Abstract: In the highway projects, more and more construction related defects and hazards are frequently encountered. One of these serious defects and hazards is the voids behind the tunnel liners. These voids would reduce the capacity of the liner and further, the presence of the groundwater in the void would induce the damage to the liner and therefore impede the safe operation of the tunnel. The conventional void mitigation method is filling the voids with cement mortar. However, the workability of the cement mortar is poor and the segregation of the coarse aggregates from the mortar is frequently encountered. Completely fill the voids is often difficult. The foam concrete, which is a relative new material with an excellent flowability and small unit weight, has been successfully applied in filling the voids behind the tunnel liners. This method belongs to the rehabilitation of the hazards on the existing tunnel and the results are satisfactory.

Key words: highway tunnel, voids behind tunnel liner, foam concrete