针对蒙华铁路万荣隧道在穿越粉细砂地层施工过程中遇到的掌子面溜坍、超前水平旋喷桩间不咬合易漏砂、初期支护钢架直接立于松散砂层上导致沉降变形大等影响工程质量、威胁施工安全的主要问题，采用阶梯法开挖支护施工技术，周边旋喷桩帷幕超前支护；在格栅钢架拱脚增设钢垫板以扩大拱脚受力面积，增加钢架的稳定，减小因拱脚砂层松动引起较大的沉降变形；利用长仰拱栈桥施工仰拱，使仰拱结构封闭成环距掌子面距离不大于20 m，二次衬砌距掌子面距离不大于50 m；采用高压水喷雾器提高空气湿度，防止粉细砂层水分散失，降低其自然稳定性；并通过初期支护背后回填注浆补强等技术措施，有效防止溜坍，将沉降变形控制在5 cm以内。现场施工结果表明，上述施工技术措施有效地保证了万荣隧道穿越粉细砂地层的施工质量和施工安全。
During construction of Wanrong tunnel of Menghua
railway crossing siltyfine 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 highpressure water sprayer
is used to improve the humidity in the air, avoid the moisture loss of siltyfine 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 abovementioned technical measures can effectively ensure the construction
quality and safety of Wanrong tunnel crossing siltyfine
sand stratum.
