Subsurface Samples

Subsurface samples should consist of undisturbed soil starting at the depth below the surface speci-fied in the SAP. Subsurface samples should not contain material from different layers of soil.

Collection of subsurface soil samples requires that the overlying soil be removed. The SAP should detail material separation and depths of subsurface sampling, including sample mixing, splitting, and representativeness.

Figure 4-2. Bucket Auger

A variety of tools, ranging from handheld me-thods to larger fuel-powered meme-thods, can be used to access unconsolidated soils. Consolidated soils can be difficult to sample because they are difficult to penetrate. Different drilling methods can be used to access consolidated soils. Truck-mounted drill rigs can gain access to soil at any reasonable depth and can penetrate hard layers.

When used with a hollow-stem auger, a drill rig can retrieve reasonably undisturbed cores for classification purposes. The following subsec-tions describe the various techniques used to

access both unconsolidated and consolidated subsurface soils.

4.6.2.1. Augers

An auger is a hole-making tool that is screwed into the soil; see Figure 4-3. The auger is used to bore a hole to a desired sampling depth and is then withdrawn. In general, augers should be used only to gain access to the soil that is to be sampled. Augers churn the soil and destroy its structure, making soil classification more difficult and causing rapid release of volatile contami-nants. Also, when augers are lowered into bore-holes, they can scrape soil off the sides of the hole so samples collected from the bottom of the hole may be contaminated with soil from shal-lower levels.

Figure 4-3. Augers

Various types of augers are available. The major types of augers, their advantages, and their limita-tions are summarized in Table 4-1.

Table 4-1. Types of Augers Sampling

Device Applications Limitations Screw

Auger

Cohesive, soft or hard soils, or residues

Will not retain dry, loose, or granular material Standard

Bucket Auger

General soil or residue

May not retain dry, loose, or granular material Sand

Bucket Auger

Bit designed to retain dry, loose, or granular material, silt, sand, and gravel

Difficult to advance boring in cohesive soils

Mud Bucket Auger

Bit and bucket designed to wet silt and clay soil or residue

Will not retain dry, loose, or wet, fibrous, or rooted soils (marshes) In situ Soil

Recovery Auger

Collection of soil sample in

Stony soils and asphalt

A hand auger can be used to expose soil as deep as 4 feet. Gasoline-powered portable augers may be able to reach up to 12 feet depending on soil conditions, but are heavy, require two people to operate, and may contaminate samples when volatile organics are to be sampled.

The hollow-stem auger is commonly used in unconsolidated soils. The hollow-stem auger column rotates as it drills into the ground and is designed to push soil up and out of the borehole along the outside of the auger. The auger itself is driven either mechanically or by a

hydraulically-powered drill rig. A plug is placed through the auger to prevent soil from rising through the hollow portion of the stem. Samples are retrieved by retracting the plug and lowering the sample collection tube through the auger. Casings and screens for access holes can be placed in the hollow stem to prevent the borehole wall from collapsing and to ensure discrete interval sam-ples. Casings also can be used to isolate near-surface contamination while drilling continues using a smaller diameter auger.

Auger Procedures

1. Clear the area to be sampled of any surface debris, such as twigs, rocks, or litter. It may be advisable to remove the first 3 – 6 inches of surface soil from an area 1 foot in diameter to prevent loose near-surface soil particles from falling down the hole.

2. Attach the auger bit to a drill rod extension, and then attach the drill rod to the T handle or the power unit.

3. Begin drilling, periodically removing, and depositing accumulated soils onto a plastic sheet spread near the hole. This practice pre-vents accidental brushing of loose material back down the borehole when removing the auger or adding drill rods, facilitates refilling of the hole, and avoids possible contamination of the surrounding area.

4. After reaching the desired depth, slowly and carefully remove the auger from the hole.

4.6.2.2. Direct-Push Techniques

Direct-push methods involve pushing a small-diameter hollow steel rod into the ground to a selected depth and can be used in most materials that can be augered or sampled with a split spoon. Direct-push techniques typically rely on a truck-mounted hydraulic ram to gain access to subsurface soils. The hydraulic ram pushes a closed-end ¾ – 2-inch diameter sampler into the soil, down to the sampling depth. The sampler contains a removable sampling tube and is capped with a retractable penetrating point. After the sampler is pushed to the desired depth, the

pene-trating point is retracted and the sampler is ad-vanced to collect the soil sample inside the re-movable tube. This sampler can reach depths of 30 – 60 feet or more in sand or fairly loose soil, but has trouble penetrating clay and hard mate-rials. As with any sampling device, hydraulic rams have both advantages and disadvantages.

The advantages of direct-push technology include the following:

 Retrieves relatively undisturbed soil cores up to 18 inches long. By collecting cores from different depths at closely adjacent locations, it is possible to develop a continuous core for soil classification purposes.

 Collects samples rapidly- one core sample can be collected every 10 minutes

 Minimizes the release of organic vapors, reducing personnel exposure hazards

 Generates little to no waste soil as compared to other drilling methods.

The primary disadvantages of direct-push tech-nology include the following:

 Requires the rental of special equipment and trained operators

 Soil cores are only ½ inch in diameter, so several core samples may have to be obtained from a single sampling location to meet the laboratory sample quantity requirements.

Direct-push technology has rapidly become the method of choice for collection of subsurface soil samples due to its speed and low cost. There are currently numerous commercial environmental drilling companies located throughout the country that can provide direct-push soil sampling servic-es.

