This paper addresses the prediction of groutable joint apertures, grout penetration limitations, and possible grout take volumes per cubic metre of rock, as a result of 5 to 10 MPa pre-injection pressures ahead of tunnels. Joints are obviously opened more than in the preceding Lugeon tests, and many rock mass properties can apparently be improved if stable micro-cement based materials are used.
Cet article concerne la prediction des ouvertures des fractures injectables, les limites de penetration du coulis et les volumes injectes par mètre cube, d'après les resultats des injections à l'avant tête des tunnels sous pressions de 5 à 10 MPa. Evidemment les joints s'ouvrent plus que dans les essais Lugeon precedents. Maintes proprietes du massif peuvent apparemment être ameliores si l'on emploi des coulis a base de micro-ciment stable.
Die Publikation widmet sich der Prognose injizierbarer Kluftöffnungen, Grenzwerte fuer Injektionsreichweiten und erreichbarer Injektionsvolumen pro Kubikmeter injizierter Fels, durch die Anwendung von 5 bis 10 MPa Vorinjektionsdruecken vor der Tunnelfront.Die Kluefte werden offensichtlich weiter geöffnet als in den vorhergegangenen Lugeon Versuchen und mehrere Felsparameter können durch die Verwendung von stabilisierten Micro-Zement basierten Injektionsmaterialien verbessert werden.
Norwegian unlined pressure tunnels took many years to reach heads of 1000m. It has also taken many years to reach 10 MPa injection pressures when pre-grouting ahead of tunnels, where inflows need to be controlled to say 1 or 5 l/m/100m, or where tunnel stability needs improvement (and both of the above).
Do we know the actual effects of this high pressure injection on the rock mass? Can effects be quantified in any way? It has been found from recent Norwegian tunnelling projects that high pressure pre-injection may be fundamental to a good result: i.e. much reduced inflow and improved stability. The pressures used are far higher than have traditionally been used at dam sites, where in Europe, Brazil and USA, maximum grouting pressures (for deep dam foundations) have been limited to about 0.1, 0.05 and 0.023 MPa/m depth respectively. (Quadros and Abrahão, 2002).
(Figure in full paper)
According to a recent report by Kluever (2000), a shallow tunnel in phyllite with 5m of cover was injected at invert level to a final pressure of 6.5 MPa, and to 5 MPa even at the shallow depth of the arch, only 5m below the surface. (However, establishment of an outer screen was advised by Kluever in such extreme situations.)
The reality is that while grout is still flowing, there is such a steep pressure gradient away from the injection holes (from logarithmic to linear depending on joint intersection angle) that 'damage' to the rock mass is limited to local, near-borehole, joint aperture increase. On at least one joint set there may be local shear and dilation. Each of these effects are probably in the region of small fractions of a mm, judging by the local grout take of the rock mass, which may be about 1 to 5 litres/m3 of rock mass (see later).
