Subsurface investigations require use of equipment to gain information below the ground surface. The equipment typically is invasive, and requires disturbance of the ground to varying degrees. Most of these exploration techniques are relatively expensive and, therefore, should be carefully planned and controlled to yield the maximum amount of information possible. It should be kept in mind that the quality of the information produced can vary significantly. If procedures are not followed carefully and data not interpreted properly, radically different conclusions can be reached. For example, poor drilling techniques could produce samples that might yield lower strength values.
Environmental ResponsibilityAfter the locations for field investigations work have been determined, routes of access to the area, and the specific sites for borings and excavations should be selected with care to minimize damage to the environment. Operation of equipment will be controlled at all times. Local laws pertaining to permissible levels of sediment flow from the site should be investigated. After the exploratory sites have served their purpose, the disturbed areas will be restored to a natural appearance. All borings and test pits should be backfilled in accordance with state environmental regulations.
The federal government has responsibility to ensure that environmental consciousness is maintained during the conduct of geotechnical investigations. Unfortunately, drilling rigs are inherently dirty. Proper maintenance of drilling rigs will minimize this problem. For exploratory drilling, drilling rigs should be steam-cleaned, and all tools, equipment and personnel decontaminated. Fluids used in drilling operations – be they hydrocarbons that have leaked from the hydraulic system or a constituent of a drilling mud – potentially are toxic, and should be controlled or eliminated wherever possible.
Major UsesBorings are required to characterize the basic geologic materials at a project. The major uses for which borings are made:
- Define geologic stratigraphy and structure.
- Obtain samples for index testing.
- Obtain ground water data.
- Perform in-situ tests.
- Obtain samples to determine engineering properties.
- Install instrumentation.
- Establish foundation elevations for structures.
- Determine the engineering characteristics of existing structures.
Borings are broadly classified as disturbed, undisturbed and core. Borings frequently are used for more than one purpose, and it is not uncommon to use a boring for purposes not contemplated when it was made. Thus, it is important to have a complete log of every boring, even if there may not be an immediate use for some of the information. If there is doubt regarding the range of borehole use or insufficient information to determine optimum borehole size, then the hole should be drilled larger than currently thought needed. A slightly larger-than-needed borehole is considerably less expensive than a second borehole.
Common MethodsMany methods are used to make borings and retrieve samples. Some factors that affect the choice of methods:
- Purpose and information required.
- Equipment availability.
- Depth of hole.
- Experience and training of available personnel.
- Types of materials anticipated.
- Terrain and accessibility.
- Environmental impacts.
- Disruption of existing structure.
Auger BoringsAuger borings provide disturbed samples that are suitable for determining soil type, Atterberg limits, Proctor testing, and other index properties, but generally they give limited information on subsoil stratification, consistency or sensitivity. Auger borings are most useful for preliminary investigations of soil type, advancing holes for other sampling methods, determining depth to top of rock, and for monitor-well installation in soils. Auger borings can be made using hand, helical, barrel, hollow-stem or bucket augers. Auger samples are difficult to obtain below the ground water table, except in clays. However, hollow-stem augers with a continuous split-barrel sampler can retrieve some unconsolidated material from below the water table.
Drive BoringsDrive borings provide disturbed samples that contain all soil constituents, generally retain natural stratification, and can supply data on penetration resistance. Drive boring is a non-rotating method for making a hole by continuous sampling using a heavy wall drive barrel. Push, or drive, samplers are of two types – open samplers and piston samplers. Open samplers have a vented sampler head attached to an open tube that admits soil as soon as the tube is brought in contact with the soil. Some open samplers are equipped with a cutting shoe and a sample retainer. Piston samplers have a movable piston located within the sampler tube. The piston helps to keep drilling fluid and soil cuttings out of the tube as the sampler advances. The piston also helps to retain the sample in the sampler tube. Where larger samples are required, the most suitable drill for this method is the cable tool rig. The cable tool rig has the capability to provide a downward driving force (drill stem on drive clamps) to make a hole and an upward force (drilling jars) to remove the drive barrel from the hole.
Cone Penetration BoringsThe cone penetration test (CPT) or Dutch cone boring is an in-situ testing method for evaluating detailed soil stratigraphy, as well as estimating geotechnical engineering properties. The CPT involves hydraulically pushing a 1.4-inch diameter probe into the earth while performing two measurements – cone resistance and sleeve friction resistance. The probe normally is pushed from a special heavy-duty truck, but also can be performed from a trailer or drilling rig. Because of the weight of the truck or trailer needed to conduct CPT borings, access to soft ground sites is limited.
Borings Are SatisfactoryTrue undisturbed samples cannot be obtained because of the adverse effects resulting from sampling, shipping and handling. However, modern samplers, used with great care, can obtain samples that are satisfactory for shear strength, consolidation, permeability and density tests, provided the possible effects of sample disturbance are considered. Undisturbed samples can be sliced to permit detailed study of subsoil stratification, joints, fissures, failure planes and other details. Undisturbed samples of clays and silts can be obtained, as well as nearly undisturbed samples of some sands.
Rock Core BoringCored rock samples are retrieved by rotary drilling with hollow core barrels equipped with diamond- or carbide-embedded bits. The core commonly is retrieved in 5-foot to 10-foot lengths. The “N” size hole (approx. 3 in.) probably is the core size most widely used for geotechnical investigations, and produces a satisfactory sample for preliminary exploration work and, in many instances, for more advanced design studies. Other hole sizes, including “B” (approx. 2.3 in.) and “H” (approx. 4 in.), also are quite satisfactory for geotechnical investigations. The decision on whole size should be based upon anticipated foundation conditions, laboratory-testing requirements, and the engineering information desired. A double- or triple-tube core barrel is recommended because of its ability to recover soft or broken and fractured zones. The use of wire line drilling, whereby the core barrel is retrieved through the drill rod string, eliminates the need to remove the drill rods for sampling and saves a great deal of time in deep borings. ND
This article is provided through the courtesy of the U.S. Army Corps of Engineers. It is excerpted from its “Geotechnical Investigations” manual.