Direct Push Sampling

Soil, groundwater, and soil gas samples can be collected by direct push (DP) technology. Both single tube and dual tube DP methods are used for sampling of soils, unconsolidated formations, and groundwater. DP methods also may be used to install monitoring wells and soil vapor sampling devices.

Related Article(s):

• Direct Push (DP) Technology
• Direct Push Logging

Contributor(s): Wesley McCall, M.S., P.G. and Tom Christy, P.E.

Key Resource(s):

• Using DP Technologies in Site Characterization and Groundwater Monitoring
• US EPA GW Sampling & Monitoring with Direct Push Report

Introduction

Direct push (DP) techniques were first used for cone penetration testing (CPT) in the 1930s. Then as the environmental industry developed it was applied initially to soil vapor sampling in the late 1980s to define the extent of volatile organic contamination. Shortly thereafter soil sampling, groundwater sampling, and groundwater monitoring methods and tools were developed.

DP Soil Sampling

Figure 1. Sampling sequence with an open-barrel, single-tube soil sampling system. The sample barrel is advanced through the open borehole to succeeding depths.

Figure 2. The outer casing of the dual-tube system remains in place to control the bore hole as the inner rod is used to trip out the sample barrel with soil core.

Common DP methods for the collection of soil samples include single-tube and dual-tube approaches. Single-tube methods (Fig. 1) utilize a single string of rods or casing to advance the sampler into the subsurface and samples are recovered incrementally as the boring is advanced. These methods are typically more quick and efficient for shallow applications (<20 ft). Samples are tripped out of the boring through the uncased borehole so formation collapse and cross contamination in non-cohesive and saturated formations may be problematic. However, closed piston, single-tube samplers can provide high integrity samples under most formation conditions. Dual tube samplers (Fig. 2) utilize concentric casings to advance the boring. The outer casing remains in place as the inner casing is used to trip out the sample as the boring is advanced incrementally. The outer casing prevents borehole collapse and generally reduces the potential for cross contamination between sampling intervals. Unfortunately, dual-tube methods are subject to formation heave when sampling in saturated sands. Under these conditions, water must be added to the outer casing as the samples are removed to prevent fluidized formation from entering the outer casing through the open cutting shoe as the sample is retrieved. Formation heave will complicate and retard the dual-tube sampling process. Under these formation conditions, a closed piston, single-tube sampler may provide higher integrity samples more efficiently. All sample contact equipment should be appropriately decontaminated between samples and locations. All borings should be properly grouted and abandoned.

DP Groundwater Sampling

Figure 3. Steps in the operation of a dual-tube groundwater sampler.

DP methods can provide a rapid and effective means to characterize the distribution of groundwater contamination in unconsolidated materials. Relatively small diameter tools can be advanced into the formation to temporarily set a screen at the desired location and depth. Installation, screen development, sampling, and device removal are often completed in 1 to 2 hours. Development of sandy formations around the screen is often performed with a simple inertial pump and groundwater yield is primarily controlled by local formation permeability. Data quality objectives are used to guide selection of the sampling device and amount of development conducted prior to sample collection. Samples can be collected for most contaminants of concern with a check valve, small bailer, a peristaltic or bladder pump. When adequately developed, these temporary piezometers may be slug tested to obtain information about the local formation hydraulic conductivity, helping to identify migration pathways and quantify risk.

Temporary DP groundwater samplers are available in single-tube and dual-tube designs. Protected screen, single-tube devices are used in many settings to obtain high quality, discrete interval samples. Dual-tube devices (Fig. 3) may be used to perform single depth, discrete interval sampling, or groundwater profiling depending on the tool design and advancement technique applied. Dual-tube systems are subject to formation heave in saturated sands which can complicate and delay progress. Under these conditions, a single-tube, protected screen device may be preferred.

Specialized multi-port groundwater profilers have been developed to provide details about the vertical distribution of groundwater contaminants and changes in groundwater chemistry. Some of these tools simultaneously provide logs (see Direct Push Logging) of variations in formation permeability and/or electrical conductivity.

Grouting with bentonite or cement slurries can be conducted bottom-up using a tremie tube and pump for many of these devices. Sometimes pressure grouting directly through the tool string is most effective. All sample contact parts should be decontaminated between locations.

DP Monitoring Well Installation

Figure 4. A DP installed monitoring well constructed with pre-packed well screens.

Figure 5. Structure of a pre-packed screen used to build monitoring wells with DP methods.

