Direct-Push Platforms

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. As opposed to drilling techniques in which soil is removed and a borehole is produced, 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.  With this no soil is removed, and only a very small borehole is created. The two major classes of direct-push platforms are cone penetrometer (CPT) and rotary hammer systems. While “CPT” technically refers only to the geotechnical cone penetrometer instruments advanced by these large 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 alone, while rotary 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 rotary 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 for direct-push vendors.

Description

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 rotary 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. 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.

ROTARY HAMMER

In contrast to their larger cousins, rotary hammer systems are usually mounted on pick-up trucks or tracks. A rotary 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 CPT systems. Geotechnical sensors used with a rotary hammer system include tip-resistance sensors (also used by CPT systems) that map soil texture 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.

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.

ROTARY HAMMER

A rotary hammer system directly pushes 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 and gravel zones. The probe is then allowed to penetrate using only static force until resistance is again encountered, at which time percussion is reapplied. Percussion is applied as required when advancing through sand, gravel, hard pan, fill material, and surface frost.

Compared to a CPT system, rotary 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 Components

CONE PENETROMETER

Unlike most rotary 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.

ROTARY HAMMER

The depth capability of a rotary hammer system depends on the amount of force of the hammer and the static weight of the vehicle in which the system is mounted. The "pushing" of tools into the subsurface depends on the hammer force (torque) of the system, which ranges from 135 to 613 foot-pounds, and the pull-down force, which ranges from 250 to 30,000 pounds. The extraction force, which is necessary to remove tools from the subsurface, ranges from 13,000 to 70,000 pounds.

Rotary 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. Rotary hammer systems have also been outfitted on track-mounted vehicles and ATVs that permit access to off-road areas.

Rotary 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 rotary hammer setup, click here.

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. Special-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 stratigraphy, relative density, and strength, (hydrogeologic conditions such as hydraulic conductivity and static and dynamic pore pressure. These instruments are briefly discussed below.

Many of the soil, soil gas, and groundwater samplers resemble the physical samplers used with rotary 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 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 stratigraphy 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. A variation on 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

ROTARY HAMMER

Rotary hammer systems also may advance a full range of soil, soil gas, and groundwater samplers and a growing list of chemical and litholologic indicator instruments. Many of the samplers resemble the physical samplers used with CPT systems; the sampler is advanced by the rod and the sample material is retrieved. This is achieved by retrieving the rod and sampler, or by physically collecting a sample through the rod (in the case of soil gas or groundwater samples).

Rotary hammer systems are designed to work in conjunction with a host of direct-push soil, soil gas, and groundwater sampling apparatus. Four main types of soil samplers are used with such systems: discreet, continuous, dual tube, and hollow-stem auger. The hollow-stem auger is used only with the most advanced rotary hammer systems that have sufficient torque to advance the auger flights. This system is capable of installing small-diameter monitoring wells or acting as a temporary casing for other direct-push samplers in collapsing soils. 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 rotary hammer systems. Groundwater samplers include (1) profilers, which are capable of collecting multiple, discreet samples during one downhole push, and (2) standard samplers, which can be driven to a desired depth, at which a screen is exposed and a sample is collected through use of a check-valve apparatus or pump. Rotary hammer systems also install prepacked monitoring wells. Several state regulatory agencies have accepted the use of these monitoring devices for contaminated groundwater sites.

Rotary 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.

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 stratigraphy, 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 rotary 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.

ROTARY 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 classify soils. The power of this approach stems from the fact that higher electrical conductivities are representative of finer-grained sediments such as silts or clays, whereas sands and gravels are characterized by distinctly lower electrical conductivities. Electrical conductivity logs are used to define zones of lower conductivity consisting of coarser-grained, more permeable sediments, that allow movement of contaminants (hydrocarbons, chlorinated VOCs, or metals) in the subsurface.

Sensors or cameras are connected to the surface by electronic cables. Sensor cables are inserted through the push rods and connected to a data acquisition system (laptop computer) at the surface.

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

• Direct-Push Geotechnical Sensors
  

• Direct-Push Analytical Instruments
  

CONE PENETROMETER

An electrical resistivity cone can be used with a CPT system to accurately profile the presence of DNAPLs in the subsurface. X-ray fluorescence and laser-induced breakdown spectroscopy units have also been developed to characterize inorganic contamination in situ. In addition, several innovative sampling systems have been developed that combine the best of sampling and analytical instruments. These closed systems convey samples from the rod tip directly to the surface, where they are analyzed by equipment such as mass spectrometers. Another example, the Hydrosparge system, which was originally developed for CPT systems, is a groundwater sampler through which nitrogen gas is passed to purge VOC from groundwater; the VOCs are carried to the surface through a closed tube that is directly connected to a gas chromatograph analytical system. Many of these systems were originally developed for either CPT or rotary hammer application but have since been adapted for both platforms. For instance, a membrane interface probe (MIP), which is a tool developed for logging but is now used with CPT systems as well. The MIP is discussed in the next section.

