Vibration technologies. A wide range of vibration systems and mechanisms are used in industry for materials handling and processing. Machines producing vibrations of different frequencies, forces and amplitudes are used to enhance the movement and storage of bulk solids, to clean surfaces, and separate materials. For example, portable vibrators which deliver vibrations through the tip of a flexible shaft are commonly used to purge air bubbles from concrete mixtures on construction sites. These have sometimes been adapted for use in vibrocore sampling, as illustrated below. On a much larger scale, hydraulic vibrating drivers are used to imbed building supports or pilings in soils and sediments. The Dutch have led in the development of submersible vibrators for offshore tower construction and anchoring systems, as well as in vibracoring equipment. Custom and commercial vibracorers. Various types of vibracorers have been improvised by investigators for wetland and shallow water core sampling. These are typically powered by small gasoline engines, electric motors or air compressors. Most of these make use of off-the-shelf hardware, such as concrete vibrators. In these systems the vibrating tip of the flexible shaft is simply clamped to the metal core tube and both are suspended from a small draw works (tripod and hoist). The main limitation for underwater coring is the flexible shaft, which tends to be stiff and heavy. To avoid using a long shaft, the vibrator clamp may be shifted up the tube as it descends. However, if the water depth is more than a few feet, this adds significant extra length to the core tube that must be used. Improvised portable vibracorers of this type have been used most often for coring in wetlands. A number of commercial vibracoring systems have been developed, which are better designed for underwater use at depths of 100 feet (30 meters) or more. For those systems powered by hydraulics and pneumatics from the surface, fluid and air lines must descend to a vibrator unit attached to the top of the core tube. In the case of submersible electrical systems, a power cord leads down to vibrator motors encased in a water-tight housing on top of the core tube. Safe and reliable connections of this sort are now routine. Even battery powered vibrocorers have been developed for deep sea work. Electrical systems have the advantage of being the most portable and convenient to use, especially from smaller boats. Deployment systems. The smallest hand vibracoring units may be deployed by an individual off the side of skiff. Nearly all vibrocorers are too heavy for hand use and require a power hoist or winch for raising and lowering the unit. Larger systems may include a submersible frame that keeps the vibracorer and tube upright at the bottom. The dead weight of the whole system can be substantial, but much greater lift capacity is needed to pull the filled core tube from the substrate, especially from cohesive materials like clay. An additional pulling force of 1,000 lbs. (450 kg) or more may be required, depending on the diameter and length of the tube, and the nature of the substrate. This is a real consideration when pulling off the side of a small boat, and for that reason pontoon or inflatable boats with a central hole in the deck are favored for portable vibracoring rigs in inland waters. Otherwise, a variety of work boats 25 feet (8 m) or longer can be used, as long as they have suitable lift capabilities. Small vibracoring units have even been deployed off of remotely operated vehicles (ROVs). A draw works, boom or lift tower is also necessary to raise the vibracorer high enough on retrieval so that the lower end of the tube can be reached on deck. As a first priority the tube may be capped and taped to make sure no sample is lost. For systems using lined (double) core tubes, the liner tube must be detached and unloaded from the bottom end. On portable rigs the lift tower itself must be strong and rigid, yet capable of being folded down or disassembled for transport. The fold-down concept is a great advantage when coring in urban harbors or rivers with fixed bridges or other obstructions. At limited access sites without any boat launch facilities it may be necessary to assemble the whole rig at the water's edge, or lift it in by crane. In other situations where no boat access is possible the vibracorer unit may be deployed from the lift cable of a boom truck. In more remote wetland areas, it may be feasible to winch or drag the boat rig across surface vegetation. Creative solutions can be found to most coring access problems. In typical shallow water (less than 100 feet) or wetland deployments, the vibrocorer unit can be dangled at the end of a cable, and kept upright during the coring operation simply by maintaining a taut line. This can be complicated somewhat by strong currents, wave action or sloping bottoms. In deeper offshore waters, vibracorers are often deployed in