Direct-push platforms have gained widespread acceptance in the environmental industry because of their versatility, relatively low cost, and mobility. Direct-push units use hydraulic pressure to advance sampling devices and geotechnical and analytical sensors into the subsurface. The weight of the truck, in combination with a hydraulic ram or hammer, is used to push the tool string into the ground. In some rigs, vibration is added to hydraulic pressure to better advance the tools.
There are two primary sampling modes. One uses a specific tool string that either performs downhole measurements or gathers a soil or water sample at a specific depth. In this mode, no soil is removed in creating the borehole. In the other mode, a dual-tube arrangement is used to take continuous soil samples for evaluation at the surface. Unlike continuous soil sampling with an auger rig, no extraneous cuttings are produced, and the samples are taken at a much faster rate.
The two major classes of direct-push platforms are cone penetrometer (CPT) and hydraulic percussion hammer systems. While CPT technically refers only to the geotechnical cone penetrometer instruments advanced by these vehicles, the vehicles themselves have come to be known by this designation. The distinction between these units is that CPT units advance the tool string by applying a hydraulic ram against the weight or mass of the vehicle, while percussion hammer units add a hydraulic hammer to the hydraulic ram to compensate for their lower mass. These platforms share the same principle of operation, similar tools, and a number of advantages and limitations. They differ in scale, application and, to some extent, the types of instruments and tools that have been developed for each. For these reasons, CPT and percussion hammer platforms fill different niches in the environmental field.
CPT systems generally are the larger of the two direct-push platforms. CPT systems usually are mounted on a 10- to 30-ton truck. Unlike a percussion hammer system, CPT systems use a static reaction force to advance steel rods and either a sampler or analytical device. The static reaction force generally is equal to the weight of the truck, which is supplemented with steel weights or, with smaller rigs, in-ground anchors. CPT systems that weigh 20 tons are common. A variety of samplers for retrieving soil, soil gas and ground water samples are used with CPT systems. Geotechnical sensors employed with a CPT system include sleeve-friction and tip-resistance sensors that map soil texture. Chemical sensors as well as downhole desorption or sampling techniques have been developed to detect, delineate and monitor sites contaminated with petroleum products, volatile organic compounds (VOCs), metals and explosives.
In contrast to their larger cousins, percussion hammer systems usually are mounted on pick-up trucks or tracks; however some equipment can be mounted on much larger vehicles. A percussion hammer system uses a combined force generated by the static weight of the vehicle on which it is mounted, and a percussion hammer to advance steel rods, and either a sampler or analytical device. A variety of samplers for retrieving soil, soil gas and ground water samples commonly are used with systems. Geotechnical sensors used with a percussion hammer system include tip-resistance sensors (also used by CPT systems) that map soil behavior types, and hydraulic conductivity sensors that map soil conductivity. As with the CPT, chemical sensors have been developed to detect, delineate and monitor sites contaminated with petroleum and VOCs. Dual-tube continuous coring also can be performed for better delineation of stratigraphy and detection of dense nonaqueous phase liquids (DNAPLs).
This article is provided through the courtesy of the U.S. Environmental Protection Agency’s Office of Super-fund Remediation and Technology.