Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 5.4
SUBSURFACE SOIL SAMPLE COLLECTION

5.4 SUBSURFACE SOIL SAMPLE COLLECTION

Designation of Decision Units for the collection of subsurface Multi Increment samples is discussed in Section 3.4."Subsurface" soil is generally considered soil that is below one foot bgs or soil that is otherwise difficult to access with standard tools used for surface samples. A subsurface DU can be thought of as a surface DU that is covered with an additional layer of soil. The fact that the targeted DU layer is covered by additional soil does not negate the need to collect high quality samples (refer to Section 4.2). The same applies to characterization of sediment that is covered by a layer of water.

Shallow subsurface soil (e.g., <12-18 inches bgs) might be accessible using a sampling tube, slide hammer, or electric drills as described in Subsection 5.3 for surface soils. Hand tools such as shovels could also be used to access deeper soil. The collection of increments and samples below this depth or from hard-packed soils will generally require the use of a push rig able to collect continuous cores. A backhoe or similar equipment can also be used for trenching or pot holing in order to gain access to deeper soil.

Overviews of push rigs and other drilling equipment are provided in ASTM Standard D 6169 (ASTM, 2005b) and ASTM Standard D 6286 (ASTM, 2006c). Direct push technologies can be used to collect samples to depths of up to 30 feet bgs or more, depending on the compaction of the soil and the presence of rocks. Auger drilling can reach depths of 100 feet bgs or more. Rotary drilling can reach depths of 1,000 feet bgs or more.

Each of these technologies is discussed in more detail below. Although included in the discussions below, auger and rotary drilling is more amenable to geotechnical investigations or the installation of monitoring wells than the collection of soil samples. This is due to the difficulty in collecting continuous cores as well as the expense, effort, and space required to operate the equipment. Drill cuttings and cores from such equipment might, however, be useful for initial screening of subsurface conditions and the need for a more intensive investigation (e.g., presence of absence of staining, potential COPCs, boundary between contaminated fill and native soil boundary, etc.; (see Section 3.4.4).

Tables 5-1 and 5-2 discuss the various drilling methods and performances.

5.4.1 DIRECT PUSH TECHNOLOGIES

Direct push technologies are a category of equipment that push or drive small-diameter hollow steel rods into the subsurface without rotating the drill rods. Direct push drilling can yield high-quality continuous cores of soil from targeted depth intervals quickly and cost effectively in the right type of soil conditions and is ideal for MI sampling strategies. Push rigs can also be used to collect soil gas or groundwater samples. Soil gas sampling is discussed in Section 7. The use of push rigs to install small-diameter monitoring wells for groundwater collection is discussed in Section 6.2. Smaller track-mounted rigs could be used for sampling areas with limited space. These rigs also normally remotely controlled and can be programmed to collect increments from a pre-established grid.

Figure 5-19. Direct Push Drill Rigs

Upper Left Photo: Truck-mounted push rig; solid drive cap and rod just prior to breaking into the subsurface. The hydraulic hammer, just above the drilling rod, moves up and down by a hydraulic piston, which can use the rig’s weight to drive the drilling rods into the ground. Stabilizing legs are used for balance as needed.
Upper Right Photo: Smaller, track-mounted rig used to access tighter areas and/or extract cores from pre-programmed grid coordinates.
Lower Left and Right:
Small push rig used to collect surface soil samples.

Figure 5-20. Drilling Rods for a Direct Push Drill Rig

Rod on left used to achieve specific sampling depth; solid drive cap penetrates into subsurface. Rod on right is the split barrel sampler. Multiple samplers may be used to delineate the entire soil column.

A hydraulic hammer is used to progressively drive the steel rods into the soil, with the weight of the drill rig used to provide a constant force on the drill string (Figures 5-19 and 5-20). Casing is advanced with a solid point held in place by an internal rod. A 1.5 to 2.25-inch diameter inner rod and core barrel are typically combined with a 3.25- to 4.5-inch diameter outer casing. As each section of the rod is advanced into the subsurface, another section of casing and rod can be attached to achieve greater depths. Two- to five-foot drive rods and samplers are typically used, depending on the depth and thickness of the targeted DU layers. Multiple drives might be required to extract the full length of core needed.

The steel rods and driving tip are pulled from the subsurface when the top of the desired soil interval (i.e., top of DU layer) is reached, with the outer casing left in place. The solid point is removed from the end of the inner rod and a split-spoon or open-barrel sampler is attached.

