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Geotechnical Investigation Procedures

Recommended policies and procedures for soil sampling

Due to the varying complexity of projects and subsurface conditions, it is difficult to establish a rigid format to be followed in conducting geotechnical investigations. However, there are fundamental, required data that should be obtained and basic steps that should be followed for any project investigation. The collected field data and assessments are the basis for all subsequent engineering decisions, and, as such, are of paramount importance to the design and success of a project. By following these requirements and steps, it will be possible to standardize procedures, and considerably reduce time and expense that would be required to return to the project site and obtain important information not obtained during the initial investigation. The following are fundamental. required data that should be obtained during a geotechnical investigation:

arrow Identification and delineation of existing soil and rock strata.
arrow Condition and performance of existing transportation structures.
arrow Qualitative and quantitative information on the character and engineering properties of the soil and rock strata.
arrow Ground water levels and environmental concerns.
arrow Slope stability condition, faults and other geologic hazards or constraints.

When it comes to soil sampling, all samples should be properly preserved and carefully transported to the laboratory to maintain sample integrity.

Disturbed Sampling

Disturbed sampling refers to methods of retrieving samples that incidentally cause the material to be remolded or at least partially altered. It should be understood that disturbed samples generally are not suitable for specialized tests requiring undisturbed soil specimens. However, disturbed samples have value for many geotechnical tests and usually are easier to obtain.

Bulk samples can provide a large amount of representative soil for compaction and sub-grade testing. Bag samples usually are obtained from test pits. In some cases, bulk samples can be obtained during auger drilling, collecting materials as they come to the surface in the flight of the auger. The auger method is less desirable because the depths of the sample cannot be accurately defined and some mixing of the soil occurs. A preferred approach when using an auger is to drill a shallow hole, then to remove the auger and collect a sample from the sidewall of the hole (after first clearing the smear zone). The quantity of a bulk sample depends on the type of testing to be performed, but can range up to 50 pounds or more. Testing performed on these samples could include classification, moisture content, compaction maximum density, R-value, pH and resistivity (corrosivity). A portion of each sample should be placed in a sealed container in order to accurately determine the natural moisture content. The size of bulk samples obtained for testing rock quality for aggregate purposes can be significant in order to process and select representative materials.

In limited cases and as a last resort, samples of cuttings can be obtained from drilling operations to augment materials collected in sampling tubes. Cuttings samples can be used to try to identify major changes in lithology during drilling operations, particularly when normal tube-sampling methods do not recover sufficient materials for this purpose. Examples of cuttings samples include material ejected from air-rotary drilling, material that is pushed to the surface on the flights of an auger, and material that is screened from the drilling mud return. Cuttings samples are highly disturbed, contaminated and sometimes altered (in size), and therefore, caution and judgment must be exercised when selecting, evaluating and classifying such samples. The depths from which cutting samples are obtained can only be roughly estimated, but these estimates can be aided by noting the depths where changes in drilling action occur.

Split-spoon samplers, also known as split-barrel samplers, are used in conjunction with the standard penetration test. The sampler is a 2-inch (O.D.) split spoon, which is driven into the soil with a 140-pound hammer dropped 30 inches. The split-spoon sampler is withdrawn, and the sample is removed after the sampler has been driven 18 inches. The sum of the number of hammer blows required to drive the sampler the second and third 6-inch increments is the standard penetration value referred to as the N-value (blows per foot). N-values can be correlated to a number of different design parameters, including relative density, angle of withdrawal, friction and shear strength. The sample should be immediately examined, logged and placed in sample jar or bag for storage. These samples are disturbed and are not suitable for strength or consolidation testing. They are suitable for moisture content, gradation and Atterberg limits tests, and are valuable for visual identification.

Undisturbed Block Sampling

Samples can be carefully carved from test pits for special testing in the laboratory. The advantage of block samples is that the test pit offers a broad area to detect critical materials and ability to obtain the exact amount of the desired material. The size of the sample should be large enough to perform planned laboratory testing. The block samples should be carefully handled, and should be protected with a moisture-proof barrier and placed within a sturdy and stable container so the sample is fully supported/constrained.

For block samples, the dimensions of the sample are controlled by the thickness of the specimen of interest and by the size of the box used to hold the sample. A column of the soil is carefully exposed so that, when the sample box is centered over the column, a 1-inch open space is left on all sides of the sample, and a half-inch space is left at the top. The empty areas then are filled with microcrystalline wax. After the wax congeals, the top of the box is attached, and the sample is carefully detached from the underlying ground with a spade. The sample is inverted, and a half-inch of material is removed. This area is filled with wax. After it congeals, the bottom of the box is attached, and the sample is ready for transport to the laboratory.

