US3406524A - Fluid-sonic pile driving - Google Patents

Description

Oct. 22, 1968 K. A. BLENKARN ET AL FLUID-SONIC FILE DRIVING Filed May 5, 1967 PRESSURE VELOCITY ARTHUR LUBINSKI PRESSURE OR VELOCITY 25 FIG. 3 KENNETH A BLENKARN INVENTORS BY PM ATTORNEY.

United States PatentO ABSTRACT OF THE DISCLOSURE In the present invention a column of liquid (ordinarily water) is confined within the pile and extends substantially to its bottom. Sonic pressure is applied to thiscolumn, preferably setting up a standing quarter wave which applies alternating stress hydraulically at the bottom of the pile. This alternating pressure changes the transverse pile dimensions in the region of maximum interest, i.e., near the bottom of the pile. This markedly decreases skin friction and permits easier driving of the pile under any type of axial driving force.

Background of the invention Structures supported on piling must necessarily have a sufliciently strong foundation to furnish support under all expected-conditions of stress. This ordinarily requires driving piles (which are usually hollow steel columns) a great distance into the ground. For example in marine piling, to support off-shore platforms in the Gulf of Mexico it is not unusual for piles to be driven to depths exceeding 200 feet below the mud line, i.e., the bottom of the water. The greater the depth of driven piling in the ground, the greater is the resistance to fiexure, which again improves the foundation properties of the piling.

However, the deeper the pile is driven the greater is the resistance to the ground to further driving. This may be due to friction along the wall of the pile or to ground bearing resistance. In the last few years a method which has achieved some prominence in pile driving, particular- 1y-.in marine applications, involves applying a cyclic force to-the piling, using a vibratonThis vibrator is rigidly attached to the pile and, for example, may consist of two counter-rotating, eccentrically mounted masses. These are arranged to transmit longitudinal or axial vibration to the pile. The speed at which these are driven to the coupled motor is adjusted to mechanical resonance of the pile.

Under these conditions, the applied force induces a pressure which is a maximum at or near the base of the pile, the pressure and the velocity changing in approximately a sinusoidal manner with distance up the pile. Such ar--.

rangement is-described, among many other places, .in US. Patent 2,350,212 of A.-G. Bodine, Jr. Asa result of this mechanical resonance, large longitudinalor axial forces are induced between the pile and ,the soil which tends to make thepile penetrate into the soil more easily than in the more conventional driving in which impacts are imparted to the pile at much lower equivalent frequenoies. I 1 g One .major difliculty which has occurred with this technique is that when any such mechanically resonating sy s the mud line. Accordingly, the mainaxial vibratory action is thus transmitted to the portion of the pile located rather close to the mud line, rather than close to the bottom of the pile Where it is most needed. On dry land also,'vibratory action is essentially limited to the upper part of the pile, in the loosened soil region. This has been shown experimentally in that it is relatively easy to drive piles by this sonic resonant system for a first, limited region,

but becomes progressively more difiicult as the depth of pile in intimate contact with the ground increases, especially in such soils as clays which display a strongresistance along the wall of the pile.

.Summary of the invention As shown immediately above, a major problem in sonic resonant driving of piles is to overcome frictional effects of the portion of the pile in intimate contact with the ground, i.e., the lowermost portion of the pile. Stated another way, it is necessary to minimize the very appreciable frictional forces in this region due to the presence of the ground, until the pile has been driven to the desired depth. We have found it is possible to overcome this difiiculty by providing Within the pile a column of a liquid having low clamping characteristics. In most cases this liquid is water. The column extends longitudinally through the pile such that it is in contact, and hence in pressureexchange relationship, with the outer walls of the pile in the region where intimate ground-pile contact exists; namely, in the lowermost portion of the pile. A cyclic stress is applied to the upper part of this liquid column at Y least approximately at a resonant longitudinal frequency of the column. Since there is little damping in this column, this produces an alternating pressure within the column which is a maximum at the bottom of the column and which varies approximately sinusoidally above this point. Thus, at the bottom of the pile there is maximum alternating pressure and zero velocity, whereas, at points above the bottom, the alternating component of pressure decreases approximately sinusoidally while the velocity increases. We ordinarily prefer to apply alternating flow at the upper end of the column at a frequency which will result in a longitudinal standing wave of a quarter wave length, so that the alternating pressure at the top of the column is very low, while the velocity is the highest. This is accomplished most easily by closing the top of the confined liquid column by a plunger or piston in contact with the water. This piston is actuated by any sort of mechanical vibrator which produces pressure in the liquid of a substantially sinusoidal nature and of a frequency which can be adjusted to make the column resonate, as stated above.

