ABSTRACT: A new technique for sealing rock by use of clay grouts and application of high frequency pressure injection is presented. The sealing properties and groutability of smectite clay are discussed and an example 1s given of how full-face drilled holes and tunnels can be sealed by grouting.
RESUME: Une technique nouvelle pour rendre etanche de. roches en injectant de l"argile par des pressions fluctuantes en haute, frequence est presentee. Les, proprietes d"etancheite at d"injection d"argile de type smectite sont discutees. Une example est donne illustrant comment des tunnels et des trous perfores par TBM peuvent être rendu etanche par injection d"argile.
ZUSAMMEMFASSUNG: Eine neue Technik fuer Bergabdichtung durch Anwendung von speciellen Tonmaterialen und von hochfrequenter Druckinjektion wird prasentiert. Die Abdichtungs-eigenschaften von smektitischem Ton wird diskutiert, und ein Beispiel wird dafuer gegeben, wie gebohrte Löcher und Tunneln mit Hilfe dieser neuen Technik abgedichtet werden können.
1 GENERAL Phase III of the Stripa Project dealing with rock sealing techniques, involves investigation of both cement and bentonite grouts. Cement has primarily been used for injection of 50 mm boreholes with the purpose of sealing fractures zones and disturbed zones around tunnels. Bentonite grouts have been injected directly from the periphery of simulated, large deposition holes by use of a "megapacker", device developed within the project. This technique can be used in all full-face drilled holes, shafts and tunnels. We, will confine ourselves here to describe bentonite grouting.
Multibarrier, concepts, for, final deposition of highly radiactive waste products usually imply that that waste canisters be surrounded by, dense smectite clay, forming a corrosion- retarding, cation-exchanging, low- permeable, and stress-relaxing environment. The, thickness of this barrier component is. often taken to be, rather small for practical and economical reasons, but its isolating capacity is still very high provided that its chemical integrity is well preserved. This requires that the temperature does not exceed a certain limit and that access to potassium in ionic form is limited, since critically high temperatures, are expected to cause cementation by silica precipitation, and continuous exposure to potassium in concentrations corresponding to that of sea- water may yield quick transformation of smectite to hydrous mica, i.e. "illite".
Technico/economical optimization suggests that the thickness of the canister- surrounding clay should be rather small and that it is accepted that some of it be chemically degraded. In order to minimize the degradation and to block migration of radionuclides that escape from the canisters due to the degradation, or due to canister corrosion, lit is essential to create stagnant groundwater in the-nearfield and this can be achieved by strategic location of impermeable plugs combined with grouting of water-bearing structures adjacent to the plugs as outlined in the paper.
2 SEALING POWER OF BENTONITE The ability of smectite clay to adsorb water so strongly that it is largely immobile under low and moderately high hydraulic gradients is very significant at high clay densities (Pusch, 1987).