Thin-wall Samplers

Undisturbed samples are required for certain tests such as peak shear, consolidation, swell potential, permeability and density tests. There are several methods available for obtaining undisturbed samples, and would depend on the investigation equipment being used and the state of the soils in situ. Care of the samples also is critical to maintaining undisturbed conditions between drilling, transportation, storage and testing.

A Shelby tube is a thin-walled steel tube, usually 3 inches (O.D.) by 30 inches long. The beveled cutting edge of the Shelby tube is slightly smaller in diameter than the inside of the tube, which allows the sample to slide easily in the tube with little disturbance. The thin wall sampler is suitable for sampling all cohesive soils. The tube is pushed 24 inches with a smooth, continuous thrust. Difficulty may be encountered in sampling very soft and wet soils that tend to drop out of the sampler. Damage to the sampling tube (resulting in a poor sample) sometimes occurs when sampling hard, cemented or gravelly soils. Good samples must have sufficient cohesion to remain in the tube during withdrawal. If sample recovery becomes difficult, i.e., the sample stays in the ground, the tube should be left in place for roughly 10 minutes to 15 minutes. During this waiting period, the sample will swell slightly to fill the sampler, increasing the likelihood of retaining the sample when the tube is retracted. This produces a relatively undisturbed sample. Care should be taken not to over-push the sample to avoid disturbance. The ends of the Shelby tube should be properly sealed immediately upon withdrawal. The sample is suitable for unit weight, triaxial, direct shear, simple shear and consolidation tests.

Stationary piston samplers have the same standard dimensions as the Shelby tube. A piston is positioned at the bottom of the thin-wall tube while the sampler is lowered to the bottom of the hole, thus preventing disturbed materials from entering the tube. The piston is locked in place on top of the soil to be sampled. A sample is obtained by pressing the tube into the soil with a continuous, steady thrust. The stationary piston is held fixed on top of the soil while the sampling tube is advanced. This creates suction while the sampling tube is retrieved, thus aiding in retention of the sample. This sampler is suitable for soft to firm clays and silts. Samples generally have a better recovery ratio than those obtained by use of the Shelby tube. Care should be taken to not overdrive the sampler to avoid disturbance.

The floating piston sampler is similar to the stationary piston sampler, except that the piston is not fixed in position, but is free to ride on the top of the sample. The soils being sampled must have adequate strength to cause the piston to remain at a fixed depth as the sampling tube is pushed downward. If the soil is too weak, the piston will tend to move downward with the tube, and a sample will not be obtained. This method, therefore, should be limited to stiff or hard cohesive materials.

The retractable piston sampler is similar to the stationary piston sampler. However, after lowering the sampler into position, the piston is retracted and locked in place at the top of the sampling tube. A sample then is obtained by pushing the entire assembly downward. This sampler is used for loose or soft soils.

Hydraulic (Osterberg) piston samplers are especially suitable for sampling soft to very soft clays and silts, and sometimes is effective in obtaining samples of cohesionless, silty sands and sands. In this sampler, a movable piston is attached to the top of a thin-wall tube. Sampling is accomplished as hydraulic pressure pushes an inner sampler head and attached sample tube until it contacts a stationary piston positioned at the top of the soil sample. The distance over which the sampler is pushed is fixed; it cannot be overpushed.

Partially Disturbed Sampling

Partially disturbed sampling refers to methods of retrieving samples that incidentally cause the material to be partially altered. Hard soil conditions might make undisturbed sampling impossible. Therefore, several methods have been developed to obtain specimens of better quality. It should be understood that partially disturbed samples do not represent in-situ conditions, and generally do not provide reliable results for specialized tests such as peak shear, consolidation, swell potential and permeability.

The Denison sampler is a large-diameter, double-tube core barrel, which is effective in obtaining 5 7⁄8-inch diameter samples of hard cohesive soils, soft rock, cemented soils, and soils containing gravel that cannot be obtained with push-type samplers. This sampler consists of a rotating outer barrel with cutting teeth on the bottom and an inner barrel with a smooth cutting shoe. The sample is captured in a very thin inner liner, which facilitates retrieval and handling. Core catchers should not be used unless absolutely necessary to retain the soil sample. Care should be taken not to overdrive the sample to avoid disturbance.

A Pitcher sampler is a double-tube core barrel, and is effective for the same soils as the Denison sampler. The primary advantage the Pitcher sampler has over the Denison sampler is that the Pitcher sampler automatically adjusts the amount by which the inner barrels lead the cutting bit as the hardness of the soil varies. The Pitcher sampler also can accept a standard thin-wall sample tube in lieu of the inner barrel/liner.

Sprague & Henwood samplers are triple-tube samplers designed for sampling overburden materials, and are an improvement over the Denison and Pitcher samplers. 

This article is provided through the courtesy of the Nevada Department of Transportation, and is excerpted from its Geotechnical Policies and Procedures Manual.

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