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10/10/2008
Pile driving dates back thousands of years. The Romans and Egyptians drove wood poles into the ground by raising and dropping a heavy weight onto the top of the pole or pile. These driven poles or piles were used to shore up bridges and building foundations. In this example we see a drop hammer being used to drive wooden poles to make a dock on a lake. The photo to the right is a close up of an ancient drop hammer.
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Ancient records show that early drop hammers were lifted by men pulling on ropes.
The energy delivered to the pile was calculated simply by multiplying the weight of the ram by how far it falls. If the pile did not move then a larger weight was used. Larger structures needed larger piles. As piles became larger and larger so did the weight and the energy required to lift and drop the hammer. Soon man-and-rope was replaced by horse or even water power.
 
Drop hammers changed dramatically in the late 1880’s with the introduction of the steam winch and then the steam drop hammer.
 
In 1952 the first diesel powered pile driver showed up in the USA. The revolutionary diesel hammer was invented in Germany and when it entered into the USA it rapidly changed the way piles were driven. The diesel hammer’s light weight and very inexpensive operation costs are why this tool is still in use today.
The diesel hammer raises and drops its weight or ram by exploding diesel fuel and air compressed by the falling ram. When the fuel explodes the ram goes up for the next stroke and repeats itself until the fuel is shut off. It is a one cylinder two-stroke engine that uses the falling piston as the ram to drive the piles.
 
Piles have become larger and larger over time. Pile hammer evolution has been based on dealing with these larger piles. The drop hammer has giving way to the steam hammer and the steam hammer has giving way to the diesel hammer. The diesel hammer is giving way to the super larger hydraulic hammers. Nevertheless, they are all devices that drop a weight on top of the pile.
INTRODUCING THE VIBRATORY PILE DRIVER/EXTRACTOR
In the 1920’s a revolutionary type of pile driving equipment emerged called the vibratory pile driver/extractor. Rather than dropping a weight on the pile, a gearbox with rotating eccentrics vibrated the pile by locking onto it with a special clamp and then shaking the pile rapidly up and down. The Russians invented the device and the French and Japanese invaded the United States with this new technology in the early 1960’s.
HOW A VIBRATORY DRIVER EXTRACTOR WORKS
The vibratory pile driver is made up of three components, the top (suppressor), the middle (eccentric gearbox), and the bottom (clamping device). These three components are further explained as follows:
The Top (Suppressor)
The top part of the vibratory pile driver is called the suppressor. This is the part that hooks to the crane line to hold the machine into position. The suppressor is made up of springs or rubber dampeners and is used to keep vibrating motion generated from the machine from transferring into the crane line. In other words, it is the shock absorber that keeps the crane line from shaking the crane when the vibrator is in operation.
The Middle (Eccentric gearbox)
The eccentric gearbox is the heart of the machine. It is a steel box with special eccentric wheels inside of it that rotate via electric or hydraulic motors. As these eccentric weights turn they force the gearbox to shake up and down. The eccentrics are paired together and timed in such a way as to combine the eccentric motion vertically while canceling each other horizontally. As these eccentrics rotate the gearbox moves up and down vertically but not sideways.
The Bottom (Clamping device)
The vibratory driver/extractor’s bottom section is a clamping device that attaches to the pile by gripping it in giant hydraulic jaws. The type of clamping jaws used depends on the type of pile being vibrated. There are jaw types to fit every kind of pile including wood, concrete, sheet, H-piles, pipe and large diameter caissons.
 
The vibratory pile driver/extractor installs or extracts piles using vibration. The device is first clamped to the pile in the hydraulic jaws. Then the eccentric weights are turned using either electric motors or hydraulic motors. As the eccentric weights begin to spin the device, and the pile it is attached to, begin to vibrate up and down in the soil. This vibrating action moves the pile up and down in the soil. The soil becomes liquid or suspended, moving up and down rapidly with the pile. The pile can be raised or lowered into the suspended soil as it is being vibrated. Once the pile has been driven or extracted the machine is shut off and the jaws are opened to release the pile.