A split-spoon sampler is a stainless steel, machined, hollow cylinder that can be opened lengthwise into two halves (Figure 5-21). Split spoons can be used with hand-operated slide hammers, push rigs or larger drilling rigs. The cylinder is fitted with threaded ends. A cutting shoe is connected to the downhole end and a driving cap is connected to the uphole end. Split-spoon samplers can be lined with a clear Teflon or polyethylene lining to help keep cores intact after the sampler is opened. Six-inch stainless steel or brass tubes are also sometimes used, although they are less amenable for the collection of Multi Increment samples.

Figure 5-21. Split Spoon Sampler

Left Photo: Assembly shown with cutting shoe to left and end cap to the right and threaded to extension rod.
Right Photo: Split spoon sampler opened; note stainless steel liner above split spoon

After the coring device is attached, the drill string is placed back into the casing and driven to the desired depth. A hydraulic hammer can be used in conjunction with the push rig for compact soils. The drive rod can be marked to help monitor depth (e.g., every six inches). A drop hammer can be used to measure blow counts as part of a Standard Penetration Test if required as part of the investigation (ASTM, 2011b). For example, a 140 lb hammer is dropped 30 inches and blows counted to advance each of three consecutive, 6-inch increments for a total of 18 inches. The resulting data are used to help evaluate structural properties of the soil, including consistency, in-situ strength, and susceptibility to liquefaction.

Figure 5-22. Continuous Cores Collected using a Push Rig

Left Photo: Push rig cores with top of acetate liner removed to access soil
Right Photo: Oil-stained interval in left core targeted for screening and testing. Entire section submitted to laboratory for processing and subsampling as a single sample.

The drill string is retracted and brought back the surface after the base of the targeted interval has been reached. A continuous and relatively undisturbed core is ideally collected within the device. The sample barrel is opened and the core exposed (Figure 5-22). The top of the plastic liner, if used, is cut away to allow access to the soil.

5.4.2 BOREHOLE CORE INCREMENT SUBSAMPLE

Each core section represents a single increment for a targeted DU layer in the same manner that a smaller core of soil collected from a surface DU with a sampling tube represents a single increment for that DU. Initial single borings might be collected to help target subsurface DU layers for more detailed testing and identify contaminants of potential concern (see Figure 5-22). If so, then the entire, suspect interval of the core should be submitted to the laboratory for processing as a single sample. The collecting of discrete soil samples from specific depths within a core rather than targeting specific depth intervals for MI sampling is not recommended (see Section 4.2.8.2 and Section 4.3).

Targeted DU layers are identified and marked in the core (Figure 5-23). The mass of an individual core increment collected with a push rig is typically too large for use in preparation of a bulk Multi Increment sample, and field subsampling of the core increments is required. Each increment for an individual DU layer is subsampled, with the extracted soil placed in a container specific to that layer. This can be accomplished by cutting a narrow wedge of soil from the entire length of the increment interval or by removing regularly spaced plugs of consistent mass from the increment (e.g., 5-10 grams; see Figure 5-24).

Figure 5-23. Identifying Targeted DU Layers in Cores

Left Photo: DU core increment placed on table for inspection and subsampling.
Right Photo: Targeted DU layer increments identified within core for subsampling and preparation of bulk Multi Increment samples.

Figure 5-24. Subsampling of Core Increments for Preparation of a Bulk Multi Increment Sample

Left photo: Removal of a continuous wedge of fine-grained soil from a core increment
Right Photo: Removal of regularly spaced, five-gram plugs of soil from a core increment using a TerraCore sampler

Under ideal circumstances the wedge method is preferred, since the resulting subsample provides 100% vertical coverage of the core increment. Removal of a continuous wedge from a core may not be possible if rocks are present or loss of volatiles may be an issue, however, and subsampling of core increments using the plug approach is most common.

Increment subsamples are combined in the field to prepare a single, bulk Multi Increment sample for the DU layer in the same manner as carried out for surface soil bulk Multi Increment sample. Note the individual soil plugs are subsamples of a single core increment and do not represent individual increments themselves. Individual subsample plugs cannot be counted towards the total number of increments collected from a subsurface DU layer since they were not collected from independent, random locations. The minimum recommended number of increments for testing of a DU (e.g., 30 to 75; see Section 4.2.2) does not apply to subsampling of core increments using subsample plugs.

The mass of soil included in a single core wedge or as the sum of plugs removed from a single core increment is dependent on target mass of soil designated for the final bulk DU layer. For example, if a 1.5 kg bulk sample is desired and 30 core increments are to be collected to represent a DU layer, then the mass for each core increment subsample should total approximately 30-50 grams. Careful consideration of the soil subsample mass collected from each core increment prior to subsampling is critical in order to ensure that mass of the resulting bulk sample will be adequate to meet target requirements (e.g., 1-2 kg) but not so large that additional subsampling in the field or laboratory will be required.

