Techniques for sealing cone penetrometer holes

Abstract

Direct penetration exploration tools such as the electric cone penetrometer (CPT) and piezocone (CPTU) are being increasingly used for site investigation studies to evaluate stratigraphy and obtain groundwater samples. Most of these techniques are considered full-displacement direct-push or direct-drive penetration methods which can leave open pathways that may allow the movement of surface or subsurface contaminants into uncontaminated layers. Sealing techniques are needed to protect groundwater conditions following the use of these types of exploration tools. A survey of industry techniques for sealing CPT holes indicates that a number of methods can be used to place grout mixtures and other seal materials into a hole either (1) after pulling the tool, (2) as the tool is pulled, or (3) simultaneously with advance of the tool. Selection of a particular method depends primarily on the soil and groundwater conditions at a given site but may also depend on other variables such as equipment availability, time, and cost. In some cases, the method of sealing may be dictated by a regulatory agency. Seal materials used for this practice typically consist of grout mixtures composed of bentonite slurries, neat cement, a combination of the two, or dry granular bentonite, which are placed to provide permanent low hydraulic conductivity barriers. A number of techniques that have been used to seal CPT holes are described, and the advantages and disadvantages of the various techniques are presented. Key words : groundwater, cone penetrometer, hydraulic barriers, contaminants.