US3498388A - Pile driving system - Google Patents

Description

March 3, 1970 A. lovls PILE DRIVING SYSTEM 2 Sheets-Sheet 1 Filed Dec. 5, 1967 March 3, 1970 A. Jovls 3,498,388

n PILE DRIVING SYSTEM Filed Dec. 5. 1967 2 sheets-sheet 2 7 FWZ AOOELEPOMETEP OUTPUT United States Patent O 3,498,388 PILE DRIVING SYSTEM Arthur Jovis, New York, N.Y. (1501 Underclitf Ave., Bronx, N.Y. 10453) Filed Dec. 5, 1967, Ser. N0. 688,184 Int. Cl. E21c 3/24; E02d 5/34; G01n 3/42 U.S. Cl. 173-2 9 Claims ABSTRACT OF THE DISCLOSURE System: automatically controlling the pile driver, utilizing Engineering News formula 2E (P S-i-C marking the penetration per hammer blow on the pile; and keeping a permanent record of operation.

This invention relates to a system including methods and apparatus to control the operation of steam or compressed air (fluid driven) pile driving hammers.

Among the principal objects of the present invention are the provision of structure for:

l) Maintaining at a constant rate the energy delivered to the pile by the pile hammer.

(2) De-activating the pile hammer when the pile has been driven to a pre-determined driving resistance; such driving resistance being a measure of the load bearing capacity of the pile.

(3) Providing a tangible and permanent record of the magnitude of successive pile displacements, and hence of the bearing capacity of the pile.

(4) Maintaining uniformity of settlement of each pile foundation in relation to the other pile foundations in the same project. Such settlement occurs when the superimposed load is applied to the pile foundation.

When a pile is driven into the earth varying factors which affect the load bearing capacity of the pile are met with. Mostly these factors are due to differing types of soil, variations in successive earth strata, and generally the total frictional resistance to the penetration of the pile into the earth. These various factors express themselves as a relation of the axial movement of the pile to the energy supplied to the pile by the pile hammer.

Various formulae have been proposed to determine the load carrying capacity of piles, but with the exception of the Engineering News formula (which directly relates energy to axial movement) they have been found impractical and have been discarded.

The Engineering News formula S-i-C was proposed by A. M. Wellington in 1888. While it is partly empirical, it nevertheless has stood the test of time, and has been almost universally adopted by the engineering profession.

In the formula:

In structural engineering, it is the practice to design a group of piles (known as a pile cluster) to support a known load, and to assign to each pile in the cluster its proportional share of the total load.

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From examination of borings, determination of test pile loads, type and size of pile etc., the engineer establishes an estimate of the safe load capacity of a pile. When this value is assigned to P in the Engineering News formula all factors in the formula, with the exception of S, become constants. Solving then for S We find:

Having previously established a value of P for a specific project and knowing the value of E for the speed and particular lmake and model hammer used in the operation, the field engineer will observe the pile driving operation and when the desired value of S (i.e. the pile penetration per hammer blow) is reached, the driving is stopped.

Driving a pile to a greater penetration per blow than prescribed (i.e. over-driving) can cause damage or crippling of the pile; driving to a lesser value of S will not develop the required bearing value of the pile and may result in undue settlement or even failure of the pile.

Any foundation not resting directly on bed rock will have some degree of primary settlement, as the load of the structure is applied. This settlement is unavoidable and is not harmful provided that it is self-limiting and that the amount and rate of settlement is uniform throughout the structure; otherwise serious damage to, or even failure of, the superstructure can result.

In the case of pile foundations, the primary settlement of the foundation units will be in direct relation to the values of S in the formula. In order therefore, to maintain the required uniformity of settlement it is important that during the driving operation the final average of the penetration of the pile per blow of the hammer should be in conformity with the computed value of S for all the piles comprising the substructure. It is one of the objects of this invention to provide such required uniformity of penetration per blow, and thus provide a uniformity of structural settlement.

