The preservation of groundwater resources is a very important issue in the design, construction, and maintenance of the highway infrastructure system. Most projects require some type of on-site subsurface geotechnical investigation to properly design the facilities. These investigations typically involve progressing smaller-diameter (<200 mm) exploratory boreholes up to depths of approximately 60 m. In some instances, cone penetrometers or instrumentation devices such as piezometers, monitoring wells, and vane shear tests are used to obtain subsurface information. By progressing exploratory boreholes that may penetrate the water table, a conduit for surface and subsurface aquifer cross-contamination may be introduced. In situations in which the investigation may lead to the contamination of groundwater resources, it would be prudent to properly abandon the borehole.

Construction of a borehole seal during decommissioning can add significantly to the time, material, and labor costs of geotechnical exploratory boreholes. Some states have regulations that mandate that all geotechnical exploratory boreholes must be sealed. When that is the case, this standard practice is of limited use, and the reader should consult references pertaining to borehole sealing. If borehole sealing mandates are not in effect, this standard practice may provide some guidelines regarding which boreholes should be sealed, as well as the procedures involved in this determination.

This standard practice addresses the permanent decommissioning (closure) of the following types of geotechnical exploratory boreholes. It is specifically intended to address the closure in situations in which hazardous material or solid wastes are determined not to be present. Included in the list of geotechnical exploratory boreholes are:

• Boreholes progressed with temporary casing for obtaining soil, rock, and groundwater information;
• Uncased boreholes progressed for obtaining soil, rock, and groundwater information using drilling mud or open-hole techniques;
• Hollow-stem flight auger boreholes;
• Observation wells for monitoring groundwater conditions;
• Instrumentation boreholes (i.e., piezometers, Borros points, etc.);
• Cone penetrometer boreholes; and
• Soil characterization tests (vane shear, dilatometer, etc.).

This practice describes the overall process of decommissioning the holes and, as such, includes a systematic approach to completing the process. The typical steps entail an evaluation of the specific site characteristics and potential problems, a review of available decommissioning options including the null option if applicable, selection of an appropriate sealant, and a method of sealing. Specific requirements for the quality control and performance of the sealed borehole are addressed. Record-keeping requirements are noted.