A) Field of the Invention
The present invention relates to a shock absorbing apparatus and method to be used in conjunction with a pile driving and/or pile pulling vibratory machine, and more particularly to such an apparatus and method which can be used effectively to isolate shocks under greatly varying load conditions imparted to the shock absorbing apparatus.
b) Background of the Invention
In the construction industry, it is sometimes necessary to drive piles into the earth to provide a proper foundation for a building or other structure. One method of accomplishing this is to place the pile in a vertical position above the earth's surface and strike the upper end of the pile repeatedly with a hammer (i.e., a metal mass which is raised and dropped on the pile) until the pile has penetrated into the ground surface a sufficient distance to provide adequate bearing. A later development was to drive piles into the ground by use of a vibrating machine which oscillates the pile from zero to 20,000 cycles per minute depending on the type of machine to cause what appears to be an almost continuous motion of the pile into the earth. Under some circumstances, such a vibratory machine can cause the pile to move into the earth relatively rapidly (e.g., as fast as ten feet per second).
A typical arrangement for such a vibratory machine is to provide a pair of weights which are mounted eccentrically for rotation about parallel axes, with the directions of rotation being opposite to one another so that the lateral forces are being cancelled out, and a net up and down vibrating force is developed by the machine. One part of the machine is coupled to the upper end of the pile, while a second part of the machine is connected through a shock absorbing device to a support member, such as a cable.
When the pile is being driven into the ground, the vibratory machine is able, in large part, to act substantially independently, in that only minimal exterior support is required, this being mainly to keep the vibratory machine properly positioned. Sometimes weights are added to the shock absorbing device to provide a greater downward force, and this gives need for effective shock absorption. Another mode of operation is when a previously driven pile is being extracted from the earth, and it is necessary to impart a tension force on the pile so as to pull it upwardly. In these circumstances, a tension force (e.g., a pulling force exerted by a connecting cable) is applied through the shock absorbing device to the vibratory machine, which in turn pulls upwardly on the pile to which it is connected. The tension force exerted by the cable can vary greatly, and can vary between two tons to one hundred tons.
For various reasons, it is desirable that the cable be subjected to a more constant load, with the rapid vibratory loads being isolated from the cable as much as possible. However, properly isolating these vibratory loads is complicated by the fact that the tension loads necessary to extract the pile can vary greatly, depending upon the size of the pile, the depth to which it is driven, and the localized resisting forces imparted by various portions of the earth material.