When the Engineering News formula first came into use, the drop hammer operating at a rate of l0 to 20 blows per minute was the only type of pile driver in general use, and it was then a simple matter for the eld engineer to determine the value of the component factors in the formula, and also to determine by observation and experience at which point to stop driving the pile so as to achieve the two partly conflicting objectives of developing the optimum load bearing capacity of the pile, while avoiding over-driving and resulting damage to the pile.

The introduction and development of the uid driven pile hammer operating at a rate of to 275 or more blows per minute, has introduced the problem of operational control.

Due to the speed at which a modern power hammer operates, it is virtually impossible for the field engineer to simultaneously:

(a) Determine at what point the pile has been driven to the required optimum bearing capacity and to deactivate the driving at this point;

(b) Maintain energy supplied by the pile 4hammer to the pile at a constant value; and

(c) Count the number of blows per minutes struck by the pile hammer and thereby determine (as explained hereinafter) the value of E in the formula.

The energy delivered bya fluid driven pile hammer is a function not only of the weight of the hammer and its length of travel (which factors are constant for a given hammer) but it is also a function of the mean effective pressure acting upon the piston in the hammer drive. Calculations based on steam or air pressure are misleading because no two set-ups are identical and it is impractical, if not impossible, to determine the mean effective pressure in the working cylinder from the pressure shown on the boiler gage. Even if this were not the case, variations in boiler pressure would prevent maintaining a constant rate of delivery of energy by the pile hammer.

Because of the aforementioned difficulties, manufacturers have conducted exhaustive tests using -highly -sophisticated test equipment and have carefully determined the energy delivered by the hammer at the point of impact, as a function of the operating rate of the hammer. The results of such tests have been tabulated by the respective manufacturers and are used in the eld to determine the value of E in the Engineering News formula.

The following is an excerpt from a table furnished by a manufacturer of pile driving hammers and states the value of E as a function of strokes per minute for a. particular model of this manufacturer.

If a constant rate of operation is maintained, and if this is accurately determined, the value of E in the formula can be established by the use of this table Without recourse to measurements of fluid pressure.

As the driving of a pile progresses the resistance to penetration increases; this results in an increase in the rebound of the hammer and therefore in its rate of operation.

It is apparent that unless some means is introduced for compensating for variations in operating rate resulting from variations in iiuid pressure and in elastic rebound, said E in the formula will not remain constant and that its proper value cannot be applied in the present state of the art.

A further object of this invention is to facilitate use of the Engineering News formula in connection with the operation of fluid driven pile hammers 'by making the values of E and S, as defined above, readily determined factors. This is accomplished by use of apparatus which makes E a constant (by holding the hammer at a xed operating speed) irrespective of changing operating conditions, and which provides a .means for accurately measuring S and uses the magnitude of S to de-activate the vpile hammer at the appropriate point. The control system involved in this invention attains these objectives and provides a permanent record of the factors E and S.

These objects and other incidental ends and advantages will more fully appear in the progress of this disclosure andy be pointed out in the appended claims.

In the drawings:

FIGURE 1 is a schematic block diagram of a preferred embodiment of the invention.

FIGURiE 2 is a graph of accelerometer output.

FIGURE 3 is a graph of rectifier output.

FIGURE 4 is a graph of monostable multivibrator output.

FIGURE 5 is a graph of lter output.

FIGURE 6 is a graph of iirst integrator output.

FIGURE 7 is a graph of change in multivibrator output in relation to hammer blow rate.

FIGURE 8 is a graph of pile penetration in relation to the present system.

FIGURE 9 is a fragmentary elevational view showing one form of pile penetration marking.