PERFORMANCE FACTORS
There basic performance factors that determine the power and effectiveness of a vibratory pile driver/extractor. These factors are listed as follows:
- Amplitude
- Eccentric moment
- Frequency
- Vibrating weight
- Cohesive soil weight added to pile
- Non-vibrating weight
Let’s review each one of these factors so we can better understand how they work.
AMPLITUDE 
Amplitude is the amount of vertical movement the vibratory pile driver/extractor can produce. Amplitude is created by the rotating eccentrics. The maximum vertical movement a vibratory pile driver/extractor can generate depends on the size of the eccentrics and the amount of weight they are moving up and down.
A vibratory pile driver/extractor’s maximum amplitude output is measured when there is no clamp attached. The only weight the eccentrics are moving up and down during the time of factory measurement is the gearbox. This is called the “starting amplitude”.
Any weight added to the gearbox reduces the net amplitude. When the clamp attachment is mounted or bolted to the gearbox the amplitude will decrease because the mass of the clamp is added to the weight being moved. Any mass added to the gearbox will decrease the overall amplitude. This includes the weight of the pile.
The pile is the largest source of weight; therefore amplitude decreases dramatically with pile weight. In addition, if the soil is cohesive clay that sticks to the pile then the pile becomes heavier, thus reducing amplitude even further.
Pile weight can be so massive that a vibratory pile driver/extractor’s amplitude is reduced to zero. In this case, a larger vibratory pile driver/extractor is required to drive or extract the pile. This is why vibratory pile driver/extractors come in many different sizes and capacities. One can calculate the amplitude output of a vibratory pile driver/extractor on a given pile size by using the following equation:
APE’s guidelines for minimum amplitude to drive the pile are .125 of an inch and our target amplitude is .250 or above. In other words, if your amplitude is less than an eighth of an inch then you may need to get a larger machine.
(Consult the factory when using this equation to calculate super heavy caissons.)
ECCENTRIC MOMENT
Eccentric moment is the measurement of the distance between the center of rotation and the center of gravity of the eccentric multiplied by the total mass of the eccentric.
Eccentric moment can be increased by making the eccentric larger or by adding more eccentrics. However, if we enlarge the eccentrics or add more eccentrics then the gearbox and associated bearings must also be enlarged which has a negative effect of increasing physical size and weight of the machine. The extra weight of a larger gearbox will add non-vibrating weight to the machine, which correspondingly reduces amplitude.
SIZE MATTERS
Productivity can be lost if the machine is too heavy to maneuver. Lighter vibratory pile driver/extractors allow the crane to reach further without need of moving the crane. Heavy machines limit the crane’s ability to reach out so time is lost during crane positioning. The larger machine may require a larger crane which costs more to purchase or rent. Heavy cranes may surpass the ability to maneuver on the jobsite. In addition, a heavier crane may exceed the load bearing capacity of the soil on the jobsite.
The trick is to add eccentric moment without increasing the size or weight of the gearbox. APE’s patented technology increases the mass of the eccentric without increasing the volume or physical size of the eccentric. Thus, more amplitude is created without increasing the size of the gearbox. This is why APE machines have a weight to power advantage.
FREQUENCY
Frequency is the rotation speed of the eccentrics per minute. Frequency is also called cycles per minute (CPM) or vibrations per minute (VPM). The number of cycles per minute or vibrations per minute is same as the revolutions per minute of the eccentrics. The faster the speed of the eccentrics the more dynamic force generated. 
The force of a vibratory pile driver/extractor is calculated using a mathematical equation known as the “dynamic force equation”.
VIBRATING WEIGHT
As stated earlier, vibrating weight is all weight put into vibration. This includes the vibrating case or gearbox, the clamp attachment, the pile, and in cases of cohesive clay, a good estimate of what the soil attached to the pile may be.