4.6.2.3. Sonic Drilling

Sonic drilling is used for continuous sampling in unconsolidated and soft/fractured bedrock. The primary benefit of this technology is that very rapid drilling rates, combined with reduced vo-lumes of secondary waste, are possible. Recent improvements in equipment design could lead to increased use in the future. A sonic rig uses an

oscillator or head with eccentric weights driven by hydraulic motors to generate significant sinu-soidal force in a rotating pipe drill. The frequen-cy of vibration (generally between 50 and 120 cycles per second) of the drill bit or core barrel can be varied to allow for the optimum penetra-tion of subsurface materials. A dual-string as-sembly allows for the advancement of casing along with the inner casing used to collect sam-ples. Small amounts of air and water can be used to remove the material between the inner and outer casings. When a drill bit is used, most of the cuttings are forced into the borehole wall. A thin-wall or split-spoon sampler can be used to obtain continuous samples. Sonic drilling also is referred to as vibratory drilling and rotosonic drilling.

4.6.2.4. Cable Tool

Cable tool drilling is well suited for areas conta-minated by hazardous substances because it does not use any circulation fluids that could potential-ly spread contamination. Auger drilling and sonic drilling are the only other drilling methods that do not use circulation fluids. Of these two, only sonic drilling has demonstrated the ability to contain contaminants as effectively as the cable tool.

There are two types of cable tool drilling methods used in the field.

 Hard Tooling (Percussion Drilling). Best used below the water table in areas where unsaturated zone soils become consolidated.

 Drive Barrel (Dry Drilling). This technique is appropriate for relatively dry,

unconsolidated soils, such as sand and gravel, often found in the unsaturated zone (i.e., the soil above the water table).

Cable tool drilling rigs operate by repeatedly lifting and dropping a heavy string of drilling tools attached to a cable into the borehole. The drilling string of a cable tool consists of five components:

1. Consolidated rock is broken or crushed into small fragments, and unconsolidated material is loosened by the drill bit or shoe.

2. The drilling stem connects the drill bit to the drilling jars.

3. The drilling jars are used to vibrate the drill bit free of the formation.

4. The swivel or rope socket connects the re-maining drill tools to the cable.

5. The cable is strung over a pulley on the mast to the drill motor.

Hard tooling is the most common form of cable tool drilling and can be used in any formation, including basalt. The percussive action (produc-ing a noise level of 53 – 115 decibels) of the drill bit crushes the formation. The cable is attached to an eccentric walking or spudding beam that also serves to mix the crushed or loosened par-ticles with water (generally 10 – 20 gallons if no water is present in the formation) to form slurry at the bottom of the borehole. Periodically, the drilling string is removed, and the slurry is pulled from the borehole using a sand pump or bailer.

The drive barrel method uses the cable tool rig to drive the drill casing into the soil. The soil is pushed inside of the casing and then collected in a split-spoon sampler or core barrel.

4.6.2.5. Test Pits

Under unusual circumstances, the excavation of a test pit may be useful in determining the depth and thickness of different types of soil or any apparent band of soil contamination. A backhoe can be used to remove sections of soil. This method probably is the most expensive method of accessing soils for sampling due to the relatively high cost of backhoe operation.

Samples should be collected from undisturbed portions of the walls or bottom of the pit, not from soil that is excavated from the pit. Before digging a test pit, four stakes should be driven into the ground far enough from the sample loca-tion that they will not be disturbed by the backhoe or other activities. The stakes should be located

such that strings attached to opposite posts will intersect directly above the sampling location.

The following procedures are used for excavating test pits or trenches for soil sampling:

 Prior to any excavation with a backhoe, it is important to ensure that all sampling locations are clear of overhead and buried utilities

 Review the site-specific HASP and ensure that all safety precautions, including installation of appropriate monitoring equipment, are taken as required

 Using the backhoe, excavate a trench approx-imately 3 feet wide and 1 foot deep below the cleared sampling location. Place excavated soils on plastic sheets. Trenches greater than 5 feet deep must be sloped or protected by a shoring system, as required by OSHA regulations.

 A shovel is used to remove a 1- to 2-inch layer of soil from the vertical face of the pit where sampling is to be done

 Samples are taken using a trowel, scoop, or coring device at the desired intervals. Be sure to scrape the vertical face at the point of sampling to remove any soil that may have fallen from above and expose fresh soil for sampling. In many instances, samples can be collected directly from the backhoe bucket if the soils in the bucket are representative of the target sampling location.

 Abandon the pit or excavation according to applicable state regulations. Generally,

shallow excavations can be backfilled with the removed soil material.

The advantages of excavating test pits include the following:

 Visibility. Provides the best information on the location of soil layers.

 Versatility. May be the only way to access subsurface soil where large rocks are present.

The disadvantages of excavating test pits include the following:

 Safety

- The backhoe may have to be decontaminated before removing it from the site

- Trench walls deeper than 3 feet may be unstable. Shoring may be required prior to having anyone enter the test pit to collect samples

- Toxic or flammable vapors may accumulate in the test pit. Any entry into a test pit that is at least 4 feet deep should be considered a confined space entry, and all applicable requirements should be imposed and detailed in the HASP. See Section 4.4.2.3 for a brief discussion of confined space entry requirements.

Note: Excavation of contaminated soil can lead to the vaporization of toxic chemicals or the generation of conta-minated dust, posing an inhalation hazard to sampling personnel. The SAP and HASP should address these potential hazards.

 Environmental

- If soil contamination is suspected, large quantities of soil may have to be put into containers such as drums and stored or disposed of as hazardous waste

- If the pit is refilled with excavated soil, contaminated surface material may be buried to a greater depth, or contaminated soil from subsurface layers may be exposed on the surface.

4.6.3. Subsurface Geological/Geotechnical