ROTARY HAMMER

An MIP is a tool developed for logging VOCs as it is advanced with a rotary hammer system. This permeable-membrane device is used to detect VOCs as it is advanced into the subsurface. As the operator advances the MIP sensor into the subsurface, a log is displayed on a screen by the field computer. This log contains information about total volatile contamination provided by a photo-ionization detector (PID), a flame ionization detector (FID), or both detectors in series. The real-time log also contains a depth and speed graph, an electrical log for the formation, and temperature log for the heated sensor. Running MIP logs on a grid or targeted pattern for a given area provides the investigator with a three-dimensional for view of the volatile contaminant distribution and the lithology.

Laser-induced Fluorescence

Direct-reading instruments that analyze organic and inorganic contamination have been designed to be advanced by CPT systems. These instruments are connected to analyzers and data loggers at the surface by data cabling that runs inside the push rods. Examples include fuel fluorescence detectors used to collect in situ measurements of hydrocarbons present in soil and groundwater. Several of these laser-induced fluorescence (LIF) units have been developed and marketed for delineating petroleum hydrocarbon plumes in situ and in near real time. LIF is the subject of another, more detailed encyclopedia entry that can be accessed using the following link: Laser-Induced Fluorescence

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 lineal 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. Soils such as impermeable clays may be more difficult and slower to penetrate than sandy soils, and may prevent soil gas samples from being collected.

ROTARY HAMMER

With a rotary 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 rotary hammer system can push tooling into the ground at about 2 inches per second. Conversely, a tool string can be retracted from the ground at about 4 inches per second. At these rates, the daily average for lineal feet pushed would be about 1,200 feet per day (based on an 8-hour work day) if tooling is both advanced into and extracted from the subsurface.

Collection times for samples vary depending on many factors, including the media being sampled, sampling depth, presence and relative abundance of groundwater, and types of soils are conducive to soil gas sampling. For example, collecting shallow groundwater and soil samples will require less time than deep soil or groundwater samples samples. Soils such as tight clays may be more difficult and slower to penetrate than sandy soils and may prevent soil gas samples from being collected.

Advantages and Limitations

CONE PENETROMETER AND ROTARY 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 rotary hammer systems will be discussed in the next section.

• Direct-push technologies do not generate "cuttings", so there is no potentially contaminated soil to dispose of. Costs of investigation are reduced, and the process is simplified.
  
  
• Unlike conventional drilling techniques, direct-push technologies do not introduce items like well casings into the sampling zone. Therefore, the potential for introducing contamination into the subsurface is reduced.
  
  
• 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 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, or other high-density barriers. Although while rotary hammer units may advance a drill bit through concrete, CPT units are not equipped to do this because they have no rotary capability. CPTs can often be advanced to depths greater than 100 feet bgs, but they cannot be used to collect deep samples as traditional augers can.
  
  
• Changes in geological density can limit the use of these technologies. The presence of soft layers overlying hard layers can alter in 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.
  
  
• CPTs systems are limited by their size and mass. They cannot be used in tight quarters or on sensitive surfaces (such as residential lawns) as readily as many of the rotary hammer configurations.
  
  
• With conventional methods such as augering, a well is installed to collect samples over time. CPT systems cannot install full-sized wells such as traditional monitoring wells or extraction wells. Mini-wells have been developed that can be set in place by CPT units, and these mini-wells must be sampled using small-diameter samplers or tubing, limiting their usefulness for some applications. Regulatory agency acceptance of mini-wells varies by state.

ROTARY HAMMER

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

• Because of the complexity of the sampling tools and sensors deployed by rotary hammer systems, their operation takes considerable experience. For this reason, rotary hammer systems should be operated by trained individuals.
  
  
• Rotary hammer systems can be used to install prepacked monitoring wells that are approved by several state regulatory agencies for use in monitoring contaminated aquifers. The costs associated with installing a prepacked well are substantially less than the costs of installing a monitoring well with a traditional drill rig.
  
  
• Because rotary 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.

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 can vary from $1,000 to $2,000 per day, not including mobilization costs. Additional analytical instrumentation and operators increase the costs. Links to on-line resources and vendors are provided below.

Applied Research Associates

Fugro Geosciences, Inc.

Gregg Drilling and Testing, Inc.

ROTARY HAMMER

Rotary 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 rotary 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, ranges from $1,000 to $1,500, not including mobilization costs. Additional costs would be incurred if analytical or geotechnical sensors were required. Links to on-line resources and vendors are provided below.

Geoprobe Systems

AMS Inc.

Precision Sampling Incorporated

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