COHESIVE SOIL ESTIMATE
It is very easy to use the amplitude equation to calculate the amplitude of all weight put into motion. It is more difficult to estimate the amount of soil that may also become weight in motion. A fast way to find out for sure what amount of soil is adding to the weight in motion is to take a piece of chalk and draw a horizontal line across the pile as it is being vibrated. This line will wave up and down correspondingly to the amplitude being delivered to the pile and soil. A skilled pile driver should repeatedly use his chalk stick to scribe a line across the pile as it is being installed or extracted so that he can compare actual amplitude to the calculated amplitude. The measured difference between the calculated and actual amplitude will tell the actual weight of soil effecting weight in motion. Calculated amplitude will be very close to actual amplitude when driving or extracting in sandy non-cohesive soils. Expect less amplitude in clay (cohesive) soils.
NON-VIBRATING WEIGHT
Non-vibrating weight is a large part of the total weight of the vibratory pile driver/extractor. The upper suspension attached to the crane is non-vibrating weight. Any bias weights added to the suspension is also non-vibrating weight. Adding non-vibrating weight to the vibratory pile driver/extractor may help drive piles but only if the vibrator has not reached its amplitude limits. Adding non-vibrating weight will not help in extreme driving conditions where the vibrator is working at maximum capacity. In fact, adding such non-vibrating weight may bog the machine down further, especially in driving in dense cohesive clays. Adding non-vibrating weight is usually a poor excuse for switching to a larger, more productive machine. In any case, more suspended weight renders the machine more bulky and less maneuverable. A good pile driver can determine whether the added bulk and reduction in maneuverability will be offset by faster installation due to a larger, more powerful machine.
DRIVE FORCE
Drive force, also known as dynamic force, is a dynamic measurement of the rotating eccentrics. Here is the equation used to calculate dynamic force:
This equation is used to calculate the dynamic force of anything rotating. The pile driving industry has renamed this standard engineering equation from “dynamic force” to “drive force” which can confuse pile drivers. Note that when calculating dynamic force the frequency is squared so one way to pump up the numbers is to increase the frequency in the equation. Increasing the frequency in an equation is simple. However, increasing the frequency of rotating eccentrics in a vibratory pile driver/extractor is much more difficult than simply changing the numbers on a piece of paper.
HIGH FREQUENCY MACHINES USUALLY HAVE LESS AMPLITUDE
When selecting pile driving equipment one must be cautious regarding frequency because manufacturers use this part of the specification to pump up the dynamic force of their machine at the expense of eccentric moment. Increasing frequency of a rotating mass such as eccentrics on a vibratory pile driver/extractor will dramatically increase loads on the bearings and related components, including the gearbox housing. Bearing life drops off dramatically as dynamic load increases and in the case of frequency the load goes up by the square of the speed. To compensate for the loss of bearing life caused by spinning the eccentrics faster the manufacturer will reduce the eccentric moment of the machine.
Higher frequency vibratory pile driver/extractors usually create less unwanted vibrations into the soils. This is mostly due to the reduced amplitude of the machine caused by reducing the eccentric moment to enhance the rotational speed. In many cases it would have been better to accept a lower frequency rather than sacrifice amplitude. This is due to the fact that less amplitude results in slower driving times when installing or extracting in most soils and especially in stiff clay or cohesive soils.
HIGH FREQUENCY OR MORE AMPLITUDE?
In loose granular soils a higher frequency machine may be a better choice, especially if the project is around old buildings or structures sensitive to ground vibration. However, high frequency can settle or consolidate soils, especially inside a coffer dam, causing unwanted tension on the interlocks of the sheet piles. In some cases, sheet pile coffer dam installation was rendered impossible to finish due to consolidation of soil during initial driving. In these cases, a drill rig had to drill relief holes around the entire inside diameter of the coffer dam to release the pressure on the sheets so they could be driven to grade. Therefore, driving with higher frequency can cause more problems then it can solve. Consequently, more amplitude is better when driving into soils that tend not to suspend or vibrate well, such as in clay or rocky soil. Less amplitude works well in volcanic soils where higher frequency can cut into rock.
PILE DRIVING AND EXRACTING IS AN ART AND A SKILL
Driving and extracting piles requires knowledge and experience. It is an art and a skill. Understanding all the parameters discussed above and how they work together is paramount. In other words, part of the skill requires knowing how these factors interrelate.
