ABSTRACT:

This paper describes the process, criteria, and considerations for design of borehole seals in rock, with a main emphasis on the hydraulic and mechanical performance of the cementitious and bentonitic seals. It is recommended that design and material selection for borehole seals be site-specific. Evaluation of sealing effectiveness should be made on the entire seal system, i.e. seal, seal-rock interface, and the surrounding rock.

1 INTRODUCTION

Borehole sealing has not received much engineering attention until fairly recently. The growing awareness of and sensitivity to environmental concerns of the technical community as well as of the public at large has resulted in an increasing recognition of the fact that these geologic penetrations may have an environmental impact. Although many government agencies have implemented regulations requiring that unused boreholes and underground excavations in rock formations be sealed, these regulations tend to be generic and broad, and rarely allow for taking into account site-specific conditions. Industry guidelines usually do not provide sufficient information about the expected performance of the emplaced seals, and hence do not allow for adequate assessment or prediction of their sealing effectiveness. This paper presents design process, criteria and considerations for sealing boreholes in rock, with a main emphasis on the hydraulic and mechanical performance of the emplaced seals. The scope of the present information is intended to cover various types of borehole and sealing materials designed for different objectives and requirements. While the main emphasis of this paper is on borehole sealing, some recommendations are applicable to larger underground excavations (e.g., shafts and tunnels). The design criteria recommended here are principally based on the results of laboratory and in situ experiments conducted as part of research effort for the US Nuclear Regulatory Commission. The proposed designs can be incorporated into the existing guidelines for specific applications, such as those given by Gray and Gray (1992) and US Bureau of Mines (1994) for sealing of abandoned boreholes in mining industry; by Smith (1986, 1990) for sealing of oil and gas wells; and by ASTM D5299 and US Environmental Protection Agency (1975) for sealing of ground water and monitoring wells. The main aspects of the design process for borehole sealing addressed here include (i) seal objectives and requirements, (ii) site characteristics, (iii) general design criteria, and (iv) material selection and placement methods. Discussions focus on three types of sealing materials: cement, bentonite, and mixtures of bentonite and crushed rock. More information on sealing of boreholes and other underground excavations in rock is given by Fuenkajorn and Daemen (1996).

2 SEAL OBJECTIVES AND REQUIREMENTS

The performance of emplaced seals has been focused on two aspects: hydraulic bond and mechanical bond. From a point of view of potential leakage, the hydraulic bond is important in order to assure that flow through the seal and along the host rock-seal interface remains within an acceptable magnitude, depending upon types of application. Providing adequate mechanical bonding assures that a seal will not be dislodged, e.g. under the action of gas or liquid pressure.

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