The alternating component of the pressure in the liquid column is, of course, transmitted to the confining walls which in this case means the wall of the pile which is in contact with the ground, at least below the mud line in the marine case. Application of this alternating pressure to the pile causes the transverse dimensions of the pile to dilate. Thus, this alternating pressure produces alternating radial deformation of the pile, which we have found results in considerable decrease of the skin friction between the pile and the contacting ground. This beneficial effect is strongest close to the bottom of the pile, where the soil compaction is the greatest, due to overburden. Accordingly, any downward force applied to the pile will cause it to penetrate the ground muchmore easily than when this resonating liquid column is not used.

The alternating pressures caused by the resonating liquid column'also act on the bottom of the pile and thus transmit an alternating force to the soil immediately below the pile. In numerous cases, this force is sufiicient to overcome the bearing resistance of the soil, especially in clays since their bearing capacity is generally small. Bearing capacity in the case of sands is considerably greater and in this case, the force transmitted to the bottom of the pile and hence to the soil due to the resonating liquid column may be insufiicient. In such case, it is necessary to add an additional longitudinal force to the pile to cause it to penetrate. As explained above, such longitudinal force will be much more effective due to the pulsating walls of the pile. This force may be a steady downward force, simply by adding suitable weight to the top of the pile. However, we prefer to apply longitudinal downward force sonically, in accordance with the systems already so used. In this case, an additional longitudinal shaking machine, which may be of the same type employed in resonating the liquid column, is applied at or near the top of the pile, for example by a separate shaking machine. Since the use of the resonating liquid column has greatly decreased the skin friction along the pile wall, the energy dissipated in thesystem driving the pile is decreased, which increases the mechanical Q of this system and greatly increases the effectiveness of this separate drive.

Sometimes, a pile is jetted during the time that it is being driven. That is to say, liquid is forced into the soil immediately below the pile, of sufficient quantity and velocity so that it considerably decreases the bearing capacity of the ground during driving. This can be very advantageously employed in conjunction with our invention. It is simply necessary to provide a restricted opening at the bottom of the liquid column through the bottom of the pile. Liquid is thus injected into the ground immediately below the pile under both the steady hydraulic head of the liquid column and the superimposed alternating pressure provided by the shaking machine acting on the piston closing the liquid column. This stream of liquid flowing into the ground below the pile is replaced by suitable addition of liquid at the upper portion of the pile so that the driving piston is in contact with the water column during the driving period.

It is not necessary that the entire bottom part of the pile be closed off (except for any jetting orifices provided) in order to achieve the advantages of this system. The pile can be made, for example, with hollow walls and the piston in this case is annular. In this case, the pile appears externally to be a thick-walled annular cylinder. It is in contact with the ground on both the outer and inner surfaces, as well as the bottom.

Brief description of the drawings This invention is illustrated by the accompanying drawings which form a part of the specification. In these drawings the same reference number in several views represents the same or a corresponding part:

FIGURE 1 shOWS in diagrammatic cross section a single cylindrical pile being driven in accordance with this invention in a marine location.

FIGURE 2 represents in graphical form the pressure and velocity of the alternating components of the stress in the liquid column shown in FIGURE 1.

FIGURE 3 shows a cross section of a second form of the invention applied to a double-walled pile.

Description of the preferred embodiments In FIGURE 1 is shown a cross section of a long tubular pile 11 which is being driven into the ground 12 below the mud line 13 in a marine location. The surface of the water 14 is above this mud line. The tubular pile is fitted with a bottom closure 15 which in this case may be simply a steel disc welded across the otherwise cylindrical opening at.the .bottom of the pile. The confined space defined by the pile 11, including its bottom 15, is filled with a column of liquid 16 (ordinarily water) which extends up to a level near the top of pile 11. Immediately resting on this liquid column 16 is a plunger or piston 17 which closely fits the inner surface of the pile 11. Preferably this piston 17 contains one or more seals 18 which may be chevron packing, O-rings (as shown), etc., to effect a substantially fluid-tight seal, preventing the liquid in column 16 from escaping from the system. Mounted on this piston 17 is a vibrator 19, capable of providing cyclic longitudinal motion to the piston 17, and hence, to the top of the column of liquid 16. Such vibrators per se are not novel. The one shown involves two weights 20 cccentrically but symmetrically mounted on a pair of meshed gears 21, rotated by an engine (not shown). The centrifugal force due to the rotation of the weights 20, produces a resulta'nt substantially sinusoidal force aligned essentially along the axis of pile 11, i.e., longitudinally. It is to be understood that any other form of shaking machine which can impart a cyclic longitudinal pressure through piston 17 to the top of column 16 may be employed. The frequency of the cyclic variation in stress should be adjusted so that this liquid column is in resonance longitudinally. In nearly all cases, this frequency will be chosen as the lowest one in which there is quarter wave resonance in the column.

