated by 68, not only greatly eases the penetration of the pile member but also conditions the soil surrounding the pile so that highly effective lodgment of the casing or pile member driven therein is possible.

The pile can be effectively driven if it is excited by elastic vibration within the pile itself, again with the characteristic wave pattern having more amplitude at the penetrating end than it has at the top. This special kind of pattern has the node (or pseudo node) elevated up high, above the center, so that the lower active portion is more than one half the total pile length. In this way the major important portion, the lower portion, is continually active. A very desirable fast penetration can be accomplished in this way.

Another advantage from my invention is that this particular wave pattern has a large acoustic impedance at the top, which is greater than the acoustic impedance at the bottom end. Fortunately the large acoustic impedance at the top end can be accomplished by this standing wave pattern, without making it necessary to taper the pile to get large acoustic impedance. The pile thus can be a straight uniform column.

By having this large acoustic impedance at thetop end I am then able to provide good impedance matching with a high impedance electroacoustic transducer,

which is useful in certain applications. Examples of such high impedance transducers are magnetostriction metals which vibrate from a pulsating electromagnetic field, or better even than this, there are electroacoustic transducers that are made from electrostrictive materials such as lead-zirconate-titanate which vibrate in response to a pulsating electric voltage.

Referring to FIG. 2, we see the elastic pile 70 with electroacoustic transducer 72 firmly connected to the top of the pile by means of hydraulic clamp 74. A heavy mass 76 on the other side of the electroacoustic transducer 72, as well as clamp structure 74, helps to provide the desired wave pattern by providing a high impedance at the top of the pile 70. Electric power source 75 provides the pulsating electric power to drive the electroacoustic transducer 72.

The feature of this invention which provides the very surprising and beneficial result comes from the frequency of the electricity from the power source 75. This frequency is chosen to generate a standing elastic wave pattern in the pile, with the nodal region N above the center, as shown by wave pattern graph 78.

As will be noted from the figure, at the operating frequency of my invention the pattern has more amplitude at the bottom than at the top. Therefore it has greater impedance at the top. This matches up nicely with the electroacoustic transducer 72, as well as having the other benefits of high performance and low noise mentioned hereinabove. I I

I claim:

1. A method of driving a pile member into the earth comprising the steps of:

placing one end of the pile against the surface of the earth with a limited amount of bias force thereagainst so that the earth will present only a moderate impedance load when the pile is subsequently driven,

driving said pile with a limited amount of vibrational force by means ofa sonic oscillator until said one end of the pile has penetrated into the earth a short distance at which lateral vibrations of the pile are damped by the earth,

operating said oscillator at a frequency such as to cause resonant standing wave vibration of the pile with a velocity antinode of said standing wave appearing near said one end of the pile and a velocity node thereof above the center so as to be more near the other end therof, and

increasing the power to said oscillator to effect the penetration of the pile into the earth to a desired depth.

2. The method of claim 1 and further including the step of attaching a member having substantial mass to the other end of said pile to facilitate the formation of a velocity node near said other end.

3. A system for use in driving a pile or the like into the earth comprising:

a column member having elastic compliance in its longitudinal dimension,

guide and support means connected to said column .member for orienting said column member for longitudinal penetration of one end thereof progressively into the earth, said guide and support means controlling the bias force of the column member against the earth,

a substantial mass connected to said column at the other end thereof,

a vibration generator connected to said column, and

means for driving said vibration generator at a longitudinal resonant frequency of said column so as to set up a standing vibration wave pattern in said column at said frequency, said standing wave having a nodal region closer to said other end of the column than the length ofa quarter wavelength of the standing wave pattern.

Claims

1. A method of driving a pile member into the earth comprising the steps of: placing one end of the pile against the surface of the earth with a limited amount of bias force thereagainst so that the earth will present only a moderate impedance load when the pile is subsequently driven, driving said pile with a limited amount of vibrational force by means of a sonic oscillator until said one end of the pile has penetrated into the earth a short distance at which lateral vibrations of the pile are damped by the earth, operating said oscillator at a frequency such as to cause resonant standing wave vibration of the pile with a velocity antinode of said standing wave appearing near said one end of the pile and a velocity node thereof above the center so as to be more near the other end therof, and increasing the power to said oscillator to effect the penetration of the pile into the earth to a desired depth.

3. A system for use in driving a pile or the like into the earth comprising: a column member having elastic compliance in its longitudinal dimension, guide and support means connected to said column member for orienting said column member for longitudinal penetration of one end thereof progressively into the earth, said guide and support means controlling the bias force of the column member against the earth, a substantial mass connected to said column at the other end thereof, a vibration generator connected to said column, and means for driving said vibration generator at a longitudinal resonant frequency of said column so as to set up a standing vibration wave pattern in said column at said frequency, said standing wave having a nodal region closer to said other end of the column than the length of a quarter wavelength of the standing wave pattern.