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YPD Floating Pile Driver

Piles are structural members driven into the ground. An end-bearing pile is a pile that is driven until its tip meets firm resistance from subsurface rock, dense sand, or gravel. The tip of the pile is the end driven into the ground. The other end is called the butt. A friction pile is a pile that, when driven into softer material without penetrating a firm bearing layer, will still develop considerable load-carrying capacity through the frictional resistance between the sides of the pile and the surrounding soil.

Pile-driving equipment consists of a pile driver that holds the pile and the hammer as the hammer drives the pile into the ground. Water-jetting equipment may also be used if allowed by the specification to displace the soil ahead of the pile and make it easier to drive the pile into the ground. A pile driver is a large piece of equipment that supports the pile and hammer in a fixed position while driving the pile into the ground. The equipment must be stable and of adequate size and capacity to lift the pile and to control both the pile and hammer during driving. A typical pile driver is composed of a crane, leads, a hammer, and other appurtenances. The term leads refers to the structure on which the hammer travels up and down and with which thehammer and pile assembly is aligned as the pile is being driven. Rails or other guides are affixed to leads to the guide the hammer as it travels up and down within the leads.

The driving force provided by these hammers is created by gravity, mechanical power (diesel, air, or steam), or both.Vibratory hammers are also used to install and extract piles. Drop (gravity) hammers are the oldest type of pile-driving hammer. A drop hammer consists of a large weight (5,000 to 7,500 kg) raised by a winch and cable to a height above the pile and then dropped onto the top of the pile. The energy generated by the free-fall of the weight creates the force that drives the pile into the ground. Power-driven hammers use gravity and mechanical power to create the force that drives the pile. In addition to diesel, steam and compressed air are also used to power hammers.

A wharf is an overall term that applies to any waterfront structure designed to make it possible for vessels to lie alongside the shore for loading and unloading. The term wharf is confined in practice to the T- and U-type marginal wharves. A marginal wharf usually consists of a timber or steel superstructure supported by a series of timber, steel, or concrete pile bents. A quay is a reinforced landing place made toward the sea or at the side of a harbor. To protect a wharf against normal wear and tear, three types of piles are used: bearing, fender, and mooring piles.

Bearing piles support the wharf or pier framework and decking. The piles should be straight and measure at least 15 cm across the top, 50 cm across the butt (bottom), and from 20 to 25 meters in length. Pile length varies according to the depth of the water and condition of the bottom. Bearing piles should be spaced from center to center 2 to 3 meters apart in one direction and 2 meters apart in the other direction.

The force of a moving ship coming in direct contact with bearing piles is enough to collapse an unprotected wharf. To protect and absorb the initial shock, fender piles are placed about 1 meter out from the centerline of the outside row of bearing piles. These piles are placed about 50 cm apart and along the sides where ships dock. Structures that are almost completely rigid, such as solid-fill quays, sometimes have fender piles backed up with heavy springs to give a combination of yield and resistance. Fender piles are driven at a slight batter (angle). Usually 1 to 12 fender piles are used along the outside edge of all rows of bearing piles, except on the extreme inshore wharf sections.

Mooring piles are aligned with the outside row of bearing piles and are spaced about 10 meters apart. This type of pile is braced along the outside row of bearing piles and usually extends to about 1 meter above the floor (or deck) of the platform. The 1-meter extension provides ample space to secure mooring lines.

Piles are made of wood, concrete, steel, poly vinyl chloride (PVC), fiber reinforced polymer (FRP),or a combination of steel and concrete. Timber piling must be treated with creosote or some other preservative compound to protect it from fungi and marine borer attacks. A heavy timber wearing ribbon, which may easily be replaced, is sometimes installed along a line of fender piles at the elevation receiving the heaviest abrasion.

Floating clusters of logs or strongly constructed rafts are called camels. In addition to absorbing impact shock and protecting fender piles from the sliding friction of a ship moving in the berth, camels may be required to breast a ship off the face of the wharf into deeper water. Dolphins are isolated clusters of piles to which a ship may be moored. The center of the cluster, called a king pile, may be a single pile or a cluster driven vertically and wrapped to act as a unit. The other piles are driven in one or more concentric rings around the king pile, each battered towards the center. The king pile is normally left somewhat longer than the others for use as a mooring post.

