Abstract: When the unexpected seepage in the deep excavation site results in the excessive deformation of the excavation supporting structure, which might seriously threaten the excavation safety. When this unexpected seepage occurs, the excessive deformation adjacent to the supporting structures is often obvious. This paper presents a case history of the mitigation of a seepage induced accident in Chengdu. The factors that might cause the unexpected seepage, seepage mitigation measures and results are also discussed in the paper. When the seepage induced hazard occurred, the seepage locations should be immediately sealed by using the pressure grouting with water glass into soils in-between the solder piles. Due to the large amount of seepage, the results of the pressure grouting might not have the expected results since the cohesive soils at the excavated walls were soaked and softened, so that the water glass could be washed away. When the large deformations at the excavation side walls, the backfilling measure should be used followed by the pressure grouting after the soil movements are stabilized. The mitigation results indicated that the pressure grouting with water glass plus the backfilling are an effective way of mitigating the unexpected seepage induced hazards. 

Key words: Deep Excavation Seepage, Double-Liquid Grouting, Foundation Pit Rescue, backfill backpressure