Direct push methods can be used to install monitoring wells (Fig. 4) that range in size from nominal ½ -inch (12 mm) to 2-inch (50 mm) inside diameter. Single-tube methods are often used with an expendable point to advance the casing to the desired depth. Alternately, dual-tube methods may be used with either a plug on the inner rod string or often by soil coring to advance to depth. Once at depth, a slotted PVC screen with riser is installed through the open bore of the casing to build the well. Because of the smaller diameter of most DP casing, prepacked well screens (Fig. 5) are often used to simplify the well construction process and save time. Annular seals are typically installed with a small tremie tube and grout pump. Some states limit use of DP wells due to the small grout annulus. The screened formation is developed with an inertial pump or surge block and samples are collected with a bailer, or peristaltic or bladder pump, depending on project requirements. Studies have found that DP installed wells provide representative samples. When properly developed, the DP wells can be slug tested to assess the local formation hydraulic conductivity.

Figure 6. Components of a direct push soil gas sampling tool string for active soil gas sampling.

Figure 7. Components of a screened implant system for long term monitoring of VOCs in soil gas.

Soil Vapor Sampling

Direct push methods are used to assess the presence, concentrations and distribution of volatile contaminants in the vadose zone (area above the water table). Common sampling approaches include both active soil gas sampling and installation of devices that passively collect volatile contaminants over a longer period (days to weeks). To develop an effective soil vapor sampling plan, project managers should understand the site specific soils, geology, and other environmental factors.

Initial soil gas surveys often involve the installation of small diameter DP rods with an expendable point (Fig. 6). Once installed at the desired depth, tubing with an adapter on the end is lowered down the rods and threaded into the back of the expendable point holder. The drive rods are retracted a fixed distance (often 6 to 12 inches) so that soil vapor from the formation may be sampled through the plastic or stainless steel tube at the surface. Usually the sample train is purged 3 system volumes before the soil gas samples are collected. Measures to test for leaks and verify sample integrity should be conducted at each location. If long term monitoring of soil vapor is required, small diameter (< 0.5-inch/12 mm) screened implants can be installed (Fig. 7). The implants are usually less than 1 ft (30 cm) in length and are made of stainless steel mesh. These devices are installed through small diameter DP rods and a mixture of fine glass beads and bentonite is used to seal the borehole as the drive rods are removed. A small well protector or similar device is placed over the installation to protect it between sampling events. The sample tube should be properly sealed between sampling events to assure sample integrity. If local regulations require a more rigorous installation, or if a longer interval must be monitored, DP monitoring wells may be used for soil vapor sampling and monitoring. Whichever device is installed, it will require purging and sampling. Once the sampling train is purged, the desired gas sampling device is placed in line and sampling is conducted. Some sampling devices include glass gas sampling bulbs, Tedlar bags, or summa canisters. These temporary borings must be properly abandoned with bentonite or cement when sampling is completed.

Summary

Direct push methods provide useful methods for sampling of soils, unconsolidated formations, groundwater and soil gas in many settings. The use of single-tube versus dual-tube methods should be guided by project data quality objectives and site specific conditions. DP methods can minimize the generation of waste cuttings and potential worker exposure to hazardous contaminants for many environmental investigations.

See Also

• Superfund Website
• General Direct Push Equipment and Tools
• General Direct Push Equipment and Tools – AMS Samplers
• Direct Push Equipment and Tools – 660 Drive Point Profiler
• Expedited Site Assessment Tools – UST
• Use of Dynamic Work Strategies Under a Triad Approach for Site Assessment and Cleanup - Technology Bulletin
• Characterization and Monitoring Technology Descriptions for Cleaning Up Contaminated Sites
• Cryogenic DP Coring
• CPT/SCAPS Thermal Desorption Soil Profiling for VOCs
• CPT/SCAPS Hydrosparge for VOCs
• Demonstration/Validation of DP Well Technology
• Summary of DP Well Comparison Study
• EPA GW Sampling & Mon. with DP Technologies
• Monitoring and Characterization
• Low Stress (Low Flow) Purging and Sampling Procedure for the Collection of Groundwater Samples from Monitoring Wells from January 19, 2010
• Sources of Variability in Groundwater Sampling LTM
• VI Brief Review and Definition
• Basic Introduction to VI *Basic Chemical and Physical Properties of TCE and Exposure Info
• OSWER Technical Guide for Assessing and Mitigating the VI Pathway from Subsurface Vapor Sources to Indoor Air
• Investigation of VI
• Utility Lines for VI
• Demonstration/Validation of Long-Term Monitoring Using Wells Installed by Direct-Push Technologies

Videos

• Dual Tube DT22 Soil and Groundwater Sampling
• Dual Tube DT325 Soil Coring Operation
• Demonstration of 3.25-inch Tools for Sampling Soil, MW Install, SPT Testing
• Soil Gas Sampling with a Tedlar Bag and Vacuum Chamber