In mechanized tunneling, due to the TBM shield's advance, an empty annular space remains behind the segment, which often filled with pea gravel and cement grout. Since about 90% of rock displacement occurs after boring at a short distance from the tunnel's face, contact grouting creates a proper substrate for installed segments and prevents stepping between them. The filling between the segments and the tunnel's wall will provide a uniformly distributed load on the segments and will increase the load bearing capacity of the concrete lining, minimize the level of ground settlement, prevent the distortion of the concrete rings, and reduce the amount of water leakage into the tunnel. Often there is no definite way to implement contact grouting as backfill. In various projects, according to the both operating conditions and trial-and-error procedure, the best method is chosen to ensure that the empty space behind the segments filled completely. In this case study, both the technical requirements and the practical issues of the contact grouting have been reviewed comprehensively for the Konjancham water conveyance tunnel in Iran. The second part of the tunnel (KT2) is 12 kilometers long with a diameter of 5.56 meters and is being bored using a double-shield TBM machine. Based on the experience of excavating about 5 km of the tunnel, the optimal method of implementing the contact grouting behind the segments is presented and its advantages and disadvantages are discussed.

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