FIGURE 2 shows a plot of the amplitude of the alternating components of pressure (line 22) and velocity (line 23) along the liquid column 16 of FIGURE 1. Since there is very little escape of liquid from the column, the velocity is essentially zero at the bottom of pile 11 and the pressure is a maximum. This peak pressure will decrease sinusoidally as the distance 1 fromthe bottom of the pile increases, and correspondingly, the peak velocity will increase sinusoidally. If the column 16 is resonated at a quarter wave length, the frequency of the cyclic vibration applied to the piston 17 is given by the equation F V/ 4L where V is the velocity of compressional waves in the liquid (approximately 5,000 feet/second in water), L is the length of the liquid column 16, and F is the applied frequency in hertz. The alternating component of pressure exerted against the walls of pile 11 alternately increases and decreases the pile diameter, i.e., dilates the pile. As stated above, since the column of liquid 16 is not subjected to appreciable damping, the pressure applied to dilate the pile walls is substantially independent of the length of pile below the mud line, i.e., below the surface of the ground, regardless of the frictional resistance of the material of which this ground is composed. Accordingly, there is radial deformation of the cylindrical pile -wall and decrease in the skin friction between the ground 12 and the pile 11. The greater the alternating component of pressure in the section of the pile in contact with the ground, the lower will be the frictional resistance of the pile as it is being driven, though this effect is by no means linear.

If jetting of the pile is to be carried out, a relatively small or constricted opening 24 is provided in the bottom closure 15 of the pile. This permits a small stream of the liquid to escape from column 16 through opening 24, as shown by the arrows in FIGURE 1. This stream of liquid tends to decrease thecohesion of the particles of the ground immediately below the pile and increase the ease of penetration.

To compensate for thhe stream escaping through the bottom of the pile 11, a makeup stream is provided through a flexible line 25, controlled by valve 26 and supplied by a pump (not shown).

To increase the speed of penetration beyond that furnished by the vibrator, as discussed above, an additional longitudinal driving force is preferably provided. Probably the simplest arrangement is shown in FIGURE 1, in which a massive cylinder 27 containing an inner abutment 28 is supported by that abutment on top of the pile 11. Thus its weight contributes extra downward driving force.

The pile is driven to a certain depth and then an additional top section is customarily Welded on to provide for additional driving. When the maximum depth has been achieved for the particular pile 11, as shown in FIGURE 1, weight 27 and the shaking equipment are removed, line 25 is disconnected and blanked off (preferably by welding), and an additional section is longitudinally mounted on pile 11, welded to it, the equipment replaced, and driving re-commenced.

FIGURE 3 shows a variation in theaformof the pile. It also illustrates driving the pile on land. This same argument can be equally well used in water-covered areas. 'In this case, the pile 11 is hollow-walled,-. consisting of an outer cylinder and an inner cylinder 29 which are preferably coaxial. These are connected by an annular closure member 30 welded to both the inner and outer members of the pile. In this case,- the-liquid column lfi is' also annular in shape, extending from the bottom of the pile to a level near its top. Resting on this liquid column 16' and closely fitting the inner and outer members of pile 11 is an annular piston 31 suitably provided with seals 32. This upper piston is shaken cyclically, longitudinally of the pile axis. The shaking machine in this case is not shown for clarity, but the direction of motion is indicated by the double arrow 33. In order to provide jetting, orifices 34 are driller in the bottom closure 30. In this case these are shown provided with check valves 35. This is simply exemplary of any suitably valving arrangement which can, therefore, govern the amount of liquid used for jetting purposes. This liquid is provided through a flexible line 25 from a pump 36 connected to a source of liquid.

In this case, a different arrangement is shown for applying additional longitudinal driving force to the pile. A collar 37 rests through abutment 38 on the top of pile 11 and is vibrated longitudinally by a shaking machine (not shown). The direction of motion of the vibration is shown by double arrow 39. This shaking machine may be identical to that used in driving annular piston 31, and in this case provides an additional alternating force which ordinarily will drive the pile more rapidly than addition of equivalent static force, as shown for example in FIGURE 1.

In some cases, it will be found preferable to drive both the annular piston 33 and the collar 37 by the same vibrating source which can be accomplished simply by increasing the length of column of liquid 16 until abutment 40 rests on piston 31 and abutment 38 makes contact with the top of pile 11 only during a portion of the stroke of a vibrator applied to collar 37. The advantage of this arrangement is its relative simplicity. Its inconvenience is that the frequency of the system can only be one value and since the speed of compressional waves through the liquid column 16 is not the same as through the pile material 11 (ordinarily, steel), use of .a single vibrating machine to drive both the annular piston 31 and the collar 37 will result in maximum effectiveness of the liquid column in radial deformation of the pile in the lower part thereof, but the most effective driving force will not be applied to the pile.

