Abstract: A case study is conducted on a doubletrack metro shield tunnel crossing underneath railway subgrade in sandypebble strata. The deformation law of the subgrade is studied using FLAC 3D numerical simulation based on field monitoring data analysis. The applicable results of ground grouting by steeltube prove the effectiveness of grouting reinforcement in controlling the subgrade settlement. The main conclusions are drawn as follows: (1) The greater the train speed and the smaller the shield burial depth, the greater the subgrade settlement, whereas the influence of train speed on subgrade settlement is far less than that of burial depth. (2) The approximate relationship among the subgrade settlement, the burial depth, and the train speed is S=-26.54+0.85z-0.01v when the doubletrack metro shield undrcrosses the railway subgrade in sandypebble strata in Chengdu with a conventional clear distance of approximately 7 m. Due to the twice disturbance of the shield tunneling, the subgrade has experienced twice sudden drops in displacement, the displacement and settlement curve shows a step shape, and the sensitivity of strata response caused by the second disturbance is faster than that of the first one, as a result, attention should be paid to the displacement caused by the second disturbance. (3) The steel tubegrouting, as a highly operable method for ground reinforcement, has a good reinforcement effect in sandycobble strata in Chengdu. The settlement of subgrade after grouting reinforcement is reduced from over 11 mm to 7 mm, which ensures the smooth and successful shield tunneling.

Key words: railway subgrade, shield undercrossing, settlement law, ground reinforcement, steeltube grouting, sandycobble strata