Cone Penetrometer Technology (CPT)
Cone penetrometer technology (CPT) is a method of providing real-time data for use in characterizing the subsurface, as opposed to older methods of analyzing subsurface conditions in the laboratory. It consists of a steel cone that is hydraulically pushed into the ground at up to 40,000 pounds of pressure. Sensors on the tip of the cone collect data. Standard cone penetrometers collect information to classify soil type by using sensors that measure cone-tip pressure and friction. CPT is often used in conjunction with Hydropunch tests, which use the CPT holes to extract groundwater for laboratory analysis. An innovation of the CPT (i.e., the wireline CPT) allows multiple CPT tools to be interchanged during a single penetration, without withdrawing the CPT rod string from the ground.
Initially developed to collect information about soil characteristics, as sensor technology was developed CPT also became a platform for collecting information about a variety of contaminants. Recent advances in sensor technology have expanded cone penetrometer capabilities to detect the presence of petroleum hydrocarbons. Sensors are being tested or demonstrated for the detection of other organics, compounds, metals, radioactivity, explosives, and soil moisture.
Generally, the advantage of using CPT is that sampling costs less and is faster than shipping samples to a laboratory. CPT locations can be altered as results are analyzed, thus providing the operator with increased flexibility. It should be noted, however, that when certain sensors are used, results may have to be validated by laboratory analysis.
Limitations and Concerns
CPT cannot be used at some sites due to high soil density.
Most sensors are now used as screening tools that provide initial site characterization data. The data is confirmed by collecting samples that are analyzed in the laboratory. This is due to limitations in sensor technology, and it will likely diminish in importance as the technology improves.
CPT is useful on sites that contain unconsolidated sediments (e.g., soil and clay that are not cemented together). On the other hand, sites with large boulders, rock or cemented layers are difficult to penetrate.
CPT sensors, such as lasers, that require a lens may be hampered by fouling of the lens due to a reaction to dust.
Decontamination may be necessary if the CPT comes into contact with contaminated material.
CPT is a characterization tool used to identify the geologic, hydrologic and chemical features of a site. It can be applied to all sites. It is not contaminant specific. CPT is generally applied to depths up to 150 feet, but it has been used as deep as 300 feet.
Technology Development Status
CPT was introduced in 1934 in the Netherlands. Actual electrical measurement sensors were not developed until 1948, and they were not widely used until the 1960s. By the 1980s, the cone penetrometer was commonly used for geo-technical applications, with at least a dozen companies routinely providing the technology. Research was initiated in the late 1980s to develop other sensors.
Other Resources and Demonstrations
A successful demonstration was performed at Sandia National Labs (SNL) in New Mexico using an integrated Laser Induced Breakdown Spectroscopy (LIBS) and Cone Penetrometer Technology (CPT) system to rapidly analyze (currently less than 24 hours) the heavy-metals content of the subsurface soils. The CPT-deployed LIBS using a high-energy laser pulse. The soil absorbs the energy and heat. Elemental analysis is conducted via an optical fiber. The CPT/LIBS system was successfully demonstrated. (LIBS technology developed by Los Alamos National Laboratories was successfully field tested to detect chromium and beryllium in surficial soils. However, one of the shortcomings of LIBS is the calibration/data analysis procedure requires a knowledge of soil type prior to effective LIBS analysis).
See http://www.osti.gov/bridge/servlets/purl/491995-LAQZsj/webviewable/ for a description of a cone penetrometer with a Raman probe used at Hanford underground tanks.
See http://www.osti.gov/bridge/servlets/purl/585514-XNxjsb/webviewable/ for determining migration and distribution of contaminants using multiple sensors on the cone penetrometer.