Pile driving is an empirical art. Certain combinations of blow energy and force pulse durations or widths (the period during which the driving force is applied to the pile) have been found to be preferred for general use in most soils and with most types of piles, including concrete, wood or mandrel driven corrugated piles. In order to increase the driving power, the repetition frequency of such blows must be increased. With increased repetition frequency, however, the weight of the hammer must be increased to compensate for the increased forces which tend to lift the hammer off the pile.

It is of course possible to increase the hammer weight. However, the weight of pile driving hammers is a critical factor in the stability of the cranes used to handle such hammers. As the hammer weight increases, the effect on the center of gravity of the crane and the crane stability relative to tipping increases. This may be compensated for either by a larger crane, or for a given crane size, by increased operator attention to stability. In either instance, increased weight reduces the mobility of the pile driver, increases the time for setting up the driver to drive each pile and consequently increases the cost of pile driving.

Due to the nature and construction of piles coming into more common usage, particularly concrete piles, which must be driven into substructure by a pile driver hammer, it has become increasingly necessary and desirable, in order to preserve the structural integrity of the pile itself, that the pile be driven initially by relatively light strokes of the hammer, while heavier blows are permissable after the pile is well-started into the substructure. Most commonly, the striker or ram of the hammer is elevated by fluid pressure delivered to an operating cylinder forming an element of the hammer, and allowed to drop to deliver a pile-driving blow by exhausting fluid from said operating cylinder.

With this type of operation, it is of course obvious that the energy delivered by each stroke of the hammer can be varied as desired by changing the elevation to which the striker is raised before it is allowed to drop, since the energy delivered is a function of the mass and velocity of the striker at the instant of the blow, and the velocity thereof is a function of the distance it is allowed to drop before delivering the blow. Methods and apparatus have in fact been previously proposed for adjusting the stroke of the striker for the purposes stated, but generally these prior methods and apparatuses have required that a workman climb to the hammer to accomplish the necessary adjustments. Since the hammer is normally suspended by a crane at considerable elevations, this operation is both hazardous and time-consuming.

The cap block provides a transformer arrangement which enables a high velocity, low weight ram to appear to the pile as a low velocity, high weight ram, thereby minimizing impact stresses in the pile, while providing efficient transfer of energy to the pile. In addition, the cap block provides a spring, which together with the ram mass and transformer controls the duration of the force pulse. The combination spring-transformer enables the driving power delivered to the pile, in terms of the product of the blow energy and the repetition frequency of a ram, to be increased without increasing the force tending to lift the hammer off the pile, or increasing the impact stresses in the pile over those normally experienced in pile driving.

The equipment is operated by a special crew, but the carpenter is present during the pile driving to direct the alignment of the piles. Piles should be straightened as soon as any misalignment is noticed. The desired accuracy of alignment varies with each job; however, if a pile is more than a few centimeters out of plumb, it should be set true. The greater the penetration along the wrong line, the more difficult to get the pile back into plumb.

When a floating pile driver is used, a frame (template) for positioning piles may be fastened to the hull. A floating template is sometimes used to position the piles in each bent. The spacing of battens is such that the centerline between them is on the pile line desired. Battens are placed far enough apart so that, as the pile is driven, the larger diameter butt end will not bind on the template and carry it underwater. A chain or collar allows the template to rise and fall with the tide. If the ends of the battens are hinged and brought up vertically, the template may be withdrawn from between the bents and floated into position for the next bent. Several templates may be used for a bent; or a single template is moved, if the pile spacing is uniform. Movement of the pile driver in the water can cause the strikes from the hammer to be out of alignment with the piles being driven, resulting in numerous cracks in the upper segment of the piles. A pile driver with legs to lift it out of the water can be used for driving that is more precise.




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Page last modified: 25-12-2019 18:43:26 ZULU