If a core wedge cannot be collected, then the target subsample mass should be collected from what is anticipated to be a representative number of points within the core increment layer. For example, if the collection of an approximately 1 kg bulk Multi Increment sample from a one-foot thick DU layer is targeted, and thirty increment cores are to be collected in the DU, then six, five-gram plugs at a spacing of two inches could be extracted from each of the 30 one-foot thick core increments for a total bulk MI sample mass of approximately 900 grams for the DU layer. The mass of soil removed from each individual core increment should be kept constant, assuming a constant DU layer thickness. Maintain consistent wedge width or plug spacing for subsampling of core increments collected from DU layers with varying thicknesses between borings (see HDOH, 2011i).

The collection of replicate Multi Increment samples to evaluate the precision of both increment subsampling and the overall sampling approach is recommended (see Section 4.2.8.2). Replicate sets of increment subsamples (e.g., triplicates) should be collected from one or more of the targeted DU layers and combined into independent bulk Multi Increment samples for testing. If the resulting data are reasonably consistent (e.g., RSD <35%) then the precision of the subsampling methods used can be considered to be good (see Section 4.2.7). Independent sets of borings are used to collect replicate samples in select DUs in order to test the precision of the overall approach, in the same manner as done for surface samples.

5.4.3 PIT OR TRENCH SAMPLE COLLECTION

The use of pits or trenches to collect Multi Increment samples might be required in situations where considerable debris, rubble, or rock create obstructions in the subsurface. Pits and trenches can also provide useful information on the nature of subsurface soils within a DU.

Pits have also been used to collect increments from multiple DU layers at sites where heavy equipment is already readily available, or a push rig would have to be brought over from another island (Figure 5-25 and 5-26). Increments are collected from pit sidewalls in the same manner as done for a continuous core (i.e. in a core-like shape). It is again important to ensure that an appropriate mass of soil is included in each increment in order to meet the targeted bulk sample mass.

Figure 5-25. Use of Shallow Pits to Collect Increments from Multiple Layers within a DU

Left Photo: Backhoe used to dig increment collection pits with large DUs at a former golf course
Right Photo: Vertical soil horizons and DU layers targeted for collection of increments and assessment of arsenic concentrations with depth (e.g., 0-6", 6-12" and 12-24"). An increment is collected across the entire targeted DU layer depth, and all increments from that same depth in multiple pits/trenches (e.g. 30-75+) are combined to prepare a bulk Multi Increment sample for that layer

Figure 5-26. Example Use of Trenches for Site Investigation

Increments collected from trench sidewalls at successive depths through DU layers in order to prepare bulk Multi Increment samples, being careful not to include collapsed sidewall material from upper DU layers.

Trenches can be strategically placed within a DU to investigate the presence of buried debris and evaluate the soil stratigraphy as well as collect Multi Increment samples. When collecting increments at multiple or at successive depths in excavations, care needs to be taken not to collect material collapsed from the sidewalls of upper soil layers.

Figure 5-27. Use of Shallow Trenches to Collect Multi Increment Samples from Exposed DU Layers

DU boundaries noted in blue. Surface Multi Increment sample (e.g., 0-6") collected prior to excavation of trenches. Increments for subsurface DU layers collected across the full vertical thickness of the exposed targeted horizon in order to prepare a bulk Multi Increment sample (layers depicted).

In Figure 5-27, a trench approximately three feet wide is excavated to a depth of three feet in order to access the sides of targeted DU layers for sample collection. An initial Multi Increment sample was collected from the surface DU layer (0 to 6 inches) using standard, soil sample collection methods. The trench was then excavated to the target depth. A Multi Increment sample was subsequently collected from exposed face of each of two targeted DU layers, situated from 6-18 inches and 18-36 inches below the ground surface. Increments were collected across the entire, vertical extent of the DU layer. With the possible exception of very narrow DUs, multiple trenches like that depicted in Figure 5-27 are typically needed to collect all increments in a systematic random fashion across DUs.

Field replicates can be collected within the same trench in order to assess the precision of the data with respect to the exposed portion of the DU layer. Independent sets of trenches within select DUs could be used to collect replicate samples in the same manner as done for surface samples and further assess the precision of the data.

