针对攀西地区昔格达组地层软弱易滑、隧道进洞易产生边仰坡开裂造成进洞困难的问题，以成昆铁路前家山隧道进口浅埋富水昔格达地层进洞施工为例，分别进行以锚固桩、旋喷桩及PBA法为主的3次进洞工法尝试，前2次方案均导致地表开裂、大尺度变形和开挖陡立面蠕动变形，第3次方案采用PBA进洞法施作桩（P）-梁（B）-拱（A）组成的明洞体系，再实施结构外回填，有效解决富水昔格达组软弱易滑地层的不稳定问题。通过实践及分析认为: 1）富水昔格达特殊地质条件的隧道进洞困难，原因不仅在于隧道双侧挖掘临空引起的横断面不稳定，而且也因为隧道进洞纵向单侧临空导致的纵向不稳定十分显著。2）PBA法处置隧道进洞纵向不稳定性有明确的力学基础，是颇具针对性的有效措施。

The waterrich Xigeda formation in PanzhihuaXichang Region is characterized by soft and easysliding and the difficult tunnel portal excavation due to slope cracking. Hence, the schemes of anchor pile, jet grouting pile and PBA (piles, beams and arches) method are tentatively used for shallowburied waterrich Xigeda formation of Qianjiashan Tunnel on ChengduKunming Railway. The surface cracks, largescale deformation and creep deformation of steep facade excavation occur when adopting anchor pile method and jet grouting pile method, while the instability of waterrich Xigeda formation can be guaranteed by adopting PBA method for opencut tunnel and cavity backfilling. The applicable and analytical results show that: (1) The difficult tunnel portal excavation in Xigeda formation are mainly induced by tunnel crosssection instability caused by bilateral empty space and tunnel longitudinal instability caused by unilateral empty space. (2) The PBA method has clear mechanical mechanism, which can guarantee instability of tunnel portal excavation effectively.