Direct-Push Platforms - Part 3
April 1, 2010
Cone penetrometer systems (CPTs) can advance a full range of soil, soil gas and ground water samplers and a growing list of analytical instruments. Specially designed samplers are used to collect high-quality ground water, soil gas, and soil samples. Geotechnical sensors provide a rapid, reliable and economical means of determining soil behavior types, which can be related to soil stratigraphy, relative density and strength. Hydrogeologic conditions such as hydraulic conductivity, static and dynamic pore pressure, and soil and water conductivity also can be collected.
Many of the soil, soil gas, and ground water samplers resemble the physical samplers used with percussion hammer direct-push systems. These samplers are advanced by the rod. Either retrieving the rod and sampler, or physically collecting a soil gas or ground water sample through the rod retrieves the sample.
Piston-type samplers are used to collect relatively undisturbed soil samples without generating soil cuttings. Several different types of samplers are used, depending on the soil type and density. The soil sampler is initially pushed in a “closed” position to the desired sampling interval, and the inner cone tip of the sampler is retracted about 12 inches, exposing a hollow soil sampler with an inner liner. The hollow sampler is pushed in a locked “open” position to collect a soil sample. The filled sampler and push rods then are retrieved. For environmental analyses, the soil sample tube ends are sealed. A longer “split-tube” sampler can be used for geotechnical sampling.
Soil gas sampling can be performed using a commercial unit such as Geoprobe vapor sampling system or a specially designed filter probe attached to a standard penetrometer tip. The former consists of a filter probe module located immediately behind the penetrometer tip to collect soil gas samples at discrete depth intervals during CPT advancement. This system has the advantage of collecting soil gas samples at multiple depth increments, while simultaneously obtaining soil behavior types with geotechnical sensors.
Several ground water sampling systems are available for use with the CPT. A typical system includes a sampler that is pushed to the proper ground water sampling zone and then withdrawn to expose an inlet screen. A small-diameter bailer or tubing with a foot valve can be lowered through the hollow-push rods and body of the sampler to collect the sample. If the sampled water comes into contact with the rods, the iron in them can have a significant effect on measured concentrations of analytes such as dissolved oxygen, iron and some trace metals, as well as changing reduction oxidation potentials. A variation on CPT ground water samplers consists of three basic components:
- A sealed filter tip with a retractable sleeve attached to
the push rods.
- An evacuated and sterilized glass sample vial enclosed in a housing
and lowered to the filter tip using a wireline system.
- A disposable, double-ended hypodermic needle that makes a hydraulic connection with the ground water by puncturing the self-sealing flexible septum in the filter tip.
The filling rate for the ground water sample vial is monitored using a pore pressure transducer attached to the vial. This monitoring shows when ground water infiltration is complete, ensuring that the pressure inside the vial is equal to the in-situ ground water pressure. These pressure measurements can be used to estimate the hydraulic conductivity of the soil.
Several more exotic ground water samplers also are used with CPT systems. One system is attached directly behind a standard CPT probe to obtain soil gas or ground water samples as the CPT probe is advanced, allowing rapid collection of samples. Another system uses an integrated pneumatic valving system to lift the sample to the surface from depths of more than 200 feet below ground surface.
Percussion hammer systems are designed to work in conjunction with a host of direct-push soil, soil gas, and ground water samplers, and a growing list of chemical and litholologic indicator instruments Three main types of soil samplers are used with such systems – discreet, continuous and dual-tube.
The discreet soil sampler is the most common of the four types. This sampler often uses a piston-activated system that can be pushed to the desired depth and then opened for collection of a sample from a discreet depth interval. Continuous soil sampler systems are very similar to the discreet sampler, but do not require piston activation systems. Dual-tube samplers create a casing around the area where soil will be collected with a continuous or discreet soil sampler.