Safety precautions are imperative to protect workers collecting samples during trenching. Note that the Hawai‘i Occupational Safety and Health Division (HIOSH) requires that any excavation that is over 4 feet in depth be shored or properly sloped when personnel will be working within the excavation (USDL, 2002). Use of the backhoe bucket to collect increments from sidewalls might be required for deep or otherwise unstable excavations.

5.4.4 HOLLOW STEM AUGER DRILLING

Figure 5-28. Hollow-Stem Auger Drill Rig

Left Photo: Large rig for deep boreholes; requires a high overhead clearance for the mast. The auger flights (right in photo, on ground) are rotated and pressure is applied from the drill rig to advance the drill string downwards.
Right Photo: Smaller, track-mounted auger rig used for shallower borings or drilling in limited clearance areas.

Hollow-stem augers were already in use for drilling and coring in unconsolidated soils for geotechnical work at the advent of environmental investigations in the 1980s. The use of auger rigs to collect soil samples has largely been replaced by more compact and efficient push rigs described above, although they are still frequently used for the installation of larger-diameter monitoring wells.

Figure 5-29. Auger Drill Bit and Drill String

Left Photo: Bit attached to bottom of auger to move soil to the side as the rotating auger advances.
Right Photo: The auger flights bring soil cuttings from the drill bit upwards to the surface. The asphalt was cut prior to drilling. Flights are added as needed as the auger advances. Plastic sheeting on the ground keeps potentially contaminated soil and water brought up by the auger from mixing with surface soils or impacting the pavement.

Auger drilling usually requires a larger drill rig than is used with direct push drilling. The entire rig can stand 20 to 40 feet and require high overhead clearance (Figure 5-28). The use of auger rigs is described in ASTM Standard D 5784 (ASTM, 2006; see also USACE, 1996; Nielsen, 2006). The rigs are capable of reaching depths of 100 feet in unconsolidated to semi-consolidated soil and even coral, but cannot normally penetrate volcanic basalt or tuff formations.

Augers used for environmental investigations typically have an outside diameter of 6 to 10 inches (Figure 5-29). As the name implies, the auger itself is hollow with a helix wound around the outside. In this manner the auger serves both as a cutting tool to advance downwards and as casing for the collection of soil samples or the installation of wells. The tip of the bottom most auger string is designed to push soil cuttings to the outside of the flights where the helix can bring the cuttings to the surface.

As pressure is applied from the weight of the rig, the flights are rotated and the soil cuttings are brought to the surface by the flights. As the flight is drilled into the subsurface new flights can be added to achieve greater depths. Each flight must be removed individually to remove the auger drill string from the boring.

Similar to a push rig, the auger flights can be left in place while a split spoon or an open barrel sampler is used to collect a soil sample or a monitoring well is installed. The sampler is attached to the end of a heavy rod and driven to the targeted depth using a percussion hammer or hydraulic hammer. The coring device is then retracted and the sample removed.

Hollow stem auger can also be used to install monitoring wells (refer to Section 6.2). The monitoring well casing is installed within the auger once the desired depth interval is reached. Clean sand is placed around the monitoring well casing as the auger flights are slowly retracted.

5.4.5 ROTARY DRILLING

A detailed discussion of air or mud rotary drilling is beyond the scope of this technical guidance. Rotary drilling is generally used for geotechnical studies or the installation of wells through bedrock rather than for collection of soil samples (Nielsen, 2006; US Navy, 2007). These rigs are less amenable for the collection of subsurface soil samples to be tested for contaminants. Aside from the expense and space required to operate the rigs, complete recovery of cores during drilling is difficult when drilling in unconsolidated and semi-consolidated lithologies, such as clays, silts, and sands. The rigs are most useful for the collection of continuous rock cores for geologic or geotechnical studies. Standard rock coring methods are summarized in ASTM guide D 2113 (ASTM, 2008). Standard rotary drilling methods are summarized in ASTM guide D 5782 (ASTM, 2006b).

Rigs are typically designed for the use of multiple drilling methods, including both auger and air or mud rotary (Figure 5-30). In the simplest type of rotary drilling a drill rod with an attached bit is continuously rotated against the bottom of a borehole in order to pulverize and break up encountered material. The pulverized fragments are carried up to the surface by air or specially formulated mud pumped into the borehole and pushed back to the surface under pressure. The mud also serves to lubricate and cool the bit during drilling. Rotary drills can also be designed to collect continuous cores of material.

Figure 5-30. Large Truck-Mounted Drilling Rig

Left Photo: Truck-mounted rig capable of both auger and rotary drilling.
Right Photo: Mud-rotary drilling operation.

