Cracking in the secondary lining of tunnels in schist was found to be common when a survey of second-ary lining integrity was conducted for the tunnels of the Gucheng-Zhuxi expressway in Hubei province. To scientifi-cally solve the problem of cracking, the following characteristics were summarized based on an investigation of the 48 tunnels along this line:1) it is very common to see cracking in the secondary lining of tunnels in schist;2) cracks are mainly located at the shallow-buried entrance and exit sections;3) numbers of cracks in the first excavated tun-nel are higher than that in the tunnel excavated later;4) the cracking pattern is mainly oblique;5) it is easier for the secondary lining to crack at places where large deformation and collapse occur;6) and cracking occurs 5 months af-ter the completion of the secondary lining. Based on the above experience, laboratory tests and site investigation, the mechanism and reasons for cracking of the secondary lining were studied and it was concluded that the schist is of-ten soft when water is encountered due to its low strength;it is difficult to complete an arch at the shallow-buried section of the entrance;asymmetrical loading is very common;the settlements at the tunnel invert are uneven;and the rock is affected by underground water and the creep effect. The following suggestions and measures are present-ed: the grade of surrounding rock will be adjusted;much attention should be paid to construction at the entrance and the design of middle rock pillar stability of tunnels with small spaces.%在对湖北省谷竹高速公路隧道二次衬砌完整性的调查过程中,发现片岩隧道普遍存在二次衬砌开裂现象.为了合理科学地治理开裂问题,文章通过对沿线48座隧道的调查分析,归纳和总结出该区段隧道二次衬砌开裂的6大特征:片岩隧道二次衬砌开裂现象普遍;开裂位置主要位于进、出口浅埋段;先行洞裂缝数量多于后行洞,开裂形式主要为斜向开裂;在发生大变形、塌方等地段二次衬砌容易开裂;开裂的时间一般在二次衬砌施作5个月后.在此基础上,通过室内试验与现场调查共同分析二次衬砌开裂的成因机理,并得出原因:片岩的强度低,遇水易软化;洞口处的浅埋段成拱性差;偏压现象普遍;仰拱不均匀沉降;岩石受地下水的影响及岩体的蠕变效应.最后,根据开裂原因提出相应的增加围岩级别、加强洞口施工及重视小净距中夹岩稳定设计等建议措施,为类似工程提供相关参考依据.
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