FIGURE 10 is a fragmentary elevational View Showing another form of pile penetration marking,

Turning to FIGURE l, this diagram is also a flow chart of the system, generally indicated by reference character 10. Elements 11 to 17 inclusive constitute the speed control while elements 18 through 23, cooperating with element 17, constitute the turn-off control and tangible recording means. Turning to the speed control, which in this system is synonymous with energy control, my control compares the actual rate of which the hammer 31 delivers blows to a pile 30 with a predetermined rate reference. An electronically controlled feed back advances or retards the energy supply throttle 17 so as to maintain a constant rate of operation of the hammer 31 as dictated by the reference signal, thereby holding constant the energy delivered to the pile by the pile hammer.

The system involves a means responsive to change in position of, or application of force, to the pile, so that at each blow of the pile hammer said means produces an output signal; said output signal is fed to a comparison means, which compares said output signal with reference signal corresponding to the desired operating speed, and produces an output related to the difference between reference signal and the signal from the means responsive to strokes of the pile hammer; said output of the comparison means is amplified to the extent necessary to drive a conventional pile hammer speed control means, typically an electrically controlled pressure throttle.

I have referred to the various means to be used, in general-terms. Thus the means responsive to changes in pile position or to application of pile hammer force, could be optical, mechanical, magnetic, electrostatic, and so forth. The output of such means could be compared with a reference by measuring phase shift or time lag between them,.by a digital comparator, and so forth; of course, different comparison means call for different appropriate reference means. I turn now to my preferred system, although I desire to claim my invention in the broader terms I have used above.

' My preferred system uses an accelerometer 11 as the force or position change responsive means because such a means is useful in connection with the other aspects of my control system described subsequently. My preferred comparison method, described below in more detail, uses the acelerometer 11 to drive electronic circuitry which produces as its output a slowly varying DC signal, the amplitude of which is proportional to the frequency with which the hammer 31 strikes the pile 30 (pile hammer frate); and then compares such DC signal with a reference DC signal proportional to the desired rate, by means of a different amplier 15.

'Ihose familiar with the art will recognize that the system is neither analog nor digital, but a hybrid of both systems. One reason I have preferred this partially digital control approach relates to another aspect of my invention. I have avoided tachometer and similar analog speed control systems that require additions to, or attachments to the main part of the drive system. My system minimally affects the main part of the hammer system and is thus more flexible than a system which would require extensive modification of existing qeuipment.

In FIGURE 1 a conventional accelerometer 11 is of the crystal, semiconductor, or magnetic type. It is preferably fastened to the pile 30 by bolting it into the drive pile cap 32, or other such means of attachment as will make it move in unison with the pile, with output shown in FIGURE 2. Rectifier 12 is a conventional circuit. Its input is the output of accelerometer 11 and it prepares the output of the accelerometer for use by the monostable multivibrator 13` by removing either the positive or negative portion of the signal (FIGURE 3) as appropriate for the type of multivibrator used. Rectifier 12 may also include a biasing means so that it passes only those signals of a specified polarity which exceed a specied threshold value. In conventional transistorized circuitry commer1 cially available at this time, the foregoing rectification and biasing are included as part of the package comprising the monostable multivibrator 13.

The monostable multivibrator 13, a conventional device which by responding to each input pulse delivered to it, in excess of a certain threshold value, produces at its output a substantially rectangular wave or pulse of fixed duration and fixed amplitude (FIGURE 4). The duration and amplitude are fixed by conventional design procedures, as is the threshold. It is customary to refer to the multivibrator as on during delivery of an output wave and off at other times, when it is quiescent. In the present system, the multivibrator is on immediately following each pile hammer blow; it remains on for a length of time determined by the designer of the circuit, and then returns to the off state. It is readily seen that for any given circuit the percentage of on time is dependent on the rate of operation of the hammer; the more frequently blows are delivered, the higher the percentage of time the multivibrator is on, and therefore the average value of the said output signal will be directly proportional to operating speed (see FIGURE 7). FIG- URE 7 is a graphic illustration of the increase in the average value of the output signal of the multivibrator 13 in relation to blows per second of the pile hammer 31, the dotted line 33 in FIGUR-E 7 indicating the average value of the output signal of the multivibrator 13.