A wide variety of ground water sampling and monitoring tools have been developed for use with percussion hammer systems. Ground water samplers include profilers, which are capable of collecting multiple, discreet samples during one downhole push, and standard samplers, which can be driven to a desired depth, at which time a screen is exposed and a sample is collected through use of a check-valve apparatus or pump. Percussion hammer systems also install prepacked monitoring wells. These wells usually are installed using a dual-tube system.
Percussion hammer systems also can deploy soil gas samplers. The samplers are designed to allow gas present in the vadose zone to be collected for chemical analysis.
Some direct-push platforms are equipped with an augerhead attachment that allows them to also use conventional hollow-stem augers. Augering is not as fast as direct-push, but can be useful when the push tool encounters refusal.
Geotechnical SensorsGeotechnical sensors used with CPT systems provide a rapid, reliable and economical means of determining the soil behavior type, relative density and strength, as well as hydrogeologic conditions such as the hydraulic conductivity and the static and dynamic pore pressure. Penetrometers house tip-resistance, sleeve-friction and piezometer sensors that are deployed and used during advancement of a borehole. Geotechnical sensors are designed for stratigraphic logging in soils as well as for identifying specific hydrogeologic properties of the subsurface. These instruments measure the amounts of resistance and friction placed on the probe as it is advanced through the subsurface, and correlates the measurements to estimate the types of soil present throughout the borehole. Although they originally were developed for CPTs, these sensors and the equipment have been adapted for percussion hammer units as well. The sensors are housed in a conical tip and cylindrical friction sleeve. The tips are about 5 inches long, and have a cross-sectional area ranging from 1.5 square inches to 2.5 square inches. Video cameras also have been developed that allow subsurface viewing.
Sensors or cameras are connected to the surface by electronic cables. Sensor cables are inserted through the push rods and connected to a multichannel data acquisition system at the surface. The multichannel data acquisition system is used to record and provide preliminary analysis of the sensor data. Video screens are used to view the signal from in-situ cameras.
A downhole soil conductivity sensor has been developed to map soil types. Soil conductivity and resistivity (the inverse of conductivity) have been used to identify changes in lithography or water quality. The power of this approach stems from the fact that higher electrical conductivities are representative of finer-grained sediments such as clay, whereas sand and gravel are characterized by distinctly lower electrical conductivities. Electrical conductivity logs have been used to identify changes in the salt content of ground water such as is found in leachate escaping from a landfill, and, in some instances, to indicate the presence of pools of non-aqueous phase liquids (NAPLs). Note that in the case of NAPL, the meter does not identify NAPL but notes an unexpected change in conductivity.
Downhole Analytical InstrumentsDirect-reading instruments that analyze inorganic and organic contamination have been designed to be advanced by CPT systems to characterize inorganic contamination in-situ. The detection limits of these techniques as deployed on a CPT tend to be high. These instruments are connected to analyzers and dataloggers at the surface by data cabling that runs inside the push rods. An example of in-situ organic detection technology, are the fuel fluorescence detectors used to collect in-situ measurements of hydrocarbons present in soil and ground water. These instruments, which provide measurements in near real time, are best used to compare relative concentrations of hydrocarbons rather than to provide absolute values.
The membrane interface probe (MIP) is a tool developed for measuring volatile organic compounds (VOCs) as it is advanced into the subsurface with a percussion hammer system. The MIP consists of a thin permeable membrane impregnated into a stainless steel screen. The screen is mounted flush to the exterior surface of the probe in an opening that allows direct contact with the medium being sampled. When the membrane is heated to between 100 degrees C and 120 degrees C, VOCs in soil or ground water migrate across the membrane and into the probe. Inside the probe, VOCs are transported to an analytical device at the surface by a carrier gas line. The carrier gas typically is nitrogen or helium. Analytical devices used with MIP include photoionization detectors, flame ionization detectors, electron capture detectors, and ion-trap mass spectrometers. Depending on the analytical equipment applied, the MIP can be used to identify VOCs present in soil or ground water at a given point or just show their relative presence.
This article is provided through the courtesy of the U.S. Environmental Protection Agency’s Office of Superfund Remediation and Technology.