The three-line transition section at exit of Xinkaotang Tunnel on Ganzhou-Longyan Railway is located in water-rich fully weathered granite stratum;and it has many characteristics,i.e.large excavation span (30.3 m), shallow cover and large excavation area (396 m2 ).The structure selection and construction method for shallow-covered large-span tunnel on three-line transition section is studied;and subsection design method for different cross-section, which suits for transition section of tunnel,is put forward.The transition section (215 m long)is divided into 6 subsections.The widening values are 0.8 m,2 m,4 m,6 m,8 m and 10.3 m respectively,with area above rail varying from 85.16 m2 to 200.02 m2 .The subsection widened by 0.8 m is constructed just like normal double-line tunnel,while subsections widened by 2 m,4 m,6 m are constructed by CRD method,double-side-wall heading method,and compound double-side-wall heading method with dilated wall respectively.The construction method of“dilated wall +enhanced arch”is put forward for subsections widened by 8 m and 10.3 m.The construction practice of Xinkaotang Tunnel shows that the design of construction methods and the corresponding support structure are feasible and rational.The results can provide reference for similar projects in the future.%赣龙铁路新考塘隧道出口处于全风化花岗岩富水地层，且处于三线渐变段，开挖跨度大，隧道埋深小，最大开挖跨度达到30．3 m，最大开挖面积为396 m2，为国内外铁路隧道工程所罕见。为安全、经济、快速修建此隧道，基于工程所处具体地形地质条件，对浅埋软岩特大跨度渐变段隧道的结构选型与施工工法进行研究，形成了应用于喇叭口渐变段的隧道内轮廓采用多个分段阶梯式变化的设计方法。渐变段共计215 m，分6段阶梯式加宽，加宽值分别为0．8、2、4、6、8、10．3 m，轨面以上净空面积从85．16 m2到200．02 m2变化。除加宽0．8 m 段按常规双线段考虑外，加宽2、4、6 m 段分别采用了四步 CRD 法、双侧壁导坑法、大墙脚复合双侧壁法，加宽8 m 和10．3 m 段采用了“靴型大边墙＋加劲拱”复合工法，且其支护参数不同。目前新考塘隧道已施工完毕并投入运营，实践证明设计的隧道结构型式及相应的工法是合理可行的。
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