Abstract: During construction of Wanrong tunnel of Menghua railway crossing siltyfine sand stratum, many problems such as tunnel face collapse, sand leakage caused by untight advanced jet grouting piles, and large deformation caused by primary support steel frame in loose sandy stratum are encountered, which would affect the project quality and safety. Accordingly, relevant countermeasures are adopted as follows. (1) The stepped excavation support method and the advanced jet grouting pile curtain are adopted. (2) The steel shims are installed at grille steel frame feet to increase stress area, and further stabilize the steel frame and reduce the deformation induced by loose sandy stratum. (3) The long invert trestle is adopted to ensure that the distance between closed ring of invert structure and tunnel face be less than 20 m and that between secondary lining and tunnel face be less than 50 m. (4) The highpressure water sprayer is used to improve the humidity in the air, avoid the moisture loss of siltyfine sand stratum, and reduce its natural stability. (5) The backfilling grouting behind the primary support is adopted to effectively reduce the collapse phenomenon and control the settlement deformation within 5 cm. The engineering practice results show that the abovementioned technical measures can effectively ensure the construction quality and safety of Wanrong tunnel crossing siltyfine sand stratum.

Key words: railway tunnel, siltyfine sand stratum, jet grouting pile curtain, step method, invert trestle, steel shim; water content