Direct-Push Platforms

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Introduction

Direct-push platforms have gained widespread acceptance in the environmental industry over the past decade 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 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 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.

The following sections provide the reader with an introduction to the two major classes of direct-push platforms. A brief description of each platform and the theory of operation for each are provided. Subsequent sections provide details on the platforms’ modes of operation; system components; types of sampling, analytical, and geotechnical equipment that have been developed for both platforms; throughput information; and a breakdown of their advantages and limitations. The final section provides links to additional on-line resources, project case studies, and contact information and a cursory cost analysis.

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Description

Cone Penetrometer

CPT systems are generally the larger of the two direct-push platforms. CPT systems are usually mounted on a 10- to 30-ton truck, as illustrated in the illustration to the right. 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 groundwater 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.

Percussion Hammer

In contrast to their larger cousins, percussion hammer systems are usually mounted on pick-up trucks or tracks; however some equipment can be mounted on much larger vehicles (see photo below). 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 groundwater samples are commonly 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 can also be performed for better delineation of stratigraphy and detection of dense nonaqueous phase liquids (DNAPLs).

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Mode of Operation

Cone Penetrometer

Because of the complexity of sensor systems and the specialized requirements for operating CPTs, their operation calls for considerable experience. For this reason, CPTs are designed to be operated by trained technicians, not the general public. Most systems are typically 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 (bgs). 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.

Percussion Hammer

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 is often required when probing near the ground surface to penetrate hard-packed soil. The probe is then 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.

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System Components

Cone Penetrometer

Unlike most percussion hammer systems, the hydraulic ram apparatus and all support systems are enclosed within the CPT truck. CPT push rods are typically 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 may also 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.

For a diagram of a typical CPT system, click here.

Percussion Hammer

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 to 35,000 pounds. The extraction force, which is necessary to remove tools from the subsurface, ranges from 13,000 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.

For a diagram of a typical percussion hammer setup, click here.

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Samplers and Analytical Instruments

Cone Penetrometer

CPT systems can advance a full range of soil, soil gas, and groundwater samplers and a growing list of analytical instruments. Specially-designed samplers are used to collect high-quality groundwater, 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 can also be collected. These instruments are briefly discussed below.

Many of the soil, soil gas, and groundwater 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 groundwater 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 are then retrieved. For environmental analyses, the soil sample tube ends are sealed with Teflon tape and plastic caps. 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 groundwater sampling systems are available for use with the CPT. A typical system includes a sampler that is pushed to the proper groundwater 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 affect on measured concentrations of analytes such as dissolved oxygen, iron, and some trace metals as well as changing reduction oxidation potentials. A variation CPT groundwater 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 groundwater by puncturing the self-sealing flexible septum in the filter tip

The filling rate for the groundwater sample vial is monitored using a pore pressure transducer attached to the vial. This monitoring shows when groundwater infiltration is complete, ensuring that the pressure inside the vial is equal to the in situ groundwater pressure. These pressure measurements can be used to estimate the hydraulic conductivity of the soil.

Several more exotic groundwater samplers are also used with CPT systems. One system is attached directly behind a standard CPT probe to obtain soil gas or groundwater 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 over 200 feet below ground surface (bgs).

Direct-push samplers are discussed in detail in the following encyclopedia entries:

• Direct-Push Soil and Soil-Gas Samplers
• Direct-Push Groundwater-Samplers

Percussion Hammer

Percussion hammer systems are designed to work in conjunction with a host of direct-push soil, soil gas, and groundwater 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 groundwater sampling and monitoring tools have been developed for use with percussion hammer systems. Groundwater 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 are usually installed using a dual tube system. Currently, outside diameters of the drive casing are available up to 4.5 inches.

Percussion hammer systems can also 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 (e.g., 4.25 inch outside diameter). Augering is not as fast as direct push but can be useful when the push tool encounters refusal.

Direct-push samplers are discussed in detail in the following encyclopedia entries:

• Direct-Push Soil and Soil-Gas Samplers
• Direct-Push Groundwater Samplers

Geotechnical Sensors

Cone Penetrometer

Geotechnical sensors used with CPT systems provide a rapid, reliable, and economical means of determining the soil behavior type, relative density, 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 were originally 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 to 2.5 square inches. Video cameras have also 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, as shown in this figure. 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.

Percussion Hammer

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 groundwater such as is found in leachate escaping from a landfill and in some instances to indicate the presence of pools of NAPL. Note that in the case of NAPL the meter does not identify NAPL but notes an unexpected change in conductivity.

Geotechnical sensors are addressed in detail in the following encyclopedia entry:

• Direct-Push Geotechnical Sensors

Downhole Analytical Instruments

Analytical systems are addressed in detail the following encyclopedia module:

• Direct-Push Analytical Instruments

Brief descriptions of some of these instruments are provided below.