In designing the multivibrator, caution must be taken that the duration of the output pulse selected be less than the minimum time possible between successive input pulses to the multivibrator. Otherwise there would be an overlapping of output pulses and the operation described here would be unsatisfactory. In the case of a pile hammer operating at 150 blows per minute, there is approximately 0.4 second between the beginning of successive strokes. Accordingly, multivibrator pulse duration must be less than 0.4 second; a suitable value would be about 0.2 second, making the multivibrator on about 50% of the time at 150 blows per minute.

The pulse (digital) output of multivibrator 13 is converted to a slowly varying DC (analog) signal by the filter 14, a conventional low-pass filter (FIGURE 5). This output signal will be proportional to on,time or hammer speed. The filter should have as short a time constant as possible without making the system unstable; this determination can be made by conventional servomechanism design techniques.

The output of filter 14 is one of the two inputs to the differential amplifier 15 of conventional type. The other input to amplifier 15 is the speed reference 16 a conventional reference circuit, such as the output from a precision potentiometer connected across a Zener diode, or standard reference cell. The setting on the potentiometer is adjusted by the field engineer to correspond to the desired operating speed (rate), as explained below. The difference amplifier produces an output which is proportional to the difference between its two inputs. By suitably proportioning the value of the signal from speed reference 16, the output of the difference amplifier will be proportioned to the difference between actual operating rate of the hammer and the desired rate.

For example, in the case referred to above where the hammer is assumed to operate at 150 blows per minute and the multivibrator 13 is designed to be on 50% of the time; if the amplitude of the multivibrator rectangular output wave (FIGURE 4) is 1 volt, the4 average value of the signal will be 0.5 volt. This will be the reference voltage, assuming a filter transfer characteristic of 100%. Any difference in the transfer characteristic will of course require corresponding adjustment in the reference voltage.

The operation of the circuit is illustrated by example. As the resistance to the driving of the pile increases, the hammer rebound also increases, causing an increase in the rate of operation of the hammer. Assuming a 5% increase in this rate of operation from 150 to 1571/2 blows per minute, the averaged signal will increase from 0.5 volt to 0.525 volt. The difference amplifier will then produce an output equal to .025 time its gain.

The output of the differential amplifier 15 is used to operate the pile hammer throttle. It is so connected to the throttle that the throttle opening is reduced if actual speed exceeds reference speed and the throttle opening is advanced if' actual speed falls below reference speed. The higher the gain of the amplifier the more sensitive the system is, but the greater the danger of instability. These two factors are accommodated by conventional design techniques.

I turn now to that aspect of the system which deals with the measurement of displacement and the use of such data to de-activate the pile hammer at an appropriate point. My method of control develops a signal proportional to S of the Engineering News formula and compares it with a reference proportional in the same ratio to the precalculated value of S, which when substituted into said formula will give the desired value of P, the predetermined load bearing capacity of the pile.

When S, as measured by my control system, falls to the value of S as precalculated, my system 10 closes the pile hammer throttle 17. This aspect of the system has four parts: (l) a displacement measuring means responsive to the magnitude of change in position of the pile or a function thereof (such as first or second derivative of position); (2) internal computation means receiving as its input the output of said displacement measuring means and capable of producing as an output a signal proportional to the sum or average of successive changes in the position of the pile for a specified number of successive displacements or during a specified time interval corresponding to such number of displacements; (3) a reference means capable of variation or adjustment to correspond to a predetermined value proportional to the value of'S which corresponds to the desired bearing strength; (4) a comparison means capable of comparing the magnitude of said reference and the output of said internal computation means, and capable of delivering suliicient output to actuate a means for stopping the operation of the pile hammer (e.g., a relay to close the throttle) when such comparison indicates equality ofthe two signals being compared.

