Abstract: In order to resolve the problem of backfill treatment of the abandoned civil air defense tunnels in the coastal soft soil area, this paper presents the construction measures of the cast-in-place foam concrete as the backfill material. The targets of the foam concrete backfill in the abandoned tunnels are defined. Throughout the laboratory material tests, the mix ratios of cement, fly ash, foaming agent and other additives were determined. Then, the 1:1 cross-section model test of the tunnel was performed to verify the degree of filling, apparent integrity, and the effectiveness on-site mixing measures. Finally, a pilot demonstration backfill tunnel was carried out to monitor the temperature and pressure changes in the backfills, the settlement of soft soil foundation during the construction process, and the long-term variation of backfill density under the groundwater. The results show that the hydration heat and the foam concrete should be properly controlled; the long-term water resistance performance during the construction is acceptable, and the environmental impact of the foam concrete is minimal.

Key words: abandoned civil air defense projects, foam concrete, backfill treatment, water resistance, environmental impact