Cone Penetrometer

Direct-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 data loggers 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 groundwater. These instruments, which provide measurements in near real time, are best used to compare relative concentrations of hydrocarbons rather than to provide absolute values. A more detailed discussion of the fluorescence instruments can be found at: Laser-Induced Fluorescence

Percussion Hammer

The membrane interface probe (MIP) is a tool developed for measuring 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 and 120^o Celsius, VOCs in soil or groundwater 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 is typically 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 groundwater at a given point or just show their relative presence.

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Throughput

Cone Penetrometer

When geotechnical instruments are advanced, the push rods are typically advanced at a controlled rate of 1 to 2 centimeters per second. CPT systems are capable of pushing CPT; groundwater, soil gas, and soil samplers; and piezometers to depths in excess of 100 feet bgs in unconsolidated material. This methodology provides detailed hydrogeologic profiling at an average rate of 400 to 500 linear feet per day, depending on the subsurface materials and conditions. Collection times for samples vary depending on many factors, including the media being sampled, sampling depth, presence and abundance of groundwater, and types of soils, as well as whether the soils are conducive to soil gas sampling. For example, collecting shallow groundwater samples through the push rods may be quick, whereas collecting deep soil samples take longer. Clay can be more difficult and slower to penetrate than most sandy soils.

Percussion Hammer

With a percussion hammer system, the throughput associated with sampling downhole geotechnical or analytical sensors is highly dependent on the material present in the subsurface and the objective of the sampling or sensing event. Under typical pushing conditions, a percussion hammer system can push about 8 to 10 30-foot holes per eight hour day. Actual throughput will vary by the tools being used and the objectives of the investigation.

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Advantages and Limitations

Cone Penetrometer and Percussion Hammer

Following are advantages and limitations associated with direct-push technologies in general and should be considered when determining whether they are appropriate for use on a site or project. Advantages and limitations specific to either the CPT or the percussion hammer systems will be discussed in the next section.

• Unless using dual tube continuous coring, direct-push technologies do not generate "cuttings" or excess soil, so there is no potentially contaminated soil to dispose of. Costs of investigation are reduced, and the process is simplified.
• Direct-push systems are quicker and more mobile than traditional drill rigs. Sampling and data collection are faster, reducing the time needed to complete an investigation and increasing the number of sample points that can be collected during the investigation. In situ emplacement of geophysical and analytical instruments allows a great deal of information about subsurface soils and contaminants to be collected in near real time. Closed sampling systems and on-board analytical instruments allow samples to be analyzed in the field, avoiding laboratory turnaround time, remobilization time, and associated expenses.
• Direct push systems do not have high masts and hence can better operate where there is conventional overhead electrical wiring.
• Direct push systems can be used to install prepacked monitoring wells. While prepacked wells are generally smaller in diameter than the conventional 2-inch outside diameter well, the largest diameter dual tube has been used to place a 2-inch monitoring well. The costs associated with installing a prepacked well are substantially less than the costs of installing a monitoring well with a traditional drill rig.
• Direct-push technologies are limited to unconsolidated materials and are limited in their penetration depths. They cannot be used to penetrate bedrock layers, concrete footings or foundations, and sometimes caliche layers and very fine grained saturated sands can cause refusal.
• Changes in geological density can limit the use of these technologies. The presence of soft layers overlying hard layers can alter the alignment of the probe and can bend, break, or refuse the rod.

Cone Penetrometer

The following are advantages and limitations associated with CPT systems in particular that should be considered when determining the technologies used on a site or project:

• Because of the complexity of the analytical methods and the specialized requirements for operating CPTs, their operation takes considerable experience. For this reason, most CPTs are designed to be operated by trained technicians.
• Most CPT systems are limited by their size and mass. They cannot be used in tight quarters as readily as many of the percussion hammer configurations.

Percussion Hammer

The following are advantages and limitations associated with hammer systems in particular that should be considered when determining the technologies used on a site or project:

• Because percussion hammer systems can be installed on numerous size platforms, with varying mobility, they are more likely (than a CPT or conventional drill rig) to access areas within buildings or off-road.

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Cost Data

Cone Penetrometer

CPT systems are usually obtained from a company that provides trained operators and analysts along with the CPT. The specialized requirements for operating a CPT and complexity of the analytical methods call for considerable experience. Typical costs for a CPT system with lithologic and sampling tools varies but generally is over $13,000 per day, not including mobilization costs. Additional analytical instrumentation and operators increase the costs. Links to on-line resources and vendors are provided below.

Percussion Hammer

Percussion hammer systems are also usually obtained from a company that provides trained operators along with system. The cost to rent a platform and the tooling depends on the size of the hammer system and the tooling or sensors required. A daily cost for an average-size rotary hammer system, outfitted with samplers and a two-man sampling crew, averages around $1,500, not including mobilization costs. Additional costs would be incurred if analytical or geotechnical sensors were required.