a bouyant frame system which stabilizes the unit on the bottom and keeps it upright during coring, even with a slack cable to the surface. This is important where wind and waves make it difficult to hold position. Core tubes, liners and catchers. Thin-walled metal tubes without liners are most effective for vibracoring because they conduct vibrational energy most efficiently. Samping requirements aside, the shorter the tube length, the better. For most vibracoring applications a 3 to 4 inch (7.6 to 10.2 cm) tube (outside diameter) with a wall thickness of 1/8 inch (3.2 mm) is standard. A common, inexpensive source of metal core tube is aluminum irrigation pipe, cut into lengths of 20 feet (6 m) or less. Steel tubes are more often used in heavy duty applications offshore, where 30-foot cores in harder substrates may be required. Metal tubes with plastic liners are often dictated in applications where sediment contamination is an issue. The disposable liner has the dual purpose of shielding personal from hazardous sediments and preventing cross-contamination of the sample by the metal tube. Both the outer tube and inner liner are usually attached with rivets to the core catcher device at the lower end. Liners are commonly made of either soft "lay flat" polyethylene tubing or hard polycarbonate or CAB (cellulose acetate butyrate) tubes of 1/16-inch (1.6 mm) wall thickness. These materials have been determined by the U. S. Environmental Protection Agency (reference) to be acceptable for most laboratory analyses of low-level contaminants in sediments. In most cases a good alternative to lined metal core tubes is to use a thicker walled (3/32 to 1/8-inch) plastic tube as the unlined core tube itself. While any plastic is theoretically less effective than metal for vibracore tubes, the practical difference in performance is usually not significant in collecting 15-foot (or less) cores of typical silty sediments. Moreover, the ability to view and process the intact core while still confined in the transparent tube has great practical advantages in the field. Plastic tubes of this dimension and length are generally not stocked by suppliers, but are made to order. Core catchers, usually of the "leaf" or finger type, are installed (by rivets) inside the lower end of the tube to help prevent sediment loss on retrieval. They may be constructed of stainless steel and designed as part of a "nose piece" or cutter tip on the tube. These are primarily for use with metal tubes, lined or unlined, and they must be cleaned for reuse between cores. A similar type of catcher made of flexible CAB plastic is used without a nose piece. It is simply rolled up and riveted inside a plastic or aluminum tube. This has the advantage of being much cheaper to make and is disposable along with the tube. Both styles of catchers work well in most sediments, but rarely, when cores are withdrawn from highly cohesive clays, the fingers will invert out the tube end, and sediment may be lost. To retain some very fluid or oily sediments it may be helpful to install two overlapping plastic catchers. Core processing tools. In document or sub-sampling cores is often desirable to open the intact core lengthwise. Various tools and techniques are useful for splitting the tube wall and the sediment column inside it. In pollutant profiling studies, it is often necessary to cut the tube wall without disturbing or contaminating the sediment within. Aluminum or plastic tubes of 1/8-inch (3.2 mm) wall thickness can be cut open with electric shears or circular saws that are set for a controlled shallow cut. Tube fragments will inevitably land on the core below it, but since this outer thin layer of the sediment core next to the wall is disturbed (by vibration), it should not be included in the samples anyway. Thinner walled plastic tubes and liners can be cut open with a vibrating saw or a hand-held knife with a hooked blade. Once the full length of the tube is cut open along opposite sides, the core itself can be bisected. The important thing here is to slice the core open by pushing in a blade across it's axis, and not by dragging the blade along the axis. This prevents horizontal zones of different material from being smeared together along the vertical axis. A handy tool for slicing cores consists of several 1-ft.x 6-in. rectangles of 18-guage stainless steel with one of the long edges turned 90 degrees as a handle. These blades are pushed into the core lengthwise, one after the other, until the whole core can be pried open all at once. Then each blade is withdrawn, dragging it laterally across the sediment and one edge of the core tube, so that the sediment surface is smoothed. This technique best reveals details of core structure. The blades should be cleaned and decontaminated between processing different cores. Additional tools for processing core samples -- spoons, mixing pans, etc. -- are more specific to each project. |
