Direct-push technology (DPT) devices are investigation tools that drive or push small-diameter rods and tools (typically not exceeding four inches in diameter) into the subsurface by hydraulic or percussive methods. Specialized direct-push probes may be used to collect in-situ geophysical, geochemical and geotechnical measurements.
Open-barrel SamplersOpen-barrel samplers have an open end, allowing material to enter at any time or depth. They also may be referred to as unprotected or unsealed samplers. The three most commonly used open-barrel samplers are open solid-barrel, split-barrel (or split-spoon) and thin-walled. Available sampler lengths range from 1 foot to 5 feet. Split-barrel and thin-walled samplers also are commonly used with hollow-stem augers.
Open solid-barrel samplers consist of a head assembly, a barrel and a drive shoe. The sampler is attached to the DPT rods at the head assembly. A check valve, which allows air or water to escape as the barrel fills with soil, is located within the head assembly. The check valve improves the amount of soil recovered in each sample by allowing air to escape. With the use of liners, samples easily can be removed for volatile organic compound (VOC) analysis or for observation of soil structure.
Split-barrel samplers, also often used with hollow-stem augers, are similar to open solid-barrel samplers except that the barrels are split longitudinally so that the sampler can be easily opened. The primary advantage of split-barrel samplers is that they allow direct observation of soil cores without the use of liners, and without physically extruding the soil core. As a result, split-barrel samplers often are used for geologic logging. Split-barrel samplers, however, may cause more soil compaction than open solid-barrel samplers because the tool-wall thickness often is greater. Although liners are not compatible with all split-barrel samplers, they may be used to reduce the need for decontamination.
Thin-walled tube samplers, similar to larger-diameter samplers known as Shelby tubes, are used with both DPT and hollow-stem augers for collecting undisturbed samples. The sampling tube typically is attached to the sampler head using recessed cap screws orrubber expanding bushings. The sampler walls, made of thin steel with a sharpened cutting edge, minimize soil compaction compared to other types of samplers. Relatively undisturbed samples are required for certain geotechnical analyses such as permeability and tri-axial shear tests.
Closed-barrel SamplersPiston samplers are similar to open solid-barrel samplers, except that the opening is sealed with a rigid, pointed piston that displaces soil as it is advanced. When the sampler has been pushed to the desired depth, the piston is unlocked by releasing a retaining device, and subsequent pushing or driving forces soil into the sampler. The assembly then can be removed and soil extracted. Piston samplers typically are air- and water-tight; however, if o-ring seals are not maintained, leakage may occur. Piston samplers also have the advantage of increasing the recovery of unconsolidated sediments as a result of the relative vacuum that is created by the movement of the piston.
Sampling Method EvaluationDPT offers many advantages for soil sampling, including fast site assessment, rig mobility and greater efficiency than conventional drilling methods. Use of available sampler lengths of up to 5 feet can allow for more continuous sampling intervals than are possible with conventional split-spoon samplers. In general, DPT is applicable in unconsolidated sediments that are conducive to withdrawing sufficient soil volume for analysis. Where sample quality or quantity may not meet sampling objectives, alternatives such as switching to a different sampling method (e.g., hollow-stem auger), or a different sampling tool (e.g., a wider diameter sampler) should be employed.
Single-tube ConsiderationsBecause only one string of rods is used, single-tube systems are not as heavy as two-tube systems, and enable quicker rod connection. Because of the lightweight rod string, situations in which a single sample is desired or with shallow sampling depths can besampled quickly and easily with few complications. However, the lack of hole casing can cause some complications when continuous sampling to depths greater than 10 feet is desired. Sampling with single-tube rods can be slower because the probehole may collapse or slough without the stabilization of an outer casing rod. A second drawback of single-tube systems is the potential for formation or saturated zone cross-contamination during continuous sampling. Because the probehole is uncased during rod retraction and reinsertion, the open probehole can serve as a conduit for potentially contaminated soils or ground water from overlying zones that may slough or migrate to otherwise uncontaminated lower zones. Cross contamination is of particular concern if NAPLs (non-aqueous phase liquids) are present that could migrate down the probe- hole. Also, when multiple samples are taken, repeated entry can deform or skew the alignment of the probehole. This can create problems when sealing the probehole if a skewed hole prevents complete insertion of a tremie pipe, or permits bridging of bentonite pellets or granules.
With no outer casing in place to guard the sampler during rod reinsertion, sample biasing may be exacerbated by probehole collapse, probehole sloughing or probehole smearing. Thus, the sampler may collect soil samples from varying zones as it is advanced to the target depth. If the displaced material is contaminated or contains analytes at levels higher than the target depth, the target sample may be biased high. Conversely, if the collapsed material is cleaner or contains analytes at concentrations lower than the target depth, the target sample may be biased low. In either case, the sample collected is not representative of the target depth.
Depending on site conditions, the collection of undisturbed samples using thin-walled samplers with DPT may or may not be feasible. In general, DPT is applicable in unconsolidated sediments that are conducive to withdrawing sufficient soil volume for analysis. Consequently, hollow-stem auger rigs are better equipped to push thin-walled sample tubes over a wider range of soil condition than DPT rigs.
Two-tube ConsiderationsThe use of the outer casing in a two-tube system has several advantages. Two-tube systems are faster than single-rod systems for continuous sampling at deeper sampling depths (i.e., < 10 ft.). Because only the inner sample barrel is removed, and not the entire rod string, reaching the target depth is not complicated by probehole sloughing and collapse. In addition, the outer casing maintains the probehole’s alignment during multiple insertions of the sampling rod. This helps assure a proper grout seal. Because the outer casing never is removed during sampling, the probehole remains sealed, reducing the potential for sloughing of contaminated soil or migration of contaminated fluids down the hole. The outer casing also protects non-sealed samplers from sample biasing caused by smearing. In addition, the outer casing enables the use of non-sealed samplers for vertical contaminant profiling above the saturated zone.
Use of the outer casing in a two-tube system also has disadvantages. It is heavier, requiring twice as much rod and a heavier rig, thus making it more cumbersome and more expensive to use. The two-tube system also is more susceptible to soil friction because of its larger diameter, slowing boring and sampling. An oversized drive shoe sometimes is used to reduce friction and buckling, but may increase the risk of contamination migration down the probehole. Even using heavier driving equipment, penetration depths often are not as great as those possible with single-rod systems due to the increased friction.
Some RecommendationsDPT is appropriate for soil sampling when:
- Its use and methodologies are consistent with the data
quality objectives of the sampling program.
- Unconsolidated sediments are to be sampled.
- Materials to be sampled contain a low percentage of gravel and
cobbles, and are not dense or highly compacted.
- Materials to be sampled are less than 100 feet in depth. DPT may be able to probe deeper in some circumstances.
If using DPT:
- Two-tube sampling should be used whenever possible. This
especially is important if NAPLs are present, or if there is a potential for
sloughing to a lower zone. If a single tube is used for vertical profilers, it
is imperative that sealed samplers are used.
- Closed-barrel samplers should be used for most applications. The
only situation where non-sealed samplers would be acceptable is with
single-sample collection events above the saturated zone.
- If recovery of samples or cave-in of the probehole is problematic,
the data quality objectives should be supplemented or different sampling
- Probeholes should be sealed using retraction grouting with a tremie tube and a liquid slurry material. However, surface pouring may be appropriate for shallow probeholes (< 10 ft. deep) in cohesive formations.
This article is provided through the courtesy of the Ohio Environmental Protection Agency’s Division of Drinking and Ground Waters. For more information, visit www.epa.state.oh.us/ddagw.