I have referred to the various means to be used in general terms, as before. I turn now to my preferred system, although I desire to claim my invention in the broader terms I have used above.

Reference is made to FIGURE l. The accelerometer 11 is the same accelerometer previously used. Its output is integrated twice by first integrator 18 and second integrator 19 (or by a double integrator), so that the output of the first integrator 18 measures the velocity of the pile (see FIGURE 6) and the output of the second integrator 19 measures the position of the pile (since a=d2s/dt2).

See FIGURE 8 which is a graph of second integrator output, and strip chart and comparator input, and where horizontal dot-dash line 33 indicates the critical value of penetration or penetration rate reference, below which the required value of S is achieved, vertical line 34 is a scale of pile penetration in inches, and horizontal line 35 is a scale of hammer blow rate (time interval). At the peaks 36, 37 and 38 the gate 24 triggers reset to zero at vertical lines 46, 47 and 48 every N hammer blows. At peak 39 the gate 24 triggers to zero reset at line 49 and comparator 23 closes throttle 17 since 2NE P The output of the monostable multivibrator 13 is used to drive reset circuitry 20 used in conjunction with the integrators 18 and 19. The reset circuitry 20 serves several functions, some beneficial and some essential. In the first place, the reset circuitry resets the first integrator after every integration or after a specified number of blows; this is a rezeroing operation which improves the accuracy of integration and decreases the effect of drift in lower quality equipment. Second, the reset circuitry resets the second integrator after every nth integration, where N is the number of blows over which S of the Engineering News formula is to be averaged, e.g., 5, 8, 101, etc. (FIG- URE 8). The rezeroing also decreases drift in this integrator as a byproduct of its principal function. Third, the reset circuitry opens an electronic gate 24 and causes the following element, comparator 23 to operate after every nth blow just prior to the resetting of the second integrator to zero. This reset circuitry 20 is essentially a commercially available scale-of-ve or scale-of-eight, etc., counter.

The penetration reference 22 is a reference voltage, similar to the speed reference 16, proportional to the desired value of S. The comparator 23 compares the said reference with the sum of n displacement (equal to 11X the average displacement, S). When the gate 24 causes comparator 23 to compare S with the reference, comparator 23 does nothing if the actual S exceeds the reference value, but it turns off the throttle 17 if Sn equals or falls below the reference. Immediately after each comparison (unless the device is turned off), the reset circuitry 20 resets the integrators 18 and 19 for a new accumulation of S and new comparison.

The strip chart recorder 21 is a recording device, such as an oscillograph or strip chart recorder, which makes a permanent record of S for such future use as may be advisable or required by the conditions of operation.

I have indicated that the above system with elements 158, 19, and 24 as described, is my preferred embodiment. It should be noted that, when S is averaged over eight blows, it is possible for the hammer to deliver more blows than absolutely necessary, perhaps seven additional blows and probably three or four. These extra blows are not usually objectionable, although an undue number of them can in some circumstances damage the pile. An alternative system may be used which averages S continuously, instead of after every nth blow.

In the alternative system the gate 24 is eliminated along with its inputs and output, and the reset circuitry 20 serves only to eliminate undue drift. An additional gate 24 is connected with its input at the output of the first integrator 18 and its output at the input of the comparaor 23. The new element is a moving-time integrator or averager (Reference: Philbrick Researches, Inc., Applications Manual for Computing Amplifiers (2d ed. 1966), p. 76; Hansen, New Approaches to the Design of Active Filters, The Lightning Empiricist, 1965, vol. 13, pp. 11, 13 (Part I) whose output is em=input voltage T0=averaging time Such a circuit produces an output which at any instant is the definite integral of its input for the last To. seconds; this in turn is equal to the average input times the number of inputs during the interval of To. It is readily seen that such circuitry is capable of giving the instantaneous value of S (multiplied by a proportionality constant). In a system embodying such circuitry, no gating of the cornparator is necessary since the output of gate 24 is always S (times a constant), unlike the output of the second integrator 19 above, which is S (times a constant) only at the close of each cycle of n integrations, just before reset takes place.

