A technique to delay hydration and maturation of borehole seals of expansive clay

Sealing of deep boreholes for site selection of repositories for high-level radioactive waste can be made by inserting perforated copper tubes containing strongly compacted smectite-rich clay. The clay expands out through the perforation holes and ultimately forms a tight “skin” around the tubes at a rate that depends on the perforation geometry and the chemical composition of the water. The placement of the seals implies that they are moved through water in the borehole which can cause erosion and loss of clay if the expansion of the clay is too fast. The paper describes a method that retards the hydration and expansion of the clay sufficiently much to complete the installation before significant loss of clay has taken place. It involves addition of fine-powdered talc to the smectite grains in preparing a coating of the tubes.

Research Highlights
► The paper describes a new technique for sealing deep boreholes by use of expansive clay.
► Today's method implies insertion of dense clay confined in perforated metal tubes. Clay moves out through the perforation and ultimately forms a homogeneous clay plug. This works to about 500 m depth but in deeper holes the clay has time to migrate out through the perforation and be lost in the placement phase, thereby giving a too low density of the clay seal.
► Delay in migration is accomplished by coating the perforated tubes with a mixture of expansive clay and hydrophobic talc, disperse quickly but lets water through at a controlled rate for initiation of the required expansion of the dense clay.
► The coating, tubes (copper or titanium) and dense clay are chemically compatible and physically stable by being confined between concrete or metal plugs.