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Virtually all vibratory pile drivers use rotating weights to produce the alternating force that mobilises and fluidises the soil. But it's necessary to use the weights in pairs to cancel out the horizontal forces that result. What if the force could be produced using a reciprocating weight, thus eliminating the horizontal cancellation requirement? In the 1970's Vulcan investigated this problem. We discuss here the two solutions it considered: the linear vibrator and the hydroacoustic driver.
The linear vibrator was the brainchild of John J. Kupka, an Austrian immigrant who had done work for MKT on their "C" series hammers and the Horn Construction "HC" hammers that Vulcan had produced. Kupka designed the linear vibrator for Vulcan and it was tested at Vulcan's West Palm Beach fabricating facility in March 1971.
Above: the Linear Vibrator, from its U.S. Patent (3,704,651) drawing. The hammer worked by applying air through the inlet 70, which was admitted through slots 56b in the ram 56. The slots were set up so that they only admitted air to one side at a time and for a short length; the rest of the ram travel upward or downward past the slots expanded the trapped air and extracted the energy from it in that way. The two air cushions at the ram ends 56a additionally bounced the ram between its two extremes and provided for transmission of the alternating force to the body, and through the primitive clamp at the bottom to the pile.
The total assembly above was 67 5/8" long; the small and large diameters of the piston 56 are 4 1/2" and 7 1/2" respectively. The outer diameter of the housing 18 was 11 1/2".
The results of Vulcan's March 1971 tests by Continental Testing Laboratories can be found here. The device worked as designed but Vulcan never pursued the technology. The primitive clamp has been noted; the suspension 40 at the top, with its Belleville washers, was already being made obsolete (along with the steel coil spring suspensions of earlier vibratories) by the rubber springs Vulcan was to use in the next decade. A more serious problem was the magnitude of the force. The peak force put out by the hammer was just shy of 5 U.S. Tons. The 400, the smallest rotating vibratory Vulcan put out, had a force of 17 tons, and the "small" 1150 (which competed with machines such as the MKT V-5 and ICE 216) 42 tons.
The basic problem with the force lay in the use of compressed air and the piston ring sealing technology used in the machine. Although Vulcan could have easily produced such a machine with the technology it used for the air/steam hammers, upscaling it to compete with vibratory hammers even in the early 1970's would have resulted in a fairly large machine. The concept of a ram/valve with expansive use of the compressed air resurfaced in the Single-Compound Hammer which Vulcan developed a decade later.
One way of getting around the size and pressure problem would have been to make the device operate with hydraulic fluid and the pressures that went with that. In the late 1970's Vulcan toyed with that idea but it was also presented with another concept, namely the hydroacoustic driver.
In 1974 the Naval Civil Engineering Laboratory issued its Technical Note N-1362, "Evaluation of a Hydroacoustic Rapid-Impacting Pile Driver" by Dr. Carter J. Ward. This report described the operation and tests on a new concept in hydraulic pile driving. Abstract for the report is as follows:
Above: Hydroacoustic driver, as tested by NCEL. The device, although compared with a sinusoidal vibrator, is actually a reciprocating version of an impact-vibration hammer in that the lower end of the hammer/valve (piston) impacts an anvil rather than transmitting force alternately. Like the impact-vibration hammer, it was capable of high blow rates, if not a great deal of energy for each blow.
The concept was presented to Vulcan in 1977-8; however, the company was going through a generational change, this compounded by the demands of the existing air/steam line (the 6300 was being designed and produced at the time.) To apply this to conventional pile driving would require some conceptual changes in the transmission of energy to the pile (i.e., lower energy per blow with higher blow rate vs. high energy per blow and low rate.) Additionally the lower energy per blow may or may not be able to move the pile past the quake point of the soil and induce plastic deformation of the soil, essential in impact pile driving. The hydroacoustic driver nevertheless remains an interesting concept for generating impact, if not pure sinusoidal vibration.