Re-injection of groundwater by pressurizing a segmental tunnel lining with permeable backfill

Recently, within a tunneling project, a method for reinjecting discharged groundwater at tunnel level has been investigated. The method was performed by pressurizing a long section of the lining with un-grouted pea gravel as backfill, in a rock mass with low hydraulic conductivity. Water was reinjected through watertight lining into a section where un-grouted pea gravel was used as backfill. The pressure response was measured both behind the lining and in the rock mass, the latter by means of several observation wells drilled from ground level. Reinjecting water into a rock mass by pressurizing a lining with a permeable backfill (such as un-grouted pea gravel) was found to be possible and feasible. Well testing methods developed for vertically drilled wells, such as a step injection test and constant head/pressure tests, were used and found to be applicable, even for testing at tunnel level. It was also found that well known analytical solutions, developed for vertically drilled wells, can be used for interpretation, e.g. transmissivity from tests performed at tunnel level and from the pressure response in two of the observation wells.