Because of the complexity of sensor systems and the specialized requirements for operating cone penetrometer rigs (CPTs), their operation calls for considerable experience. Most systems typically are deployed with a three-person crew and a geologist. Two people are needed to handle the push rods and operate the hydraulic press, and a third person operates the sensor systems, if applicable.
The principle behind CPT technology is fairly straightforward. A hydraulic ram is used to push the penetrometer tip and push rods into the subsurface, often to depths in excess of 100 feet below ground surface. The depth of penetration is limited by the structure of the subsurface formation. The technology can be used only in unconsolidated material. Hard layers – partially cemented sediments, and rocks and boulders – limit penetration.
A percussion-hammer system directly drives sampling tools and sensors into the subsurface; drilling is unnecessary to remove soil in order to make a path for the tool. The system relies on a relatively small amount of static weight combined with percussion to provide the energy for advancement of a tool string. Probing tools depend on soil compression or rearrangement of soil particles to permit advancement of the tool string.
Probing tools are advanced as far as possible using only the static weight of the carrier vehicle. Greater depth is achieved using the combined effect of the vehicle weight and hydraulic hammer percussion. Percussion often is required when probing near the ground surface to penetrate hard-packed soil. The probe then is allowed to penetrate using only static force until resistance is again encountered, at which time percussion is reapplied.
Compared to a CPT system, percussion hammer systems require far less training and experience, however, it is essential that the operator be familiar with the limitations and operations of the system, and have a complete understanding of the sampling tools associated with the system prior to operation.
System ComponentsUnlike most percussion hammer systems, the hydraulic ram apparatus and all support systems are enclosed within the CPT truck. CPT push rods typically are 1 meter long, and are flush-threaded so that additional lengths may be added as greater depths are reached. Additional rod sections are stored on-board for easy addition during probe advancement. Built-in grout systems allow the remaining boreholes to be filled while the rods are retracted, and most systems also have an integrated decontamination system that cleans the rods with hot water or steam as they are being withdrawn into the vehicles.
A variety of samplers are carried in the CPT truck. Geotechnical sensors and analytical instruments also may be included in the system. These instruments are attached to data acquisition systems inside the CPT truck by data cables inside of the probe rods, allowing acquisition and analysis of data to be conducted within an enclosed, protected work space.
The depth capability of a percussion hammer system depends on the amount of force the hammer can deliver and the static weight of the vehicle in which the system is mounted. The pushing of tools into the subsurface depends on the drive-down force, which ranges from 250 pounds to 35,000 pounds. The extraction force, which is necessary to remove tools from the subsurface, ranges from 13,000 pounds to 70,000 pounds.
Percussion hammer systems are outfitted on a number of platforms capable of accessing areas within a building. Some platforms are small enough to pass through a standard doorway.
These systems also have been outfitted on track-mounted vehicles and ATVs that permit access to off-road areas.
Percussion hammer systems are capable of directional drilling into the subsurface at up to 37.5 degrees. Most systems are equipped with a standard cylinder capable of advancing 54- and 66-inch-long tools into the subsurface; however, some systems are designed for stroking up to 12-foot lengths.
This article is provided through the courtesy of the U.S. Environmental Protection Agency’s Office of Superfund Remediation and Technology.