Drilling fluids, including air, are selected based on the anticipated nature of subsurface conditions, including soil or rock type, depth, temperature, and pressure. The HEER Office does not typically allow the use of any fluid other than air for drilling carried out as part of an environmental investigation. Drilling mud can compromise the representativeness of samples collected as well as introduce contaminants into the targeted formations. Use of a foam suppressant composed of a surfactant and polymer mud might be required, however, in rare cases where there is a need to core into a zone where potentially flammable product is present.

Figure 5-31 shows a compressed air drilling setup. Caution should be taken with air rotary rigs to prevent particles (or exhaust) from entering the borehole during drilling, particularly if samples are to be collected for contaminant analysis. On occasion, potable or distilled water may need to be added, but only in the situation that cuttings cannot be brought to the surface by air alone. Drilling under these circumstances should be discussed with the HEER Office before implementation.

Figure 5-31. Pressurized Air Discharge from an Air Rotary Assembly

Left Photo: Compressed air is pumped down through the drill bit and drives cuttings up through the outside of the core barrel. Cuttings and compressed air are directed to a cyclone mechanism (on right) that dissipates air pressure and velocity and allows for cutting collection in a drum below cyclone.
Right Photo: Rotary cutting bit.

Rotary core barrels typically range from 1 to 10 inches in diameter. A single tube core barrel is rarely used due to poor sample recovery and sample disturbance. A double tube core barrel is most frequently used in rock core sampling for geotechnical engineering applications. A triple tube core barrel is used in zones of highly variable hardness and consistency. In this design a separate, non-rotating liner is added to the double tube core barrel in order to improve sample recovery and minimize sample handling and disturbance (refer to US Navy, 2007).

A variety of coring bits, core retainers, and liners are used in various combinations to maximize the recovery and penetration rate of the selected core barrel. Example cores and coring bits are illustrated in Figure 5-32. Bits are fitted with diamond or carbide teeth to facilitate cutting. The outer barrel rotates to allow the bit to penetrate the formation. As the outer barrel is advanced, the sample rises and is retained in an inner liner, which in most drill string assemblies does not rotate with the outer barrel. The bit cuts an annular groove in the formation to allow passage of the drilling mud or air. Cuttings are forced up the outside of the core barrel. A split-spoon barrel is typically used to collect five-foot cores.

Figure 5-32. Example Recovered Cores and Rotary Drill Coring Bits

Left Photo: Rock Cores Collected Using Air Rotary Drilling.
Right Photo: Rotary Drill Coring Bits.

Table 5-1. Overall Performance Estimation for Various Types of Drilling/Excavation Methods
  Direct Push Hollow-Stem Auger Rotary Drilling Trenching
Relative Site Impact Low- Moderate Moderate High Low-Moderate
Relative Cost Low- Moderate Moderate High Low-Moderate
IDW Generated None to Minimal Moderate to High Volume High Volume Moderate to High Volume
Depth Potential Shallow (to approximately 30 feet) Moderate (to approximately 100 feet) Deep (to 1,000 feet) Shallow (to approximately 20 feet)
Soil Condition Limitations* Un consolidated Semi-Consolidated Unconsolidated Consolidated Semi-Consolidated Unconsolidated
Monitoring Well Installation Size Small (1 inch well casing) Medium (2 to 6 inch well casing) Large (2 to 24 inch well casing) None
Types of Sampling Soil Gas, Soil, and Groundwater Soil and Groundwater Soil and Groundwater Soil and Groundwater
Vehicle Requirements Truck mounted Specific Heavy Rig required Specific Heavy Rig required Excavator or pick/shovel
Other Issues Vehicle access Vehicle access, overhead clearance Vehicle access, overhead clearance Open area required, access for excavator
Notes:*
Also See Table 5-2: Relative Performance of Commonly Utilized Drilling/Excavation Methods Versus Commonly Encountered Substrate

Table 5-2. Relative Performance of Commonly Utilized Drilling/Excavation Methods Versus Commonly Encountered Substrate
Subsurface Formation/ Material Direct Push Auger-Hollow Stem Trenching Air Rotary
Sand E G G NR
Loose sand and gravel E G G NR
Loose boulders in alluvium NR P G NR
Clay, silt G E E NR
Coralline Limestone with and without fractures NR NR NR E
Tuff NR P to G NR E
Basalts-thick layers NR NR NR G
Basalts-highly fractured NR NR NR P
Notes:
E = Excellent
G = Good
P = Poor
NR = Not Recommended
Adapted from (US Navy, 2007)