Performance of fracture sealing with bentonite grouting.

Abstract

A bentonite grouting technology has been developed to meet the requirements of fracture sealing for nuclear waste isolation and environmental engineering. The work reported presents an experimental evaluation of the performance of the bentonite fracture sealing in densely welded tuff. Included are the study of grout stability and resistance to erosion, theoretical and numerical analyses of the grout penetration process, pressure distribution in the fracture during grouting, grout delivery distance, breakthrough pressure and rheological characterization. Bentonite suspensions with solids content of 18, 20 and 22% by weight have been injected into single fractures in rock samples at 0.07 to 3.45 MPa pressure. Bentonite grouting reduces the fracture hydraulic conductivity by 7 to 9 orders in magnitude, to the level of 10⁻⁷ to 10⁻⁹ cm/s. Grout stability and erosion have been studied with long-term flow testing at up to 121 kPa hydraulic pressure. After 1,000 to 5,000 hours of flow testing, at pressure gradients of up to 91 cm/cm, the hydraulic conductivity of the grouted fracture does not increase with test time. The hydraulic conductivity of the grouted fracture decreases with increasing pressure gradient up to the wash-out pressure. Analytical solutions have been derived for pressure distribution during grouting in wedge shaped fractures and in fractures with constant aperture. Influence factors on pressure distribution in a fracture have been identified. They are yield stress, fracture shape, original aperture, deformation properties of the rock mass and the fracture, mixing of air in a grout and sample size. A mathematical solution of grout delivery distance under a given injection pressure has been developed. The breakthrough pressure has been studied, based on the mechanical deformation of the fracture applied by a normal stress across the fracture and by the grout pressure within the fracture during grouting. Results indicate that bentonite grouting is a promising fracture sealing technology. Bentonite grout has great hydraulic conductivity reduction and chemical physical stability. Some disadvantages of bentonite grout have been identified, such as the formation of hydraulic fractures in the grout and the washing away of grout particles when the applied pressure gradient is high enough.