The alternating pressure maximum which takes place at the bottom of the liquid column 16 produces stress in the pile wall which must be maintained within the elastic limit of the material forming this wall. It is simply necessary to compute the hoop stress and determine from this what maximum allowable fluid pressure can be used.

An example will further illustrate some of the quantities taken into account in this arrangement for pile driving. Assume a pile made of steel, approximately 30 inches in diameter with a wall thickness of 1 inch, thearrangement being as shown in FIGURE 1. Assume a length of water column of 150 feet. For this pile a convenient allowable fluid pressure is 100 psi. For a quarter wave resonant condition, the resonant frequency is approximately 8 hertz. To achieve the necessary pressure at the bottom, an amplitude of 0.38 inch is applied sinusoidally to the piston, that is, the total piston stroke is twice this figure, or 0.76 inch. The peak force on the piston is 5.2 pounds and the corresponding average power expended in this sinusoidal motion of the piston is 0.0075 horse power. The peak velocity orfiow of the liquid at the top of the column is approximately 1.6 ft./sec. It is seen that a relatively insignificant amount of power is used in this shaking machine to minimize frictional effects onwalls the pile and improving its driving characteristics. A pressure sensor at the'bottom of the pile in the water pears obvious, it has not been illustrated in the drawings.

Such a sensor can, if desire'cl, be usedto controlthe amplitude, or the frequency, or'both, of the shaking Illachine used at the top of the column to apply this alternating pressure.

It should be emphasized that any type of driving source can be employed with this invention. Those illustrated are but two out of the several immediately available. For example, if desired, the arrangement shown in co-pending patent application Ser. No. 504,609 Lubinski offers another means for driving, as does the conventional impacting source known for many years.

It is apparent that variations from the forms of apparatus and the precise steps of the methods given will be apparent to those skilled in this art. The essential novelty still lies in the incorporation in the pile of a liquid column driven substantially under resonant conditions to change cyclically the pile dimensions and minimize skin friction of the ground to the pile during driving, substantially as provided by the appended claims.

We claim: 1. A method of driving a pile comprising the steps of confining a column of liquid within said pile extending substantially to the bottom thereof, said column being in pressure-exchange relation with the outer walls of the pile below its contact with the ground,

applying cyclic flow to the upper part of said liquid column at least approximately at a resonant longitudinal frequency of said column to produce cyclic dilation of said pile below said contact with the ground, and

applying force axially to said pile.

2. A method in accordance with claim 1 in which said flow is substantially sinusoidal and of such a frequency that the alternating pressure at the bottom of saidc'olumn is essentially greater than the pressure at any point above said bottom.

3. A method in accordance with claim 2 in which said force includes both a downward component and a cyclic component, and in which said column is at least approximately in quarter wave resonance.

4. A method in accordance with claim 3 in which said cyclic component of said force is at least approximately at the same frequency as the frequency of said sinusoidal stress.

5. A method in accordance with claim 2 including hydraulic jetting of the ground immediately below said pile.

6. Apparatus useful in pile driving comprising a pile confining a liquid column extending substantially to the bottom of said pile, at least the lower portion of said column being confined by the outer wall of said pile,

a vertically movable piston on top of said column and in contact therewith,

means for applying cyclic force to said piston at least approximately at substantially a quarter-Wave resonant longitudinal frequency of said column to produce radial deformation of said pile below the earths surface, and

separate means for applying axial force to said pile.

7. Apparatus in accordance with claim 6 in which said pile is hollow with a closure at the bottom and is filled with said liquid.

8. Apparatus in accordance with claim 6 in which said pile has hollow walls defining an annular volume closed at the bottom and filled with said liquid. 7

L 9. Apparatus in accordance with claim 6 in which said column is in liquid communication with the ground immediately below said pile through a constricted opening in the bottom of said pile, whereby jetting action occurs at said bottom of said pile, and in which means are provided for adding liquid to said column to maintain its height as said jetting action takes place.

10. Apparatus in accordance with claim 6 in which 1 said means for applying axial force provides both a downward component and a cyclic component.

References Cited UNITED STATES PATENTS 3,289,774 12/1966 Bodine 17519 FOREIGN PATENTS 280,033- 5/1913 Germany. 408,590 1/ 1925 Germany. 741,833 12/1955 Great Britain.

JACOB SHAPIRO, Primary. Examiner.

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