Equipment and methods |
Vibration technologies. A wide range of vibration systems and mechanisms are used in industry for materials handling and processing. Machines producing vibrations of different frequencies, forces and amplitudes are used to enhance the movement and storage of bulk solids, to clean surfaces, and separate materials. For example, portable vibrators which deliver vibrations through the tip of a flexible shaft are commonly used to purge air bubbles from concrete mixtures on construction sites. These have sometimes been adapted for use in vibrocore sampling, as illustrated below. On a much larger scale, hydraulic vibrating drivers are used to imbed building supports or pilings in soils and sediments. The Dutch have led in the development of submersible vibrators for offshore tower construction and anchoring systems, as well as in vibracoring equipment. Custom and commercial vibracorers. Various types of vibracorers have been improvised by investigators for wetland and shallow water core sampling. These are typically powered by small gasoline engines, electric motors or air compressors. Most of these make use of off-the-shelf hardware, such as concrete vibrators. In these systems the vibrating tip of the flexible shaft is simply clamped to the metal core tube and both are suspended from a small draw works (tripod and hoist). The main limitation for underwater coring is the flexible shaft, which tends to be stiff and heavy. To avoid using a long shaft, the vibrator clamp may be shifted up the tube as it descends. However, if the water depth is more than a few feet, this adds significant extra length to the core tube that must be used. Improvised portable vibracorers of this type have been used most often for coring in wetlands. A number of commercial vibracoring systems have been developed, which are better designed for underwater use at depths of 100 feet (30 meters) or more. For those systems powered by hydraulics and pneumatics from the surface, fluid and air lines must descend to a vibrator unit attached to the top of the core tube. In the case of submersible electrical systems, a power cord leads down to vibrator motors encased in a water-tight housing on top of the core tube. Safe and reliable connections of this sort are now routine. Even battery powered vibrocorers have been developed for deep sea work. Electrical systems have the advantage of being the most portable and convenient to use, especially from smaller boats. Deployment systems. The smallest hand vibracoring units may be deployed by an individual off the side of skiff. Nearly all vibrocorers are too heavy for hand use and require a power hoist or winch for raising and lowering the unit. Larger systems may include a submersible frame that keeps the vibracorer and tube upright at the bottom. The dead weight of the whole system can be substantial, but much greater lift capacity is needed to pull the filled core tube from the substrate, especially from cohesive materials like clay. An additional pulling force of 1,000 lbs. (450 kg) or more may be required, depending on the diameter and length of the tube, and the nature of the substrate. This is a real consideration when pulling off the side of a small boat, and for that reason pontoon or inflatable boats with a central hole in the deck are favored for portable vibracoring rigs in inland waters. Otherwise, a variety of work boats 25 feet (8 m) or longer can be used, as long as they have suitable lift capabilities. Small vibracoring units have even been deployed off of remotely operated vehicles (ROVs). A draw works, boom or lift tower is also necessary to raise the vibracorer high enough on retrieval so that the lower end of the tube can be reached on deck. As a first priority the tube may be capped and taped to make sure no sample is lost. For systems using lined (double) core tubes, the liner tube must be detached and unloaded from the bottom end. On portable rigs the lift tower itself must be strong and rigid, yet capable of being folded down or disassembled for transport. The fold-down concept is a great advantage when coring in urban harbors or rivers with fixed bridges or other obstructions. At limited access sites without any boat launch facilities it may be necessary to assemble the whole rig at the water's edge, or lift it in by crane. In other situations where no boat access is possible the vibracorer unit may be deployed from the lift cable of a boom truck. In more remote wetland areas, it may be feasible to winch or drag the boat rig across surface vegetation. Creative solutions can be found to most coring access problems. In typical shallow water (less than 100 feet) or wetland deployments, the vibrocorer unit can be dangled at the end of a cable, and kept upright during the coring operation simply by maintaining a taut line. This can be complicated somewhat by strong currents, wave action or sloping bottoms. In deeper offshore waters, vibracorers are often