Although the foregoing system permits instantaneous and continual comparison of actual S with the reference, it is not my preferred embodiment because the circuitry is more complex and the determination of appropriate constants more difficult. In some applications, however, the added complexity may be warranted.

Turning to FIGURE 9 and 101 alternative or supplemental recording means for the determination of the value of S (the penetration of the pile per blow of the hammer)v is provided by solenoidal activated markers 51 and 61 (FIGURES 9 and l0). The solenoids 52 and 62 are powered by the output of the monostable multivibrator 13 the signal being properly amplified or otherwise modified.

These markers when operated in conjunction with wood piles in the case of marker 51 has a sharply pointed armature 53 which is driven against the pile 30 co-ordiuately with each blow of the pile hammer 31 thereby producing an indentation 54 on the pile. Each indentation corresponds to one blow of the hammer, the space between each successive indentation corresponds to a value of S. T o obtain an average value of S for N blows, the total measurement taken from the last mark on the pile to the preceding (N-I-1)th mark divided by N gives the desired value of S.

Since the pointed marker is ineffective against a steel pile 40 a pressured paint spray 66 with the spray valve 65 activated by the armature 63 of the solenoid -62 (FIG- URE 10) is more suitable. This produces a paint mark 64 corresponding to each blow of the hammer. In all other respects the determination of the value of S is the same as for the pointed marker.

I wish it to be understood that I do not desire to be limited to the exact details shown and described for obvious modifications Will occur to a person skilled in the art to which the present invention relates.

I claim:

1. In a pile driving control system, having a throttle, the improvement comprising: governing means for maintaining a constant rate of hammer action connected to said throttle; and sensing means responsive to movement of the pile, and connected to said governing means, whereby the energy delivered by the hammer to the pile is maintained within closely predetermined limits.

2. Structure in accordance with claim 1, said sensing means including an accelerometer associated with said pile.

3. Structure as claimed in claim 1, in which the sensing means measures acceleration of the pile per hammer blow, and means to convert said acceleration measurement of the sensing means into a measurement of pile penetration (S of the Engineering News formula) per hammer blow.

4. Structure in accordance with claim 2, said sensing means being an accelerometer providing an output voltage in correlation to the pile movement, means providing a reference voltage, and a comparator; said accelerometer voltage and said reference voltage being fed to said comparator, the output of said comparator being connected to control said throttle, Awhereby when the voltage per pile movement is below the reference voltage the pile driver is deactivated.

5. Structure in accordance with claim 2 including means for visibly marking the pile at each blow, said marking means being actuated by said accelerometer.

6. Structure as claimed in claim 3 having a recorder and in which the output of the means to convert is `fed to said recorder to provide a tangible record of pile penetration per blow.

7. Structure as claimed in claim 3 having means to totalize the value of a predetermined number of said measurements of pile penetration; means to provide a penetration rate reference; means comparing said totalized value with said penetration rate reference; and when the totalized value is less than the reference value, to act on the throttle and thereby deactivate the pile driver, whereby in a plurality of piles, each pile having been driven to a uniform degree of penetration per hammer blow, supports an equal share of the load carried by all of the piles in said plurality with consequent uniform settlement of a structure supported thereby.

8. Structure as claimed in claim 3 including means to mark the pile per hammer blow, said pile-marking means being xed in position relative to said pile.

9. Structure in accordance with claim 3 including electrical gating means for rendering said comparator operative at intervals corresponding to a group of hammer blows.

References Cited UNITED STATES PATENTS 2,580,299 12/1951 Hunicke 73-84 3,353,362 ll/1967 Lubinski 61-53.5

ERNEST R. PURSER, Primary Examiner U.S. Cl. X.R.

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