THE INTERRELATIONSHIP OF PERFORMANCE FACTORS
The performance factors: amplitude, eccentric moment, frequency, vibrating weight, soil weight, and non-vibrating weight all relate to each other and this interrelationship has a lot to do with the success of the vibratory unit on any given job. An interrelationship of these factors may seem complicated but it is quite similar to an impact hammer where ram weight, ram stroke, hammer speed, and pile weight all effect driving results.
Usually the test of the best pile driver is the one that is lightest and fastest at putting the pile into the ground. With a vibratory driver, performance tests show that pile penetration rate is directly proportional to amplitude. The higher the amplitude, the better the penetration rate should be. Therefore, increasing amplitude will increase pile penetration rate.
The rotating eccentrics generate the vibratory force which creates the amplitude of vibration into the pile. The more eccentric moment, the higher the amplitude should be, unless the non-vibrating weight was dramatically increased to compensate for the larger eccentrics. Increasing eccentric moment does not always increase amplitude. It depends on the engineering design relationship between eccentric moment and non-vibrating weight. Therefore, it is wise to not focus on eccentric moment without comparing it with the amount of amplitude the machine delivers.
The pile is pulled up or down by the rotating eccentrics. It is the rotating eccentrics that create the amplitude. Frequency does not change the eccentric moment so therefore speed does not change amplitude. It may seem or look like more amplitude is generated when the vibratory machine is slowed down but this is an illusion. The human eye can see the movement of the pile when the eccentrics are moving slower but the amplitude is exactly the same. Decreasing frequency does not change amplitude. In fact, decreasing frequency reduces drive force because the eccentrics are turning slower and drive force goes up by the square of the speed.
WHY REDUCING SPEED MAY SPEED UP DRIVING OR EXTRACTING
Frequency changes the dynamics of how the pile is moved. If the pile is on top of a log for example, moving the frequency up or down may catch the log at just the right vibration speed to cut throw it. Vibrating at the wrong speed may cause the wood log to bounce off the end of the pile with no harm or ability to cut into it. Pipe piles bend and absorb vibration energy and then release this energy in varying ways depending on the wall thickness and length of the pile. Timing the speed of the vibrations to best utilize this stored energy as it is reflected back is crucial. Piles that refuse to drive at one speed will drop like butter if the speed is adjusted. H-Piles that refuse to go down will fall into the soil if the right frequency is found. Changing frequency while the vibratory pile driver is working can break through hard soil layers and a good pile driver usually moves the speed up or down when driving in hard layers. Turning the machine to full throttle and waiting for something to happen is a good sign you need a new pile foreman.
The same is true for extracting concrete piles. Higher frequencies may damage the pile while slowing things down will move the pile and break the friction. A skilled pile driver watches the pile and learns what works and does not work. The first step is to understand that frequency can be changed by moving the engine rpm up or down. What works on one type of pile may not work at all on another type of pile. Soil conditions change with location and layer so keep an open mind and be prepared to speed up or slow down the frequency to tune the pile to the right speed to get the job done.
VIBRATORY EXTRACTION
The vibratory pile driver/extractor revolutionized the way piles are pulled out of the ground. Prior to the existence of vibratory pile driver/extractors piles were yanked out of the ground by brut force using all the crane line capacity available. Using brut force to extract piles is dangerous because the crane line could snap and cause the crane boom to go over backwards. Converted air/steam hammers turned upside down to create an uplifting force were used to extract piles with limited success, followed by specially made impact extractors also powered by air or steam. When the vibratory pile driver/extractor entered the marketplace all the old ways of extracting piles became history.
The advantage of a vibratory driver/extractor is its ability to go from driver to extractor without any equipment changes. For driving, the crane line is left slack. This enables the pile to move downward. For extraction, the crane line is put in tension. The vibration breaks the pile loose and the crane pulls it up.
If you have any questions please give me a call at 206 498-9400.
Sincerely,
John L. White
President, APE
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