deployed in a bouyant frame system which stabilizes the unit on the bottom and keeps it upright during coring, even with a slack cable to the surface. This is important where wind and waves make it difficult to hold position. Core tubes, liners and catchers. Thin-walled metal tubes without liners are most effective for vibracoring because they conduct vibrational energy most efficiently. Samping requirements aside, the shorter the tube length, the better. For most vibracoring applications a 3 to 4 inch (7.6 to 10.2 cm) tube (outside diameter) with a wall thickness of 1/8 inch (3.2 mm) is standard. A common, inexpensive source of metal core tube is aluminum irrigation pipe, cut into lengths of 20 feet (6 m) or less. Steel tubes are more often used in heavy duty applications offshore, where 30-foot cores in harder substrates may be required. Metal tubes with plastic liners are often dictated in applications where sediment contamination is an issue. The disposable liner has the dual purpose of shielding personal from hazardous sediments and preventing cross-contamination of the sample by the metal tube. Both the outer tube and inner liner are usually attached with rivets to the core catcher device at the lower end. Liners are commonly made of either soft "lay flat" polyethylene tubing or hard polycarbonate or CAB (cellulose acetate butyrate) tubes of 1/16-inch (1.6 mm) wall thickness. These materials have been determined by the U. S. Environmental Protection Agency (reference) to be acceptable for most laboratory analyses of low-level contaminants in sediments. In most cases a good alternative to lined metal core tubes is to use a thicker walled (3/32 to 1/8-inch) plastic tube as the unlined core tube itself. While any plastic is theoretically less effective than metal for vibracore tubes, the practical difference in performance is usually not significant in collecting 15-foot (or less) cores of typical silty sediments. Moreover, the ability to view and process the intact core while still confined in the transparent tube has great practical advantages in the field. Plastic tubes of this dimension and length are generally not stocked by suppliers, but are made to order. Core catchers, usually of the "leaf" or finger type, are installed (by rivets) inside the lower end of the tube to help prevent sediment loss on retrieval. They may be constructed of stainless steel and designed as part of a "nose piece" or cutter tip on the tube. These are primarily for use with metal tubes, lined or unlined, and they must be cleaned for reuse between cores. A similar type of catcher made of flexible CAB plastic is used without a nose piece. It is simply rolled up and riveted inside a plastic or aluminum tube. This has the advantage of being much cheaper to make and is disposable along with the tube. Both styles of catchers work well in most sediments, but rarely, when cores are withdrawn from highly cohesive clays, the fingers will invert out the tube end, and sediment may be lost. To retain some very fluid or oily sediments it may be helpful to install two overlapping plastic catchers. Core processing tools. In document or sub-sampling cores is often desirable to open the intact core lengthwise. Various tools and techniques are useful for splitting the tube wall and the sediment column inside it. In pollutant profiling studies, it is often necessary to cut the tube wall without disturbing or contaminating the sediment within. Aluminum or plastic tubes of 1/8-inch (3.2 mm) wall thickness can be cut open with electric shears or circular saws that are set for a controlled shallow cut. Tube fragments will inevitably land on the core below it, but since this outer thin layer of the sediment core next to the wall is disturbed (by vibration), it should not be included in the samples anyway. Thinner walled plastic tubes and liners can be cut open with a vibrating saw or a hand-held knife with a hooked blade. Once the full length of the tube is cut open along opposite sides, the core itself can be bisected. The important thing here is to slice the core open by pushing in a blade across it's axis, and not by dragging the blade along the axis. This prevents horizontal zones of different material from being smeared together along the vertical axis. A handy tool for slicing cores consists of several 1-ft.x 6-in. rectangles of 18-guage stainless steel with one of the long edges turned 90 degrees as a handle. These blades are pushed into the core lengthwise, one after the other, until the whole core can be pried open all at once. Then each blade is withdrawn, dragging it laterally across the sediment and one edge of the core tube, so that the sediment surface is smoothed. This technique best reveals details of core structure. The blades should be cleaned and decontaminated between processing different cores. Additional tools for processing core samples -- spoons, mixing pans, etc. -- are more specific to each project. |
